Can anyone explain this

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Dennis Alwon

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Chapel Hill, NC
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Electrical Contractor
We had a service call. Easy fix no problem. Here is the situation. Kitchen lights were on a gfci breaker because it apparently fed some outside receptacles, The lights were tracks with low voltage heads. Every time the switch was turn on it would trip the GFCI.

My guys removed the track heads and everything was fine. Install all the units one by one and everything worked except for one track head. They installed the last head without the bulb and the circuit held. They put the bulb in and the circuit tripped.

These are low voltage and I thought the gfci would not protect the secondary side of the trany. My guess is there is a short on the primary side that only shows up when there is a load. I have never witnessed this before.
 

big john

Senior Member
Location
Portland, ME
Did your guys try it in reverse order? Or only with that last fixture?

My guess is also that the load itself was contributing to leakage. I would not expect a GFCI to be able to read the load on a transformer, as it is an SDS. But if there was a high-resistance neutral-ground fault on the primary side, as you increased the load, and increased neutral current, you're increase the voltage through that fault.

-John
 

Dennis Alwon

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Location
Chapel Hill, NC
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Electrical Contractor
Yes, let me explain-- It is not a track but 2 monopoints with track heads. If only the defective unit is installed without the bulb the circuit is fine. Install the low voltage bulb and the GFCI trips. They switched bulbs and the same problem. The bulb is obviously connecting two parts of the trany together thru the filament but it is still on the LV end of the transformer.
 

realolman

Senior Member
I can't see why it would trip because it was a GFCI ... Do you suppose it would trip if it was just a normal breaker? If it were tripping because of the GFCI wouldn't the current have to be unbalanced on the neutral and hot?
 

Hv&Lv

Senior Member
Location
-
Occupation
Engineer/Technician
I can't see why it would trip because it was a GFCI ... Do you suppose it would trip if it was just a normal breaker? If it were tripping because of the GFCI wouldn't the current have to be unbalanced on the neutral and hot?
I wonder if the neutral was shorted against the defective head? That would lead to some return currents going back through the ground.
 

Joethemechanic

Senior Member
Location
Philly Pa burbs
I wonder if the neutral was shorted against the defective head? That would lead to some return currents going back through the ground.

Why would the primary neutral even be anywhere near the the track lighting head?


Do you think somebody grounded the secondary through a high impedance path that may include the primary wiring?

Or there may be a primary to secondary fault in the transformer with the only path to ground being this one defective lamp head?

There is no chance of this being an autotransformer is there?
 

Sierrasparky

Senior Member
Location
USA
Occupation
Electrician ,contractor
Could be a problem earlier in the circuit that the load as increased causes the problem. I'd rule out the fixture by adding a greater load to the switch leg at the fixure outlet to see if it trips. Also check the fixture tranny for fault.
 

Dennis Alwon

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Location
Chapel Hill, NC
Occupation
Electrical Contractor
Here is what I don't get. Why when the fixtures are connected it works fine. It only trips the gfci when the bulb is inserted into a particular track head.

I didn't see the track heads so I am not sure if they were electronic or magnetic
 

hillbilly1

Senior Member
Location
Atlanta,Ga
With some of the electronic low voltage power supply's, they do not turn on until there is a load detected. Makes it a pain in the butt to troubleshoot, because you will not get any voltage at the socket with the lamp out. So probably what was happening was the ground fault was not showing up until the electronics kicked in.
 

Dennis Alwon

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Staff member
Location
Chapel Hill, NC
Occupation
Electrical Contractor
With some of the electronic low voltage power supply's, they do not turn on until there is a load detected. Makes it a pain in the butt to troubleshoot, because you will not get any voltage at the socket with the lamp out. So probably what was happening was the ground fault was not showing up until the electronics kicked in.
Now that makes sense and is what I originally stated- a load caused the short on the primary. Not sure how or why but it does seem to be what is happening.
 

grich

Senior Member
With some of the electronic low voltage power supply's, they do not turn on until there is a load detected. Makes it a pain in the butt to troubleshoot, because you will not get any voltage at the socket with the lamp out. So probably what was happening was the ground fault was not showing up until the electronics kicked in.
That's what I was thinking, too. :)
 

Joethemechanic

Senior Member
Location
Philly Pa burbs
I think that would be a violation

411.5 Secondary Circuits.
(A) Grounding. Secondary circuits shall not be grounded.
(B) Isolation. The secondary circuit shall be insulated
from the branch circuit by an isolating transformer.

Well I don't think Dennis would install substandard Chinese made lighting. But this is from what I understand a service call,,,,,

I don't trust a lot of stuff on the market now to be what it is supposed to be. There are quite a few "grey market" items out there anymore.
 

Dennis Alwon

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Location
Chapel Hill, NC
Occupation
Electrical Contractor
Actually Dennis didn't install it at all. and I wouldn't have had them on a gfci breaker either. We did an addition on the house a few years back and I have been doing their work since then. I believe the track heads were Halo.
 

charlie b

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Location
Seattle, WA
Occupation
Electrical Engineer
Just out of curiosity, can you distinguish between a trip initiated by the GFCI electronics within the breaker and a trip initiated by the breaker's instantaneous trip? I am wondering of the GFCI stuff is irrelevant, and if all you have is a short circuit.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Electrical Contractor
Just out of curiosity, can you distinguish between a trip initiated by the GFCI electronics within the breaker and a trip initiated by the breaker's instantaneous trip? I am wondering of the GFCI stuff is irrelevant, and if all you have is a short circuit.
Not sure but either way it is interesting that it is only with the bulb inserted. I guess we could have tried a standard breaker but the fact was the head needed to be replaced so we left it at that.
 
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