Applying cable limiters to short services 1

[Morrand]

I'm reviewing some of our old distribution system design practices at the moment, and one that I'm looking at has to do with the application of cable limiters on our secondary network. In short, at 208V it calls for cable limiters on all of the main distribution cables, and on services when there isn't enough available fault current to burn a fault clear.

What is odd about this practice is that it contains this phrase: "Limiters shall be installed on the source end of 500 kcmil services except for cable shorter than 15 feet." It goes on to suggest that limiters aren't required on the load end of those cables either, although it doesn't explicitly say that (yet).

I've been going through the references that I have on hand and can't come up with a good, reasonable explanation why short services would not require limiters. Nobody kept any notes in the file to explain why this was originally written in (though we do have the original, typewritten copy from 1972) so I don't have any direct source for this practice. Neither my 1941 Standard Handbook for Electrical Engineers nor our copy of the 1957 EEI Underground Systems Reference Book are helpful in deciphering this particular rule.

Is there any good general reason why a 500 kcmil service lateral that is less than 15 feet from a grid network source to a main switch should not have cable limiters applied to it, when a similar but longer lateral should?

 

[dpc]

Short service conductors allow more fault current to pass due to lower impedance.

 

[magoo2]

Limiters were so named because they limit damage to secondary cables from fault currents. I suspect the installation cost for limiters for a service lateral less than 15 feet exceeds the value of the service lateral.

 

[Morrand]

magoo,

If it's a short, heavy service (3+ cables), then that makes some sense: at upwards of $2000 per set of cable, the limiters don't provide much of a savings over letting a faulted cable burn off. The fault current divides across the other sets and doesn't damage them, if there are enough sets, so there's no particular reason to use the limiters. That's at least half of the explanation.

The other thing I can think of is that, if the fault current is below burnoff, something else (like the upstream main) finally has to burn out and clear it. But with available fault current that low, there should be limiters on the mains already, and those should clear before the main cable gets roasted. That doesn't seem like a very good plan (dropping the service bus this way would drop other customers unnecessarily, after all), but perhaps the other half of the thinking is that such a short cable poses too little risk of faults to be worth protecting against?

 

[Morrand]

It turns out the 15-foot rule is a part of our rates, nothing more (at least, nothing that I could find). It figures, but at least that's enough to justify keeping it in the practice for now.

LPS for magoo, though. I compared the costs of replacing a two-set service lateral against replacing one set and six limiters, and it turns out that we need a service longer than 115 feet to save any money by installing limiters. It sounds like something for our next rate case.