Equipment SCA and protection

[farmape]

Electrical equipment often has a “Short Circuit Amps (SCA)” rating. If the available SCA exceeds the equipment’s rating, but the upstream protective device to this equipment is sized to operate at a value less than the equipment’s SCA rating, is this acceptable?
Thanks

 

[dpc]

In general, no. Because you don't know which device will operate first. If the upstream device and the device in question have been tested together and are UL (or similar) listed as a tested combination, AND the tested combination has an interrupting rating exceeding your available fault current, then it is acceptable.

 

[lansford]

I agree with DPC for Low Voltage Equipment, but I have seen may studies by equipment manufacturers on Medium Voltage equipment where the upstream devices clear before donwstrem protective devices and the studies considers this acceptable. One example is for low intrrupting bayonette fuses in a padmount transformer, which are ovrdutied and the upstream breaker clears the fault.
This is contrary to the UL series rating rules on Low Voltage equipment. I have not gotten what I consider good scoop on this but I have seen it a number of times in studies from these reputabe companies ?
JIM

 

[farmape]

Thanks for the info. I have also seen this anomaly with MV equipment. LV systems, especially old systems with new engineering studies, sometimes have equipment rated less than the available calculated SCA. What can be done immediately (without massive capital investment)? For adjustable trip units on CBs the Inst. can be set below the equipment rated SCA in the interim, but, from what I'm hearing, this is not a permanent answer?

 

[magoo2]

You might be able to apply current-limiting fuses ahead of devices with insufficient short circuit capability. That may be one of the easier fixes to consider without too high a price tag.

Another option is installing current-limiting reactors. This lowers your available short-circuit current, increases your voltage regulation and is a more significant cost to consider. It will require you to rework your short-circuit study too.

 

[dpc]

Adjusting the trip units is not even a temporary solution. That does nothing to reduce the magnitude of the fault current.

You can install current-limiting fuses upstream but again they must be a test combination with the breakers you are trying to protect. If the fuse is too large, it won't help. The breaker manufacturer should be able to give you information on what current-limiting fuses would work.

 

[Zogzog]

You can also upgrade you breakers for increased interuption ratings. We do alot of those.