Coordination of Motor Overload

[timm333]

I have a question about the coordination of low voltage (LV) motor circuit protector (MCP) with overload. When we use MCCB, the overload element is included in the breaker, so it is not a problem. But for motor circuit protector (MCP), an external electronic overload element is used. The software libraries don’t have the TCC’s of some electronic overloads. If the TCC of electronic overload is not available, will it be acceptable to use the TCC of a class 20 thermal overload to mimic the TCC of external electronic overload element? Thanks.

 

[David Castor]

Since both the MCP and the OL relay are protecting the same motor, there's no real coordination issue between the two. If you want to see the curve, it should be possible to add the TCC to the library of TCC data used by the software you are using.

 

[timm333]

Yes that makes sense. When we use MCCB, we coordinate the overload element of MCCB with the motor starting curve and also with the LTPU of the upstream main breaker of MCC. (The overload of MCCB has to be to the right of motor starting curve, and to the left of the LTPU of upstream main breaker of MCC.)

When we use motor circuit protector (MCP), should we still plot the TCC of the external electronic overload element to be sure that it is to the right of the motor staring curve, and to the left of the LTPU of upstream main breaker of MCC? Or will it be Ok that we just plot the TCC of MCP, and eliminate the TCC of external overload element?

 

[jim dungar]

You should show both. The MCP curve usually looks like an L shape because it does not a thermal portion.

 

[topgone]

Yes that makes sense. When we use MCCB, we coordinate the overload element of MCCB with the motor starting curve and also with the LTPU of the upstream main breaker of MCC. (The overload of MCCB has to be to the right of motor starting curve, and to the left of the LTPU of upstream main breaker of MCC.)

When we use motor circuit protector (MCP), should we still plot the TCC of the external electronic overload element to be sure that it is to the right of the motor staring curve, and to the left of the LTPU of upstream main breaker of MCC? Or will it be Ok that we just plot the TCC of MCP, and eliminate the TCC of external overload element?

AFAIK. MCPs are instantaneous while the OLR are on the long-time portion of the protection. There are no overlaps between the two.

 

[David Castor]

You can plot the OL relay curve if you want to but in practice it should always coordinate with MCC main or feeder, unless the motor is excessively large. (Assuming the OL relay is sized correctly). It's also important to make sure the OL can carry the maximum expected motor running current, including any service factor.

For coordination with the upstream devices, you would only need to consider the largest motor in the MCC. If it coordinates, the smaller motors should coordinate also.

 

[Jraef]

If the TCC of electronic overload is not available, will it be acceptable to use the TCC of a class 20 thermal overload to mimic the TCC of external electronic overload element?

To answer this question, YES, the SSOL mfrs will emulate the standard I²t OL curve that they would supply with their thermal OL relays. Whether it is Class 10 or 20 is going to be dependent upon what you select. Some SSOLs offer one or the other based on the part number, others let you select it in programming.

 

[timm333]

Thanks. How about motors operated by VFD? We use MCCB in the MCC to protect the branch circuit from MCC to VFD. This MCCB has both short circuit and overload protection, and also it acts as the disconnect switch. In addition to this MCCB, the VFD has its internal fast acting fuse (for short circuit protection) and internal electronic overload (for overload protection).

For coordination: I think we should only coordinate the MCCB (in MCC) with the upstream device; and we can ignore the fuse (and electronic overload) which is inside the VFD. What do you guys thing about it?

 

[topgone]

Thanks. How about motors operated by VFD? We use MCCB in the MCC to protect the branch circuit from MCC to VFD. This MCCB has both short circuit and overload protection, and also it acts as the disconnect switch. In addition to this MCCB, the VFD has its internal fast acting fuse (for short circuit protection) and internal electronic overload (for overload protection).

For coordination: I think we should only coordinate the MCCB (in MCC) with the upstream device; and we can ignore the fuse (and electronic overload) which is inside the VFD. What do you guys thing about it?

Just make sure the downstream breaker coordinates with the source breaker. If there are no other loads in parallel with the downstream load, you don't have to worry about coordination, whether the source breaker trips of the downstream breaker trips ahead, you've lost the same load!