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Title: Test your grounds regularly!
Post by: Jonathan Johnson on November 22, 2014, 02:56:47 PM
We test our smoke detectors.

Our cars do a self-test on the airbags every time we start the engine. We make sure the brake pads and tires aren't worn out.

We inspect fall arrest equipment regularly.

We inspect fire extinguishers annually.

We make sure our first-aid kits are kept stocked.

We test or inspect other safety equipment that we use, on a regular basis.

But we never test the safety grounds in our electrical systems, until there is an unfortunate incident.

Because the safety ground does not normally carry current, a failure of the system will not be obvious until someone receives a shock, is injured or killed, or there is equipment or property damage.

Since the National Electrical Code, electrical inspectors, and audio/video/computer engineers recognize the importance and safety of proper grounding, doesn't it stand to reason that it's a safety device that should be inspected and tested on a regular basis, just like every other safety device we use? But yet we treat it as "set-it-and-forget-it."

Test those grounds as if your life depends on it, because it does.
Title: Re: Test your grounds regularly!
Post by: Jonathan Johnson on November 22, 2014, 02:58:58 PM
Hmm. I suppose that means I just gave myself a job, checking the grounds of all the receptacles on my farm.
Title: Re: Test your grounds regularly!
Post by: James Wright on November 22, 2014, 04:18:25 PM
We do in the UK.

http://en.wikipedia.org/wiki/Portable_appliance_testing

Title: Re: Test your grounds regularly!
Post by: Steve M Smith on November 22, 2014, 05:17:06 PM
We do in the UK.

http://en.wikipedia.org/wiki/Portable_appliance_testing (http://en.wikipedia.org/wiki/Portable_appliance_testing)

Well, some of us do.  Everything which comes into our factory has been tested but I have never turned up at a venue with a band or PA and been asked by the owner if any equipment has been tested and it's not a legal requirement.

And it really annoys me that it gets referred to as a PAT test when the T in PAT is for test.  That's as annoying as PCB board!


Steve.
Title: Re: Test your grounds regularly!
Post by: Mike Sokol on November 22, 2014, 07:21:47 PM
Well, some of us do.  Everything which comes into our factory has been tested but I have never turned up at a venue with a band or PA and been asked by the owner if any equipment has been tested and it's not a legal requirement.

And it really annoys me that it gets referred to as a PAT test when the T in PAT is for test.  That's as annoying as PCB board!


Steve.
Or SPL level?  :o
Title: Re: Test your grounds regularly!
Post by: Ray Aberle on November 22, 2014, 09:25:37 PM
Or SPL level?  :o
From the Department of Repetitive Redundancy Department
PIN Number
ATM Machine
Tuna Fish
Title: Re: Test your grounds regularly!
Post by: Stephen Swaffer on November 22, 2014, 10:37:08 PM

Test those grounds as if your life depends on it, because it does.

Easy to say and hard to disagree with-but IMO much harder to effectively do.  I don't consider a 3 light tester or a NCVT adequate for "safety" testing-they are good diagnostic tools and can be effective in assuring that everything is as we expect it to be-but if I am putting myself in harms way, I will use something more robust as a primary test-opening a breaker/switch that I can verify disconnects, or any of a dozen other things.

To play devils advocate, I could place a 100mA fuse in an EGC and it would test fine all day long-until an incident happens.  True, a very unlikely scenario, but a corroded or loose connection could create the same issue.  I am for safety testing-but I detest testing that merely makes me (or management) feel good.  Any suggestions on a ground test that would be reasonably robust, while still being convenient?
Title: Re: Test your grounds regularly!
Post by: Mike Sokol on November 22, 2014, 11:15:23 PM
Easy to say and hard to disagree with-but IMO much harder to effectively do.  I don't consider a 3 light tester or a NCVT adequate for "safety" testing-they are good diagnostic tools and can be effective in assuring that everything is as we expect it to be-but if I am putting myself in harms way, I will use something more robust as a primary test-opening a breaker/switch that I can verify disconnects, or any of a dozen other things.

To play devils advocate, I could place a 100mA fuse in an EGC and it would test fine all day long-until an incident happens.  True, a very unlikely scenario, but a corroded or loose connection could create the same issue.  I am for safety testing-but I detest testing that merely makes me (or management) feel good.  Any suggestions on a ground test that would be reasonably robust, while still being convenient?

An INSP-3 or SureTest Analyzer would do this test nicely, but unfortunately won't find an RPBG without a secondary test such as measuring for EGC voltage referenced to a known external ground using a wire run to the service panel's ground or a NCVT which references earth ground through your own body. These ground impedance testers cost about $300 which is too expensive for casual usage but certainly affordable for a certified inspection. 

In all cases the test gear is only as good as the person doing the test. So training a technician what to look for and how to interpret the results is just as important as having the right (calibrated) gear.
Title: Re: Test your grounds regularly!
Post by: Debbie Dunkley on November 22, 2014, 11:30:54 PM
From the Department of Repetitive Redundancy Department
PIN Number
ATM Machine
Tuna Fish

Here in the U.S. folks say "cod fish" too.  In the UK folks just say "cod".
Title: Re: Test your grounds regularly!
Post by: Lyle Williams on November 23, 2014, 01:23:04 AM
PAT testing verifies that RCD/GFCI works.  On site "trip button" testing of RCD/GFCI tells you that ground isn't open circuit.  Not foolproof though.
Title: Re: Test your grounds regularly!
Post by: Scott Holtzman on November 23, 2014, 04:38:45 AM
No, people from the South (sorry deb NC is still the South)  say Cod Fish.  I grew up in the South and used to day it myself until I moved to the North and can order Broiled Cod.  I also learned that soda is fizzy water (aqua sin gas) and Pop is a carbonated flavored bevorage.
Title: Re: Test your grounds regularly!
Post by: Mike Sokol on November 23, 2014, 07:41:29 AM
No, people from the South (sorry deb NC is still the South)  say Cod Fish.  I grew up in the South and used to day it myself until I moved to the North and can order Broiled Cod.  I also learned that soda is fizzy water (aqua sin gas) and Pop is a carbonated flavored bevorage.
All of which you can enjoy while sitting on your davenport...  8)
Title: Re: Test your grounds regularly!
Post by: Kevin Graf on November 23, 2014, 09:11:54 AM
PAT testing verifies that RCD/GFCI works.  On site "trip button" testing of RCD/GFCI tells you that ground isn't open circuit.  Not foolproof though.
I'll skip the Portable Appliance Testing part as I'm not aware of anything like that in the US.
Now as to the GFCI "trip button" testing,  this test has nothing to do with the EGC Safety Ground circuit.  It only looks at the current in the Hot and Neutral conductors.
Title: Re: Test your grounds regularly!
Post by: John Roberts {JR} on November 23, 2014, 09:21:24 AM
No, people from the South (sorry deb NC is still the South)  say Cod Fish.  I grew up in the South and used to day it myself until I moved to the North and can order Broiled Cod.  I also learned that soda is fizzy water (aqua sin gas) and Pop is a carbonated flavored bevorage.
After I moved to MS I teased relatives in NC that now they were yankees... :o

Regarding fizzy (carbonated) water, and it has been decades since I studied latin in HS, "aqua sin gas" sounds like still water, or water "without" gas, water with gas would be "aqua cum gas",,,, While gas does not sound like an actual latin word.

JR
Title: Re: Test your grounds regularly!
Post by: Debbie Dunkley on November 23, 2014, 09:46:55 AM
No, people from the South (sorry deb NC is still the South)  say Cod Fish.  I grew up in the South and used to day it myself until I moved to the North and can order Broiled Cod.  I also learned that soda is fizzy water (aqua sin gas) and Pop is a carbonated flavored bevorage.

I wondered that Scott because when I lived in CA, I don't remember them saying cod fish.
I say fizzy water - always did in the UK but no-one here knows what I mean. They say soda here in NC and CA but I understand its pop in the mid west.

BTW……how does a thread go from testing grounds regularly to food and drink references???
It is one of the reasons I love this forum…..

Sorry Jonathan for the thread hijack…...
Title: Re: Test your grounds regularly!
Post by: Lyle Williams on November 23, 2014, 09:53:08 AM
I'll skip the Portable Appliance Testing part as I'm not aware of anything like that in the US.
Now as to the GFCI "trip button" testing,  this test has nothing to do with the EGC Safety Ground circuit.  It only looks at the current in the Hot and Neutral conductors.

The test button shorts active to earth through a resistor.  If there is no earth, there should be no active neutral imbalance and hence no trip.
Title: Re: Test your grounds regularly!
Post by: Steve M Smith on November 23, 2014, 10:53:21 AM
I'll skip the Portable Appliance Testing part as I'm not aware of anything like that in the US.

It's quite a good idea.  Despite not being obliged legally, most companies will not allow any electrical equipment brought onto their premises to be used without a test.

This goes equally for equipment bought or hired for company use or something such as the power supply for a laptop a visitor might bring.

The test is quick and does not need to be done by a qualified electrician... and you get a little green sticker with a date on which says it's due for a retest in a year's time so you don't need to have it done for every visit.

The machine does all the work.  You just plug it in and press go!

(http://img-europe.electrocomponents.com/images/R512571-01.jpg)


BTW……how does a thread go from testing grounds regularly to food and drink references???

In this case, I have no idea.  However, I am often guilty of sending forum threads off on tangents.  Speaking of tangents...

Steve.
Title: Re: Test your grounds regularly!
Post by: Kevin Graf on November 23, 2014, 12:17:13 PM
The test button shorts active to earth through a resistor.  If there is no earth, there should be no active neutral imbalance and hence no trip.
No, what is does is bypass the GFCI sensor circuit. Safety Ground EGC/PE has nothing to do with the test.  GFCI receptacle's are often used with older 2 wire circuits (it allows the use of 3 wire plugs) and even with no ground/earth connection the GFCI works.

The only question the GFCI asks is:
Are the Hot & Neutral currents identical and opposite in phase (±5mA).  The unit does not ask 'why is there a current difference?'.
Title: Re: Test your grounds regularly!
Post by: Jonathan Johnson on November 23, 2014, 12:42:29 PM
Regarding fizzy (carbonated) water, and it has been decades since I studied latin in HS, "aqua sin gas" sounds like still water, or water "without" gas, water with gas would be "aqua cum gas",,,, While gas does not sound like an actual latin word.

When you order plan bottled water in Russia, you must specifically request "voda biz gaza" (water without gas) else you will get carbonated water. Apparently they prefer the gassy stuff there.
Title: Re: Test your grounds regularly!
Post by: Lyle Williams on November 23, 2014, 01:50:02 PM
No, what is does is bypass the GFCI sensor circuit. Safety Ground EGC/PE has nothing to do with the test.  GFCI receptacle's are often used with older 2 wire circuits (it allows the use of 3 wire plugs) and even with no ground/earth connection the GFCI works.

The only question the GFCI asks is:
Are the Hot & Neutral currents identical and opposite in phase (±5mA).  The unit does not ask 'why is there a current difference?'.

Yes, that turns out to be correct.  I have an old RCD where the test button is active-to-earth, but does seem that all my contemporary ones test without going to earth. 

My external (plug in) RCD tester works by shunting active to earth.
Title: Re: Test your grounds regularly!
Post by: Ray Aberle on November 23, 2014, 10:38:54 PM
BTW……how does a thread go from testing grounds regularly to food and drink references???
It is one of the reasons I love this forum…..

My Bad. I was going on the whole "TLA that then has a word after that's also one of the letters in the TLA." thing, and mentioned Tuna Fish. Tuna... is a fish.... duhh....
Title: Re: Test your grounds regularly!
Post by: Jonathan Johnson on November 23, 2014, 10:58:45 PM
Tuna... is a fish.... duhh....

Or a Tuna Boat. (But that's used for catching Tuna... fish.)

You can tuna fish, but you can't teach it to sing.
Title: Re: Test your grounds regularly!
Post by: Mike Sokol on November 24, 2014, 01:00:32 AM
The only question the GFCI asks is: Are the Hot & Neutral currents identical and opposite in phase (±5mA).  The unit does not ask 'why is there a current difference?'.

Yes, it's a common misconception that the GFCI considers the ground wire current. As you note, it's only looking for equal current in the Hot and Neutral wires. Now, if some of the Hot current is diverted to the Ground (EGC) wire, that will cause the GFCI to trip. But it's not tripping because of the current in the EGC Ground, it's tripping because of the H-N current imbalance caused by this diversion. You need to keep this in mind when troubleshooting GFCI tripping. Here's a link to a GFCI primer and graphic on how this works. http://www.noshockzone.org/rv-electrical-safety-part-viii-gfci/ (http://www.noshockzone.org/rv-electrical-safety-part-viii-gfci/)
Title: Re: Test your grounds regularly!
Post by: Lyle Williams on November 24, 2014, 02:50:48 AM
I never thought GFCI/RCD considered ground current.  I thought that the simplest way to implement the test button and create a active/neutral imbalance was to shunt current to ground, but that isn't the way it is done these days.
Title: Re: Test your grounds regularly!
Post by: Steve M Smith on November 24, 2014, 03:05:04 AM
I never thought GFCI/RCD considered ground current.  I thought that the simplest way to implement the test button and create a active/neutral imbalance was to shunt current to ground, but that isn't the way it is done these days.
If all you have is live, neutral and ground, how else can you do it?


Steve.
Title: Re: Test your grounds regularly!
Post by: Steve Bradbury on November 24, 2014, 07:32:23 AM
The device doesn’t care what causes the imbalance in current. If you connect a resistor from the input supply to the load a current will bypass the device and, depending on the resistance value, trip the device. Testing in this manner means that the device can be checked without relying on an external earth.

Regarding the PATest, the current (sorry) standards specify that the stickers should not have a next test date on them simply the date of the latest test.

This is to clarify that there is no specified test period and because testing companies were taking advantage of the confusion and over testing equipment. I recently (6 months ago) went on a general health and safety course run by my local authority and the guy running the course got this wrong. It made me wonder what other errors he made, but it got me a certificate that says I have core H&S knowledge.  I digress…

The test period should be risk assessed. A computer sitting in a server room which is never turned off is not subject to the same abuse as an amplifier that is dry hired to a different customer every day. The test periods should reflect the type of use and abuse that the equipment is subject to. With the latter prior to every hire would be sensible.

Someone mentioned tests not straining equipment to the same level as actual faults. With some PAT equipment (generally mains powered) the earth bond test can be set up to 25A although IT or business equipment is tested at 100mA. Most testers also hold the test for a short preset period of time. Standard insulation tests were usually done at 500V but increasingly, due to concern about damaging electronic equipment, 250V can be selected.

Interestingly most literature I read suggests that most faults are picked up on by the visual check.

Disclaimer: I did electrical engineering at university a long time ago but as I approach senility I consider myself more as an informed enthusiast. Several years ago I attended a PAT training course run by Seaward.  The level of electrical knowledge required to pass was minimal.
Title: Re: Test your grounds regularly!
Post by: Lyle Williams on November 24, 2014, 11:49:41 AM
If all you have is live, neutral and ground, how else can you do it?


Steve.

The test button and resistor either sits diagonally across the current imbalance sensor, from live on the load side to neutral on the supply side allowing some current to bypass the sensor,
  or
The test button and resistor are connected to an additional winding in the current imbalance sensor.
Title: Re: Test your grounds regularly!
Post by: Lyle Williams on November 24, 2014, 12:03:10 PM
The Australian PAT standards do specify durations, but they are somewhat risk based.  Equipment in a residential environment might be allowed five yearly testing, while hire gear needs monthly visual and three-monthly electrical tests.

The standards also have a few options for earth continuity testing within an appliance, including a high current test.  The high current test methodology isn't mandatory, so is rarely done in practice.

All of that assumes that there is an earth available from the wall socket.
Title: Re: Test your grounds regularly!
Post by: John Roberts {JR} on November 24, 2014, 12:52:00 PM
If all you have is live, neutral and ground, how else can you do it?


Steve.
The value of GFCI/RCD is that they work independently of ground. I installed GFCI in my bathroom and kitchen precisely because my house wiring does not have solid safety grounds.

An external test would logically have to divert current elsewhere, and internal test button can imbalance the internal detection mechanism.

JR
Title: Re: Test your grounds regularly!
Post by: Steve M Smith on November 24, 2014, 01:44:14 PM
I think I have it now.  The test switch must place a load diagonally. i.e. from one conductor at the input to the other one at the output so its current only appears in one half of the sense circuit causing the imbalance.


Steve.
Title: Re: Test your grounds regularly!
Post by: Mike Sokol on November 24, 2014, 01:50:47 PM
I think I have it now.  The test switch must place a load diagonally. i.e. from one conductor at the input to the other one at the output so its current only appears in one half of the sense circuit causing the imbalance.


Steve.
Yup, these work like a balanced XLR input on a mixing board using current instead of voltage. If anything happens to disturb the perfect null of the current sensing transformer, and it reaches over 6 mA of imbalance, then the trip circuit is triggered. So leaking current from one of the legs around the transformer creates this imbalance.

A GFCI/RCD will also trip if you provide outside current on the neutral, say from multiple branch circuits with their neutrals connected together or even the neutral and ground wires swapped in a receptacle (yes, that happens). Another good reason that interconnecting neutrals is a code violation.
Title: Re: Test your grounds regularly!
Post by: Bill McKelvey on November 24, 2014, 03:58:07 PM
From the Department of Repetitive Redundancy Department
PIN Number
ATM Machine
Tuna Fish
VIN number
Title: Re: Test your grounds regularly!
Post by: g'bye, Dick Rees on November 24, 2014, 03:59:27 PM
VIN number

VIN ordinal or vin ordinaire?
Title: Re: Test your grounds regularly!
Post by: Bill McKelvey on November 24, 2014, 04:07:57 PM

Since the National Electrical Code, electrical inspectors, and audio/video/computer engineers recognize the importance and safety of proper grounding, doesn't it stand to reason that it's a safety device that should be inspected and tested on a regular basis, just like every other safety device we use? But yet we treat it as "set-it-and-forget-it."

Test those grounds as if your life depends on it, because it does.

I replace the receptacles in my stage boxes every other year as I do a lot of outdoor events and the outlets corrode making a poor connection. Found out the hard way the first time I had a corroded outlet.
Title: Re: Test your grounds regularly!
Post by: Kevin Graf on November 25, 2014, 09:27:13 AM
The instructions for a GFCI receptacle or circuit breaker state that the unit must be tested regularly.  Yet the only times that they are tested is when the button is tripped by accident.
Title: Re: Test your grounds regularly!
Post by: Mike Sokol on November 25, 2014, 10:55:53 AM
The instructions for a GFCI receptacle or circuit breaker state that the unit must be tested regularly.  Yet the only times that they are tested is when the button is tripped by accident.
I hate it when I trip a GFCI receptacle that's upstream on the branch circuit, and nobody can tell you where that receptacle is located. Usually it's hiding behind some cabinet or piece of furniture, so nobody has ever seen (or tested) it.   
Title: Re: Test your grounds regularly!
Post by: Frank DeWitt on November 25, 2014, 10:58:24 AM
The instructions for a GFCI receptacle or circuit breaker state that the unit must be tested regularly.  Yet the only times that they are tested is when the button is tripped by accident.

The only one of mine I don't test regularly is the one on my automatic start generator. Press the button. the GFCI trips, the load goes away and the generator stops,  Press the reset button,  No.  GFCIs need AC in order to reset.  Bummer.
Title: Re: Test your grounds regularly!
Post by: Guy Holt on November 25, 2014, 12:07:42 PM
An external test would logically have to divert current elsewhere, and internal test button can imbalance the internal detection mechanism.

In fact it is the opposite, the test circuit adds current passing through the CT to create an imbalance. As you can see in the illustration of a GFCI test circuit in the schematic below it is a pretty simple circuit consisting of a contact switch and Current Limiting Resister. How it works is that one conductor of the test circuit runs through the CT while the other does not, which means that the current drawn by the Current Limiting Resister when the test button is depressed adds to the current passing through the CT on the return side but not the supply creating the imbalance that trips the GFCI.

(http://www.screenlightandgrip.com/images/generators/SB_GFCI_Diagramdetail.jpg)

As such, GFCI test circuits are misleading when they are used on Floating Neutral generators, like the Honda EUs, because they can produce a false positive.  On a Floating Neutral generator, even though there is no ground fault circuit for fault current to go to, the Current Limiting Resister will still draw current on the Hot outside the CT and return it through the CT on the Neutral. The discrepancy caused by the Current Limiting Resister in the test circuit (illustrated above) will initiate the GFCI to trip even though there is in fact no Ground Fault Circuit for Fault Current to go to if there were a Fault. The false positive received by GFCI test circuits on Floating Neutral generators does nothing to eliminate hazardous conditions.

Guy Holt, Gaffer
ScreenLight & Grip
www.screenlightandgrip.com
Title: Re: Test your grounds regularly!
Post by: John Roberts {JR} on November 25, 2014, 12:26:31 PM
In fact it is the opposite, the test circuit adds current passing through the CT to create an imbalance. As you can see in the illustration of a GFCI test circuit in the schematic below it is a pretty simple circuit consisting of a contact switch and Current Limiting Resister. How it works is that one conductor of the test circuit runs through the CT while the other does not, which means that the current drawn by the Current Limiting Resister when the test button is depressed adds to the current passing through the CT on the return side but not the supply creating the imbalance that trips the GFCI.

(http://www.screenlightandgrip.com/images/generators/SB_GFCI_Diagramdetail.jpg)

A semantic distinction with no real difference. 

The internal test button does indeed imbalance the internal detection mechanism as the schematic shows. 
Quote


As such, GFCI test circuits are misleading when they are used on Floating Neutral generators, like the Honda EUs, because they can produce a false positive.  On a Floating Neutral generator, even though there is no ground fault circuit for fault current to go to, the Current Limiting Resister will still draw current on the Hot outside the CT and return it through the CT on the Neutral. The discrepancy caused by the Current Limiting Resister in the test circuit (illustrated above) will initiate the GFCI to trip even though there is in fact no Ground Fault Circuit for Fault Current to go to if there were a Fault. The false positive received by GFCI test circuits on Floating Neutral generators does nothing to eliminate hazardous conditions.

Guy Holt, Gaffer
ScreenLight & Grip
www.screenlightandgrip.com

Semantics aside the GFCI/RCD will trip whenever current does not follow the proscribed current return path. It does not really matter where the current strays to, anywhere other than where it should be flowing is a fault and safety hazard.

The only kind of shock hazard that returns 100% of the hot current to neutral is if the human gets across the hot and neutral. These devices will not protect against that.   

JR
Title: Re: Test your grounds regularly!
Post by: Guy Holt on November 25, 2014, 01:13:02 PM
... GFCI/RCD will trip whenever current does not follow the proscribed current return path. It does not really matter where the current strays to....

In a double fault situation current can stray and yet not trip a GFCI/RCD.  In an IATSE Local 481 workshop that I give on ground fault protection, I demonstrate this by simulating a double fault situation (like what would happen if a defective cord was dragged through the mud) with an unbounded, ungrounded Honda EU6500.
 
(http://www.screenlightandgrip.com/images/generators/SB_Header.jpg)

We first create a fault in the Hot by attaching a jumper cable from the Hot pocket of a little patch box I made up to the input of a Fault Simulator that basically consists of a variable resister. So that we can obtain precise measurements of leakage current, we attach a second jumper from the output of the Fault Simulator to the input of a Fluke 1587 Insulation Multimeter, and then a third jumper cable from the output of the Fluke 1587 to one of two ground rods we drive.

(http://www.screenlightandgrip.com/images/generators/481_GFCI_Wkshp_Demo_Set_Up_Sm.jpg)

To create a second fault we jumper from the neutral pocket of the patch box to the input of a box with just a switch and another jumper from the output of the switch box to another ground rod. We start by opening the switch on the switch box and closing the Fault Simulator (maximum resistance) so that we can regulate the current leaking to earth.

(http://www.screenlightandgrip.com/images/generators/481_GFCI_Wkshp_Gen_SetUp3.jpg)

When we fired up the generator and supply power to our fault circuit, we see immediately on the Fluke 1587 that there is 4.9mAmps leaking to earth. Clamping a Megger DCM300E Leakage meter onto the jumper going from the Neutral pocket of the patch box to the second ground rod, we see that the Fault Current is returning to the generator's windings through the second fault that we established on the Neutral (as depicted in the illustration below.)

(http://www.screenlightandgrip.com/images/generators/SB_Test_Double_Fault.jpg)

As we gradually reduce the resistance of the Fault Simulator by turning its' rheostat, the leakage current begins to rise. When the Fault Simulator is all the way open (minimum resistance) we get a clear Ground Fault of 7.9mAmps (according to the readout of the Fluke) and the breaker does not trip. Closing the switch on the Switch Box so that there is only one fault in our system, the current leakage to earth stops. Opening the switch on the switch box, the leak begins again.

The purpose of this demonstration is to show that a double fault - one in the Hot, and a second in the Neutral - creates a potentially hazardous situation because a path (circuit) now exists for fault current to return to the generator windings even though there is no neutral to ground bond. If an individual comes into contact with this ground fault circuit created by the two faults, fault current will travel through the individual on its' way back to the generator's windings (use this link - http://www.screenlightandgrip.com/html/481_GFCI_Workshop.html (http://www.screenlightandgrip.com/html/481_GFCI_Workshop.html) - for  a complete summary of my IA workshop on Ground Fault Protection.)

Guy Holt, Gaffer
Screenlight & Grip
www.screenlightandgrip.com
Title: Re: Test your grounds regularly!
Post by: Lyle Williams on November 25, 2014, 01:30:32 PM
I hate it when I trip a GFCI receptacle that's upstream on the branch circuit, and nobody can tell you where that receptacle is located. Usually it's hiding behind some cabinet or piece of furniture, so nobody has ever seen (or tested) it.

Yes, that totally sucks.  For testing portable RCD/GFCI with the external (shunt to ground) tester I need an isolation transformer to prevent upstream trips.
Title: Re: Test your grounds regularly!
Post by: John Roberts {JR} on November 25, 2014, 01:57:40 PM
As I already said a hot to neutral fault will not be protected.

I do not have any perspective on how common such a double fault is. In grounded systems neutral is already connected to earth so a hot shock hazard (to earth) exists normally, while hopefully protected by GFCI. In consumer electronic products, ungrounded mains power is generally double insulated because of the increased shock hazard.

This reinforces my personal preference for local GFCI protected outlet strips for back line use, and grounding generator neutral (before the GFCI/RCD). While no system is completely human-proof and I am not working in the trenches like you guys.

If the generator neutral was well bonded to earth before the GFCI/RCD, the neutral to earth fault could trip a sensitive GFCI before any humans were put at risk due to neutral current flowing outside the GFCI loop via the earth path. That said earth may not be reliably low enough resistance to always trip the GFCI during light duty power distribution and/or full system GFCI/RCD may use higher trip currents to avoid annoyance trips. 

Sounds like another good reason to ground your generators, I hope I don't have a double fault in my bathroom or kitchen that would circumvent my GFCI outlets, but my neutral is grounded to earth (I know it is from the last time I got a shock in my yard from a miswired extension cord via the earth return path.).   :o 

JR
Title: Re: Test your grounds regularly!
Post by: Stephen Swaffer on November 26, 2014, 10:50:27 AM
Current code requires GFCIs to be accessible for testing purposes-of course we all know code is always followed-and of course there are the grandfathered ones to deal with.

I understand the premise of the false positive on an ungrounded genny and agree with JR that is another good reason to ground gennys every time.  I still say better to have a GFCI that gives a false positive than no GFCI-does it give a false sense of safety?  Personally, you need to use the same care and attention to safe practices on a GFCI protected circuit as you do on a non GFCI protected circuit-for the same reason you don't point a gun at someone whether the safety is on or off -the GFCI and the safety are additional lines of defense not the primary safety device.
Title: Re: Test your grounds regularly!
Post by: Guy Holt on November 28, 2014, 08:09:12 PM
The value of GFCI/RCD is that they work independently of ground.

There is an ongoing debate on this point. Some people believe that GFCIs will function regardless of the grounding arrangement of the power source, while others believe the opposite. To settle the debate, I developed an exercise for the Ground Fault Protection workshop I teach for Local 481 to see what effect earth grounding of generators has on the operation of GFCIs. For this exercise we set up a double fault situation and then test the GFCI’s response level when the system is grounded and ungrounded. The results were that earth grounding of bonded generators greatly increased the effectiveness of GFCIs in double fault situations.

(http://www.screenlightandgrip.com/images/generators/SB_Test_Bonded_Full_Grd.jpg)

As illustrated in the crude schematic of our set-up above, the ground rod creates a definite Ground Fault Circuit, splitting the leak current, and making it impossible for balance to be restored to the system by all of the Fault Current returning to the Neutral through the second Fault before passing back through the GFCI. By diverting some of the Fault Current around the second fault, the ground rod assures that there will be an imbalance in the current traveling through the GFCI on the return side that will make the GFCI trip.

Increasing the effectiveness of GFCIs in double fault situations is the most compelling argument I know for earth grounding generators (at least in instances where GFCIs are being used.) To learn more about this exercise use this link  - http://www.screenlightandgrip.com/html/481_GFCI_Workshop.html  (http://www.screenlightandgrip.com/html/481_GFCI_Workshop.html)- for my Ground Fault Protection workshop.

Guy Holt, Gaffer
ScreenLight & Grip
rentals@screenlightandgrip.com
Title: Re: Test your grounds regularly!
Post by: Rob Spence on December 09, 2014, 03:42:06 PM
I hate it when I trip a GFCI receptacle that's upstream on the branch circuit, and nobody can tell you where that receptacle is located. Usually it's hiding behind some cabinet or piece of furniture, so nobody has ever seen (or tested) it.

I hate that the GFCI receptacle is only $15 but the one in the panel is $35.


Sent from my iPad using Tapatalk HD
Title: Re: Test your grounds regularly!
Post by: Rob Spence on December 09, 2014, 03:44:59 PM
The only one of mine I don't test regularly is the one on my automatic start generator. Press the button. the GFCI trips, the load goes away and the generator stops,  Press the reset button,  No.  GFCIs need AC in order to reset.  Bummer.

Where is the GFCI in your system?
My standby unit has a GFCI receptacle on it but a regular 90a two pole breaker for the load.


Sent from my iPad using Tapatalk HD
Title: Re: Test your grounds regularly!
Post by: Steve Bradbury on December 09, 2014, 05:15:11 PM
I’m not sure if this has been covered (apologies if it has), but with a combined neutral-earth system a break in the neutral can cause a situation where equipment becomes live and will not be protected by a RCD/GFCI. The picture below shows the situation.

(http://i62.tinypic.com/2cr98xk.jpg)

The yellow ring represents the RCD and because any current flowing through the earth cable and case also flows through the RCD neutral circuit it will not trip. The break in the neutral also stops the device working so it could be assumed that the circuit is dead. Anyone touching the case will form a potential divider with the load impedance. It is something to be aware of and once again a non-contact voltage detector would show the case was live.
Title: Re: Test your grounds regularly!
Post by: Jonathan Johnson on December 09, 2014, 05:39:59 PM
I’m not sure if this has been covered (apologies if it has), but with a combined neutral-earth system a break in the neutral can cause a situation where equipment becomes live and will not be protected by a RCD/GFCI. The picture below shows the situation.

(http://i62.tinypic.com/2cr98xk.jpg)

The yellow ring represents the RCD and because any current flowing through the earth cable and case also flows through the RCD neutral circuit it will not trip. The break in the neutral also stops the device working so it could be assumed that the circuit is dead. Anyone touching the case will form a potential divider with the load impedance. It is something to be aware of and once again a non-contact voltage detector would show the case was live.

The problem with your diagram is that it assumes the EGC (Equipment Grounding/Earthing Conductor) and GEC (Grounding Electrode Conductor/earthing system) do not have a single, common bonding point, as required by the National Electrical Code in the United States. (Your diagram does not even show the GEC.)

With a single, common bonding point, a break in the neutral upstream of the bonding point will not be a hazard, because the equipment chassis and the dirt (or building structure, etc.) will be at or near the same potential because they will remain bonded. If the EGC remains bonded to the dirt, the GFI won't trip because of this fault, but there is no hazard unless there is also fault in the GEC.

Without a common bonding point, you have a bootleg ground. This highlights one reason why bootleg grounds are more dangerous than no ground at all.

In order for this to be a hazard in a properly wired system, it would require a physical break in the neutral/grounding bus of the service entrance!

Like I've said before, nearly all electrical incidents require TWO faults, one in the primary system and one in the safety system. Fault number one is the broken neutral (primary). Fault number two is the bootleg ground (safety). (I consider an intentional miswire to be a fault.)
Title: Re: Test your grounds regularly!
Post by: Steve Bradbury on December 09, 2014, 08:09:33 PM
Quote
The problem with your diagram is that it assumes the EGC (Equipment Grounding/Earthing Conductor) and GEC (Grounding Electrode Conductor/earthing system) do not have a single, common bonding point, as required by the National Electrical Code in the United States. (Your diagram does not even show the GEC.)

Most countries are not covered by the United States National Electrical Code. In some regions, it is common for the electricity distributer to use a common earth and neutral between the substation and the consumer’s premises. At the point of entry the circuit protective conductors are separated (TN-C-S). Whilst these are usually bonded to water and/or gas pipes, which form a secondary, parallel, local earth, it cannot always be guaranteed. A break in the supply neutral external to the premises could result in the situation above.

It is not a common fault, but neither is it an intentional miss-wire. A TN-C-S system doesn’t have a separate ground conductor which would be referred to as a TT system where the earth is provided locally, which is why I didn’t include one in the diagram.
Title: Re: Test your grounds regularly!
Post by: Stephen Swaffer on December 10, 2014, 01:06:15 PM
Steve,

What do the initials stand for in TN-C-S and TT?  Probably obvious but I am drawing a blank.

Typically in the US covered by NEC, the only wires the POCO brings to the premisis are hot (energized) and neutral (grounded conductor)-three wires for a single phase service/4 for a 3 phase (usually).  The Safety or earth ground is done locally (though the POCO typically ties their neutral to ground in many locations during distribution.
Title: Re: Test your grounds regularly!
Post by: Steve Bradbury on December 10, 2014, 04:02:49 PM
T = Earth (from French Terre)
N = Neutral
C = Combined
S = Separate
I = isolated
CPC = circuit protective conductor (earth wire)
PME = Protective multiple earthing
DNO = Distribution Network Operators

So TN-C-S = Earth/Neutral-combined-separate.
A TN-S would have separate earth and neutral conductors back to the substation. The earth would typically be the armoured shield of the supply cable.

The IET (formerly IEE) have a leaflet that explains it all. If you type “earthing your questions answered” into Google you should find it. When you click on the link it just downloads a pdf so I couldn’t find a direct link.
Title: Re: Test your grounds regularly!
Post by: Tim McCulloch on December 10, 2014, 04:12:58 PM
T = Earth (from French Terre)
N = Neutral
C = Combined
S = Separate
I = isolated
CPC = circuit protective conductor (earth wire)
PME = Protective multiple earthing
DNO = Distribution Network Operators

So TN-C-S = Earth/Neutral-combined-separate.
A TN-S would have separate earth and neutral conductors back to the substation. The earth would typically be the armoured shield of the supply cable.

The IET (formerly IEE) have a leaflet that explains it all. If you type “earthing your questions answered” into Google you should find it. When you click on the link it just downloads a pdf so I couldn’t find a direct link.

http://electrical.theiet.org/wiring-matters/16/earthing-questions.cfm?type=pdf
Title: Re: Test your grounds regularly!
Post by: Kevin Graf on December 12, 2014, 09:39:47 AM
Jim Brown has a 3 page summary:

POWER SYSTEM ARCHITECTURES IN EUROPE AND THE AMERICAS
Five different systems are used for the distribution of electric power in public 'low voltage'
systems, and these may also be used in private systems. These systems differ primarily
in how they treat the grounded conductor (neutral) and it's relationship to the
Protective Earth (PE) conductor. ('Low voltage' in this context means, in practice, systems
with phase voltages between 100 V and 240 V.)


In his paper:

Power and Grounding for Audio and Video Systems
A White Paper for the Real World – International Version
Jim Brown
Audio Systems Group, Inc.

http://www.audiosystemsgroup.com/SurgeXPowerGround.pdf

More Jim Brown papers:

http://www.audiosystemsgroup.com/publish.htm
Title: Re: Test your grounds regularly!
Post by: John Roberts {JR} on December 18, 2014, 11:13:01 AM
I don't know if this is the right thread, but I got a little tingle the other day. I was working on my RO filter under my sink and had a trouble light plugged into a GFCI outlet nearby, while that outlet has no ground connection, since my house wiring is old.

While reaching across to grab my water faucet, my arm was touching the grounded bail from my trouble light, I felt the unmistakeable tingle of electrons and holes moving through my arm. It was pretty low level but I quickly released and decided to meter between the two exposed metal parts. My high impedance VOM measured 106V, I didn't bother to test for current and ASSume it was less than 6mA required to trip a GFCI. I finished the project at hand, but a day later decided to revisit to investigate further. To my surprise a day later I only measured 3V between the same grounds.

My suspicion is that something plugged into that GFCI circuit is leaking to the ground line. I tried to recreate exactly the same conditions but a day later low voltage.  It is not unusual to measure voltage on a floating ground with high impedance meter, but it is usually too little current to feel.

I still feel safe with the GFCI in place and am not quite sure why the sink was grounded since I replaced my steel water main from the street with ABS plastic a couple years ago (maybe there's more iron in my water than I thought. The water sediment filter is usually red). The panel in my laundry room may ground the plumbing there.

I am almost tempted to ground my kitchen outlet to the cold water pipe, while I am not sure where it is getting ground. I guess I could run a ground wire from my panel, but I don't feel like rewiring my  house.

JR
   
Title: Re: Test your grounds regularly!
Post by: Stephen Swaffer on December 18, 2014, 12:47:11 PM
If you have metallic plumbing in the house it should be grounded to the panel-which should have an grounding electrode/ground rods-might not hurt to make sure they didn't just rely on a metallic water supply pipe that no longer exists.  If the water system is grounded, it wouldn't be  an entirely bad idea to ground the GFCI receptacle there-since no ground is acceptable, any ground should be OK.

Another interesting test would be with a NCTV to see which part was hot (might show both)-which would help diagnosis.

Another thought, is a poor neutral connection in your panel can potentially energize anything that should be grounded-depending on grounding electrode/soil conditions and loads turned on in the house. 
Title: Re: Test your grounds regularly!
Post by: John Roberts {JR} on December 18, 2014, 01:53:07 PM
If you have metallic plumbing in the house it should be grounded to the panel-which should have an grounding electrode/ground rods-might not hurt to make sure they didn't just rely on a metallic water supply pipe that no longer exists.  If the water system is grounded, it wouldn't be  an entirely bad idea to ground the GFCI receptacle there-since no ground is acceptable, any ground should be OK.

Another interesting test would be with a NCTV to see which part was hot (might show both)-which would help diagnosis.

Another thought, is a poor neutral connection in your panel can potentially energize anything that should be grounded-depending on grounding electrode/soil conditions and loads turned on in the house.

I will look into this further... but a day later the voltage was only 3V so kind of odd...

JR
Title: Re: Test your grounds regularly!
Post by: Jonathan Johnson on December 18, 2014, 01:57:24 PM
I don't know if this is the right thread, but I got a little tingle the other day. I was working on my RO filter under my sink and had a trouble light plugged into a GFCI outlet nearby, while that outlet has no ground connection, since my house wiring is old.

While reaching across to grab my water faucet, my arm was touching the grounded bail from my trouble light, I felt the unmistakeable tingle of electrons and holes moving through my arm. It was pretty low level but I quickly released and decided to meter between the two exposed metal parts. My high impedance VOM measured 106V, I didn't bother to test for current and ASSume it was less than 6mA required to trip a GFCI. I finished the project at hand, but a day later decided to revisit to investigate further. To my surprise a day later I only measured 3V between the same grounds.

My suspicion is that something plugged into that GFCI circuit is leaking to the ground line. I tried to recreate exactly the same conditions but a day later low voltage.  It is not unusual to measure voltage on a floating ground with high impedance meter, but it is usually too little current to feel.

Possible scenario (don't have time to draw a diagram):

Upstream outlet is fed with a 2-wire cable, no ground.

There is a 3-wire cable (hot/neutral/ground) from the upstream outlet to the GFCI. The ground wire is connected to both the upstream outlet and the GFCI. (Maybe there isn't a 3-wire cable between the two outlets in your home, in which case the rest of this scenario is bunk.)

A device plugged into the upstream outlet with a three-prong cord has a hot-to-chassis fault, energizing the "ground" wire that runs between the upstream outlet and the GFCI. Because this ground wire has no connection to the neutral in the main panel or elsewhere, there is no path for the return current so the breaker does not trip, until...

You plug in your trouble light with a 3-wire cord into the GFCI, and the ground wire is bonded to the metal shell. You touch the shell and the faucet at the same time, and receive an electric shock. The GFCI does not trip, because the fault is upstream of the GFCI, and there is no imbalance measured between hot and neutral by the GFCI, and the GFCI does not monitor for current or voltage in the ground wire.

A day later, you unplugged whatever was upstream, or maybe it (the refrigerator?) wasn't operating at the time so the fault was not energized, resulting in only a 3V reading.
Title: Re: Test your grounds regularly!
Post by: Jonathan Johnson on December 18, 2014, 02:05:11 PM
A day later, you unplugged whatever was upstream, or maybe it (the refrigerator?) wasn't operating at the time so the fault was not energized, resulting in only a 3V reading.

P.S. -- I don't know if it was you or someone else who wired your kitchen, but the more I think about this (without enough information  :) ), the more I'm suspecting a fault in the compressor of the refrigerator. Here's why.

It's not uncommon for electricians to put the refrigerator outlet as the first in the circuit, then feed a GFCI on the countertop. You generally do not want the refrigerator on a GFCI (to prevent loss of food if it accidentally gets tripped). So maybe the electrician did that, but the home run he tapped into to energize that refrigerator receptacle happened to be a 2-wire cable. Since the new cable he installed between that and the GFCI would have been 3-wire, he dutifully connected the ground wire at both ends.

Then, your refrigerator compressor motor developed a hot-to-chassis ground fault. Because the ground wire in the circuit is not connect to neutral (at least isn't not a bootleg ground!), the fault does not trip the circuit breaker. Because the refrigerator is upstream of the GFCI, it does not trip the GFCI. And because it's a refrigerator, it cycles on and off -- giving you the varying readings from 106V to 3V.
Title: Re: Test your grounds regularly!
Post by: John Roberts {JR} on December 18, 2014, 02:35:26 PM
Possible scenario (don't have time to draw a diagram):

Upstream outlet is fed with a 2-wire cable, no ground.

There is a 3-wire cable (hot/neutral/ground) from the upstream outlet to the GFCI. The ground wire is connected to both the upstream outlet and the GFCI. (Maybe there isn't a 3-wire cable between the two outlets in your home, in which case the rest of this scenario is bunk.)

A device plugged into the upstream outlet with a three-prong cord has a hot-to-chassis fault, energizing the "ground" wire that runs between the upstream outlet and the GFCI. Because this ground wire has no connection to the neutral in the main panel or elsewhere, there is no path for the return current so the breaker does not trip, until...

You plug in your trouble light with a 3-wire cord into the GFCI, and the ground wire is bonded to the metal shell. You touch the shell and the faucet at the same time, and receive an electric shock. The GFCI does not trip, because the fault is upstream of the GFCI, and there is no imbalance measured between hot and neutral by the GFCI, and the GFCI does not monitor for current or voltage in the ground wire.

A day later, you unplugged whatever was upstream, or maybe it (the refrigerator?) wasn't operating at the time so the fault was not energized, resulting in only a 3V reading.
Found it..... the leakage path is a 3 wire plug electric kettle plugged directly into the GFCI outlet. I unplugged the  kettle to probe between the outlet ground and the sink...

Today with the kettle unplugged I measure 5V between grounds, with the kettle plugged in I measure 106V. Since I am on my way out to go shopping I think I will get a new kettle for Christmas.  ;D ;D ;D

Problem solved for now and hopefully if the shock was strong enough the GFCI would trip... We can stick a fork in this one... as always makes sense after the fact.

JR

Title: Re: Test your grounds regularly!
Post by: Mike Sokol on December 18, 2014, 02:43:33 PM
Found it..... the leakage path is a 3 wire plug electric kettle plugged directly into the GFCI outlet. I unplugged the  kettle to probe between the outlet ground and the sink...

Today with the kettle unplugged I measure 5V between grounds, with the kettle plugged in I measure 106V. Since I am on my way out to go shopping I think I will get a new kettle for Christmas.  ;D ;D ;D

Problem solved for now and hopefully if the shock was strong enough the GFCI would trip... We can stick a fork in this one... as always makes sense after the fact.

JR

This is exactly the sort of thing that can happen to an RV that has an open EGC on its shore power cord. Any inside appliance with a grounded plug can energize the RV's electrical ground plane, and feed through the entire electrical system grounds. It's what I call a Reflected Hot Skin Voltage since I like to come up with names for fails like this.
Title: Re: Test your grounds regularly!
Post by: Jonathan Johnson on December 18, 2014, 02:54:17 PM
We can stick a fork in this one...

Sorry, no can do. All my forks are 4-prong, and all my outlets are 3-prong.
Title: Re: Test your grounds regularly!
Post by: Mike Sokol on December 18, 2014, 03:56:12 PM
Sorry, no can do. All my forks are 4-prong, and all my outlets are 3-prong.

Guess I should have predicted this, but there's a band named "Fork In An Outlet" and their tunes are available at an on-line radio.
Title: Re: Test your grounds regularly!
Post by: Chris Hindle on December 18, 2014, 04:00:45 PM
Sorry, no can do. All my forks are 4-prong, and all my outlets are 3-prong.
Get thee a TRIDENT. ;D
Title: Re: Test your grounds regularly!
Post by: John Roberts {JR} on December 18, 2014, 04:14:07 PM
While far from definitive, my rat shack VOM measures off scale MOhms of resistance from either neutral or hot to ground, but does measure 0.27 nF from one pin and 0.13 nF from the other to ground so some minor capacitance but well low on the shock hazard scale.

======

I just returned from the store with my brand spanking new electric kettle. The best that $12.92 can buy at Walmart. I am pleased to note that my new kettle has a two wire line cord so should be pretty thin leakage to my ground now.

JR

PS: Don't plug in a fork with your bare hands... if a metal fork you could be shocked.
Title: Re: Test your grounds regularly!
Post by: Stephen Swaffer on December 18, 2014, 06:02:45 PM
Silly question-did you test the GFCI?  Don't need to lose any forum members!
Title: Re: Test your grounds regularly!
Post by: John Roberts {JR} on December 18, 2014, 06:37:59 PM
Silly question-did you test the GFCI?  Don't need to lose any forum members!

I appreciate the concern but no..

Since you asked I just tested both of them... and they both dutifully tripped and then let me reset them.  ;D

The .2 nF capacitance would be way lees than a mA (more like 10 uA).. Surprised I could feel the tingle at all, and at first I wasn't sure it was electrical (I'm old and sometimes nerves just fire for no reason) but I measured 100V so did not grab it again.

I feel it was pretty harmless capacitance in the old kettle, but the new one is better with no ground lead at all to leak current into all my other appliances.

JR
Title: Re: Test your grounds regularly!
Post by: Mike Sokol on December 18, 2014, 07:20:18 PM
I used to do this a long time ago to check for leakage current in test gear, so perhaps it's time to dust it off for testing hot chassis fault current. If you put a 1,000 ohm resistor between the suspected hot-chassis and an earth/ground connection, then measure the AC voltage across the resistor,  you'll read approximately 1 volt per mA of fault current.

This is an important thing to know since these hot-chassis conditions can be divided into three different categories. High-Z/Low-Current, Mid-Z/Mid-Current, and Low-Z/High-Current.

A High-Z/Low-Current hot-to-chassis fault may have only a few mA of ground fault current available, and sometimes less. This is what a lot of electricians call a phantom or ghost voltage because it disappears when measured with an analog meter that loads down the hot chassis voltage. Even a properly operating appliance with an open EGC can exhibit a High-Z/Low-Current fault since virtually all power supplies have small amount of leakage. And a stinger cap in an old guitar amp can provide a High-Z fault as well.

A Mid-Z/mid-current fault can have tens to hundreds mA of leakage current available. Old guitar tube amp transformers and leaky capacitors can easily cause this condition. This fault can give you a good jolt and perhaps even cause heart fibrillation, but it's not enough to trip a circuit breaker. However, it should be sufficient to trip a GFCI.

A Low-Z/high-current fault is essentially a short circuit between the hot side of the line and the chassis. Could be caused by a pinched wire, shorted stinger cap, or burned transformer windings. It can supply up to the circuit breaker current to the fault path. However, a human is typically 1,500 ohms or so (hand to hand) so a shock victim will typically only draw around 100 mA from a Low-Z fault. The bad news is that just 30 mA for a few seconds is the danger area where ventricular fibrillation can occur.

The thing to realize is that ANY voltage (more than 2 or 3 volts) between the chassis and earth is a warning sign that the EGC path has been compromised. Once that occurs, anything goes. 
Title: Re: Test your grounds regularly!
Post by: Steve M Smith on December 19, 2014, 02:58:42 AM
Found it..... the leakage path is a 3 wire plug electric kettle plugged directly into the GFCI outlet

First, the obligatory warning: !! Don't try this at home !! (even though I did).

Many years ago I had a washing machine which would more often than not, trip out the power.  As the machine was merely about twenty five years old and was a hand me down from my mother when she bought a new one and it had only been repaired by me half a dozen times before, I was loathe to replace it with a new one.

I traced it to a high resistance/low leakage from the heating element to the grounded outer cover.  Whilst I had the machine open, A thought crossed my mind: "what if the leakage was at the live end of the element?".  So I pulled off the wires from the terminals and swapped their positions.

It turned out that I was probably right as I had now moved the bad area closer to neutral.  The machine continued working for another five years before it was finally retired.


Steve.
Title: Re: Test your grounds regularly!
Post by: Stephen Swaffer on December 19, 2014, 01:39:21 PM
Heating elements are a common source of "leakage" to ground.  I suspect they are responsible for most of the "deaths by microphone" that happen in baptismal pools.  The 2014 NEC now requires that dishwashers be connected to a gfci circuit-I was told that the reason was that manufacturers could not make the dishwashers safe for non-GFCI circuits.  I am guessing this is likely due to heating elements and too much plastic in the design to provide an effective ground fault path.  Since nothing makes a homeowner upgrade to GFCI,  I wonder how many older homes getting new dishwashers will have the circuit upgraded to GFCI?

As more and more plumbing systems become entirely plastic, heating elements are creating more potential hazards.
Title: Re: Test your grounds regularly!
Post by: John Roberts {JR} on December 19, 2014, 02:29:20 PM
It seems to me that a much cheaper implementation of GFCI could be accomplished by original equipment manufacturers if designed in from scratch. If heating elements are a known problem engineer that problem away.

I have mixed emotions about government mandating such changes, and while the calculus sounds a little cold, how many housewives have been electrocuted by their dishwasher? If one brand kills a bunch of housewives, the class action lawyers will sniff them out and make bad design more expensive than good.

That same calculus applies to my recent pet project of electrocuted guitar players who also sing... clearly not enough for a class action.

JR
Title: Re: Test your grounds regularly!
Post by: John Roberts {JR} on December 19, 2014, 04:30:10 PM
I used to do this a long time ago to check for leakage current in test gear, so perhaps it's time to dust it off for testing hot chassis fault current. If you put a 1,000 ohm resistor between the suspected hot-chassis and an earth/ground connection, then measure the AC voltage across the resistor,  you'll read approximately 1 volt per mA of fault current.

This is an important thing to know since these hot-chassis conditions can be divided into three different categories. High-Z/Low-Current, Mid-Z/Mid-Current, and Low-Z/High-Current.

A High-Z/Low-Current hot-to-chassis fault may have only a few mA of ground fault current available, and sometimes less. This is what a lot of electricians call a phantom or ghost voltage because it disappears when measured with an analog meter that loads down the hot chassis voltage. Even a properly operating appliance with an open EGC can exhibit a High-Z/Low-Current fault since virtually all power supplies have small amount of leakage. And a stinger cap in an old guitar amp can provide a High-Z fault as well.

A Mid-Z/mid-current fault can have tens to hundreds mA of leakage current available. Old guitar tube amp transformers and leaky capacitors can easily cause this condition. This fault can give you a good jolt and perhaps even cause heart fibrillation, but it's not enough to trip a circuit breaker. However, it should be sufficient to trip a GFCI.

A Low-Z/high-current fault is essentially a short circuit between the hot side of the line and the chassis. Could be caused by a pinched wire, shorted stinger cap, or burned transformer windings. It can supply up to the circuit breaker current to the fault path. However, a human is typically 1,500 ohms or so (hand to hand) so a shock victim will typically only draw around 100 mA from a Low-Z fault. The bad news is that just 30 mA for a few seconds is the danger area where ventricular fibrillation can occur.

The thing to realize is that ANY voltage (more than 2 or 3 volts) between the chassis and earth is a warning sign that the EGC path has been compromised. Once that occurs, anything goes.

To put my recent experience in perspective my high impedance meter that indicated 106V was surely a phantom or ghost voltage since only uA of current available.  I can imagine similar NCVT giving false indications.  We need to be careful about further testing of phantoms since some may be real, perhaps a led with a few K resistor would help parse between uA and mA or more, but be careful with such testing.

OK some more testing... today I measured again and my phantom voltage was back,,, it looks like the problem follows some power strips I have plugged in. These power strips are cheapest Walmart or Lowes but one says it is protected whatever that means.  I measure about 250 to 350 uA of current between my outlet ground and kitchen sink with several power strips and appliances plugged in so my old kettle may not have been the only smoking gun. I do like my new kettle better through.  ;D

Just for chuckles I got a high efficiency red LED and a diode and 3k resistor in series. I can see the LED light up very dimly from hundreds of uA between the grounds and not at all with no power strips plugged in.

caveat al

JR
Title: Re: Test your grounds regularly!
Post by: Jonathan Johnson on December 19, 2014, 05:41:49 PM
Many years ago I had a washing machine which would more often than not, trip out the power.  As the machine was merely about twenty five years old and was a hand me down from my mother when she bought a new one and it had only been repaired by me half a dozen times before, I was loathe to replace it with a new one.

I traced it to a high resistance/low leakage from the heating element to the grounded outer cover.  Whilst I had the machine open, A thought crossed my mind: "what if the leakage was at the live end of the element?".  So I pulled off the wires from the terminals and swapped their positions.

FYI, here in the states, "washing machine" almost always refers to a clothes washer. We use the term "dishwasher" for that machine in the kitchen that cleans the dishes (and I don't mean the family dog).
Title: Re: Test your grounds regularly!
Post by: Mike Sokol on December 19, 2014, 09:43:45 PM
To put my recent experience in perspective my high impedance meter that indicated 106V was surely a phantom or ghost voltage since only uA of current available.  I can imagine similar NCVT giving false indications.  We need to be careful about further testing of phantoms since some may be real, perhaps a led with a few K resistor would help parse between uA and mA or more, but be careful with such testing.

Yes, if a NCVT indicates voltage on a backline amp or kitchen appliance, then there's almost certainly an open EGC (Equipment Grounding Conductor or Safety Ground). Of course, if you have a GFCI circuit then that's allowed as a code exception, and the same goes for double-insulated appliances that don't have a grounded power cord. But in any case it could be a high-z (low current) ghost voltage, or a low-z (high current) dead short, or something in between. The key is that you really don't know what it is without further fault current testing, and that can be dangerous if you're not careful.

Last year one of my former live sound students stopped by the cafeteria to tell me the NCVT I gave him as a graduation present found a "hot" mixing board at a barn stage where he was mixing. But rather than believe the hot-chassis was real, he then touched the mixing board with his hand as a test and said got a really good shock. Further investigation showed they had supplied him with an extension cord that had a broken off ground pin. This was likely a mid-z fault with maybe 10 mA of potential shock current available, but it could just as easily been an open EGC combined with a direct hot-to-chassis short with many amperes of fault current ready to kill you.

So PLEASE, if any of you suspect a hot-chassis condition either using a NCVT, DMM to earth measurment, or a guitar player reporting a shock, DO NOT use your own hands to test it. The problem with high-z faults is that they can turn into low-z faults at any time. And that can be deadly.
Title: Re: Test your grounds regularly!
Post by: John Roberts {JR} on December 19, 2014, 10:37:25 PM
That's what I like about my LED... the difference between uA of leakage that glows dimly, and melting the LED is hard to miss...

JR
Title: Re: Test your grounds regularly!
Post by: Stephen Swaffer on December 19, 2014, 11:15:16 PM

But rather than believe the hot-chassis was real, he then touched the mixing board with his hand as a test and said got a really good shock.


Hmmm-he received a graduation present so presumably he passed-do you really want to claim him as a student?  Of course, maybe he cut class the day you taught "What to do when the NCVT shows hot"??

Any suggestions on a good, low impedance DMM?  A low impedance mode on a DMM would be a nifty feature at times.
Title: Re: Test your grounds regularly!
Post by: Mike Sokol on December 20, 2014, 07:45:28 AM
Hmmm-he received a graduation present so presumably he passed-do you really want to claim him as a student?  Of course, maybe he cut class the day you taught "What to do when the NCVT shows hot"??

Sadly, he not only attend my regular practicum classes, he also was there for a No~Shock~Zone clinic where I showed them how to use a NCVT. Nobody seems to believe that a shock can kill them. 
 
Quote
Any suggestions on a good, low impedance DMM?  A low impedance mode on a DMM would be a nifty feature at times.

I think I've seen low-impedance test probes somewhere. It could be as simple as a 10K ohm / 1-watt resistor in an adapter across the input jacks.
Title: Re: Test your grounds regularly!
Post by: Mike Sokol on December 20, 2014, 09:19:33 AM
I think I've seen low-impedance test probes somewhere. It could be as simple as a 10K ohm / 1-watt resistor in an adapter across the input jacks.

This Fluke T+Pro tester could be a good solution. It's a new spin on the old solenoid tester design and appears to load the line to about 5 mA, so it should shunt out any "ghost voltages". While an old solenoid tester does have a similarly low impedance for this test, if something goes wrong (gets across too high AC voltage in an industrial setting) they can explode, launching metal shrapnel at you. These new designs are safer and have the same form factor for simple operation by electricians, but also include a DMM and GFCI cycle test. http://www.amazon.com/Fluke-T-PRO-Electrical-Tester/dp/B000VRHD4S/ref=sr_1_1?ie=UTF8&qid=1419084477&sr=8-1&keywords=Fluke+T%2BPro (http://www.amazon.com/Fluke-T-PRO-Electrical-Tester/dp/B000VRHD4S/ref=sr_1_1?ie=UTF8&qid=1419084477&sr=8-1&keywords=Fluke+T%2BPro)

JR - What do you think?
Title: Re: Test your grounds regularly!
Post by: John Roberts {JR} on December 20, 2014, 09:39:16 AM
Sorry I'm not familiar with solenoid testers and I can't tell a lot from the description.

I could see what I want to see with a high impedance VOM with an added shunt resistor across the input (worry that using current measurement scale with out a series resistance could damage the meter.

One could design a go/no-go tester using an LED with a shunt resistor across the LED so the LED doesn't light up until a few mA. While I like the faint LED light from hundreds of uA like I see in my kitchen outlets.

JR
Title: Re: Test your grounds regularly!
Post by: Mike Sokol on December 20, 2014, 10:36:37 AM
Sorry I'm not familiar with solenoid testers and I can't tell a lot from the description.

Old-school electricians like to use them. They consist of a magnetic winding with a plunger on a spring. So when they detect voltage across the leads they "buzz" and there's an approximate voltage scale that lines up with the plunger. The further it pulls in, the higher the voltage. Of course, if you accidentally get across an 11KV line in an industrial plant, the solenoid launches like a rocket and can kill you. That's why Fluke and others make modern versions that look and operate like a solenoid tester, but have modern electronics.

As I noted earlier in this thread, a 1K (10 watt) or 10K resistor (1 watt) resistor on a banana plug that fits into the meter connectors would work perfectly, but it needs a recess for the safety collar that's on all modern meter lead male ends. I seem to remember someone building one of these, but can't find it with a casual search. Seems like a $5 add-on solution if you can find one already manufactured.
Title: Re: Test your grounds regularly!
Post by: Rob Spence on December 22, 2014, 01:42:28 PM

OK some more testing... today I measured again and my phantom voltage was back,,, it looks like the problem follows some power strips I have plugged in. These power strips are cheapest Walmart or Lowes but one says it is protected whatever that means.  I measure about 250 to 350 uA of current between my outlet ground and kitchen sink with several power strips and appliances plugged in so my old kettle may not have been the only smoking gun. I do like my new kettle better through.  ;D


caveat al

JR

I suspect the power strip was protected by the plastic packaging it came in. Once you removed it, all bets were off :-)

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Title: Re: Test your grounds regularly!
Post by: Craig Hauber on December 23, 2014, 12:22:06 AM
Sorry I'm not familiar with solenoid testers and I can't tell a lot from the description.
More commonly called a "wiggy"
they put a decent enough load on the line to help reveal bad connections.  And they have a tactile vibration as well as a clacking sound so you don't always have to be looking at them when testing many lines in cramped conditions.