short circuit rating

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jcapp

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I have a problem at a facility. The oringinal service was install with a short circuirt rating
of 35k, my calculation shows a need for at least 45k (I don't have the exact number right now). I'm guessing the Utility beefed up the infrastruture over the last 40 years. What are my choices for remediation. A new 3000 amp service will be costly. One suggestion was to put a smaller transformer with a higher impedance, I don't want to reduce my kva. Do they make any device that will eliminate the SS issue? Maybe an arrester or something that adds resistance?
 
Can you rerun the service feeder to make it longer?
You could install a series reactor.
You could see if there any manuf. series ratings for the equipment you already have.
You could replace the transformer with one that has a higher impedance but same kVA.
 
ron said:
Can you rerun the service feeder to make it longer?
You could install a series reactor.
You could see if there any manuf. series ratings for the equipment you already have.
You could replace the transformer with one that has a higher impedance but same kVA.
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You also might look into adding a fusible main if the manufacturers will series rate the existing with a current limiting fuse.

We had the same problem at a school recently and the engineers decided on a series reractor, but some of the old gear was 5k so they may not have had many options.
 
jcapp said:
. . . my calculation shows a need for at least 45k . . .
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Start your calculations over. Contact the serving electric utility to get the actual expected available fault current. Use the software from Bussmann or similar company to get closer to the actual amount at the service. In my opinion, you should enlist the help of a registered professional engineer that specializes in this type of work and not get the software.

The reason for all this is that most electric utilities will give you the maximum available fault current based only on the impedance of the transformers serving your facility. This results in the maximum that assumes a bolted fault and an infinite bus; neither will happen. It is my bet that nothing needs to be done other than to do your due diligence. :)
 
I thought of longer feeder but it's not practical

I'll check on a series reactor ( i don't know what it is where would i find info?

The equipment is old, I checked for upgrades, and I checked if the manufacturer could certfiy the bracking and a higher rating--no good

The rating on the trans is 6, how high do the go?I will look into a fusible main and a series reactor

I did consult an engineering company, in fact they surfaced the problem. I acquired the fault current number from the Utility. I?m not confident that SS is the engineers specialty, mine either. Are you saying that the Utility may have protection in place that my change my options?
 
jcapp said:
I have a problem at a facility. The oringinal service was install with a short circuirt rating
of 35k, my calculation shows a need for at least 45k (I don't have the exact number right now). I'm guessing the Utility beefed up the infrastruture over the last 40 years. What are my choices for remediation. A new 3000 amp service will be costly. One suggestion was to put a smaller transformer with a higher impedance, I don't want to reduce my kva. Do they make any device that will eliminate the SS issue? Maybe an arrester or something that adds resistance?
Click to expand...

Ok, step 1, follow Charlies advice. If you still have a problem post your equipment types, some are diret swap for higher AIC's and pretty easy. If that dosent work a current limiting reactor is your best bet.
 
You know, I don't have a problem with a transformer with a higher impedance; however, isn't this an electric utility that is supplying the service? It is my experience that a crew is going out to replace a transformer and the dispatcher orders the transformer to be taken out from the yard. The supply truck picks up the requested kVA and voltage transformer and delivers it to the site. I assure you that the supply truck driver doesn't know what impedance looks like on the nameplate and he will pick up the next one in line at the yard.

Bottom line, unless this is a very special order and you are working very closely with the proper utility engineer, you are not going to get the impedance you would like to have on your new transformer. :smile:
 
All,

Reducing the size of the transformer with a higher Z%, did reduce the AIC to an acceptable number--Thanks--

But now the engineer is saying that the downstream motors will produce fault current and in series they may produce fault current that will blow my bracing. Is this accurate? How do I determine the fault current? After the transformer is replaced am I still in code violation.--------------the Utility wants to replace the trans, free of charge, they say it will save them power.
 
jcapp said:
All,

Reducing the size of the transformer with a higher Z%, did reduce the AIC to an acceptable number--Thanks--

But now the engineer is saying that the downstream motors will produce fault current and in series they may produce fault current that will blow my bracing. Is this accurate? How do I determine the fault current? After the transformer is replaced am I still in code violation.--------------the Utility wants to replace the trans, free of charge, they say it will save them power.
Click to expand...

For a rule of thumb, use 6x FLA as motor contribution to the available fault current. And another rule of thumb, for your guesses and for ignoring X/R ratios; your equipment needs to be rated 20% greater than your calculated available fault current, or you need to know exactly all impedances and sources in your system (i.e. don't use rules of thumb).
 
JWCELECTRIC said:
Jim on your rule of thumb for motor contributed loads, are you using 6 x Locked Rotor Amps, or 6 x FLA of the motors?

- JWC
Click to expand...
For short circuit contribution:use either 6X FLA or 1X LRA. Induction motors typically cannot produce more power than they consume.
 
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