In an existing building the utility is replacing transformers (for an addition which will have a new separate main disconnect). The new transformers have a much higher short circuit let through current. The original building service is a 50 year old ITE switchboard.
The let through current is over 80,000A.
The short circuit rating of the main circuit braker is 50,000A. (the transformers are about ten feet away from the switchboard indoors in a room next to the electric service room)
Any suggestions (other than replacing the entire switchboard)??? The main circuit breaker and switchboard are rated at 1600A.

http://www.ferrazshawmut.com/products/pdf_107/A4BQ.pdf

try current limiting fuses in a appropriate enclosure between the transformer and the main.

Anything put between the transformer and the existing equipment would have to have a tested series rating. It is all but impossible to get a calculated series rating unless the existing equipment are "power circuit breakers".

Have you thought about running the incoming cables around the building a few times? This will add impedance and lower the available fault current.

You only have a few choices. By mearly adding a current limiting device between the transformer and the 50kAIC breaker doesn't help. They must be series rated together, which they generally are not for older equipment.
You can add a reactor downstream of the transformer, and it will limit fault current, but be careful of voltage drop problems.
Validate the fault current that you are being told, at least with an infinite primary calculation, and don't forget to add contribution from downstream motors.
Wire can reduce current significantly, so run lots of wire as mentioned by JBD.

Tie the wire in a knot.

Is the transformer something close to 3750kVA with a 480V secondary?

The Circuit Breaker can be the first or second element in series....

Install L or T type current limiting fuses with an appropriate rating: have an EE design it.

If the bussing permits, install a different breaker.

With such an old piece of gear: scrap it.... (I can't believe that old breaker is still passing its tests.)

Alternately, feed the existing service from the new installation. Re-engineer it.

Blame the extra on the Poco. Not your fault.

Perhaps you could just run each phase in its own conduit.... (*ducking*)

-Jon

Jon,
That would help, but be sure that PVC conduit is used.

Check the fault current as given. If you were given a fault current with an infinite source the actual value will be less. You will need to recaculate the fault including the utiltiy system impedance. The utility may be reluctant to give this information.

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.

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

The utility has already said that they will not put in different transformers. Mostly because if they have to replace, the low impedance transformers are what they will have availible "on the shelf".
As for extra service entrance length - this is only feasible if we install a pad mount transformer outside instead of replacing the inside transformers which is not that bad of an option - BUT what would be the cost of 200 feet of a 2500 amp (or so) secondary. And we would still have a 50 year old switchgear.

Replace the switchgear.

Ron
EXACTLY!

With the parameters set by the OP, it really seems like a new gear is going to be installed.

I see a few options from an installers view.

1) Replace the gear, continue as planed.

2)Do not touch the gear or the transformers, have the power co bring in a second service with it's own transformer to power the addition. (AHJ can allow it)

3)Replace the old 1600 amp gear with a new 2500 amp gear and feed both buildings from this new gear.

4)Bring the service to the addition and feed the original 50 year old gear from the additions service. This would add impedance to the circuit ahead of the old gear.

Anyway you want to go we would be happy to do the work, lots of gear and copper.

Bob

Changing the switchgear has always been the number one option. I am just looking for any lower cost option so I can tell the owner that we looked for lower cost options. The switchgear has the 1600A MCB plus about 12 200A CBs and a few smaller CBs.
Any option that cost nearly the cost of replacing the switchgear is (IMO) not really an option as it they would still have the 50 year old switch gear.
So far the options are:
1. Replace the switchgear.
2. Current limiting fuses before the switchgear if it can be shown that the fuses will limit the let through current to less than 45K at any fault current over 45K up to the available. (I am still not sure this is an option)
3. As mentioned above, a separate service for the addition. I wonder if the utility would install a second set of transformers for the addition in the same location instead of replacing the existing with larger?

I apologize beforehand if my question is awfully stupid, but is it not true that the circuit let through current limit will be 50000 amps (the one of the main breaker, as stated by “cgw”)?

Why is important to consider the transformer short circuit current if it is higher than the breaker one?

Regards,

Joe.-

Joe,

The deal is that the "50,000 Amp" short-circuit rating of the breaker does not mean that it will limit the current to 50,000 Amps. Rather, it means that the breaker is designed to reliably open and quench the resulting arc when up to 50,000 amps is flowing through it. If there is more than 50,000 amps, the breaker may fail to open. It might never be able to quench the arc, or it might fuse solid in the closed position, or might fail through any number of other failure mechanisms.



[This message has been edited by SolarPowered (edited 04-17-2006).]

Which is a very gentle way of saying it could flame out big time or more likely explode, damaging the buss work and causing further chaos!!

I've seen in person the end result of this, it's not pretty!!

Somewhere here on ECN (I think it was the Photos or Violation forums..) there was a picture of what that end result looked like.
(IIRC the thread was titled "I wonder what he's thinking?")