transformer question

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matt

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Utah
A few days ago our company received a service call. While digging the new service to a building next door, the power company broke a primary feeder cable (7200 Volts to ground) single phasing the service to our customer.

Our customer has a 120/208 3 phase service. A piece of equipment required 480 volts. Installed was a 30 kva 480v delta - 120/208v wye transformer reversed. The installer tied the center tap of the "primary" to the building steel. The 480v "secondary" was ungrounded.

When the building was single phased smoke came out of the transformer. Upon investigation the wire (6 AWG thhn) from the center tap to the case of the transformer was fried.

I know the center of the primary is not supposed to be bonded but had a difficult time explaining to the customer why not; and also the reason the wire fried upon single phasing. After all, there have been no apparent problems until today..

Can anyone help me explain this?

Thanks,

Matt

[ May 15, 2003, 10:52 PM: Message edited by: matt ]
 
Re: transformer question

Normal operation there is no current flow from the star point to ground. Loss of one phase will cause high load current to flow on the star point to ground.

The source star point is grounded, and the primary, of this transformer is grounded, there is a complete circuit.
 
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