On Mon, 25 Sep 2006 06:04:47 GMT Salmon Egg wrote: | On 9/24/06 9:17 PM, in article firstname.lastname@example.org, | " email@example.com" wrote: | |> Suppose you are wiring up a luminaire using a transformer to change the |> voltage down to 120 volts. The light socket will be an Edison screw base |> type, so the shell of that socket will be wired to a grounded conductor. |> |> Which of the follow are allowed ways to connect the ground? |> |> 1. The conductor attached to the socket shell is grounded by means of |> connecting it to the grounded conductor of the circuit supplying |> power to the transformer primary. |> |> 2. The conductor attached to the socket shell is grounded by means of |> connecting it to the grounding conductor of the circuit supplying |> power to the transformer primary. |> |> You might want to consult 210.6(C)(2). What is you interpretation? | | I am not up on my NEC but I would be pretty sure that you are not allowed to | have a direct ground to the shell. The shell should be connected to a | NEUTRAL conductor that in turn is grounded according to other requirements. | Protective grounds should function as protection only and not for normal | power transfer.
If you are making a separately derived system from the transformer secondary then you select which of the 2 wires in a 2 wire secondary is to be the groundED conductor, and bond it to ground, which is provided by the groundING conductor (the one wearing green if it's not naked). That's what option 2 would create. If you think this is unsafe, then please describe how a grounded separately derived system is unsafe.
Since the transformer secondary is an isolation, grounding one of the secondary wires to the circuit grounding wire is NOT going to introduce any current on the ground wire. The only time you would have such current is if there is a fault between the ungrounded wire of the separate system, and something grounded elsewhere. That should then provide the fault current to trip the OCPD on the transformer secondary.
If the selected wire on the transformer secondary is attached to the supply groundED wire, I'm not sure what that makes. Maybe it will be considered to be an autotransformer, despite there being to windings.
Either approach is safe, I think. But I would feel safer with the groundING conducture being used.
That's assuming grounding is even done at all. For low voltage lighting, the secondary is not even to be grounded at all.