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Joined: Apr 2002
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Explain the parallel fuses ??

Explain the parallel fuses please
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Joined: Jun 2004
Posts: 1,273
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1200A can (barely) be seen next to each bus -- down low.

The 600A frame size fuses are ganged because -- at one time -- that was a popular size limit for 250VAC fuses (blade style)

The operative assumption was that such matched sets would blow in sequence and that they'd always be replaced in pairs -- if not the entire sextet.

IIRC, this disconnect feeds a major retail establishment -- and the power flow is backwards: line is at bottom, load is at top.

It feeds a distribution panel on the other side of the wall... just as ancient. Consequently, these fuses did not have more than a prefunctary purpose. The many smaller fuses just a few feet away would protect the branch conductors and all sub-feeders.

These (MAIN) fuses could then only blow if some anarchist placed a bomb at the distribution board. (!)

You'll note that they did their duty for decades on end.

It was installed generations ago. (30s to 40s?)

The NEC at one time required disconnecting means exterior to any structure.

(In the very beginning the switch was to be thrown OFF when a building became unoccupied.)

This safety switch was probably installed per the Fire Marshall's specifications. Being in an urban setting, the line side power was coming in from below. Rather than fuss and bother, the power flow through the switch was simply reversed.

Obviously, none of it would fly today: exposed bussing, wrong way power flow,... even the fuses are standard blow -- not slow blow. Also note the extremely wide single layer copper bussing. It's simply not done anymore.

Today the Fire Marshall would be satisfied by a shunt trip MAIN for his crews. The entire box would never be necessary.


Last edited by Tesla; 05/26/14 11:18 PM.

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