In article , clare at snyder dot ontario dot canada wrote:
On Wed, 23 Jul 2008 12:21:52 GMT, (Doug Miller)
wrote:

In article cgFhk.374$DS3.189@trnddc01, Grant Erwin
wrote:
Doug Miller wrote:
In article , 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.
My application will never require the switch to break the
current - that will be done by the dryer's timer.

Also, in a closed-contact situation, I'm thinking that paralleling 2
sets of contacts will double (more or less) the current capacity.


Assuming you're talking about North American standard wiring, that is NOT
correct. The two 120V legs that comprise a 240V circuit are not in
parallel;
they are in *series*, and the current is the same at all points in the
circuit.

I think you may be misunderstanding Bob's application.

Not at all. I think you didn't read his post carefully enough.

Suppose you had
a single 15A switch in line with L1. When you toggle it, it will (obviously)
make or break 15 amps.

To clarify: it will make, break, or carry up to 15 amps *safely*. If 21 amps
is applied to it, it will make, break, or carry 21 amps -- until it fails.

Now suppose you put *another* 15A switch in parallel
with the first switch. Now if both switches are on, half the current would
flow through each. It's the switches that Bob is thinking of as being in
parallel, not the L1/L2 legs.

In the part you snipped, he wrote:

"So, if I use a 2 pole 15A rated switch, parallel its poles, & never
break the current with it, could it carry 21A?"

He's not even talking about two switches. He's talking about *one* switch,
and thinking that the L1 and L2 legs are in parallel. They are not.

Nope. He's talking about a DPDT switch. That is ONE switch physically,
but 2 electrically.

There is one critical difference: a DPDT switch disconnects both sets of
contactors simultaneously. That doesn't happen with two physically separate
switches.

Also, FWIW, using two switches in parallel as you suggest is flatly prohibited

by the National Electrical Code.

Except it is NOT 2 switches. Relays are used this way all the time.

Please read before posting. The guy I responded to here explicitly said two
switches: "suppose you put another 15A switch in parallel with the first
switch".

On 24 Jul 2008 01:55:16 GMT, (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.

Michael A. Terrell wrote:
Bob Engelhardt wrote:
Michael A. Terrell wrote:
Do you expect each set of contacts to have the eaxact same resitance?
I don't know, ... My intuition is that they would be "close enough".

If they are not equal, you have different current in each set of
contacts. It might be "Close Enough" in a fresh production switch, but
it will change as the switch ages. At some point the imbalance can be
enough to cause excessive current to flow trough one set of contacts,
welding them, or causing them to overheat and destroy the switch. I've
seen it too many times to ever do it myself. Every time the switch is
used, it expects a small spark to clean the contacts. Always switching
under no load makes them deteriorate even faster. ...

Thanks - I've ordered a switch that won't have to be paralleled, but
your info is good to have.

Bob

Curt Welch wrote:
[a discussion of unequal resistance in contact sets ] ...
So in practice, I think your solution would work fine - as long as it
doesn't get switched much under load. ...

Thanks. The majority opinion seems to be "It's not a great idea, but
you should be OK", with the minority being "NO!"

I feel the need to go wire a few 15 amp switches in parallel and see what
type of current balance I actually get.....

Yeah, yeah ... confuse us with facts BG

Bob

Curt Welch wrote:
... I did a quick test ... to see what would happen.

... 40% 60% balance of current.

I suspect with the contacts of a 15 amp switch, the distribution could
easily be less balanced even with a new switch, and would only get worse
over time as one switch would no doubt wear down faster than the other.
....
But this just demonstrates how parallel low resistance paths don't tend to
split the current evenly because the resistance is never very close to
equal.

....

That's surprising, and very good to know. A limited sample size (!), to
be sure, but it might not even be the worst case. I'm glad that I went
with the rated switch.

Thanks for the input,
Bob

Don Foreman wrote:
To pick a nit here ....

I'll see your nit and raise you one:

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.

According to http://www.powerstream.com/Wire_Size.htm, 12 ga wire has a
resistance of 1.6 ohms/1000', or .0016/ft, or .0004/3". I, too, have
problems with numbers of decimal places. BG

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.

Good point. My default approach would be to run tiny jumpers between
the switch poles, but if I used significant lengths of wire to each, I
could minimize the effect of different pole resistances. Say a foot of
#16 (good for 22A "chassis wiring")in series with each pole. That would
be .004 ohms. Now, what is the typical contact resistance in a switch?

Thanks,
Bob

Bob Engelhardt wrote:

Michael A. Terrell wrote:
Bob Engelhardt wrote:
Michael A. Terrell wrote:
Do you expect each set of contacts to have the eaxact same resitance?
I don't know, ... My intuition is that they would be "close enough".

If they are not equal, you have different current in each set of
contacts. It might be "Close Enough" in a fresh production switch, but
it will change as the switch ages. At some point the imbalance can be
enough to cause excessive current to flow trough one set of contacts,
welding them, or causing them to overheat and destroy the switch. I've
seen it too many times to ever do it myself. Every time the switch is
used, it expects a small spark to clean the contacts. Always switching
under no load makes them deteriorate even faster. ...

Thanks - I've ordered a switch that won't have to be paralleled, but
your info is good to have.

Bob


You're welcome.


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On Thu, 24 Jul 2008 09:30:30 -0500, Don Foreman
wrote:

On 24 Jul 2008 01:55:16 GMT, (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 --

Safety states never parallel switches or wires for added current.

If all is perfect you would share equal currents. The world isn't perfect.

I'd have a switch throw a relay. A nice auto start relay will be good for
hundreds of amps. If you need two hot lines - use two - just wire the controls
in parallel.

This way the switch can be in a lighter cable or long life condition as well.

The relay or contactor can be burnt up and replaced from time to time as needed.

Martin

Martin H. Eastburn
@ home at Lions' Lair with our computer lionslair at consolidated dot net
TSRA, Endowed; NRA LOH & Patron Member, Golden Eagle, Patriot's Medal.
NRA Second Amendment Task Force Charter Founder
IHMSA and NRA Metallic Silhouette maker & member.
http://lufkinced.com/


Grant Erwin wrote:
Doug Miller wrote:
In article , 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.
My application will never require the switch to break the current -
that will be done by the dryer's timer.

Also, in a closed-contact situation, I'm thinking that paralleling 2
sets of contacts will double (more or less) the current capacity.


Assuming you're talking about North American standard wiring, that is
NOT correct. The two 120V legs that comprise a 240V circuit are not in
parallel; they are in *series*, and the current is the same at all
points in the circuit.

I think you may be misunderstanding Bob's application. Suppose you had
a single 15A switch in line with L1. When you toggle it, it will
(obviously)
make or break 15 amps. Now suppose you put *another* 15A switch in parallel
with the first switch. Now if both switches are on, half the current would
flow through each. It's the switches that Bob is thinking of as being in
parallel, not the L1/L2 legs.

Bob, I doubt you can get 30A out of a 15A DP switch this way, though.

Grant Erwin


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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.

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?

Thanks,
Bob


Found this contactor on surplus site...


http://www.isesurplus.com/index43.htm

--

Richard

(remove the X to email)

Doug Miller wrote:
In article , Bob Engelhardt wrote:
Doug Miller wrote:
Think this through a little farther. You don't have one wire carrying 240V as
you illustrated. You have two: L1 and L2.

Finish your drawing -- show where you're going to connect the L1 and L2 leads
to that switch.
Sigh:

/
L1 --- _________/ ___________
| / |
|__/ __|
240 LOAD

L2 --- _________________________

Sigh:

So L2 will always be supplying 120V to the load, even when the switch is off.

Does that sound safe, prudent, or wise to you?


That is exactly how all electric ovens I have ever seen control the
elements...

They use relays or SSR's, not switches but only one leg is switched &
the other is hot at all times.

MikeB

Using parallel switches is bad. If either fails it fails totally.
If one is slower than the other the other fails.

The L2 is typically used for the 'other side' and to power to GND electronics.

In equipment - it is a bad plan. It is dangerous to have a hot line
all of the time on equipment. Use 2 phase or dual leg switches or contactors
or large circuit breakers.

Use one in each leg. Each to handle the load plus safety factor.

Martin

Martin H. Eastburn
@ home at Lions' Lair with our computer lionslair at consolidated dot net
TSRA, Endowed; NRA LOH & Patron Member, Golden Eagle, Patriot's Medal.
NRA Second Amendment Task Force Charter Founder
IHMSA and NRA Metallic Silhouette maker & member.
http://lufkinced.com/


bq340 wrote:
Doug Miller wrote:
In article , Bob
Engelhardt wrote:
Doug Miller wrote:
Think this through a little farther. You don't have one wire
carrying 240V as
you illustrated. You have two: L1 and L2.

Finish your drawing -- show where you're going to connect the L1 and
L2 leads
to that switch.
Sigh:

/
L1 --- _________/ ___________
| / |
|__/ __|
240 LOAD

L2 --- _________________________

Sigh:

So L2 will always be supplying 120V to the load, even when the switch
is off.

Does that sound safe, prudent, or wise to you?


That is exactly how all electric ovens I have ever seen control the
elements...

They use relays or SSR's, not switches but only one leg is switched &
the other is hot at all times.

MikeB


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