NON FUSIBLE SAFETY SWITCH SCCR WHEN PROTECTED BY MCP

engineerintraining12

Senior Member
Location
Saint Louis
Hey everyone,

Not trying to clog this forum with too many posts, I just have a big project I'm trying to complete and wanted to raise a question I had about the SCCR (Short Circuit Current Rating) of a non fusible disconnect when it is protected by a motor circuit protector (instantaneous only circuit breaker). I have a Siemens non fusible disconnect (Catalog #HNF363J) being fed by an Allen Bradley 140MG-G motor circuit protector located in a motor control center. I know that the fusible disconnect has an SCCR of 10 kA when protected by any UL listed circuit breaker, but is this the case when it's fed by a motor circuit protector that does not have the same UL listing as a circuit breaker?

Best Regards
 

kwired

Electron manager
Location
NE Nebraska
I am going to assume this may be a local disconnect at the motor location which generally would be required by code?

If so you also need to consider the fact that the available short circuit current at the motor is going to be less than the available short circuit current at the MCC because of impedance of the circuit conductors between those two points. Unless short distance to the motor, often times the available current will likely be low enough you don't have a problem with this. With no other information I believe the thing should still be rated for at least 5kA SCC in all cases.
 

engineerintraining12

Senior Member
Location
Saint Louis
I was thinking the same thing. Siemen's 10 kA rating for their non fused disconnects is only valid when it's being protected by a UL listed circuit breaker. In my scenario, the non fused disconnect is protected by a motor circuit protector, which is not UL listed. The available fault current at the disconnect in question is only around 2000 amps, which is low enough to pass the minimum 5 kA rating assumption.
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Most NF disconnects I have looked at are actually 10kA WITHOUT fuses or a circuit breaker ahead of them. Those that have higher ratings with breakers are always with SPECIFIC breakers and always from the same mfr as the NF Disconnect (because who would pay to test with a competitor's products?).

But I see the statement you are looking at for the Siemens disconnects. Interesting catch. I would interpret that to say if the entire assembly (meaning the factory assembled Combination Motor Starter with an MCP) is UL listed, that would qualify as a "UL listed circuit breaker". If not, then that would mean that anyone using a Siemens MCC that had buckets with MCPs in their starters would have to use a Square D or C-H disconnect down stream!
 

engineerintraining12

Senior Member
Location
Saint Louis
Most NF disconnects I have looked at are actually 10kA WITHOUT fuses or a circuit breaker ahead of them. Those that have higher ratings with breakers are always with SPECIFIC breakers and always from the same mfr as the NF Disconnect (because who would pay to test with a competitor's products?).

But I see the statement you are looking at for the Siemens disconnects. Interesting catch. I would interpret that to say if the entire assembly (meaning the factory assembled Combination Motor Starter with an MCP) is UL listed, that would qualify as a "UL listed circuit breaker". If not, then that would mean that anyone using a Siemens MCC that had buckets with MCPs in their starters would have to use a Square D or C-H disconnect down stream!
I agree with your point about the entire assembly being UL listed, I would definitely want to verify with Siemens what their interpretation is based off the wording on their disconnect switch catalog. Interestingly enough, Eaton has gone out of their way to put a statement on one of their technical documents saying, "UL does not permit series combination ratings with motor circuit protectors."

Check out the attached
 

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