Grounding control transformer

[Rob442]

I have a 100va control transformer I am replacing in a Siemens MCC starter bucket. 480v pri to 120v sec. The previous transformer had the X2 grounded to the frame of the bucket. On the new xfmer I am picking up voltage(With a non contact voltage detector)on the X2 so it is not 0V. By testing it out and grounding X2 to the frame the whole frame becomes live (with ncvd). Can anyone help me out on how I’m supposed to properly run a ground wire for this xfmer?

 

[augie47]

If you connect one terminal ( X2 ) to ground (the enclosure) you will be in compliance with 250.30 and it will not put voltage on the frame. That is the nature of a separately derived system (power transformer)

 

[kwired]

NCVT is picking up capacitance - that is what they function on.

Don't know why it is acting like it is but if you are certain you have a good connection between X2 and the frame, and the frame is indeed grounded something odd is happening with your NCVT. Test with a 'contact' type low impedance meter to eliminate capacitive coupling giving you any misleading readings.

Unknown open circuit condition in what is supposed to be a grounded conductor can give you readings you weren't expecting, with regular meter as well as NCVT.

 

[Rob442]

Didn’t think about hooking up the primary ground for testing

Didn’t think about hooking up the primary ground for testing

NCVT is picking up capacitance - that is what they function on.

Don't know why it is acting like it is but if you are certain you have a good connection between X2 and the frame, and the frame is indeed grounded something odd is happening with your NCVT. Test with a 'contact' type low impedance meter to eliminate capacitive coupling giving you any misleading readings.

Unknown open circuit condition in what is supposed to be a grounded conductor can give you readings you weren't expecting, with regular meter as well as NCVT.

I didn’t think about landing the primary ground as I was testing on my bench.I did and now all conditions are reading ok.

Thanks for your help

 

[kwired]

I didn’t think about landing the primary ground as I was testing on my bench.I did and now all conditions are reading ok.

Thanks for your help

So you were standing on different potential than the item you were testing and assuming was grounded - makes sense you might have some capacitance between the two points and could make a NCVT give a voltage indication.