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Posted by Taxlover on January 31, 2008, 4:29 am
> Taxlover wrote:
>> I bought a double pole switch for my router table. It says it can handle
>> 20a/240v or 35a/120v.
>>
>> Why would each pole be able to handle only 20a on 240v (with 120v on each
>> pole) but 35a on 120v? Or does it assume you are connecting the 120v to
>> both poles to get the greater capacity.
>>
>> I wrote the company but got a reply that didn't really say anything.
>>
>
> My guess is the reason is if the poles start to open at very slightly
> different times you will have a 240V arc across the pole that opens
> earlier. A higher voltage arc is harder to extinguish.
>
> For the same reason you can't reliably parallel the 2 poles for 120V. If
> one pole opens slightly early the other pole will interrupt the full 35A.
>
> AC only switches (not intended for DC use) intentionally operate slowly so
> zero crossings of the AC supply voltage help extinguish the arc. Slow
> operation probably makes it more likely the poles will open at slightly
> different times.
>
Okay, that makes sense. Thanks for explaining it simply.
My old tablesaw came with a 1hp relay and a 2hp motor. To help it along
they wired two poles in parallel, but eventually one fused. I suppose that
is the same problem you are describing.
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