Rating of Circuit Breaker for motor Protection

[tayyab1020]

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

 

[busbar]

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.

 

[rbulsara]

If the breaker is a inverse time thermal magnetic breaker with a trip unit (a regular molded case circuit breaker) than it should be sized not more than 2.5 times the FLA of the motor accroding to NEC. (there is an exception to go higher for loads requiring exceptionally higher starting torque)

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.

 

[gordonl]

Be aware that most CB's don't limit fault current, where as fuses do, this can result in increase damage to motors and MCC's during heavy faults. Also be wary of the fault ratings of the breakers, all to often I've seen MCCB in MCC's come in under rated for Isc despite the clear specification of fault level required.

(From a reliability point of view I prefer fuses as well)

 

[REDDOG]

Why do want to replace fuses with circuit breakers?
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.

 

[busbar]

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.]

 

[jbartos]

Suggestion: There are more sophisticated motor protection devices being offered, e.g.

http://www.pkz2.com/

http://www.startco.ca/fallnews2002.pdf

etc. for more info

 

[tayyab1020]

The reason for replacing the fuses is their bolted ends which make their replacement very difficult. The breaker that I have selected has a thermal and instantaneous unit. The value of unit is 10 times the rated current.

I want to know that with this value of shortcircuit current what nominal rating circuit breaker can be used.

 

[advidana]

Changing from a fused motor control system to Circuit breaker system is not that easy.

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

 

[swgrmfg]

Tayyab1020:

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

 

[jbartos]

Suggestion: Visit

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

 

[illiterate]

You need to Know the Incoming Fault Rating ie 35KA
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.