ECN Forum Forums General Topics of the Trade General Discussion Area New Transformers/New Short Circuit Current

Another thought is an isolation transformer, with the output impedance appropriate for your needs.

Bob,

Many utilities want you to use their maximum design level short circuit currents for equipment selection so that this type of problem does not happen the next time they change transformers.

"Tie the wire in a knot."

Won't that make the electrons dizzy and create harmonic distortion?

JBD
I am aware of what you said. Thats why I said the utilities are reluctant to give out this information. Most utilities use the infinite method for fault current.
It seems to me there are 3 choices.
1. Install reactors to reduce the fault or
2. Wrap the service around the building til
the fault current is at a safe level.
3. Replace the main panel. Bet someone
didn't plan on that.

Bob,
The utilities in this area do not simply use an infinite bus. They determine the maximum size transformer that may ever be installed at the site (this may be due to a system upgrade or an emergency replacement) and they use the lowest transformer impedance that they contract to purchase. They call this their design level.

My situation is that the transformers are located in a vault right next to the service room. There is no real impedance between the trans formers and the main circuit breaker. There is not really room in the service room to add disconnects ahead of the switchgear and I can not put the main disconnect in the transformer vault.
Can anything be put between the transformers and the main disconnects on the service entrance (e.g. fuses)?
(This is 208V)

The only benefit fuses will offer, would be if they are part of a UL listed series combination with the under-rated equipment.

Contact Bolt-Switch and have them make a custom assembly suitable with T type fuses.

The way I read it:
If series rated circuit breakers are used, the circuit breakers must be listed for the use. Obviously I am not going to fined a main circuit breaker that will be listed for 50 year old branch circuit breakers.
On the other hand, current limiting devices can be used to protect the down stream circuit breakers.
Agree or disagree?

If the above is true, the next question is - can current limiting fuses be installed in the service entrance before the main disconnect (see 230.82(1)) and can they be used to calc. the available short circuit current at the circuit breakers in the main switch gear?

No current limiting device (including fuses) may be used to protect molded case circuit breakers unless they have been tested for a series rating.

It is all but impossible for calculations to be used on any protective device except power/iron frame/drawout style breakers.