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

JBD
So (for example) if there were a current limiting device between the service transformer and the main disconnect that limited the 100,000 amps to say 60,000 amps. But the availible short circuit current at the main disconnect would be calculated as if the current limiter were not there?

cgw,
Yes.

Points to ponder: http://www.iaei.org/subscriber/magazine/05_e/nema.htm

-Jon

Thanks winnie.
That is helpful. It addresses exactly what I am talking about.
The analytical method is possible but pretty iffy.

Not only iffy, but unlikely that you will get a PE to sign off on it.

According to the referenced article.
"The primary difficulty in meeting the analysis criteria is the fact that circuit breaker contacts open very rapidly, especially when the fault current is higher than the rating of the circuit breaker. "

Which leads me to the question of 'What does it mean for a fuse to be current limiting?'

If the breaker is rated to 50kA, and the fuse is 'current limiting' to 40kA, then the breaker should never 'see' more than 40kA...but if, in the initial moment of a short you actually see more than 50kA of current, until the 'current limiting' effect kicks in, then the speed of the breaker is quite relevant.

What differentiates the following:
1) Limiting fault current via 'current limiting' fuses.
2) Limiting fault current via the resistance of extra service/feeder conductor.
3) Limiting fault current via adding line reactors.
4) Limiting fault current via coiling the service/feeder conductors up to form 'air core' inductors.

The consensus seems to be that 1) is 'hard' and that a PE would need to sign off on this application, but that 2) is easy, just part of normal fault current calculations, simply that you would need quite a bit of conductor. What about 3) and 4)?

-Jon

Jon,
"Limiting fault current via 'current limiting' fuses." is different than "Limiting fault current via the resistance of extra service/feeder conductor.", "Limiting fault current via adding line reactors." and "Limiting fault current via coiling the service/feeder conductors up to form 'air core' inductors."

BTW, another option is to request the utility company to provide a high impedance service transformer, so the resulting fault current will be lower.

The limiting fault current via 'current limiting' fuses is dependent on the amount of fault current. The fuse is an inverse time type device, so that if anything downstream or upstream should lower the fault current by their actions dynamically, the fuse will perform differently (generally slower).
The other methods of fault current reduction, reduce fault current no matter what the devices may do.

Winnie,

A current limiting protective device has two primary selection values, one of which is Ip, the peak current that flows in the circuit before the fuse begins to melt(according to the NEC this must be substantially less than without the device). The other is the Tc or total current clearing time (UL allows 1/2 cycle maximum).

Install a reactor ?! are you mad - what about all that radiation protection you would need - all those lead sheets going to cost ya.

Rugged