Grounding requirements

[Howard59]

Is grounding required on a 240v to 120v control transformer where the primary is single phase of a 3 phase 120/240 high leg delta. Does it make a difference if the high leg is used or not.
The way I read it if, there is greater than 150volts to ground on any of the primary conductors (the high leg would qualify, but not the others) then groundig of the secondary is required.

 

[tom baker]

Re: Grounding requirements

What size is the transformer? For a control power transformer of 1 kw and under a, grounding electrode is not required, but grounding is always required for a system as given in 250.20, including a mid point grounded delta.

 

[Howard59]

Re: Grounding requirements

Let me rephrase. Do I need to ground the secondary of this transformer to the metal enclosure of the machine that it is in?
I don't know for sure, but I suspect that it's much less than one kva.

 

[augie47]

Re: Grounding requirements

From 250.30 (A) (3):

Exception No. 3: A grounding electrode conductor shall not be required for a system that supplies a Class 1, Class 2, or Class 3 circuit and is derived from a transformer rated not more than 1000 volt-amperes, provided the grounded conductor is bonded to the transformer frame or enclosure by a jumper sized in accordance with 250.30(A)(1), Exception No. 3, and the transformer frame or enclosure is grounded by one of the means specified in 250.134.