Transformer Bonding

[wireddd]

We have to install a 30 KVA 3 phase transformer
to step 240 v down to 220 for a new CNC machine.
On the secondary is it required to bond X4 to the grounding conductor at the transformer even if we are not going to use the neutral?
The supply is 240 volt three wire, secondary is 220 volt three wire.

 

[jim dungar]

Re: Transformer Bonding

Yes, it is required. Transformer secondaries have to be grounded when the resultant phase to ground voltage would be 150V max.
Anyway it is probably a good idea. Most electricians usually expect to see a ground reference. Also there may be noise/harmonic mitigation advantages to a grounded wye system feeding CNC machines.

edited consistency of response

[ August 22, 2004, 09:27 PM: Message edited by: jim dungar ]

 

[iwire]

Re: Transformer Bonding

If you did not bond the transformer secondaries a ground fault on the secondaries would not operate the overcurrent protection.

This is good sometimes but I believe you need ground fault indicators to run it this way.

 

[jim sutton]

Re: Transformer Bonding

I too would ground the transformer so that the overcurrent device's would operate in case of a ground fault. But I do not interpret it to be required due to 220.20 (B)(1). Even if you ground the transformer, wouldn't the voltage to ground still be above 150v?

But if you don't ground it you should install ground detectors to alarm in case of a fault.

If you ground one leg, you have to identify the grounded conductor throughout the system just like any other grounded system. (200.6)