Cable limiters

[NEU2005]

My experiences with cable-limiters is in reducing cable degradation due to potential subsequent cable fault conditions and to provide greater system reliability in electrical distribution network-protector system designs.

I am aware of the fact that cable-limiters may also provide reduced available fault-current values to downstream electrical equipment. I am not familiar with how to accomplish this. I do not know how affective or to what degree they may reduce the fault currents. I suspect there are a lot of variables to consider (conductor type, size, length, number or paralleled conductors per phase, available fault currents versus allowable fault currents).

Any assistance would be much appreciated in the application of cable-limiters for the purpose of reducing fault-current values is systems less than 600 volts.

 

[dpc]

The cable limiter is basically a current-limiting fuse but is intended to provide only short-circuit protection, not overload protection. If you understand how a current-limiting fuse limits fault current, you understand cable limiters.

The Bussman or Ferraz Shawmut websites can provide a lot of detailed information on the functioning of current-limiting fuses.

Like a fuse, the cable limiter can only limit current by melting and clearing a fault within the first 1/2 cycle. For longer clearing times, it is not current-limiting. Also, for very high amperage feeders with multiple conductors per phase, the limiters will probably not help much in reducing downstream fault current.

Also, if you are trying to use a cable limiter to protect a downstream circuit breaker, this is only possible if the cable limiter and breaker are a tested combination, and I don't think there are any tested combinations involving cable limiters, so from a Code compliance point of view, you are probably out of luck.

 

[JRSherbundy]

I have used cable limiters one time, many years ago. Cable limiters are fuses, but are specified by cable size rather than ampere rating. I believe you need at least three cables in parallel to make cable limiters work for the application. Basically, they are intended to remove a faulted cable from a parallel group of cables. Say a fault developes midway along one of the cables. The fault current will fed the fault from both ends of the cable, causing the limiters to open at both ends of the cable. I believe you need three or more in parallel to allow the fault current to divide between cables and keep the unfaulted cable limiters from opening while the faulted cable limiters will have more fault current. To me, it seems like the fault condition would weaken the surviving limiters. Maybe the expectation is that the limiters should be replaced in the event of an operation regardless of outcome. Also keep in mind that taking the faulted cable out of service leaves the remaining cables carrying more than their original share of the load. My other recollection is that cable limiters can be though of as really big lugs on the end of the cable. Fitting such an animal into a piece of gear may compromise wire bending space too.

Just a few thoughts, hope it helps.