Cable Limiter Lugs

[Dumbo2929]

For those of you who do Short Circuit Studies, what is the current thinking in considering cable limiter lugs installed by the utility on their transformers? Should the study be done with taking into account any current limiting ability for a worst-case condition? Is there anything written on including or excluding limiter lugs in studies? Thanks

 

[davidbeach]

Cable limiters should not be counted on as a means of reducing downstream fault currents. The limiters are intended as cable protection for the individual conductors in parallel. From that stand point, ideally the limiters would not operate for any fault downstream of the cables. Even if the limiters did open that portion of the system, I've never seen a series rating where cable limiters were the part of the listing.

 

[Dumbo2929]

David,
That was the answer i was looking for. Thanks

 

[cuky2000]

CABLE LIMITER APPLICATIONS
* Service entrance conductors
* Between transformer or network bus and busway
terminal boxes
* Large feeders with three or more conductors per phase.

LIMITER FEATURES
 Current-limiting characteristics provide protection to
conductor insulation and reduce damage when faults occur.
 Properly applied cable limiters may permit the use of
equipment with reduced withstand ratings

See the enclose link for additional information.

http://www.bussmann.com/library/bifs/1042.pdf

 

[Dumbo2929]

So Cuky, you say that they are suitable for current-limiting to downstream panels, enough to lower the rating of those panles below the available SC current witout the limiters? That seems to be in contradiction with DavidBeach.

 

[davidbeach]

Dumbo2929, it is possible that the limiters will, mathematically, limit downstream fault currents in specific applications, but as far as I know you will not find a listed series rating that includes cable limiters. Without that listing, the mathematical reduction in fault current won't let you reduce the rating of any of your downstream equipment.

 

[cuky2000]

Even though current limiting fuses and limiters have different characteristic to respond to short circuit, still they have some similarity in term of limiting fault current as described in the following example:

* Limiters protect cable by “limiting” the thermal damage of the cables insulation under fault conditions.
* In spot network transformers, limiters often coordinate with the secondary fuse.
* Their current-limiting feature can be used to minimize arc flash hazards by reducing the magnitude of the arc flash current and the time of the arc flash exposure according with Copper/Bussmann publication.

For coordination purposes, see a sample of TC characteristics curve below and the let through current in the previous link