26KVA for 1 sec Fault rating

[MCCdesigner]

A stupid question,
I was asked what (in the field of control panels and MCC's) what 26Kva for 1 sec was, I know its a fault rating I wonder if any of you would be so kinda to tell me what it means, I thought it was something to do with the max KVA a given setup (busbar system) would take before failing.

and how do you measure it?

 

[davidbeach]

26kVA (not Kva) for one second would be nothing, certainly not a fault. 26kA might be a fault rating (not a standard rating though) with 1 second as a clearing time. But 26kVA on a 480V MCC is less than 32A.

 

[jraef]

LOL, good catch davidbeach. I was about to answer that it is a fault bracing or a withstand rating, which it would be were it not for the "V".

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[davidbeach]

Then there is the possibility that it is 26kV for 1 second as a dielectric test. But certainly the quantity in the original post is not correct.

 

[jraef]

I doubt that it's kV david, for an MCC that would be extremely high unless it was a medium voltage MCC.

MCCdesigner,
In the context of a low voltage MCC, there are two ratings that it (26kA) could pertain to.

Bus bracing, which is the ability of the bus bar supporting structure to keep the bars from coming together under a fault condition of that magnitude. However, that is usually not expressed with a time duration, it is just the magnitude, and most MCCs on the market are not built for any less than 42kA that I know of.

Withstand rating, which is the rating of the entire system, including all devices inside, to withstand the forces of 26,000 amps attempting to flow through a fault before an upstream device has a chance to clear it. So if there is a circuit breaker feeding the MCC, they know that the breaker has a "let through" of 26kA, and a clearing time of under 1 second, so they want to ensure that everything downstream from that breaker can withstand that much current for that long without becoming a projectile. In an MCC then, either you have a current limiting main breaker or fuses that would limit fault current to levels that individual downstream components can handle. An alternative would be for every downstream device to be capable of that fault level.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[waross]

Just a guess fellows but could it be the length of time the mcc could withstand a 26ka fault before the heating started to degrade the insulation system?
respectfully.

 

[REDDOG]

Could be the interupting rating of a MCC bucket containing a MCB or MCP in combination with a starter

 

[jpcqub]

Think you've typed it in wrong. If its 26kA for 1 sec then its a short circuit capacity of 26000 amperes rms symmetrical available at incoming line terminals of the MCC.