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Determining Short Circuit rating of MV fused switch

Determining Short Circuit rating of MV fused switch

Determining Short Circuit rating of MV fused switch

(OP)

I wanted to know how the overall all short circuit rating of a MV fused switch was determined when both the switch and fuses had different SC current ratings.

Most MV switches that I have come across have a listed 25kA Short Circuit Rating with a 40kA momentary or C&L rating. When these switches are fused I typically see the fuses having a 50kA rating with a 80kA momentary rating or something higher.

My question comes to how these ratings are applied against the avaliable short ciruict current avaliable at the switch. For the "Equipment Evaluation" to ensure that the switch is is rated properly do you use the short circuit values of the switch itself or the fuses?

For example lets say that the avaliable short circuit current available at the switch is 35kA Interrupting and maybe say 55kA Momentary. Would the switch fail since the 35kA and 55kA short circuit values are higher than the switches short circuit and momentary values respectively, or would the switch pass since the short circuit values fall below the fuses short circuit and momentary values inside the switch?

I am familiar with applying interrupting and momentary ratings to MV breaker but and not quite sure with an MV switch that has both a switch and fuse rating.


RE: Determining Short Circuit rating of MV fused switch

I'd use the lowest value unless the switch manufacturer has test data showing a higher rating for the switch and fuse as a tested combination. I doubt this will be the case for expulsion fuses, but maybe for current-limiting type fuses there could be some type of combined rating.

RE: Determining Short Circuit rating of MV fused switch

You can compare the curves if you have enough different switch current withstand data point to create a curve and you have the fuse clearing curve. Otherwise, use the lowest rating.

RE: Determining Short Circuit rating of MV fused switch

(OP)
So unless there is published data showing that the swich is series rated with the fuse or there is proof that the fuse protects the switches thermal damage curve then you must use the lesser of the two interrupting ratings.

I guess a similar example would be in the case of a Westinghouse DSL breaker where the fuse limitor increases the interrupting rating of the breaker and is therefore combination rated with the breaker. This works by having the fuse interrupt for high fault levels in order to prevent the breaker from opening. This would be similar with the MV switch except the switch does not interrupt faults so the fuse would just be protecting the thermal rating of the switch?

RE: Determining Short Circuit rating of MV fused switch

The switch data points likely include an instantaneous rating and then something like 3 cycle and/or 1 second and/or 3 second ratings. So, the fuses would have to current limit below the instantaneous rating and this gives your fault rating. To protect the switch thermally, you ensure the fuses or protection relay clear in less time at the time-currents points given.

RE: Determining Short Circuit rating of MV fused switch

Don't we also have to look at the close and latch rating? I visualize a switch closing into a faulted circuit with the fault current melting or welding the contact's leading edge before the switch is fully seated and latched or the fuse clears.

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