Bob Engelhardt wrote:
I'm in the process of modifying an electric clothes dryer. I want to
switch 21A, 240v current, but I don't have, and can't find, a switch
with that capacity.
But, as I understand it, switches are rated for the current that they
can *break*. My application will never require the switch to break the
current - that will be done by the dryer's timer.
The current breaking ability of the switch is certainly a big part of the
rating, but the current carrying is certainly in there as well. You can't
push 100 amps through a 1 amp switch. I bet however that you would be
fairly safe pushing 21 amps through the 15 amp switch.
Also, in a closed-contact situation, I'm thinking that paralleling 2
sets of contacts will double (more or less) the current capacity.
So, if I use a 2 pole 15A rated switch, parallel its poles, & never
break the current with it, could it carry 21A?
It seems to me that the resistance would be extremely hard to control in
that situation. The resistance is so low, that I strongly suspect you
would get a large imbalance between the two polls. If the alignment of the
contacts is only slightly different in the two polls it seems to me you
could get a 10 to 1 ratio in the resistance. Such as .00001 vs .000001 in
the two contacts and if that's so, you get a 10 to 1 imbalance in the
current. Your wires leading to the switch, and the contact you make in
connecting the wires to the switch all play important factors in
determining the total resistance of each path and in turn control the
current. If you just tighten the screw a little more on one pole and get
twice the surface area for the wires connected to one pole vs the other
because you flattened the wires more on one pole I think it could lead to a
2 to 1 resistance ratio and a matching 2 to 1 current flow. All in all,
it's not safe to assume the resistance would be "close enough" to create a
reasonable balance.
However, because switches that can carry 15 amps have to be a bit
over-rated to be safe, I think it would in practice work just fine for your
21 amp load even with a single pole. And if you put them in parallel, you
would only make it better, no matter how imbalanced the resistance was. So
even if it was as bad as 10 to 1 out of balance with the resistance, it
would still reduce the current to about 19 amps in one pole and 2 amps in
the other bringing it that much closer to the 15 amp rating.
So in practice, I think your solution would work fine - as long as it
doesn't get switched much under load. With the strong magnetic load talked
about in the other post, there's a _HUGE_ back emf voltage spike when you
break the circuit which the switch has to be designed to take or else it
will wear down very quickly. With your resistance only dryer load, that
would not be an issue so at worse.
In practice, I think it would work fine, and at worse, you would see a
reduced life span on the switch. If the switch didn't have to be used
much, it would last even longer in practice.
To be safe however, you of course shouldn't do it. The odds of it melting
down and starting a fire or hurting someone only go up when you play games
like that. It's a good thing you found a switch rated for the task at a
reasonable price...
I feel the need to go wire a few 15 amp switches in parallel and see what
type of current balance I actually get.....
Thanks,
Bob
--
Curt Welch
http://CurtWelch.Com/
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