Is it a NEC requirement to bond an aluminum channel letter to AC ground when using a low voltage 3 Vdc LED lighting system such as GE Gelcore? My concern is that LED systems are DC and bonding to the AC side of the powers supply essential sets up a third potential at the letter. It would potentially interfere with the DC fault protection circuits on the output side of the power supply.
LED CHANNEL LETTERS & NEC 250.4
- Thread starter jimmyi
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hbiss
EC, New York NEC: 2017
- Location
- Little Falls, New York NEC: 2017
- Occupation
- EC
Re: LED CHANNEL LETTERS & NEC 250.4
No. I believe it would fall under Art. 411.
-Hal
No. I believe it would fall under Art. 411.
-Hal
- Location
- Illinois
- Occupation
- retired electrician
Re: LED CHANNEL LETTERS & NEC 250.4
What about 250.4(A)(2)?
What about 250.4(A)(2)?
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- Semi-retired engineer
Re: LED CHANNEL LETTERS & NEC 250.4
I am not convinced it is required but I would not be worried about AC versus DC ground. The bonding wire is the same wire.
I am not convinced it is required but I would not be worried about AC versus DC ground. The bonding wire is the same wire.
Re: LED CHANNEL LETTERS & NEC 250.4
Within the power supply, the DC ground is isolated via the step down transformer and full wave bridge. I connected a 12 Vdc permlight power supply to a load in the shop and measured a 60 Vac difference between DC and AC ground. Thus the third potential that would be applied to the dead metal.
Within the power supply, the DC ground is isolated via the step down transformer and full wave bridge. I connected a 12 Vdc permlight power supply to a load in the shop and measured a 60 Vac difference between DC and AC ground. Thus the third potential that would be applied to the dead metal.
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- Semi-retired engineer
Re: LED CHANNEL LETTERS & NEC 250.4
now you could have a potential difference between ac neutral and dc common IF the dc power supply is isolated, but that is not the same thing as ground.
no offense but there really is no such thing as ac and dc ground. its the same point.
now you could have a potential difference between ac neutral and dc common IF the dc power supply is isolated, but that is not the same thing as ground.
Re: LED CHANNEL LETTERS & NEC 250.4
You are right, I should have typed neutral for the AC side. On the DC side the external signals are referred to as positive and negative. Internal to the circuit the negative node is referred to as DC ground. Power supply designers (me) think in terms of DC ground and virtual ground. It's all relative.
BTW, thanks for the feedback on my 250.4 question.
You are right, I should have typed neutral for the AC side. On the DC side the external signals are referred to as positive and negative. Internal to the circuit the negative node is referred to as DC ground. Power supply designers (me) think in terms of DC ground and virtual ground. It's all relative.
BTW, thanks for the feedback on my 250.4 question.
hbiss
EC, New York NEC: 2017
- Location
- Little Falls, New York NEC: 2017
- Occupation
- EC
Re: LED CHANNEL LETTERS & NEC 250.4
Within the power supply, the DC ground is isolated via the step down transformer and full wave bridge. I connected a 12 Vdc permlight power supply to a load in the shop and measured a 60 Vac difference between DC and AC ground. Thus the third potential that would be applied to the dead metal.
That doesn't make sense. If there is isolation by the transformer it really doesn't make any difference if there is a FWB or not, there should be no potential from the primary hot or neutral to any point in any circuitry on the secondary side.
I think what you are seeing is "phantom" voltage caused by the high input impedance if your DVM.
-Hal
Within the power supply, the DC ground is isolated via the step down transformer and full wave bridge. I connected a 12 Vdc permlight power supply to a load in the shop and measured a 60 Vac difference between DC and AC ground. Thus the third potential that would be applied to the dead metal.
That doesn't make sense. If there is isolation by the transformer it really doesn't make any difference if there is a FWB or not, there should be no potential from the primary hot or neutral to any point in any circuitry on the secondary side.
I think what you are seeing is "phantom" voltage caused by the high input impedance if your DVM.
-Hal
- Status