Re: power-grid physics

["James R. Frysinger" <frysingerj@COFC.EDU>]

On Friday 2003 August 22 17:24, you wrote:
> Thanks for the switching information. I was wondering how they did that!
> Those big high-current Frankenstein movie-type knife switches have to be
> opened & closed when there's no load, or else they'll just go *poof* or
> *BANG* and weld themselves shut, often with substantial damage to the
> switch operator. NOT pretty.
....

        Are you all aware that the switches feeding your secondary cribs have
explosives in them? You have probably seen the workers opening and shutting
those switches quickly (atop the poles) with long fiberglass poles. But the
receptical for the switch's blade contains an explosive wrapped with wire.
When the secondary crib is overloaded, the proportional current through the
wire heats the explosive up high enough to cook it off. The gasses that
result are used to expell the blade of the switch very quickly. When your
neighborhood loses power, you often hear a "bang" as the lights go out.
That's not a spark; it's the explosive going off and opening the switch
quicklyin order to minimize the arcing. Remember that the secondary crib is
what provides the split 235 V power to the houses, which represent a fair
amount of inductive loading (appliance motors, e.g.).

        Literally, those are blown fuses!

Jim

--

James R. Frysinger
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Senior Member, IEEE

http://www.cofc.edu/~frysingj
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j.frysinger@ieee.org

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