switchgear short circuit ratings 1

[mikeengurs]

We have an old medium voltage Allis-Chalmers switchgear system. The breakers are rated for 1200A, and 29KA interrupting rating. The e-tap model shows a 3-phase bus fault value of 40KA. This is all contribution at the bus these breakers are connected to. My question is, when we are considering bus fault, should we be looking at bus or breaker short circuit rating?

 

[dpc]

You need to look at the maximum fault current that can flow through each breaker (from either direction). The fault duty can be different for each breaker. So if you look at a bus fault report, you would (in general) subtract the fault current flowing into the bus from that breaker to figure the breaker fault duty.

Some programs can compute the individual breaker duties automatically. I don't know about ETAP.

Since it appears your breakers may be underrated or at least close to it, you may also need to consult ANSI C37 to make sure your software is calculating the fault duty in accordance with the ANSI requirements for that vintage breaker.

It looks like there may be some problems, based on the data you have provided.

 

[mikeengurs]

dpc, the 40KA value is the total sum from all contributions including the utility. The SC going through each breaker is less than 29KA. I guess, I need to look at the situation where fault is at load side of the breaker, and look at the source bus contribution to consider the other direction.

 

[dpc]

Right - you need to look at both directions, if applicable.

If you're within 80% or so of the breaker rating, you should probably do an ANSI C37 calc. Maybe that is what ETAP is giving you - not sure. But it is not a straight Ohm's Law network reduction.

 

[Zogzog]

Are these 500MVA breakers? FB or FC's perhaps? There are upgrades available if you find you exceed your breaker ratings.

 

[mikeengurs]

dpc, yes, etap outputs both directions. It faults all buses and calculates SC KAs through all breakers one bus at a time. Zogzog, these are old MA250-C1, rated at 29KA interrupting. Almost all SC current is coming from the Utility. If we install an current limiting fuse there, we'd lose coordination with downstream breaker. If we use a series reactor, we'd have to make sure MV motors can start. Is anyone familiar with reactor manufacturer? Are they expensive? Do they have a long lead time?

 

[dpc]

Check with Trench about reactors - but there are others.

If you're really up against it you could check with these guys about their CLIP device:

http://www.gwelec.com/current-limiting-protector-clip-p-102-l-en.html

But the best solution is to upgrade the breakers.

 

[mikeengurs]

I'll check them out. It seems like a good solution. I agree with you that upgrading breakers is the best option, but will be expensive. At least we have some options to consider. Thanks a lot!

 

[Zogzog]

Upgrading breakers will not be an expensive as you think, I have about 50 MA-350C-1's in stock an can give credit for your old ones after the swap. Not only do you upgrade MVA you also have newly refirbished breakers.

 

[ScottyUK]

What is the bus fault rating in this board? Installing higher rating breakers only helps if the bus bracing is good for the higher fault level. I don't know North American gear very well, but this is certainly a consideration when uprating British gear.


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If we learn from our mistakes I'm getting a great education!

 

[mikeengurs]

ScottyUK you are correct, I'm not sure what the bus fault rating is. I need to verify this before deciding what the bext solution is. ZogZog, thanks for your comments. We might decide to go that route. In that case, I'd contact you. Do you know what lead time we should expect?

 

[Zogzog]

Depends on how many breakers you need.

http://www.cbsnuclear.com

 

[mikeengurs]

A question that came to mind. These breakers are rated for 29KA, but 250 MVA. The study shows bus as 33.1KA at 4.16KV. That is equivilant to 238 MVA. Does that mean they're still under the SC rating?

 

[dpc]

Probably not. You have to use the current ratings - the MVA ratings are nominal. This is addressed in the ANSI C37 procedure.

You might want to get a copy of A Practical Guide to Short Circuit Calculations by Conrad St. Pierre. He covers all of this in detail.

As I mentioned before, this is not a straightforward calculation.

 

[mikeengurs]

To add a conclusion to this, I found out that Breakers were rated with MVA prior to 1999, and KA ratings get adjusted based on rated and operating voltages. E-tab and other programs does the adjustment. After 1999, the KA rating given by the vendor is constant, and no MVA adjustment is done. As first, it was confusing that e-tap adjusts some breakers and not all, but it was doing the right thing.

 

[Zogzog]

Yep, everything changed in 1999, here is a paper that goes into more details.