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Author Topic: Test your grounds regularly!  (Read 26279 times)

Lyle Williams

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Re: Test your grounds regularly!
« Reply #40 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.
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John Roberts {JR}

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Re: Test your grounds regularly!
« Reply #41 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
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Stephen Swaffer

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Re: Test your grounds regularly!
« Reply #42 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.
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Guy Holt

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Re: Test your grounds regularly!
« Reply #43 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 GFCIís 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.


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 - for my Ground Fault Protection workshop.

Guy Holt, Gaffer
ScreenLight & Grip
rentals@screenlightandgrip.com
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Rob Spence

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Re: Test your grounds regularly!
« Reply #44 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
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rob at lynxaudioservices dot com

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Rob Spence

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Re: Test your grounds regularly!
« Reply #45 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.


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rob at lynxaudioservices dot com

Dealer for: AKG, Allen & Heath, Ashley, Astatic, Audix, Blue Microphones, CAD, Chauvet, Community, Countryman, Crown, DBX, Electro-Voice, FBT, Furman, Heil, Horizon, Intellistage, JBL, Lab Gruppen, Mid Atlantic, On Stage Stands, Pelican, Peterson Tuners, Presonus, ProCo, QSC, Radial, RCF, Sennheiser, Shure, SKB, Soundcraft, TC Electronics, Telex, Whirlwind and others

Steve Bradbury

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Re: Test your grounds regularly!
« Reply #46 on: December 09, 2014, 05:15:11 PM »

Iím 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.



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.
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Jonathan Johnson

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Re: Test your grounds regularly!
« Reply #47 on: December 09, 2014, 05:39:59 PM »

Iím 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.



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.)
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Steve Bradbury

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Re: Test your grounds regularly!
« Reply #48 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 consumerís 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 doesnít have a separate ground conductor which would be referred to as a TT system where the earth is provided locally, which is why I didnít include one in the diagram.
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Stephen Swaffer

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Re: Test your grounds regularly!
« Reply #49 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.
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Steve Swaffer

ProSoundWeb Community

Re: Test your grounds regularly!
¬ę Reply #49 on: December 10, 2014, 01:06:15 PM ¬Ľ


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