Available Short Circuit for MV Switch

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Keri_WW

Keri_WW

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I am trying to determine the proper kA rating for a 5kV S&C switch. The actual voltage is 4160V. Do MV switches have typical impedance values similar to transformers and if so are they listed anywhere? The serving utility indicated that they can deliver 2.0 MVA to the switch with their current feeder. This equates to ~ 277A. The facility's standards list kA values of 25kA, 40kA, 50kA, and 63kA, so I am wondering how to determine the impedance factor to plug in to this formula, or is it safe to assume that since they can only deliver 2.0MVA, that the lowest standard value of 25 kA is acceptable?

A = VA / (V x %z)

Thanks,
Keri
 
I think you might be confusing the utility feeder's continuous loading capacity (2 MW) with the available short circuit current. Two different things. You'll have to ask the utility for the available short circuit current at 4160 V. Then check that against the momentary rating of the S&C switch (and fuse if there is one).
 
Keri_WW said:
I am trying to determine the proper kA rating for a 5kV S&C switch. The actual voltage is 4160V. Do MV switches have typical impedance values similar to transformers and if so are they listed anywhere? The serving utility indicated that they can deliver 2.0 MVA to the switch with their current feeder. This equates to ~ 277A. The facility's standards list kA values of 25kA, 40kA, 50kA, and 63kA, so I am wondering how to determine the impedance factor to plug in to this formula, or is it safe to assume that since they can only deliver 2.0MVA, that the lowest standard value of 25 kA is acceptable?

A = VA / (V x %z)

Thanks,
Keri
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It depends on how stiff your PoCo is. The service drop conductors drastically drop that short-circuit value if your switch is located far from the POCC. Best to assume 500 MVA available SCMVA (69kA) at the connection point and then include in your calculations the service drop length and the service wire size to arrive at a conservative figure of available SC current at your switch. Your 2 MVA load has nothing to do with the short-circuit current available from the PoCo.
 
Typically, a Utility statement saying they are able to supply 2 MVA of load usually means they only have 2 MVA left of capacity on a particular feeder based on the maximum MVA capacity they design for their circuit breakers at the substation. In my Utility, we use 10 MVA as the maximum capacity before we add an additional circuit breaker and run a new line or split the existing line load. This is only one limiting condition; line configuration, other customer load factors and voltage regulation are but a few additional ones considered.

Fault duty at 2 MVA is not very likely since at that low level voltage regulation would be very hard to control. Last but perhaps not applicable to your utility, our design limit for our substations is 20,000 amps at the buss. If your utility is similar (most are) this places the upper limit for fault duty at about 144 MVA. Not sure how "topgone" arrived at 500 MVA for the short circuit MVA but I suspect the 25kA S&C units will work for you. To be safe, contact the utility and request the short circuit duty at you point of contact. Just a pure guess here but from what is a typical three phase line serving close to the 10 MVA load limit I spoke of; I would think a fault duty level close to 8kA or about 58 MVA. Let us know what you get since I'm curious about my guess.
 
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