Rating of Circuit Breaker for motor Protection
Rating of Circuit Breaker for motor Protection
(OP)
I want to replace fuses of my Motor Control Centre with motor rated Circuit Breakers. The current rating of installed fuses is 2.5 to 3 times the rating of the motor.
What rating of the motor rated circuit breakers should I use, either equivalent to the rating of the motor or maintain the same philosophy 2.5 to 3 times.
M. Tayyab Khan
What rating of the motor rated circuit breakers should I use, either equivalent to the rating of the motor or maintain the same philosophy 2.5 to 3 times.
M. Tayyab Khan





RE: Rating of Circuit Breaker for motor Protection
In ANSI regions, circuit breaker short-circuit-only {instantaneous trip} can be applied to magnetic starters where winding overload protection is provided by a relay integral to the starter. One style is a Cutler-Hammer MCP device, described at: http://www.cutler-hammer.eaton.com/unsecure/cms1/SA12068.PDF
Sizing/setting is specified in NEC Article 430.
RE: Rating of Circuit Breaker for motor Protection
The MCP, motor circuit protector which is a molded case switch with instantaenous trip only, is also permissible. Make sure that the instantaneous trip is set higher than the LRA of the motor and no more than 800% according to NEC table 430.52.
RE: Rating of Circuit Breaker for motor Protection
(From a reliability point of view I prefer fuses as well)
RE: Rating of Circuit Breaker for motor Protection
The fuse should be of thr dual element time delay type.
The fuse should be 1.75 * the Full Load current of the motor.If the fuses are sized correctly, why are they blowing?
Opening of any type of overcurrent device, that is tripping of a CB or opening of the fuse should be investigated as to the cause.. before reclosing or replacing.
RE: Rating of Circuit Breaker for motor Protection
This is an anecdotal opinion, but a tradeoff for fuse-versus-breaker "false" operation should be weighed against the relative damage to overload-relay elements; i.e., heaters opening like fuses in cases where the let-though I²t is excessive. [Trying to point our a potential weakness without blatant finger pointing.]
RE: Rating of Circuit Breaker for motor Protection
http://www.pkz2.com/
http://www.startco.ca/fallnews2002.pdf
etc. for more info
RE: Rating of Circuit Breaker for motor Protection
I want to know that with this value of shortcircuit current what nominal rating circuit breaker can be used.
RE: Rating of Circuit Breaker for motor Protection
Generally the reason most people use fuses is because they can interrupt high valves of short circuit at a low cost, which is the type that are bolted in. These fuses are rated 100,000 amps (AIC) and are current limiting.Standard breaker start at 10,000 amps and are available at higher AIC for additional cost, with curent limiting features the cost sky rockets.
Have an electrical engineer look at your system and perform fault calc's to help select the correct breakers. Be aware that these circuit breaker with current limiting could cost over a thousand dollar each or 10 times or more than the cost of fused disconnects. And for values over 800 amps they may not be available.
If you use the wrong type of Circuit Breaker they can explode during a fault. It nothing to play around with, especialty if you are the guy closing the breaker into the fault.
If you don't understand short circuit rating and fault ratings then for sure don't change it out.
good luck and be careful
RE: Rating of Circuit Breaker for motor Protection
Some new high efficiency motors have very high inrushes. That, coupled with allowable voltage fluxuation by utilities (+5%), DC offsets, and starting a motor near voltage zero crossing prompted manufacturers of MCP's to recommend the following rule which never fails. (You know that the MCP is a magnetic only breaker). 10 times the continuous rating of the breaker should be greater than or equal to 13 times motor FLA. 10 times being the full magnetic adjustment range of the MCP, and 13 times FLA takes into consideration all the inrush criteria mentioned above.
GH
RE: Rating of Circuit Breaker for motor Protection
http://www.danfoss.com/Journal/4_00/Industrial.htm
http://www.danfoss.com/Journal/4_00/dj4_00.pdf
for: New Current-Limiting Circuit Breaker Range
etc. for more info
RE: Rating of Circuit Breaker for motor Protection
Then a fault limiting Mccb may be selected for the MCC Main Incomer
Manufacturer supplied Cascading tables are then used to
select the Type MPCB (motor protection circuit breakers)
and from then the MPCb is sized for the motor.