Reducing Utility Fault Levels on LV System

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pgonski

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We have a project where an existing facility is going to be replacing their utility transformer and the LV main breaker.

During a short circuit analysis, it was discovered that given the fault current from the utility, the existing panelboards are not properly rated, given the short circuit levels on the bus.

Any suggestions on how to reduce the short circuit from 24kA down to below 22kA? So far I was thinking of using reactors, or additional cable lenghts, but if there are any other options such as current limiting fuses/breakers, I'm all ears.

Thanks!
 
pgonski said:
We have a project where an existing facility is going to be replacing their utility transformer and the LV main breaker.

During a short circuit analysis, it was discovered that given the fault current from the utility, the existing panelboards are not properly rated, given the short circuit levels on the bus.

Any suggestions on how to reduce the short circuit from 24kA down to below 22kA? So far I was thinking of using reactors, or additional cable lenghts, but if there are any other options such as current limiting fuses/breakers, I'm all ears.

Thanks!
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Current limiting fuses or breakers would need to be engineered into a system, so they are often not a good option for retrofit. If the utility has not already placed the transformer, ask them it they can provide one with a higher impedance (Z). If not, I've found that for small amounts of reduction like that, increasing the cable length ends up being the least expensive route. It won't take much.
 
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Phil Corso said:
PGonski...
What is voltage level?
Regards, Phil Corso
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Voltage level is 120/208 on our side.

Unfortunately the utility is unable to provide me with a better assumption than using an infinite bus on the transformer primary, but without better info, I have to use that :-(
 
pgonski said:
Voltage level is 120/208 on our side.

Unfortunately the utility is unable to provide me with a better assumption than using an infinite bus on the transformer primary, but without better info, I have to use that :-(
Click to expand...
That's pretty lame on the utility's part. I'll second Jraef's suggestion above. The best solution in this case would be to get the actual available fault from the utility. Since that doesn't seem to be an option, see if you can get a transformer with a higher than normal percent impedance (%Z). If the utility won't go for that option, or if the new transformer's already been installed, then it's time to start looking at adding some length to your conductors coming from the transformer secondary. At 120/208 the fault level should drop fairly rapidly as you add length.
 
SparkyHC said:
So your service is 208/3P, how big in amperes?
400 ampere RK5 fuses will drop 25k to less than 10K...
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however, you can only use this to protect things other than OCPD downstream. OCPD have to be specifically tested with each other to get a higher AIC rating of the combination.
 
petersonra said:
however, you can only use this to protect things other than OCPD downstream. OCPD have to be specifically tested with each other to get a higher AIC rating of the combination.
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Bob, not sure I understand what your saying...for fun, if the let through of the fuse is less than 10kA are saying I am "not" allowed to connect a 10kA rated sub-panel?
 
SparkyHC said:
... if the let through of the fuse is less than 10kA are saying I am "not" allowed to connect a 10kA rated sub-panel?
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That is absolutely what he is saying.

There is no major fuse manufacturer that has not warned against using fuses to 'reduce the fault current' to molded case breakers, for alt least 30years.

Over the past decade, language has been added to the NEC effectively requiring fuse and molded case breakers to be actually tested for a series-combination rating.
 
SparkyHC said:
Bob, not sure I understand what your saying...for fun, if the let through of the fuse is less than 10kA are saying I am "not" allowed to connect a 10kA rated sub-panel?
Click to expand...

Suggest you check out what Jim is saying and find out why this is true.
 
SparkyHC said:
Bob, not sure I understand what your saying...for fun, if the let through of the fuse is less than 10kA are saying I am "not" allowed to connect a 10kA rated sub-panel?
Click to expand...

That is exactly what I am saying.

Short of actual testing of that specific combination under UL auspices.
 
pgonski said:
Voltage level is 120/208 on our side.

Unfortunately the utility is unable to provide me with a better assumption than using an infinite bus on the transformer primary, but without better info, I have to use that :-(
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In your case, that probably makes little difference since your transformer % Z is so small. If the transformer is very big or you are a long way from the substation, it can start making a difference.

See an example of transformer size differences and the maximum with and without infinite bus here:
http://forums.mikeholt.com/showthread.php?t=152307&p=1473982#post1473982
 
pgonski said:
We have a project where an existing facility is going to be replacing their utility transformer and the LV main breaker.

During a short circuit analysis, it was discovered that given the fault current from the utility, the existing panelboards are not properly rated, given the short circuit levels on the bus.

Any suggestions on how to reduce the short circuit from 24kA down to below 22kA? So far I was thinking of using reactors, or additional cable lenghts, but if there are any other options such as current limiting fuses/breakers, I'm all ears.

Thanks!
Click to expand...

2K is not much. But when it gets that close, I subtract 10% from the transformer impedence since it can vary 10% from the label.

If you know the voltage and wire size supplying the transformer, you could move the infinate bus from the transformer primary, back some distance. Say for example, to the property line.

Its probably pretty safe to assume the POCO will never build a substation that can supply infinite current directly on your clients property.
 
don_resqcapt19 said:
240.86.
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If you use 240.86, IIRC, series ratings cannot be used if:
"1.) Motors are connected on the load side of the higher rated overcurrent device and on the line side of the lower-rated overcurrent device, and
(2) The sum of the motor full-load currents exceeds 1 percent of the interrupting rating of the lower-rated circuit breaker." (240.86(C))

240.86 offers two options:
  1. 240.86(A) Selected Under Engineering Supervision in Existing Installations and the provisions mentioned above OR
  2. 240.86(B) Tested Combinations and the provisions mentioned above!
 
let's back up a step...

let's back up a step...

How did my earlier comment morph into a "series rated combination"?
The OP would like to reduce 24kA to less than 22kA...my comment was somewhat general as we
have not been advised of the transformer size, the impedance, the ampere rating of the main overcurrent device, I recommended
the use of a fuse with a reduction in let through current. The example I gave was a 400 ampere, RK5 fuse which reduces a 24kA
line side short circuit current to less than 10kA...
Assuming the OP's gear is rated at least 10kA (most are) this would enable a fully rated system, not a series combination....
 
another step...

another step...

Also, for the few interested...I'm reading an "Electrical Protection Handbook" from Cooper Bussman which has seven pages
of series combinations of line side fuses and load side circuit breakers. Of course Bussman fuses are line side with load side circuit
breakers from Square D, Cutler-Hammer, General Electric and Siemens...
These charts can be used by an electrician without requiring some engineer to sign off as they are "Tested Combinations"....
 
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