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Question, main breaker tripping for VFD driven motor

Question, main breaker tripping for VFD driven motor

Question, main breaker tripping for VFD driven motor

(OP)
We have a 1400amp/480v main breaker (not branch) that keeps nuisance tripping. This feeds a couple VFD's that power a few 200hp motor/pumps. To my surprise I found out this is a instantaneous trip breaker. WHY would an instantaneous breaker be used on a main??

RE: Question, main breaker tripping for VFD driven motor

To limit damage perhaps. Or for no reason because no one evaluated it.

Is it really tripping on INST ? Is there enough current to trigger INST? Did you check actual current against settings?

Is breaker/trip unit tested?
 

Rafiq Bulsara
http://www.srengineersct.com

RE: Question, main breaker tripping for VFD driven motor

Most breakers need INST function to meet their UL rating for SCCR.

Only power breakers with short time rating can get away without INST element.

Rafiq Bulsara
http://www.srengineersct.com

RE: Question, main breaker tripping for VFD driven motor

Quote:

WHY would an instantaneous breaker be used on a main??

Because it is cheaper. It's a very common practice.  As Rafiq indicated, standard molded case circuit breakers must be provided with an instantaneous element to protect themselves.

 

"Theory is when you know all and nothing works. Practice is when all works and nobody knows why. In this case we have put together theory and practice: nothing works... and nobody knows why! (Albert Einstein)

RE: Question, main breaker tripping for VFD driven motor

Semantics abound... You may need to further clarify.

A "standard" breaker used as a main would have BOTH thermal trips AND instantaneous (magnetic) trips; it's a normal minimum function of a protection scheme. It can have MORE trips than that, such as Short Time trips and Ground Fault, but it cannot hae LESS than that. With that said, a common Thermal-Magnetic Circuit Breaker will have adjustable instantaneous trip settings, but the thermal trips are factory set and not visible from the outside. So just because you don't see a setting dial for them doesn't mean they are not there.

There is also a type of circuit breaker that has ONLY instantaneous trip elements; often referred to by the original Westinghouse brand name of Motor Circuit Protector (MCP) or as a "mag-only" breaker or the Siemens / ITE designation of "instantaneous-only" CB. They have no thermal or long time trip elements. These are ONLY to be used in LISTED factory assembled motor controller assemblies, because the idea is that a motor controller will have its own thermal sensing and tripping element dedicated to that single circuit for ONE motor.

So if you have a breaker that ONLY has mag trips, and someone has used it for something OTHER than a single factory assembled motor controller, then it is illegal and must be replaced with the proper TMCB.


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