Fusible vs. Non Fusible Disconnect Swiches

[mstrlucky74]

Is there a general rule or instances when one is used over another? Motors for example most of time have unfused disconnects. Thanks.

 

[Jraef]

If you already have the proper OCPD in a circuit, you don't need to duplicate it. For example a dedicated feeder CB in a panel going out to a machine, if the CB fits the protection requirements of the machine, but you want a local disconnect, you can use a non-fused disconnect.

One potential pitfall however is that NF disconnects are rarely rated for using on a circuit with more than 10kA fault current (i.e. they have an SCCR of 10kA), unless there are fuses ahead of them and they are the same fuses you would have used had it been a Fused disconnect. SOME NF disconnect manufacturers have had their series listed with their own circuit breakers, but even if so, it is a very specific series listing.

 

[augie47]

In "the day" I leaned toward fusible for a few reasons:
(a) occasionally equipment calls for fuse protection (no allowance for breakers)
(b) occasionally the equipment would call for protection other than what was originally spec'ed and the proper breaker was slow shipping or higher priced than a set of fuses.
(c) a lot of my industrial customers wanted fusible ahead of equipment such as motor starters for the current limiting advantages.

 

[infinity]

Is there a general rule or instances when one is used over another? Motors for example most of time have unfused disconnects. Thanks.

For the most part the disconnects you're referencing are used as maintenance disconnects so someone can work on the motor. Typically it would be a waste of money to use a fused disconnect for this application.

 

[mstrlucky74]

Thanks guys.

 

[kwired]

If you already have the proper OCPD in a circuit, you don't need to duplicate it. For example a dedicated feeder CB in a panel going out to a machine, if the CB fits the protection requirements of the machine, but you want a local disconnect, you can use a non-fused disconnect.

One potential pitfall however is that NF disconnects are rarely rated for using on a circuit with more than 10kA fault current (i.e. they have an SCCR of 10kA), unless there are fuses ahead of them and they are the same fuses you would have used had it been a Fused disconnect. SOME NF disconnect manufacturers have had their series listed with their own circuit breakers, but even if so, it is a very specific series listing.

Keep in mind available fault current at the end of a long run isn't the same as it is at the start of the run, and the smaller the conductors are the more limiting effect they will have.

 

[ggunn]

Keep in mind available fault current at the end of a long run isn't the same as it is at the start of the run, and the smaller the conductors are the more limiting effect they will have.

Generally unfused switches are rated at 100% of load current while fused discos are not. For example, if I am designing a PV system where the inverter output is 30A, I can use a 30A unfused disco if I have OCPD between it and the service, but if I have to have a fused disco it must be a 60A switch.

 

[kwired]

Generally unfused switches are rated at 100% of load current while fused discos are not. For example, if I am designing a PV system where the inverter output is 30A, I can use a 30A unfused disco if I have OCPD between it and the service, but if I have to have a fused disco it must be a 60A switch.

I run into that with motors as well. Partly because that motor that needs a 40 amp fuse to allow it to start would need a 60 amp fused switch to be able to accept the 40 amp fuse, but the motor current x 1.25 is still less then 30 so a 30 amp non fused disconnect will work unless the hp rating of the switch is too low (generally it isn't though).