Current Limiting Fuse Application

[fhp]

I intend to use a 4,000A current limiting fuse to be installed right after the load terminals of a power CB with a 75kAIC rating in an existing switchboard (there is no space available to install the CLF at the line terminals of the breaker). Will the CLF protect the breaker for a 110kA symmetrical RMS available fault current considering the let-through of this fuse is between 57 to 65 kA (depending on the brand and type of fuse; based on the let-through curve)? Based on my understanding on the operation of the CLF, the CLF will not allow a current higher than this level, i.e. 57 or 65kA. and thus protecting the breaker. The CLF will be installed right after the breaker where the possibility of a fault between the breaker and fuse is almost nil.

 

[gordonl]

This would certainly do the trick, a breaker with an interupting rating of 75kA has a momentary withstand of much higher.

Another possibility is installing the fuse ahead of the breaker removing even the slight chance of a high magntude fault having to be interupted.

If this is the main feed to a plant or etc. you may think of installing a current limiting fuse in parallel with a reactor at the incomming. During the fault the fuse blows, fault current is limited by the reactor, fault is cleared by a downstream breaker, power to the rest of the facility is maintained through the reactor after the fault is cleared. During normal operation there are no voltage regualtion problems with the reactor as all current passes through fuse.

 

[MDom]

fhp,I think that if the max. breaking current of the fuse
is 57-65 kA and the available fault current is 110 kA,the
CLF may be destroied during the interruption and will not protect the CB.In case of the short curcuit current is higher that rated breaking current of the fuse,indicated by the manufacturer, the arc may appear immidiately after the melting, evaporation of the fuse's wire of CLF. The current will pass over the arc and will not be interrupted.

 

[gordonl]

MDOM,

The fuses peak let through was 65kA, fhp never shared the interupting rating of the fuse which I assume is over the fault level.

 

[GordS]

I think I would be a bit reluctant to say the proposed solution was acceptable as presented. I think the breaker and fuse combination need to be checked and approved as a combination, not catalogue engineered in the field. I was going through a paper from the 1991 PCIC entitled "SC Ratings, Labels and Fault Withstandability of Moded case and Insulated Case CB and Combination Motor Starters" which states "Past manufacturer application data recommended the application of current limiting fuses to limit fault currents to motor control centers. The application used current limiting fuses in the MCC circuit breaker to reduce let thru current below the MCC interrupting rating. Later testing of this application for UL approval of the combination found that the practice was unacceptable. This confusion occurred because the manufacturer used the up over and down method and applied the let thru currents determined by this analysis to the interrupting rating and not its withstand rating... Since the withstand ratings of molded case and insulated case ciruit breakers are not published the manufacturer must be consulted for the proper fuse-breaker applications". Sorry if I have misinterpreted the proposed application.

 

[fhp]

fhp

As a follow-up question, neglecting the possibility of a fault occuring between the breaker and the fuse, the situation described above would therefore protect the power CB? To: MDOM; fuse has a 200,000 AIC rating: To GordS: Thanks, but per catalogue of power CB, withstand rating is 75kA.

 

[gordonl]

fhp

With molded case circuit breakers there has been some concern with series combinations because there have apparently been cases where the circuit breakers have been faster than the fuse, and/or the molded case breakers can't handle the energy let through of the fuse.

If your dealing with a power circuit breaker you should make sure that the breaker interupting rating is more than the peak let through. The breaker withstand is what the breaker can pass through without trying to open, the interupting is what the breaker can interupt.

If the breaker interupting is less than the peak let through of the fuse at your fault level then it could conceivably happen that the breaker would try to interupt more than it's rating. (Low level trip which develops to high level at contact parting)

 

[REDDOG]

Be advised that the CB Interrupting Rating is based upon all 3 poles opening for a symeterical fault, not a phase to ground or phase to phase fault. Check with the manufacture for line to line or line to ground interrupting rating! Fuses are tested and listed differently than circuit breakers. The fuse will limit the current to the values indicated, however this is based upon a fixed value of impedance in the faulted circuit. A CB may display dynamic impedance,(force of magnetic field caused by fault current, may cause the contacts to part slightly against their holding springs)this caues the circuit impedance to be increased causeing fhe prospective fault current to be reduced pulling the fuse out of current limit. The fused does not interrupt 200000 amperes but current that has the rate of rise of 200000 amperes.