Motor Overload vs Circuit Breaker

Can someone explain the differences? Do I still need to protect behind a contactor/overload combo?


Thank you!

a circuit breaker protects the circuit from being overloaded


an overload protects the motor/T leads (wires from contactor to motor) from overheating


typically, a circuit breaker is ahead of the contactor, and an overload is attached or directly behind a contactor/motor starter in the circuit.


NEC Article 430

Motor protector circuit breakers are available which cover the CB/fuse and motor overload needs in one convenient device.

https://www.automationdirect.com/ad...ters_-z-_overloads/fuji_manual_motor_starters

Circuit breakers are also for instantaneous short-circuit protection. Motor overloads can usually be set in a range to match your motor's full-load amperage (FLA) and will trip if your motor draws too much current for too long.

Motor Overloads don't open the power connection, only the status contacts. CB's amd MCP's do.

I'll echo what is explained so far.

MCB = before contactor
Overload = after contactor

Overloads typically come with pins to attach to the contactor.

Motor circuit breakers can often have a link module or kit that will attach it to the contactor to create a combination starter, but they don't come that way. I also recommend a coil module, if the coil is on top of the contactor, and you will usually need a line-space adapter for the MCB to meet a combination starter rating.

Keep in mind - the AUX contacts of the overload and circuit breaker are reversed from each other. If a MCB trips, you would use the N.O. contact for the contactor coil to turn it off since the MCB's tripped state is 'off'. When an overload trips, it activates, so you would use the N.C. contact to turn the contactor coil off.

Also note that an overload doesn't really provide any SCCR protection, you will want to use a fuse in front of the contactor/overload. Many MCB / combos on the other hand are rated around 35k-65k SCCR and you don't need fusing if the rating is sufficient.

And one final thing, Motor Overloads/Motor Breakers usually also trip on 'single phasing'. That is when one phase is lost either in the supply or in the connection to the motor. Three phase motors will run with only two phases and depending on the load may start, though the direction of rotation isn't certain.

So why/when do I need to add motor overloads? Some applications just have breakers or fusing on some motors. What scenarios would make me want to use them also?

So an Overload is NEVER used in place of a CB, correct?



Thank you guys for the help!

Timeismoney08 said:

So why/when do I need to add motor overloads? Some applications just have breakers or fusing on some motors. What scenarios would make me want to use them also?

So an Overload is NEVER used in place of a CB, correct?



Thank you guys for the help!

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You always want overload protection.

The motor-circuit-breaker can be a replacement for the overload.

As listed above, the advantage of a motor circuit breaker vs overload is that you don't need fusing, and it opens when tripped. Overloads do not open when tripped, they just provide aux contacts that do.

If you want a great publication and you use Bussman products, search online for "Bussman SPD". It is very lengthy, but provides tables for thier fusing and a whole section on different requirements for motor circuits.

Rson has it right. You need both. I use this diagram in my classes.

If you put too much load on a motor it will get hot, if it gets hot enough it may burn out or even cause a fire. A small overload will increase the temperature gradually, a large overload will increase the temperature quickly. If you remove the electric supply before the motor gets too hot you can save it from damage. Fix the reason for the overload and then restart the machine.

The ideal way with a motor is to directly measure the motor temperature using thermistors within the motor windings. But it is relatively expensive and so isn't generally used on small motors. The less ideal way is to use a Motor Thermal Overload or Motor Breaker which tries to simulate the temperature rise within the motor by heating up bimetallic strips within the overload.

A miniature circuit breaker doesn't have the thermal component of a motor breaker, it can see sudden large over currents, but not small long lasting over currents. It also doesn't see the missing phase that the others will detect.

A Thermal Overload doesn't have the sudden large over current protection that a miniature circuit breaker does. Generally a Thermal Overload doesn't switch the mains, it just has contacts that open another device that switches off the mains. I haven't used separate thermal overloads for years, not since the introduction of motor breakers.

A motor breaker combines the best of an MCB and a Thermal Overload, but is often more expensive that MCBs or Thermal Overloads individually. I always add the auxiliary pack that allows it to also switch off the mains via a contactor.
Not everyone likes motor breakers, some prefer a combination of thermal overload and fuses. That gives long slow overload, single phasing and sudden large overload protection.

Some applications just have breakers or fusing on some motors

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It may depend on the type of motor, but most likely depends on the designer not allowing for or not caring about an overheating motor. If the motor is small and in a place where it doesn't matter if it burns out, then you might decide not to protect it. If the motor is very small the overloads become less accurate and useful, the motor may burn out before the overload reacts.

BryanG said:

A miniature circuit breaker doesn't have the thermal component of a motor breaker, it can see sudden large over currents, but not small long lasting over currents. It also doesn't see the missing phase that the others will detect.

Click to expand...

I believe that most modern motor circuit breakers have differential tripping. If one phase is missing, it still trips at the proper (approximated) winding termperature. It is done with clever mechanical linkage in the trigger mechanism in the circuit breaker. For example Siemens 3RV motor circuit breakers all have this functionality.

edit: Reading your post again, I think that is actually what are are saying.
If you use a regular MCCB in the entry of a motor starter, it does not have the functionality of tripping earlier on a missing phase.
If such a motor starter is combined with an overload, then the over load will have the missing phase detection function.

The ONE place I suggest Overload Relays in place of MCP's is when running multiple motors on the output of 1 VFD.

We have these situations fairly regularly on fans and conveyors among others. The thing you don't want to happen on the output of a VFD is for the circuit to open while it is running. The resulting voltage spike can damage the VFD.

So, the gang of O/L's protecting the individual motors have their N/C contacts wired in series to an input to the VFD to trip it out on 'External Fault'.

Just to be clear:

I am mentioning circuit breakers meant for loads or motors such as an Allen Bradley 140M series, not supplimental or mini-circuit breakers such as the Allen Bradley 1492. One is very different from the other and as such rate for different applications.

The 1492 series would replace the fuse if you want, but you would still need an overload, as it won't provide proper protection for the motor. In fact, if you tried to size these to protect the motor, you will get nusance tripping - you often have to over size them due to the start-up current of a motor.

The 140M series replaces the overload and fuses and is made specifically for motor applications.

Thank you all! I understand now and appreciate all the information.