GFCI, Subpanel and switch questions

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  #1  
Old 09-06-13, 07:37 AM
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GFCI, Subpanel and switch questions - any more THOUGHTS?

I installed a new subpanel. The feed from the main (8-3 AWG UF) is on a double pole 30 Amp GFCI breaker at the main panel. The subpanel has two separate 15 amp circuits protected by two 15 amp GFCI breakers (I know overkill) in the subpanel. I had one circuit on at the subpanel breaker and one circuit turned off at the subpanel breaker while I was installing a light fixture on the de-energized circuit. While I was installing the light I touched the neutral and ground wires together and the GFCI breaker in the Main tripped.

How is this possible if the circuit I was working on was off at the subpanel?


Also, not sure if this is related, but I was curious so I checked the circuit at the same spot that tripped the GFCI (which is after a light switch):

subpanel Breaker on, switch on = 120 V
subpanel Breaker on, switch off = 3 V
Subpanel Breaker off = 20-100 mv

I don't know if this means anything or if its normal to have a voltage reading after a switch in the off position?

Thanks Kevin
 

Last edited by kmalone; 09-06-13 at 09:07 AM.
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  #2  
Old 09-06-13, 07:56 AM
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on a double pole 30 Amp GFCI breaker at the main panel. The subpanel has two separate 15 amp circuits protected by two 15 amp GFCI breakers (I know overkill) in the subpanel
Not just overkill, Sometimes using two GFCIs can cause problems.

I had one circuit on at the subpanel breaker and one circuit turned off at the subpanel breaker while I was installing a light fixture on the de-energized circuit. While I was installing the light I touched the neutral and ground wires together and the GFCI breaker in the Main tripped.
At the subpanel is the neutral bar isolated and the ground bar bonded.

Are you using a digital multimeter. Some have trouble rejecting induced voltages so sometime you will get a small reading on a circuit that has no actual voltage.
 
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Old 09-06-13, 08:12 AM
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While I was installing the light I touched the neutral and ground wires together and the GFCI breaker in the Main tripped.
You were woring on a GFCI protected circuit. Regardless of whether the subpanel branch breaker was on or off, when a ground and neutral touch downstream of an energized GFCI device, the device will trip.
 
  #4  
Old 09-06-13, 08:25 AM
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GFCI in main and subpanel are not causing problems under normal operation, so I am inclined to leave them. They have never tripped other than when I was installing a new light fixture and touched wires together. Once the light was installed, everything works fine. I was just perplexed as to how a GFCI breaker could trip in the main if the subpanel breaker was off on the circiut I was working on.

Subpanel is in detatched building. Neutral is isolated and ground bar is bonded. Also have ground rods installed.

Yes digital multimeter and I know they have limitations. The reason I think the 3 volt reading is real is that it drops to millivolts when I switch breaker off
 
  #5  
Old 09-06-13, 08:29 AM
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Normal?

thanks - so just to clarify its "normal" to have a GFCI breaker trip in the main panel when I touch neutral/ground together - even if I did this downstream of a subpanel breaker in the OFF position?
 
  #6  
Old 09-06-13, 10:28 AM
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Reposted - GFCI and voltage readings

I installed a new subpanel. The feed from the main (8-3 AWG UF) is on a double pole 30 Amp GFCI breaker at the main panel. The subpanel has two separate 15 amp circuits protected by two 15 amp GFCI breakers (I know overkill) in the subpanel. I had one circuit on at the subpanel breaker and one circuit turned off at the subpanel breaker while I was installing a light fixture on the de-energized circuit. While I was installing the light I touched the neutral and ground wires together and the GFCI breaker in the Main tripped.

How is this possible if the circuit I was working on was off at the subpanel?


Also, not sure if this is related, but I was curious so I checked the circuit at the same spot that tripped the GFCI (which is after a light switch):

subpanel Breaker on, switch on = 120 V
subpanel Breaker on, switch off = 3 V
Subpanel Breaker off = 20-100 mv

I don't know if this means anything or if its normal to have a voltage reading after a switch in the off position?

Thanks Kevin - sorry this was reposted since I think it was closed. I didnt get many replies

Read more: http://www.doityourself.com/forum/el...#ixzz2e8PE7hFs
 
  #7  
Old 09-06-13, 10:35 AM
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Threads merged; previous thread was not closed. Can you explain what you don't understand about the switch voltages. Did you see the last paragraph of my post #2?
 

Last edited by ray2047; 09-09-13 at 08:14 PM.
  #8  
Old 09-06-13, 10:39 AM
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sorry - I think I understand the voltage reading. You are saying that I am using the wrong meter and the reading may not be actual?

I was curious about clarifying that its "normal" to have a GFCI breaker trip in the main panel when I touch neutral/ground together - even if I did this downstream of a subpanel breaker in the OFF position?

Just trying to make sure neither of these items are a red flag of a dangerous situation
 
  #9  
Old 09-06-13, 11:04 AM
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It only takes only 4-6 milliamps to trip a GFCI. A switch is never on the neutral side of a 120 volt circuit.

Not the wrong meter but you must understand the limits of the meter accuracy. Some meters such as analog or solenoid tester (Wiggins) with lower impedance are less likely to be affected by induced voltages. Expensive digitals usually do a good job. About equal to a cheap analog so why not use a cheap analog. An analog also doesn't need a battery for voltage and amp reading so you won't ever get a false reading due to low battery like you can with even an expensive digital.
 
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