Motor circuit protector

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Can a motor circuit protector be fuse and can it be used in fused disconnects? If yes then link to manufacturer of fuse motor circuit protector?
I don't see any reason why you could not fuse a motor circuit protector. But I don't know what the point would be.

I also don't see any way to install a motor circuit protector in a fused disconnect. A motor circuit protector is basically a molded case circuit breaker with only an IT element installed. It's not going to fit in a fuse holder.
 

hhsting

Senior Member
I don't see any reason why you could not fuse a motor circuit protector. But I don't know what the point would be.

I also don't see any way to install a motor circuit protector in a fused disconnect. A motor circuit protector is basically a molded case circuit breaker with only an IT element installed. It's not going to fit in a fuse holder.

The question is not asking if you can fuse motor circuit protector.

I am aware that motor circuit protector can be breaker. Can the motor circuit be fuse only Not breaker?
 

paulengr

Senior Member
I think what you are looking for is either a fused disconnect or just add fuse blocks.

Let’s be clear here. A motor circuit requires a minimum of 3 components: a lockable disconnect switch, a short circuit protection device, and an overload device (may be an overload relay or motor thermal switch). A motor circuit protector is a brand name of a device that does the first two functions. It is however Listed as a component. Similar to say motor lead wire it is only legal to be installed as a replacement part or as part of a Listed assembly (part of a combination starter, MCC, etc.). This is why for instance you don’t see them sold as an “MCP-disconnect”. There is no such type Listing in the UL white book because it’s missing the third function so it’s not an assembly.

A fuse performs the second function. So if you add a fuse block after an MCP it does the same thing. So it’s not really illegal just pointless to do this on a new install. However it should always trip before the MCP so it just turns the MCP into a switch, although there is no technical guarantee of this and no series ratings for MCPs exist so if you are really worried about Listjng this one is dead on arrival.

New equipment (combination starter or MCC bucket) is going to come with either a fused disconnect (cheapest, most common) or with an MCP. Transformer or other device feeder buckets for MCCs use standard breakers or fused disconnects. Disconnects come either fused or infused. There are enclosed breakers (single breaker in a box) as well. But as I said no MCP-only or MCP fused disconnects.

If you are building something yourself either as a panel assembly (UL 508A) or as a field assembly there are a couple obvious options. First one is a fused disconnect. They are often even marked with horsepower ratings. You can order them with “extended cabinets” from some manufacturers which comes essentially as a combination starter cabinet minus the starter assembly. The second component-level (but listed assembly) device is the “Fuserbloc” which is offered by a few manufacturers (private labeled a lot). It is a fuse block with an integral disconnect with several handle and fuse options that is very easy to install into just about any enclosure.

If you like the idea of a “breaker style” device I suggest you look at manual motor starters. An MMS traditionally looks very much like just a switch and was available for small motors. It incorporates the functions of a disconnect, short circuit protection, and optionally overload protection. This one device does everything. If you attach a contactor you have a complete starter. These have gotten much larger over time and are now available up to over 100 HP. Since it is a Listed assembly instead of a component it can be used in custom equipment unlike the MCP.

I have been down this road many times. The typical scenario is the customer has an MCC or a combination starter and wants to upgrade to a soft starter or a VFD. The soft starters are pretty hardy (good AIC) but not the VFDs in the past. So often we remove the contactor and overload leaving only the MCP or fused disconnect. Fuses added as needed (AIC concern). I don’t like doing this but I’ve seen others leave the contactor and sometimes even the overload in place feeding the soft starter/VFD (very hard on the precharge circuit) or use the contactor essentially as an oversized isolation relay. I’ve seen this done with the main contacts as signals (not a good idea, not enough power to provide cleaning action) or using the auxiliary contacts only. Either way I’m not a fan of this approach. At worst I’d leave it as a “manual” bypass.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
See 430.52(C)(7). The motor short-circuit protector (motor circuit protector) is a fuse device and it has the same restrictions as an instantaneous trip breaker...that is that it can only be used as part of a listed combination motor starter. They cannot be used in a stand alone fused disconnect.

However if you are asking about a standard type fuse in a motor circuit, they can be used to provide the required short circuit and ground fault protection and the required overload protection. This application does not require the fuses to be part of a listed combination type starter. It would only require the addition of some type of motor controller, which could be the fused disconnect.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
I don't see any reason why you could not fuse a motor circuit protector. But I don't know what the point would be.

I also don't see any way to install a motor circuit protector in a fused disconnect. A motor circuit protector is basically a molded case circuit breaker with only an IT element installed. It's not going to fit in a fuse holder.
While we typically call that type of breaker a motor circuit protector, the code only uses the term instantaneous trip breaker. 430.52(C)(3)
The code does use the term "motor short-circuit protector" for a fused type device that provides the same type of protection as an IT breaker 430.52(C)(7).

It gets kind of confusing where the field used terms and the code used terms are so far apart in their meaning. The code does try to help with this by using Informational Notes in both of those sections.
 
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hhsting

Senior Member
See 430.52(C)(7). The motor short-circuit protector (motor circuit protector) is a fuse device and it has the same restrictions as an instantaneous trip breaker...that is that it can only be used as part of a listed combination motor starter. They cannot be used in a stand alone fused disconnect.

However if you are asking about a standard type fuse in a motor circuit, they can be used to provide the required short circuit and ground fault protection and the required overload protection. This application does not require the fuses to be part of a listed combination type starter. It would only require the addition of some type of motor controller, which could be the fused disconnect.

Oh dear! I have drawings where ahead of fire pump controller they are using fused disconnect with fuse in them. The fused disconnect is a service disconnect. The fuses are sized based on locked rotor current.

Overload protection is not allowed for Fire Pump.

Let me understand what you are saying the drawings cannot you fused disconnect with fuses ahead of fire pump motor controller since fuses have overload and short circuit protection? It has to enclosed circuit breaker?
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Oh dear! I have drawings where ahead of fire pump controller they are using fused disconnect with fuse in them. The fused disconnect is a service disconnect. The fuses are sized based on locked rotor current.

Overload protection is not allowed for Fire Pump.

Let me understand what you are saying the drawings cannot you fused disconnect with fuses ahead of fire pump motor controller since fuses have overload and short circuit protection? It has to enclosed circuit breaker?
If the fuses are sized to carry the locked rotor current, they are not sized to provide overload protection. Overload protective devices must be sized per 430.32. Just the type of protective device does not make it an overload device. It is only an overload device where sized to provide overload protection.
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
If the fuses are sized to carry the locked rotor current, they are not sized to provide overload protection. Overload protective devices must be sized per 430.32. Just the type of protective device does not make it an overload device. It is only an overload device where sized to provide overload protection.
AND, since it is a Fire Pump, you CANNOT HAVE overload protection.
 
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