Minimum Clearance Between Parts 1

[DURY]

As part of the installation of a 13.2kv Y/480/277v dry type transformer, the electrician installed an insulated (600V) ground conductor in contact with one of the windings. Although the windings were insulated, a failure took place between the high voltage winding and the insulated ground, within one year, requiring the transformer to be replaced. The installation instruction of the transformer gave no instruction on any required clearances. Are there any standards for field installed wiring related to transformers? I don’t think NEC 490.24 applies since both the windings and the grounding conductor was insulated.

 

[alehman]

In a standard dry type transformer the primary windings have a very thin varnish insulation intended to insulate only for the turn-to-turn voltage (a few tens of volts), not for voltage to ground. That combined with the 600V insulation on the ground wire was able to withstand the 7600 volts for only so long before it failed.

The manufacturer should be able to provide recommended clearances. But for 13.2kV, 95kvBIL switchgear, you normally need at least 5 inches to rigid components. I would want a lot more for a wire that's not tied down (it wasn't tied to the windings was it?). The 600V insulation on the wire provides no value in reducing the clearance requirement.

Regardless of manufacturer's instructions, any competent electrician should know this is not acceptable installation practice for any transformer (regardless of voltage). The electrician owes you a new transformer.

 

[busbar]

If a field installation practice reduced the transformer-BIL ratings through conductor placement {not of the manufacturer} then it could be considered poor workmanship.

 

[GusD]

Hi Dury

I have installed transformers in the field as well as rewound some Hi-voltage transformers Dry or Wet.I don't recall a single time when the ground wire or cable is run near the windings.As a rule, the Hi-voltage coils surround the inner Low voltage coils.At the factory or at repair shop,the Star point/s/or Grounds are brought to Bushings or studs with suficient clearances for the operating voltages.
It is true that the high- voltage coils have very little voltage between each other,consequently the insulation of the coils are also rated for lower voltage.The problem is that the Transformer Phase leads(UVWXYZs)are at full voltage potential and who ever did the job might not have known that.
I cannot comment on the workmenship,I do not know the circunstances of why anyone would do that,however;it still a bad practice under any circunstances.

GusD

 

[jbartos]

Suggestion: Adhere to:
OSHA, ANSI C2 standard - NESC, etc.

 

[DanDel]

Dury, as GusD said, it is unusual to have a ground wire near the windings, as typically any core ground is connected to the frame at the bottom of the assembly. To what was the ground wire connected?

 

[DURY]

The ground conductor was used to bond the primary conductor shields to the transformer frame.

 

[DanDel]

Then the primary conductors were fed into the compartment from the top?
The ground wire should have been routed as far away from the windings and connections as possible. Even using a cable with the correct system insulation might not have lasted, since the voltage potential could not have been equalized around the insulation by using shielding.