We had the incoming line to the complex blow a fuse yesterday. Single phased the place for over an hour until the power company got there.

My question is how are those things sized?

why do I ask? Because they are 125 amp fuses on a 7200 volt line. After changing them out the lineman measured the load it was 92 , 106 and 110. That 110 sure sounds a bit to close to me but I know next to nothing about HV stuff.

Now 125 amps at 7200 volts would be 1875 amps at 480 volts correct?

There is going to be a meeting with the power company and my engineer boss to try and solve this problem.

The characteristics of medium voltage fuses are different than low voltage fuses. Assuming that you refer to an E style (125E) med. volt. fuse is designed to protect the down stream transformer, and can handle significantly more full load current for more time, than the equivelent situation for low voltage.

I figured something like that.

The catch is there is no downstream transformer but rather downstream transformers.

Each building gets its own drop with fuses and a transformer. NONE of those blew, the main into the complex blew.

Does this make sense to the high voltage / medium voltage guys?

As an example, 100-ampere “K” or “T” universal-link 15-38kV fuses are intended to continuously withstand their rated current, and not open at 200-240 amperes for 300 seconds. [The next standard size listed for ANSI Std C37.41 fuses is 140 amperes.] Some load balancing on the 480V side might help.




[This message has been edited by Bjarney (edited 06-10-2003).]