Voltage drop and GFCI

[Worlds Gr8est Electrician]

I had a customer today call and tell me one of her outside gfi's were not working. She said she had another electrician replace it twice already in the lst 6 months. She wanted to see if I could find anything wrong with the circuit he may have missed.

The gfi was being fed from one of the lighting circuits. It had 113v under a 15 amp load. This is not low under 15 amp load, but I was wondering would voltage drop cause a GFI to go bad over time.

I did not find anything else wrong with her circuit that is was being fed from. The bad gfi was leveton weather proof. I replaced it with a cooper which I prefer. Everything was up and running after that. The only other thing I noticed and corrected was her main lug panel had the neutral and ground bonded with a bonding screw which I removed.

 

[Dennis Alwon]

So is there normally 15 amps on this device or on the circuit.

 

[gk351]

Just curious if the neutrals and ground were seperated as well? Was this a sub-panel or main whole house panel? Was there an outside CB Disco? Meter main combo? just being nosy!

 

[hurk27]

I would rule out voltage drop as the cause, I have seen GFCI's installed on tec power supply systems that only have 60 volts to ground and they work fine.

The problem is there are a few things that can cause a GFCI to fail, some will jump out at you and some will depend upon how honest the home owners is in telling you when asked a question.

Here are a few things I have found that can cause the electronics to fail in a GFCI:

Near by lightning strike.

Water leaking in behind the cover or house was power washed.

Bad connection on a MWBC neutral or the service neutral, if it looses connection for just a few seconds it can over voltage this circuit and many others.

Surge from utility that exceeded the rating of the MOV in the GFCI in voltage or duration of spike.

Also I have had a few fail after a fault from hot to ground, some will take a hit like this a few time then some will fail on the first fault.

The reason I suspect is that a bolted fault between the hot and ground puts way more current through the current transformer that provides the voltage signal for the electronics to trip the GFCI out, current transformers voltage will keep rising even higher as the current passing through it gets higher, a bolted fault can be in the upwards of a 1,000 amps in most branch circuits, but with the higher impedance of this circuit (.46Ω) it will be about 242 amps, still that is quit high to be flowing through the current transformer.

The reason I know the above problem with a hot to ground fault is I had a customer who had hit his extension cord while mowing his yard it tripped the breaker and he reset the breaker but the GFCI would not reset, so I just replaced it, a week later he was trimming his hedges and again cut the extension cord, again it tripped the breaker but the new GFCI would not reset, so I tried a brand new GFCI using a switch caused a hot to ground fault and sure enough the GFCI failed to reset after resetting the breaker, after this I put in a GFCI breaker and never got a call back again, now after reading on how most GFCI's operate I know now why the breaker tripped before the GFCI, it is because most GFCI's have a 3 cycle delay from when the first ground fault is detected and the electronics trigger the relay to shut the GFCI off, this delay allows the GFCI to pass the full fault to the breaker through the current coil in the GFCI, but the electronics is still seeing this higher voltage coming from the current coil for 3 cycles, enough to do some damage.

I hope the reason I haven't received any more calls from him is because he learned not to cut his cords, but who knows LOL

 

[hurk27]

Now on the point of removing the bonding screw.

If this is in fact a sub panel with a service disconnect located elsewhere, it's true the grounds and neutrals need to be kept separate in this panel, but one word of advice, make sure there is a ground fault path between this panel and the service disconnect, I have run into sub panels fed with SE cable and removing the bonding jumper would remove the fault current path, and if a fault was to occur it would put 120 volts one all the grounding in the house, this could be very dangerous, you must always make sure there is a low impedance ground between the sub panel and the service disconnect where the main bonding jumper should be before ever removing a bonding jumper.

 

[ramsy]

Thank you

Thank you

Very informative work Wayn, glad I stopped by.

 

[andyselec@aol.com]

I have seen alot damaged by Ants.

 

[jumper]

The reason I know the above problem with a hot to ground fault is I had a customer who had hit his extension cord while mowing his yard it tripped the breaker and he reset the breaker but the GFCI would not reset, so I just replaced it, a week later he was trimming his hedges and again cut the extension cord, again it tripped the breaker but the new GFCI would not reset, so I tried a brand new GFCI using a switch caused a hot to ground fault and sure enough the GFCI failed to reset after resetting the breaker, after this I put in a GFCI breaker and never got a call back again, now after reading on how most GFCI's operate I know now why the breaker tripped before the GFCI, it is because most GFCI's have a 3 cycle delay from when the first ground fault is detected and the electronics trigger the relay to shut the GFCI off, this delay allows the GFCI to pass the full fault to the breaker through the current coil in the GFCI, but the electronics is still seeing this higher voltage coming from the current coil for 3 cycles, enough to do some damage.

I hope the reason I haven't received any more calls from him is because he learned not to cut his cords, but who knows LOL

Are you sure you are not talking about me.

I am notorious for damaging my cords when doing yard work, especially with a hedge trimmer.

 

[ptonsparky]

We are finishing up a project that is combining high humidity with condensation. At least two of the GFCIs have failed already with a third highly suspect. Leviton TPWR. Possiblility you have a similar situation? A leaking cover perhaps?

 

[kwired]

I would rule out voltage drop as the cause, I have seen GFCI's installed on tec power supply systems that only have 60 volts to ground and they work fine.

The problem is there are a few things that can cause a GFCI to fail, some will jump out at you and some will depend upon how honest the home owners is in telling you when asked a question.

Here are a few things I have found that can cause the electronics to fail in a GFCI:

Near by lightning strike.

Water leaking in behind the cover or house was power washed.

Bad connection on a MWBC neutral or the service neutral, if it looses connection for just a few seconds it can over voltage this circuit and many others.

Surge from utility that exceeded the rating of the MOV in the GFCI in voltage or duration of spike.

Also I have had a few fail after a fault from hot to ground, some will take a hit like this a few time then some will fail on the first fault.

The reason I suspect is that a bolted fault between the hot and ground puts way more current through the current transformer that provides the voltage signal for the electronics to trip the GFCI out, current transformers voltage will keep rising even higher as the current passing through it gets higher, a bolted fault can be in the upwards of a 1,000 amps in most branch circuits, but with the higher impedance of this circuit (.46Ω) it will be about 242 amps, still that is quit high to be flowing through the current transformer.

The reason I know the above problem with a hot to ground fault is I had a customer who had hit his extension cord while mowing his yard it tripped the breaker and he reset the breaker but the GFCI would not reset, so I just replaced it, a week later he was trimming his hedges and again cut the extension cord, again it tripped the breaker but the new GFCI would not reset, so I tried a brand new GFCI using a switch caused a hot to ground fault and sure enough the GFCI failed to reset after resetting the breaker, after this I put in a GFCI breaker and never got a call back again, now after reading on how most GFCI's operate I know now why the breaker tripped before the GFCI, it is because most GFCI's have a 3 cycle delay from when the first ground fault is detected and the electronics trigger the relay to shut the GFCI off, this delay allows the GFCI to pass the full fault to the breaker through the current coil in the GFCI, but the electronics is still seeing this higher voltage coming from the current coil for 3 cycles, enough to do some damage.

I hope the reason I haven't received any more calls from him is because he learned not to cut his cords, but who knows LOL

He probably watched you do it enough times he figured he could save a little money and change it himself.

 

[G._S._Ohm]

high humidity with condensation. At least two of the GFCIs have failed already with a third highly suspect.

Maybe somebody makes GFCIs with a conformally-coated PC board.

 

[G._S._Ohm]

it is because most GFCI's have a 3 cycle delay from when the first ground fault is detected and the electronics trigger the relay to shut the GFCI off, this delay allows the GFCI to pass the full fault to the breaker through the current coil in the GFCI, but the electronics is still seeing this higher voltage coming from the current coil for 3 cycles, enough to do some damage.

UL943 calls for a trip time in seconds of equal to or less than T = (20/I)^1.43 with I in mA. Three cycles would be about 50 mS.
Is this three cycles a spec somewhere or just engineering practice?
Thanks for your info.

 

[Worlds Gr8est Electrician]

Just curious if the neutrals and ground were seperated as well? Was this a sub-panel or main whole house panel? Was there an outside CB Disco? Meter main combo? just being nosy!

The neutrals and grounds were separated but were being tied together with the bonding screw. There was a main breaker outside.

All of the obvious like water leaking on the gfi was not happening. I noticed nothing unusual at all except the neutrals and grounds being bonded at the sub panel. The gfi I replaced appeared to be brand new.

Dennis the circuit did not have 15 amps on it at all times. I do not think voltage drop was the issue anymore. I am not sure what caused it. It just seems odd that she had them going bad so often.

 

[ELA]

it is because most GFCI's have a 3 cycle delay from when the first ground fault is detected and the electronics trigger the relay to shut the GFCI off, this delay allows the GFCI to pass the full fault to the breaker through the current coil in the GFCI, but the electronics is still seeing this higher voltage coming from the current coil for 3 cycles, enough to do some damage.

UL943 calls for a trip time in seconds of equal to or less than T = (20/I)^1.43 with I in mA. Three cycles would be about 50 mS.
Is this three cycles a spec somewhere or just engineering practice?
Thanks for your info.

I was assuming hurk27 meant to say 3 (1/2) cycles and that would then be consistent with UL943. (less than or equal to 25msec) for a fault greater than 0.25A. Being a life safety device I would think that life protection drove the timing and that the device would be considered expendable to assure that.

I was not aware of any intentional design criteria that were used to "coordinate" the tripping of the GFCI with the circuits upstream breaker.
If there is I also would like to learn of it.

The LM1851 data sheet is pretty detailed as to the timing elements involved in the trip time. (See page 5 of pdf) According to that document, in theory, the GFCI could possibly trip within (1/2) cycle if the fault was very large and the integration time was swamped out.
Page 4 also shows a graph of fault current vs. trip time hinting at trip times less than 20 msec.

 

[G._S._Ohm]

I was assuming hurk27 meant to say 3 (1/2) cycles and that would then be consistent with UL943. (less than or equal to 25msec) for a fault greater than 0.25A. Being a life safety device I would think that life protection drove the timing and that the device would be considered expendable to assure that.

I was not aware of any intentional design criteria that were used to "coordinate" the tripping of the GFCI with the circuits upstream breaker.
If there is I also would like to learn of it.

The LM1851 data sheet is pretty detailed as to the timing elements involved in the trip time. (See page 5 of pdf) According to that document, in theory, the GFCI could possibly trip within (1/2) cycle if the fault was very large and the integration time was swamped out.
Page 4 also shows a graph of fault current vs. trip time hinting at trip times less than 20 msec.

The fastest power handling relay [in the 10A range or so] I ever measured for clearing the arc with a resistive load was about 15 mS, depending on current draw and voltage. But inductive and motor loads would draw out the arc.

 

[ELA]

The GFCI does not use a relay. It is a spring loaded contract tripped by an SCR fired solenoid, more like a breaker trip mechanism then relay.

Here is a capture of a Leviton 7599 Smartlock Pro GFCI tripping on a 0.27A RMS ground fault ( hot to ground).



The short was created using a phase fire unit near the peak of the line. When triggered near zero cross the trip time was 10.5 msec. for the same current.

 

[G._S._Ohm]

And the formula would allow 24 mS max at this current.

Some people some of the time may present this low 440 ohm load across 120v.

Thanks.

 

[kwired]

The GFCI does not use a relay. It is a spring loaded contract tripped by an SCR fired solenoid, more like a breaker trip mechanism then relay.

Here is a capture of a Leviton 7599 Smartlock Pro GFCI tripping on a 0.27A RMS ground fault ( hot to ground).

View attachment 7799

The short was created using a phase fire unit near the peak of the line. When triggered near zero cross the trip time was 10.5 msec. for the same current.

When a GFCI stops working is it because the "contract" had expired, or was otherwise voided for some reason?