Current Limiting Fuse Application
Current Limiting Fuse Application
(OP)
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.






RE: Current Limiting Fuse Application
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.
RE: Current Limiting Fuse Application
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.
RE: Current Limiting Fuse Application
The fuses peak let through was 65kA, fhp never shared the interupting rating of the fuse which I assume is over the fault level.
RE: Current Limiting Fuse Application
RE: Current Limiting Fuse Application
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.
RE: Current Limiting Fuse Application
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)
RE: Current Limiting Fuse Application