Fuses as motor overload protection

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mbrooke

mbrooke

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When are fuses instead of heaters desired or used in motor protection? Sane to attempt between 5-200HP? Advantages vs disadvantages? What about a thermal mag breaker at 115%?
 
romex jockey said:
RK fuses, from an economic view.....

~RJ~
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All agree with you on that, and hence the attractiveness. But, if I may >>>


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The breaker has a coil (short circuit protection) and thermal elements for capable of providing overload protection ;)
 
I should have asked:

Are you referring to motor overload protection or Motor feeder, branch circuit protection?

It the latter two, we use fuses almost entirely in the Pump Panels we purchase. They are suitable for use as service equipment.

MCCs would typically have a motor circuit protector as in your picture above.
 
mbrooke said:
All agree with you on that, and hence the attractiveness. But, if I may >>>


The breaker has a coil (short circuit protection) and thermal elements for capable of providing overload protection ;)
Click to expand...

It's likely that those are MCPs, which do not have a thermal element.
 
ptonsparky said:
I should have asked:

Are you referring to motor overload protection or Motor feeder, branch circuit protection?

It the latter two, we use fuses almost entirely in the Pump Panels we purchase. They are suitable for use as service equipment.

MCCs would typically have a motor circuit protector as in your picture above.
Click to expand...


Motor overload protection, which is why I mentioned overload when I started the thread :) Which it is true that overload protection also prevents the wire from over heating.


But- here is my question. Why would you need heaters in an MCC when in theory the breaker can do the same?
 
It is my understanding that motor overload heaters react to current differently and more like the motor windings would react...hence giving a more accurate protection for the motor. The heater in a breaker reacts more similar to how a conductor would react. I'd have to do some study to give a more detailed answer, but this is what I have been taught and it seems to make sense to me.
 
It’s all about the trip curves. Yes you CAN technically use fuses as running OL
protection. But the tricky part is that in order for the fuse size to fit within the rules for that running OL protection for the motor, the SC protection ends up being lower and usually results in the motor starting current clearing the fuse. CAN it be done? Yes, I’ve seen it work. But as I said, it’s a tricky balancing act between those two criteria and in most cases it never works. The only place I have seen it work is below 15A where the rules on SC protection become looser because the minimum 14ga wire size allows for wider allowable SC protection. Run some numbers using the OL and SC protection rules against motor starting current curves and you will see what I mean.

That never works for T-M circuit breakers with fixed trips by the way. The curves are just too tight. But it will work with T-M breakers that have adjustable thermal trips, because that thermal trip element is exactly the same as a normal thermal OL relay. But UL did not allow adjustable trip T-M breakers to be used as the Branch OCPD until recently unless there was another listed Branch OCPD ahead of it, which made it pointless.

That UL listing issue changed about 15 years ago, so what they call those breakers now, in an attempt to avoid confusion, is Motor Protective Circuit Breakers or MPCBs (the Europeans call them MCPs, but that’s confusing here because MCP means something different to us). In fact you can also use an MPCB as a Manual Motor Starter and most of them are UL listed as such. So technically though if you need remote / automated control, you can make a fully listed combination starter using just an MPBC + a contactor; totally legitimate. It’s just a little too unusual for the control mfrs to market them heavily. I’ve used them a lot when money is tight, but to be honest, I usually have to spend more time explaining it to Engineers and Inspectors to be worth it.
 
RD35 said:
It is my understanding that motor overload heaters react to current differently and more like the motor windings would react...hence giving a more accurate protection for the motor. The heater in a breaker reacts more similar to how a conductor would react. I'd have to do some study to give a more detailed answer, but this is what I have been taught and it seems to make sense to me.
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Makes perfect sense, but I wonder to what extent in reality. I mean, in theory, motor overloads must also react the same way as the supply wires.
 
ptonsparky said:
Relying on fuses for overload protection results in single phased motors. My experience. Coordinating them to allow for the starting current and overcurrent is pretty tough.
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You can get motor rated fuses. We used those plus a thermal O/L unit.
 
Jraef said:
It’s all about the trip curves. Yes you CAN technically use fuses as running OL
protection. But the tricky part is that in order for the fuse size to fit within the rules for that running OL protection for the motor, the SC protection ends up being lower and usually results in the motor starting current clearing the fuse. CAN it be done? Yes, I’ve seen it work. But as I said, it’s a tricky balancing act between those two criteria and in most cases it never works. The only place I have seen it work is below 15A where the rules on SC protection become looser because the minimum 14ga wire size allows for wider allowable SC protection. Run some numbers using the OL and SC protection rules against motor starting current curves and you will see what I mean.

That never works for T-M circuit breakers with fixed trips by the way. The curves are just too tight. But it will work with T-M breakers that have adjustable thermal trips, because that thermal trip element is exactly the same as a normal thermal OL relay. But UL did not allow adjustable trip T-M breakers to be used as the Branch OCPD until recently unless there was another listed Branch OCPD ahead of it, which made it pointless.

That UL listing issue changed about 15 years ago, so what they call those breakers now, in an attempt to avoid confusion, is Motor Protective Circuit Breakers or MPCBs (the Europeans call them MCPs, but that’s confusing here because MCP means something different to us). In fact you can also use an MPCB as a Manual Motor Starter and most of them are UL listed as such. So technically though if you need remote / automated control, you can make a fully listed combination starter using just an MPBC + a contactor; totally legitimate. It’s just a little too unusual for the control mfrs to market them heavily. I’ve used them a lot when money is tight, but to be honest, I usually have to spend more time explaining it to Engineers and Inspectors to be worth it.
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Well put.

However, what about dual element fuses?

Fixed breakers- some do have a high magnetic pickup of 10-20x... would those work?


I guess what I'm asking is how many times the motor inrush vs the FLA?

I like the idea of MPCBs and MCPs.
 
dkidd said:
MCP has stood for Motor Circuit Protector for as long as I have known them, and is still used by manufacturers.
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Right. It does for us in North America, but in Europe, "MCP" and "Motor Circuit Protector" is the name sometimes given to the versions with adjustable THERMAL trips, which we don't have on "our" version of an MCP. That's why we had to come up with a different name for the MPCBs here when they became legal. For years, the only thing we could call them (MPCBs) here was Manual Motor Starters, which itself means something different too.
 
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