Equipment Derating 2

[Binwork]

For low voltage system 480/277V 3Phase 4 wire system.
When utility short circuit X/R is higher than equipment test X/R.
Then Equipment will face derating.
I am in NYC, I have see many buildings have 200kA 480/277 3Phase incoming short circuit with high X/R, then service equipment with 200kAic withstand raiding/ interrupt rating are not good due to derating. And I have not see any equipment are large than 200kAic.

So how do you handle this scenario? is there a solution to fix the code violation?

If there is not a solution. then how do you explain this scenario to your customer since it is violation.

 

[waross]

The impedance of the supply conductors will often bring the ASCC at the equipment down to an acceptable level.
I have seen a minimum length of supply cable specified to bring the ASCC at the equipment down to an acceptable level.
The extra cable was doubled back in a cable tray.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[dpc]

Cable limiters are generally used on each of the incoming conductors ahead of the main device. Also I agree with Bill, the actual available fault current at the main switchboard will often be quite a bit less than the fault current the utility states. In addition, the utility will have network protectors.

 

[davidbeach]

Don’t count on the network protectors to do anything. They’re quick to isolate reverse problems but very slow (generally) for forward faults, letting cable limiters and other devices have first crack at trying to clear the fault. 208V area network faults may also be allowed to simply burn themselves out.

When one this sentence into the German to translate wanted, would one the fact exploit, that the word order and the punctuation already with the German conventions agree.

-- Douglas Hofstadter, Jan 1982

 

[Binwork]

The utility company provide short circuit at the Point of Entry, and usually utility company's cable connect to the service switchboard directly. Therefore, when doing power system study, the service switchboard will have ASCC higher than equipment withstand rating and/or OCPD (breaker or fuse) AIC rating.
For example. utility company provide short circuit 200kA x/r=8. Then the service switchboard usually will have 210kA short circuit current.

And no matter what, as long as study show short circuit current higher than equipment Aic rating. then it is violation. So how can you explain this/ convince customer about this scenario?

 

[waross]

Translation of a notice over the door of a small jail in Latin America;
"The Law is hard, but it is The Law."

--------------------
Ohm's law
Not just a good idea;
It's the LAW!