Short Circuit Question, please help

[trobbins313]

An 800 Amp Busduct rated at 22,000 Amps is protected by an 800 Amp Class L fuse. The available short circuit RMS current is 100,000 Amps. Is the Bus Rating selection acceptable by using the UP OVER DOWN METHOD?
Thanks.

Tim

 

[mayanees]

Get the data sheet for the actual fuse, and use the published let-through number that corresponds to the 100kA source.
John M

 

[trobbins313]

Thank you so much.

 

[ron]

Dynamic impedance of downstream protection make it such that you cannot use the up / over down method.

22kA is not acceptable.

 

[rcwilson]

Dynamic impedance of downstream protection make it such that you cannot use the up / over down method.

22kA is not acceptable.

Ron - I would agree if the bus duct had plug-in breakers.

But if the bus duct is not a plug-in duct and only connects to a downstream breaker or fused switch that is series rated with the current limiting supply fuse, wouldn't the up-over-down method be acceptable? The bus duct should not add any dynamic impedance durng a fault that might affect the current limiting function of the fuse.

 

[ron]

Ron - I would agree if the bus duct had plug-in breakers.

But if the bus duct is not a plug-in duct and only connects to a downstream breaker or fused switch that is series rated with the current limiting supply fuse, wouldn't the up-over-down method be acceptable? The bus duct should not add any dynamic impedance durng a fault that might affect the current limiting function of the fuse.

It is not the busway adding dynamic impedance, it is the downstream protection and the speed in which it opens.

The series rating for the busway is with the upstream proteciton. The downstream protection efects how much fault current the upstream device sees during a downstream fault.