Sizing A Motor Protective Circuit Breaker

[Smart $]

I checked out the CSA file and class number for the Eaton device. According the the class number for that device covers:

"Manual across-the-line and reduced voltage starters, rotary switches, drum controllers and cam type controllers, open type or enclosed, with horsepower ratings. Starters and controllers may include in their assembly protective devices (e.g. overcurrent and/or overload devices) installed in accordance with the requirements of the Canadian Electrical Code Part I and NFPA 70 National Electric Code (NEC)."

I think this device may be able to be used as both overload and short protection for my branch circuit. The Canadian Electrical Code Part 1 is the code that contained the rule of having the max instantaneous SCPD being no more than 1300%. Sounds like since this has been already certified under that electrical code I might not have to take it into consideration.

...

As I noted in Post#10, the Eaton PKZM series is not considered as SC protection by Codes unless UL 508 Type E/F listed controller. I did not follow up to see if it is listed as such... but if it is not, you will be required to have an additional SC protection device.

 

[templdl]

The way that I have always viewed motor protection has been done with a combination starter. Without being combination tested it's defaut sscr of 5ka or 10ka depending upon the starter size. Remember that contactor's are not intended to interrupt faults but currents up to the motor LRA. However, if tested in a listed enclosure with an MCP you will find that the enclosure itself will have a label affixed to it with the combination sscr as an assembly with the MCP clearing the fault. It is also to be pointed out that an MCP being mag only is a UL component listed device and does not have a kaic rating on the label.
It is the motor starter that includes the motor overload protection that protects the motor from overload. The MCP is not intended to protect the motor but the motor circuit. Being a mag only device its pickup setting should be set per NEC art 430-52. But with exception #1 allowing for up to 1700% of the FLC if I' m not mistaken. Since energy efficient motor have a much higher inrush a higher mag setting is allowed to combant nuisance tripping. When it comes down to it being a mag only instantaneous device there may not be a big difference between 11x,13x, or even 17x when the device trips anyway unless the fault is arcing which can open up another discussion as to what the characteristics of an arcing fault is.
The MCP's job is to detect a motor winding failure which often ocvures when a winding goes to ground. The MCP will respond to instantaneous current in excess of the magnetizing current that is inherent when a motor is started. I have always suggested that an MCP's pickup setting be set just about the magnetizing current of the motor but not cause nuisance tripping. When an MCP trips there is a motor winding failure. The intent is to take the motor off line is reduce the magnetude of motor winding damage which may allow for the rewinding and rrpair of the motor.
MCPs were designed originally by Westinghouse in about 1970 to address fires as a result of motor winding failures as it was determined that about 19% of fires were as the result of motor failures per the IAEI news at that time.