Re: OT - toggle switch current rating
- From: Bruce L. Bergman <blnospambergman@xxxxxxxxxxxxxxxxx>
- Date: Thu, 24 Jul 2008 12:39:35 -0700
On Thu, 24 Jul 2008 09:30:30 -0500, Don Foreman
<dforeman@xxxxxxxxxxxxxxxxxxxx> wrote:
On 24 Jul 2008 01:55:16 GMT, curt@xxxxxxxx (Curt Welch) wrote:
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.
To pick a nit here ....
if two switches are connected in parallel using 3" of 12-gage wire on
each leg, the resistance ofeach wire leg is about .004 ohms. So, if
one contact is .0001 ohm and the other negligable, the branch
resistances would be .0041 and .004 so the currents would be very
nearly the same.
But if the 240V starts arcing across the contacts of one of the
switches in parallel, the arc won't get 'split' between the two sides,
it will keep going on one - that's why regular light switches aren't
rated to break a motor load. A "Quiet" switch opens slowly with the
action of the handle, and it's really easy to start an arc by opening
it slowly.
Motor Rated toggle switches are snap-action and have some sort of an
arc chute designed in. This is the only kind that can live very long
breaking higher currents.
When you are breaking current, you need to have some sort of an arc
chute or magnetic arc blowout device there. If the arc doesn't blow
out within a few cycles, the switch will - and it can be violent.
If you want this design to have a long life with low downtime, go
get a dedicated purpose contactor to switch the main load. Use your
control switch and the timer as pilot duty switching only, and they'll
live forever.
The appliance builders should design them all that way, and they do
for commercial equipment that gets used 80 hours a week for decades.
But consumer grade appliances can get away with switching the main
load in the timer of a clothes dryer only because they are designed
with cost as an overriding concern. People buy on price, and if the
timer lasts them 5 years before those switching contacts burn up they
let the customer pay for a service call to repair it.
--<< Bruce >>--
.
- References:
- OT - toggle switch current rating
- From: Bob Engelhardt
- Re: OT - toggle switch current rating
- From: Curt Welch
- Re: OT - toggle switch current rating
- From: Don Foreman
- OT - toggle switch current rating
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