Ungrounded controll transformers

[Moparmyway]

I am working in NYC on an old building and mostly every motor controller has its own controll transformer. 480/120. The x1 and x2 terminals on the secondary side read 128 volts when tested to each other. There is no grounded conductor to the x2 neutral terminal (some older transformers have x1 as their neutral).


Taking readings from the fused secondary terminal (usually x1, but sometimes on older transformers its x2) to ground gives inconsistant readings. I believe there could also be a situation where a ground fault on the secondary side would energize the metal components in contact with the ground fault, but would not blow the fuse due to the ungrounded neutral secondary.

Should I install a grounded conductor to the controll (secondary) transformer neutral ??

If I install a grounded conductor to the neutral terminal will the controll circuit operate correctly, or will there be some ill effects ??


I believe it would operate fine, unless there is a fault to ground, and these controll transformers were installed prior to the NEC becoming aware of the importance of grounding and bonding.

 

[don_resqcapt19]

There is no safety issue with the use of ungrounded control power transformers for motor control circuits. A single ground fault creates a grounded system, it does not energize the metal that it is faulted to.
I didn't look, but I seem to recall an exception that would permit these control circuits to be ungrounded.

 

[Smart $]

...
I didn't look, but I seem to recall an exception that would permit these control circuits to be ungrounded.

I didn't look as well, but as I recall, the transformer (secondary being an SDS) has to be rated 1kW or less.

 

[Moparmyway]

Thanks Dudes,

Would there be any advantage in grounding the neutral terminal ??

Was this wired/installed this way for any specific reason, or just because of the age of the installation ??

Thanks for your help and insight

 

[Smart $]

Thanks Dudes,

Would there be any advantage in grounding the neutral terminal ??

Was this wired/installed this way for any specific reason, or just because of the age of the installation ??

Thanks for your help and insight

The advantage in grounding the secondary would be personnel safety... so you have to ask just how much safer personnel would be by grounding. The disadvantage would be possible loss of control circuit on first fault.

Age probably contributes in the control circuit being ungrounded... but stating how much is better left to others :roll: