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Author Topic: Test your grounds regularly!  (Read 26253 times)

Mike Sokol

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Re: Test your grounds regularly!
« Reply #30 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.
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Bill McKelvey

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Re: Test your grounds regularly!
« Reply #31 on: November 24, 2014, 03:58:07 PM »

From the Department of Repetitive Redundancy Department
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g'bye, Dick Rees

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Re: Test your grounds regularly!
« Reply #32 on: November 24, 2014, 03:59:27 PM »

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Bill McKelvey

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Re: Test your grounds regularly!
« Reply #33 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.
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Kevin Graf

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Re: Test your grounds regularly!
« Reply #34 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.
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Mike Sokol

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Re: Test your grounds regularly!
« Reply #35 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.   
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Frank DeWitt

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Re: Test your grounds regularly!
« Reply #36 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.
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Guy Holt

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Re: Test your grounds regularly!
« Reply #37 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.


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
« Last Edit: November 25, 2014, 12:12:48 PM by Guy Holt »
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John Roberts {JR}

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Re: Test your grounds regularly!
« Reply #38 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.


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
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Guy Holt

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Re: Test your grounds regularly!
« Reply #39 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.
 

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.


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.


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.)


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 - for  a complete summary of my IA workshop on Ground Fault Protection.)

Guy Holt, Gaffer
Screenlight & Grip
www.screenlightandgrip.com
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Re: Test your grounds regularly!
« Reply #39 on: November 25, 2014, 01:13:02 PM »


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