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switch rating

 
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jrstone(at)insightbb.com
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PostPosted: Fri Jan 27, 2006 8:23 pm    Post subject: switch rating Reply with quote

Can someone educate me on switch ratings?
I just red Bob's article on the subject and I have the following question.
In his article he states that one can use the same amp rating that is written on the side of the switch for 115vac in our 14vdc airplanes. My switches have a rating of 15 amps at 250vac. So do I have a 7amp switch or a 30 amp swithch if used in my 14vdc airplane.
The switch btw is an avionics switch in the normal feed wire to the E-bus and downstream of the diode. My E bus is essentially my avionics bus minus one radio and has 10 items on it. I'm concerned the switch will be overloaded and perhaps I should add a contactor or the s704-1 relay from B+C.
Any thoughts?
Thanks,
Jim
Harmon Rocket II
95% wired


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harley(at)AgelessWings.co
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PostPosted: Sat Jan 28, 2006 4:45 am    Post subject: switch rating Reply with quote

Morning, Jim...

>>My switches have a rating of 15 amps at 250vac. So do I have a 7amp
switch or a 30 amp swithch if used in my 14vdc airplane.<<

You have a 15 amp switch! In fact, it is also a 14 amp or a 13 amp or a
5 amp or a 1 amp switch at anything less than 250 volts.

The ratings listed on a switch are the MAXIMUM use for that unit. If
you want to use a smaller voltage, fine...it is still capable of
handling 15 amps. You would want to use a smaller rated switch if you
wanted to save money or weight or size.

A similar example is fuses. Say that a fuse for your auto or aircraft
is rated for 15 amps and 32 volts. Just because you use it on a 28 volts
system, or a 14 volt system doesn't change it's amperage rating. It'll
still trip at or about 15 amps.

It's been awhile since I taught electronics, but there is a bit more to
it all than that. But basically you won't run into any trouble if you
treat the ratings on switches as MAXIMUMS, not REQUIRED.

Harley Dixon
Jim Stone wrote:

Quote:


Can someone educate me on switch ratings?
I just red Bob's article on the subject and I have the following question.
In his article he states that one can use the same amp rating that is written on the side of the switch for 115vac in our 14vdc airplanes. My switches have a rating of 15 amps at 250vac. So do I have a 7amp switch or a 30 amp swithch if used in my 14vdc airplane.
The switch btw is an avionics switch in the normal feed wire to the E-bus and downstream of the diode. My E bus is essentially my avionics bus minus one radio and has 10 items on it. I'm concerned the switch will be overloaded and perhaps I should add a contactor or the s704-1 relay from B+C.
Any thoughts?
Thanks,
Jim
Harmon Rocket II
95% wired








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nuckollsr(at)cox.net
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PostPosted: Sat Jan 28, 2006 5:19 am    Post subject: switch rating Reply with quote

At 11:21 PM 1/27/2006 -0500, you wrote:

Quote:


Can someone educate me on switch ratings?
I just red Bob's article on the subject and I have the following question.
In his article he states that one can use the same amp rating that is
written on the side of the switch for 115vac in our 14vdc airplanes. My
switches have a rating of 15 amps at 250vac. So do I have a 7amp switch
or a 30 amp swithch if used in my 14vdc airplane.
The switch btw is an avionics switch in the normal feed wire to the E-bus
and downstream of the diode. My E bus is essentially my avionics bus
minus one radio and has 10 items on it. I'm concerned the switch will be
overloaded and perhaps I should add a contactor or the s704-1 relay from B+C.

Keep in mind that switch ratings are based on electrical
and mechanical stresses over tens of thousands of cycles.
Further, the ratings speak to currents that the switch must
make and break along with the nature of the load's dynamic
responses (resistive, inductive, lamp loads, etc).

The catalog ratings are strongly influenced by environmental
and utilization numbers. I've often observed that more
switches in small aircraft die for LACK OF USE than for
OVERSTRESSED USE.

Generally speaking, ANY full sized toggle switch you
pick up with a rating of 7A or more at any AC voltage
will probably run the lifetime of your airplane in ANY
location in the system up to and including boss-hog
pitot heaters with 30A inrush and 10A running currents
as long as they're regularly called upon to switch those
loads. I.e, cycle every switch, every flight cycle.
10 years from now at one flight/day your switches will
have less than 4,000 cycles on them.

If your e-bus is wired per a Z-figure, it has TWO
independent power pathways (a design goal). So failure
of any one pathway does not generate an 'emergency'.
If the AeroElectric Connection has any agenda, it's to
plant and nurture the seeds of an idea: "No single failure
of any component should be more than a maintenance event."
This includes ANY switch on the panel.

I'll suggest the concerns you cited above are overblown.
The reason one might consider adding the relay at the
battery bus for larger e-bus loads (greater than 5A) . . .

http://www.aeroelectric.com/Pictures/Schematics/E-BusFatFeed.gif

. . . is to create a mini-battery contactor AT THE BATTERY
bus to account for the fact that the always hot feeder
is protected at greater than 5A. Such feeders are worthy
of disconnection AT THE BUS instead of running the wire
all the way to the panel. This is a crash safety convention
borrowed from the TC aircraft world and has nothing to do with the
RATINGS of the switch. One could run a 15A e-bus alternate
feed through a 7A rated toggle switch with very little
concern for service life and zero concern for failure.

Bob . . .


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jrstone(at)insightbb.com
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PostPosted: Sat Jan 28, 2006 8:01 am    Post subject: switch rating Reply with quote

Good explanation
Thanks Harley .
Jim
do not archive
---


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