Short Circuit Rating

[conmgt]

Hello,

I need to put a disconnect between a 30kVA transformer and a SqD QGA circuit breaker. The circuit breaker has a 65kA rating and is mounted in a 800A service panel fed directly from a dedicated utility transformer.

I'm looking at non fusible disconnects and SqD states:
"Above 10 kA, When applied on systems with greater than 10 kA short circuit current available, the UL Listed short circuit current rating for SqD non fusible switches is based upon the switch being used in conjunction with fuses or SqD circuit breakers or Mag Gaurd motor circuit protectors."

Does that mean that the SqD breaker in the service panel is providing the short circuit rating for the disconnect?

 

[jim dungar]

Does that mean that the SqD breaker in the service panel is providing the short circuit rating for the disconnect?

Effectively there is a listed series combination SCCR between the upstream device and the non-fused switch.
You can only use the breakers which have been tested. I do not see a listed combination using Q-frame breakers.

 

[conmgt]

Actually, the breaker has yet to be purchased. I did notice that no Q breakers were in that list. I'm going to see what other frames are available for that panel.

 

[augie47]

You might check with your utility. With a 30kva transformer and a decent length of wire, your may very well not have over 10k available at your switch.

 

[conmgt]

If I'm reading SqD's chart correctly, R, J, L, T, and H frames are appropriate.

Yeah, I may not need over 10k...not sure. I have about 150' from the utility pole transformers to the service panel and then another 30' from that panel to this switch.

 

[conmgt]

Or maybe install a fusible disconnect instead of a non fusible one and let the fuses provide the protection. I find fuse selection to be overwhelming. Any thoughts/advice on how to select fuses?

 

[JoeStillman]

If that transformer is 208V, 3?, then it would need to have an impedance of 0.8% to provide 10,000 amps at the sencondary. I have never seen one with that low an impedance. A normal dry-type is 5.75%. Oil filled are in the range of 1.5-2.5.

I wouldn't even worry about the AIC rating of the switch.

 

[david luchini]

If that transformer is 208V, 3?, then it would need to have an impedance of 0.8% to provide 10,000 amps at the sencondary. I have never seen one with that low an impedance. A normal dry-type is 5.75%. Oil filled are in the range of 1.5-2.5.

I wouldn't even worry about the AIC rating of the switch.

He's working on the primary side of the transformer, not the secondary.

 

[conmgt]

Yep, I'm working on the primary side...between the main service panel and the 30k transformer.

Does anyone know of a good resource to learn more about the types of and selection of fuses?

 

[ron]

Does anyone know of a good resource to learn more about the types of and selection of fuses?

http://www.cooperindustries.com/con...ry/selecting_protective_devices_handbook.html