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jonlaury
Joined: 06 Nov 2006 Posts: 336
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Posted: Mon Jul 19, 2010 11:54 am Post subject: Redundant Power switching |
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If I bring separate bus power legs to a DPST to feed a single load, is the current shared (i.e. halved) on the two poles/switches?
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retasker(at)optonline.net Guest
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Posted: Mon Jul 19, 2010 5:25 pm Post subject: Redundant Power switching |
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Shared, yes. Halved, not likely. How much of the load each pole of the
switch provides depends on the resistance in each leg. Furthermore, if
you are trying to get by with a switch that is not rated per pole for
your total load you will be overloading it. It is very unlikely that
both poles will switch simultaneously so there will be momentary full
loads on one of the poles. Depending on how many times you plan to
operate the switch and what the load is you may have a problem.
If you are doing it with a properly rated switch to provide a redundant
source of power, you have just created a single point of failure - the
switch.
My $0.02.
Dick Tasker
jonlaury wrote:
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joemotis(at)gmail.com Guest
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Posted: Mon Jul 19, 2010 8:14 pm Post subject: Redundant Power switching |
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Either leg of the switch has the ampacity of the overcurrent protection device rating available, when either or the sum of both legs reach that value, the overcurrent device will trip.
It is still the sum of the total load
that will trip the breaker, not how you divide it downstream of the overcurrent protection.
On Mon, Jul 19, 2010 at 12:54 PM, jonlaury <jonlaury(at)impulse.net (jonlaury(at)impulse.net)> wrote:
[quote]--> AeroElectric-List message posted by: "jonlaury" <jonlaury(at)impulse.net (jonlaury(at)impulse.net)>
If I bring separate bus power legs to a DPST to feed a single load, is the current shared (i.e. halved) on the two poles/switches?
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jonlaury
Joined: 06 Nov 2006 Posts: 336
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Posted: Tue Jul 20, 2010 9:59 am Post subject: Re: Redundant Power switching |
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Dick and Joe, thanks for your input.
Dick, I will have two switches. The plan is to use the DPST to switch legs of 6-7A and 2A. One switch is a Honeywell AML 34, rated at 125v/15a (at) 60% power factor. The back-up is a Honeywell AML 31, 125v/10a (at) 60% In a similar chart for another switch, Honeywell specifies "Resistive", along with % power factor so I'm assuming that the %power factor is for inductive loads. The 6-7 A load is for electronic fuel injectors which I believe are solenoids and hence, inductive loads.
Joe, I didn't quite follow how the loads on the switch legs are additive. Each pole/switch is separate from the other in the switch and are fed from fuses on separate buses.
J
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user9253
Joined: 28 Mar 2008 Posts: 1907 Location: Riley TWP Michigan
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Posted: Wed Jul 21, 2010 5:31 am Post subject: Re: Redundant Power switching |
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Quote: | If I bring separate bus power legs to a DPST to feed a single load, is the current shared (i.e. halved) on the two poles/switches? |
The simple answer to your question is, no, not necessarily.
I agree with the other posts above. The problem is that it is not clear exactly how the circuit is wired and what is trying to be accomplished. If you can post an electrical drawing of your circuit, it will be helpful. Do not be too concerned about switch ratings as long as they are in the ballpark. If a switch handles 10 percent more current than its rating, that does not mean that it will be destroyed. Its life will be shortened but that might not matter. Supposing that the life of a switch is rated at 10,000 cycles and due to higher than rated current, it only lasts for 5,000 cycles. It will still last as long as the airplane. Now I am not saying that switch ratings should be ignored. But a switch will not blow up if slightly overloaded. I would use a switch with the proper rating if available.
Joe
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jonlaury
Joined: 06 Nov 2006 Posts: 336
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Posted: Thu Jul 22, 2010 9:42 am Post subject: Re: Redundant Power switching |
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Joe G,
My question was rendered moot by going with two DPST switches, one leg of which powers the EFI ECU and the other, the Injectors, per Dick Tasker's suggestion to eliminate the single point of failure.
I was being kind of myopic wondering if a DPST providing dual power sources to a single load would see 1/2 the load on each leg.
Thanks,
John
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