Circuit Breaker Raing and Motor FLA

[paul2gh]

Hi everyone,

We are asked to use circuit breaker to protect a 6kV motor, not a fuse. So how to decide the rating of Circuit breaker?
I use 1.2 x full load current of motor as a minimum to choose CB rating for LV system. For MV sytem like 6kV, what is the criteria?

Paul2gh

 

[rbulsara]

You cannot just use the voltage ratio and select ratings for MV system protection. There only a few standard ratings (amp wise) for MV breakers, like minimum may be 400A and then next higher is 1200A. Also remember that what sets the over current protection characteristics are CTs and the relay used in conjunction with the MV CB. A MV CB by itself is just a dumb switch. Then there are standard rating of CTs as well to choose from.

In MV the rating selected are much higher than equivalent LV ratings. You adjust the protection using the relay settings.

 

[paul2gh]

Rbulsara,Thanks for your prompt reply.



Because of the inrush of Motor starting current( 6 x FLA), do I need to use a larger CB to protect a motor?
For example, if the motor's FLA is 630A, can I use a CB rated at 630A to protect it?

Paul2gh

 

[rbulsara]

The short answer is No, it has be to somewhat greater than FLA.

At 6kV it won't matter as minimum CB rating you can get is 1200A. To answer an hypothetical question, you really need to check the duty rating of the breaker with mfr. But certainly you need not use 6xFLA rated breaker, may be 2X. NEC is says "suitably" rated device..so I think it will be a matter of experience and professional judgment based on information the mfr can provide.

This again is only for MV system.