ECN Forum Forums General Topics of the Trade General Discussion Area double throw disco

Has anyone used a double throw safety switch. Just wondering what applications this had been used for.

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I would have thought one of the most common uses would be to select different power sources, e.g. utility power or generator.

Manual transfer switch, as Paul says.

We use them all over the place in the factory that I work at to reverse 3-phase motors. Matt

Another use is to select which load gets power when the same source runs one OR another load.

Note that nowadays there are two styles of ‘double-throw’ XOP switches. See http://ecatalog.squared.com/catalog/173/html/sections/03/17303014.html
A newer style has two sets of movable blades and two sets of fixed jaws. Older is one set of movable blades and two sets of fixed jaws. (Note upper vs. lower photos.)

[I believe Cutler-Hammer has a similar variation.]

AFAIK, the 2 different variations in construction are a result of fusible or non-fusible design.

The extra length needed for 2 sets of fuses requires 2 separate interlocked switch mechanisms, while a non-fused switch can be made with a single set of blades.

NJ, that is a good point. I installed one fusible double-throw switch in a service-entrance application in the 1970s to accommodate a standby genset. One set of fuses was normally hot through the utility feed [with the switch open.] At the time there was an NEC exception that clearly allowed the practice.

By job completion, a prominent label warned of the condition, and a piece of clear acrylic secured with non-metallic machine screws covered the {utility-powered} set of fuses.

Two C-H double-throw switch versions [one set of movable blades versus two] are illustrated on page 26 of www.eatonelectrical.com/unsecure/cms1/TB00800001E.PDF {Aaak! 5MB file}

Note also the two applications on page 24 fig. 8-7 — “two sources” versus “two loads”.





[This message has been edited by Bjarney (edited 04-05-2004).]

should it be called a three posion insted of a "double trough"?