POCO changing transfomer

[tom baker]

A POCO changes a transformer and installs a larger transformer, with a higher available fault current. The customers service does not have a high enough AIC, who would pay to replace the customers service?
Would this be determined by the utilities tariffs?

[ June 06, 2005, 08:18 PM: Message edited by: tom baker ]

 

[mdshunk]

Re: POCO changing transfomer

That sounds more like a question for a lawyer. Most of the time if the transformer is changed, you or your customer would never know about the impedance or available fault current change anyhow.

 

[charlie]

Re: POCO changing transfomer

We post on the Internet our Goldbook with its drawings. The drawing GB7-060 gives the standard fault current available for the transformer used. If this information is used, a larger transformer will not normally have enough fault current to "blow up" the equipment. Our reasoning is that the parameters given are, "The standard calculated available fault currents are given in amperes, RMS symmetrical, at the secondary bushings of the Company's transformer, assuming an infinite bus and a bolted fault." We are assuming that we will not have the lowest impedance transformer, we do not have an infinite bus, and the fault will not be bolted. If the engineer or electrician moves in on those margins and the system blows apart, it will not be the fault of the serving electric utility but will be the fault of the designer even if we replace the transformer bank.

Normally, when a service is upgraded, everyone knows what the new bank is going to be and may plan for the increased fault duty. Under these conditions, the building owner may not wish to replace the existing equipment and would not be required to replace it under most laws. In some cases, that is where 240.86(A) comes in. It may be applied if the circumstances permit.

 

[physis]

Re: POCO changing transfomer

amperes, RMS symmetrical

Does that mean no neutral current?

 

[charlie]

Re: POCO changing transfomer

No, under fault conditions, the fault current is asymmetrical for the first few cycles of the fault. Even though the fault is not symmetrical, the ratings of electrical withstand is given in RMS symmetrical.

 

[mc5w]

Re: POCO changing transfomer

I have noticed that Home Depot and Lowe's have stopped peddling class H fuses and instead stock class RK5. When my sister bought a house she had me go in and fix a few problems such as dirty fuse clips that caused the mains to nuisance blow. Stuck in class RK5 fuses while I was at it. It was not a UL listed refit but better than not doing it.