# What is the point of having a fused disconnect on a motor circuit?



## Bird dog (Oct 27, 2015)

Fuses can blow faster than breakers can trip. Also, fuses can be current limiting & breakers can't do that. IIRC Fuses can be used for running motor overload instead of heaters, but, I wouldn't do that..


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## just the cowboy (Sep 4, 2013)

*Couple of reasons*

Local disconnects at the motor should not be fused, but many people do.

Some reasons you might fuse the main disconnect.

Service to the disconnect may be sized for max size of disconnect, but fuses make it fit a smaller load.
May need fast acting fuses for a VFD for a motor. 
May need time delay fuses for a motor.

Cowboy


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## MDShunk (Jan 7, 2007)

Fuses can be bad news on the load side of a VFD. Some VFDs will give up the ghost if they suddenly have no load. That's why local disconnects on VFD controlled motors should have an aux contact breaking the enable circuit. Fuseholders, if used, should be the circuit indicating type that will also break the enable circuit.


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## Navyguy (Mar 15, 2010)

Fuses protect wire, heaters protect motors. The question is do you really need multiple fuses / breakers to protect the same wire?

Cheers
John


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## MDShunk (Jan 7, 2007)

In factory work, stuff like this happens because "it was handy", i.e. We had it and used it so we didn't have to order an unfused disco.


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## telsa (May 22, 2015)

What are you supposed to do when the EE put it in the contract?

You build it per the print, and don't attempt to re-engineer the dang thing.


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## MDShunk (Jan 7, 2007)

telsa said:


> What are you supposed to do when the EE put it in the contract?
> 
> You build it per the print, and don't attempt to re-engineer the dang thing.


You always have the option to RFI and see if the draftsman clicked on the wrong symbol.


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## Forge Boyz (Nov 7, 2014)

I wired a small kiln that the specs called for a fused disconnect on the 480v power circuit and the 120v control circuit that were to be run to it. Didn't make sense to me why it was necessary.

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## MDShunk (Jan 7, 2007)

Probably unnecessary or wasteful but not really hurting anything. You can get fuses with more botique "trip profiles", but a kiln doesn't seem like the type of the thing that would require it.


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## gpop (May 14, 2018)

Have you ever considered that the disconnect is there because the fuses are there. The disconnect makes it safer to service the fuses.

Ive seen them in the past installed like a over load device for items that run 24/7 and do not have a starter (mainly supply/extraction fans). Sometimes these were installed with over sized wire and daisy chain down a bunch of small motors so the fuses also acted to de-rate the load side of the disconnect.

Mcc space can be at a premium so its not surprising.


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## 99cents (Aug 20, 2012)

Navyguy said:


> Fuses protect wire, heaters protect motors. The question is do you really need multiple fuses / breakers to protect the same wire?
> 
> Cheers
> John


Yes, fuses are usually there for fault current. Fuses and overloads serve two different functions.


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## Cow (Jan 16, 2008)

MDShunk said:


> Fuses can be bad news on the load side of a VFD. Some VFDs will give up the ghost if they suddenly have no load. That's why local disconnects on VFD controlled motors should have an aux contact breaking the enable circuit. Fuseholders, if used, should be the circuit indicating type that will also break the enable circuit.



I've heard the opposite but never actually seen it. 



If an open switch on the load side of the VFD is suddenly closed into a motor load. 



Fortunately, I've only seen them fault so far and they've all been Allen Bradley.


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## paulengr (Oct 8, 2017)

Fuses and breakers can't substitute for an overload relay...Overloads are very specialized with a very long curve. The overload trips overloads. Breakers and fuses trip on faults. Between them both breakers and fuses limit current although both can be modified to be more aggressive about it. Fuses are faster than breakers and can withstand far more fault current. It's not unusual to see breakers with backup fuses in large switchgear so you can have a 35 ka vacuum breaker with a 100 ka rating with the fuses. gsBut MOST fuses have crpppy accuracy and they are single phase devices, and breakers and electronic overloads can respond to far more issues. There are smart fuses where you can supply an external relay that can trip the fuse giving you the features of both. Fuses are also often used in disconnects where they aren't needed because price and availability often drives silly things. Unlike NEMA, if designed right, IEC starters can be Type II (no damage) but the short circuit protection must be a current limiting fuse. Finally you can just reset an overload and go back to work but by OSHA regulation you have to fault find before resetting a breaker or replacing fuses. Molded case breakers require visual inspection (NEMA AB 4) before resetting. The whole starter requires inspection in IEC Type 1 starters but not Type 2. Manual motor starters are also irritating...it's both an MCP and an overload so you must do fault finding no matter what the cause even if it's Type 2, and it's impossible to tell if it's an electrical fault or an overload.

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## JRaef (Mar 23, 2009)

The “OCPD” at the panel is technically the “Feeder” protection, providing Short Circuit AND Over Current protection for the conductors going from the panel to the motor starter. Because the code can’t assume what you are doing with that feeder circuit, there are separate requirements for “Branch” SC and OC protection. In a combo starter, the fuses or CB or MCP in that starter are providing ONLY** the BRANCH SC protection, the OL relay then provides ONLY the OC protection.

Can you use the Feeder protection device in the panel as the Branch SC protection for the motor circuit? Yes you can, in which case you can use a non-fused disconnect at the motor starter. But the problem is, non-fused disconnects have very low SCCR listings if no fuses are included, so the listing of the starter may get degraded to 10kA, as opposed to 65kA typical of a combo starter with a CB or 100kA with fuses. So yes, it’s possible, but often not practical and it’s usually simpler to just have local fuses or a CB in the starter. 

If you were referring to using a fuses disconnect AT the motor as a LO/TO point, that’s almost never necessary and having multiple SCPDs in the same circuit just increases your troubleshooting time, which increases your down time. I highly recommend against that practice.

**Technically it is remotely possible to size a fuse to provide OL protection too, but I don’t recommend it because fuses only come in certain sizes so if you do end up falling into a size that will technically work, it’s usually a compromise that can result in either nuisance clearing or motor damage.


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## paulengr (Oct 8, 2017)

If fuses came in the right sizes they could be an overload but the Bussmann manual has great charts showing why it won’t work. Basically the problem is you can’t slow them down enough.

Disconnects on the motor side are common when the motor is a long way from the starter like with kilns and paper machines that are several hundred feet long.




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