Unfused disconnects....

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mb5412

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Hello, can someone explain what circumstance(s) dictate the use of an unfused disconnect vs a fused one and the authority that permits this?

I do understand that article 240 specifies that a conductor is to be protected from overcurrent/shorts where it receives its supply w/exceptions but I don't recall reading about unfused disconnects.

Thanks,

LB
 
Re: Unfused disconnects....

If the conductors and the equipment are properly protected by the branch circuit OCPD, the unfused disconnect is just being used as a "disconnect switch" and there is no reason to use a fused disconnect.
Don
 
Re: Unfused disconnects....

Don is correct. Motor disconnects, air-conditioning disconnects, appliance disconnects, etc., are all examples of disconnects that don't necessarily have to be fused.
Jim T
 
Re: Unfused disconnects....

Many pieces of equipment require fuses for overcurrent protection (in paticular, those marked with "maximum fuse size".) Circuit breakers can't be used as a substitute. That's one reason why you see so many fused disconnects. So there is often a circuit breaker at the panel protecting the wire, and a fused disconnect at the equipment protecting the equipment.

Steve
 
Re: Unfused disconnects....

We used un-fused disconnects for the following reason: If you have a piece of equipment that is being fed from an Electrical Control Room that may be some distance from the motor or what ever piece of equipment that you are dealing with, we would install a un-fused disconnect at the equipment site for maintenance purposes. Who ever is working on that particular piece of equipment could pull the switch and lock and tag out right at his work site. It was a big time-saver and also a good safety feature that ensured the person working on the equipment would lock and tag out.
 
Re: Unfused disconnects....

Thanks for the response. Just had a few more questions.

1)When selecting an unfused disconnect, should attention be paid to the short circuit rating of the disconnect?

2)Is the reason that some equipment disconnects still calls for fused disconnects is to provide selective coordination?

In either case, is there any reference out there to help determine what a particular installation calls for.

Thanks
 
Re: Unfused disconnects....

mb5412
Yes yes yes on the AIC rating. Unfused disconnects are typically 10,000 amps AIC.
Requiring fused disconnects most likely is to limit the fault current and allow the downstream equipment to have a lesser withstand rating.
 
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