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Is this switch sufficient to be an Avionics Master Switch

blueflyer

blueflyer

Well Known Member
I have decided I want an avionics master switch. Foregoing the debate as to whether this is a good idea or not, my question is about the switch I have selected. Specifically, do you think this switch is appropriate for an avionics master switch. My avionic breakers add up to 26 amps. If not, do you have a recommendation for an avionics switch?

http://www.alliedelec.com/nkk-switches-sw3821/70192259/#tab=specs

Here are the specs on it:
Body Style : Rectangular
Contact Form : DPST
Current, Rating : 30 Amps
Mounting Hole Size : 23 x 37 mm
Mounting Type : Panel
Number of Poles : 2
Number of Positions : 2
Operation : On-None-Off
Standards : UL CSA
Termination : Screws
Type : Rocker
Voltage, Rating : 250 VAC

 
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All I can say is that the SPEC you list is for AC voltage. You have DC voltage in your aircraft. ANY switch that has a larger DC amperage than the sum of all the DC current will work.

The PDF link to the DATASHEET has a DC rating at 30 Volts that equals the 125 VAC current resistive rating so it appears that it will be ok.
 
A general rule of thumb is that any SNAP action AC switch can handle the same current at 12 VDC that it can handle at 120 VAC. The actual loads should be added up, not the breaker sizes.
 
I have used switches similar to this, I used a jumper between the two sides to turn it into what is effectively a SPST switch---------the two sides share the load.
 
Mike S said:
I have used switches similar to this, I used a jumper between the two sides to turn it into what is effectively a SPST switch---------the two sides share the load.
Click to expand...

Mike, this doesn't always work. One, the current will only split 50-50 if the two switches have exactly the same resistance. Two, the critical part for a dc switch is opening (disconnect) when there's a tendency to draw an arc. One switch will nearly always open first, putting the entire opening load on the other.
 
a few people have brought up things I hadn't considered, so maybe I should sit down and read a little more before I decide on an appropriate avionics master switch.
 
Or you could just use the switch to enable a continuous duty contactor. Then the switch current isn't important. The 30A is switched by the contactor not the switch. Then you can pick a switch that matches your panel.
 
Read these articles by Bob Nuckolls. Avionics Master Switches: Really Necessary? If you really have to have an avionics switch, Bob Nuckolls offers a safe way to wire it without a single point of failure.
How the avionics switch was born
Bob Nuckolls is an engineer who has worked for major aircraft companies. He wrote the book on how to wire airplanes. The majority of E-AB airplanes are wired according to his book. Bob Nuckolls has served as an expert witness in aviation accident lawsuits. I admire his electrical and electronic knowledge.
 
Joe, thanks for posting the links. I read the PDF and it answered a question I had about my RV4. My electrical bus has an avionics master switch, and also the alternated feed that the pdf describes. What's most interesting is that Bob Nuckolls (AeroElectric connection) did the wiring diagrams for my airplane! So its interesting, since Bob was involved, that I have an avionics master switch, but maybe the builder insisted. I will also note that my install does not have the diode that Bob says is necessary. I had the same concern about back feeding, and the pdf you linked supports this.
 
I will also note that my install does not have the diode that Bob says is necessary.
Click to expand...
The diode prevents engine cranking current from flowing from the battery through the e-bus circuit to the starter when the e-bus switch is inadvertently left on. The diode also prevents the e-bus from powering the main bus when the master contactor is opened. Here is a DIODE to consider.
 
Forgive my ignorance, I'm just getting started on the idea of building a plane...

Wouldn't it be prudent to use lighter duty switches which control higher power relays for your switches such as this?

Or is the consensus that a relay is another point of failure and avoided because of that?
 
Welcome to VAF!

rdrcrmatt said:
Forgive my ignorance, I'm just getting started on the idea of building a plane...

Wouldn't it be prudent to use lighter duty switches which control higher power relays for your switches such as this?

Or is the consensus that a relay is another point of failure and avoided because of that?
Click to expand...

Matt, welcome to VAF.

Using a relay is common-------but not universal, as not everybody likes them. One of those choices you get to make when doing a homebuilt :D

In my case, yes I have one. http://www.bandc.biz/spdtsealedrelay12v20a.aspx
 
Wouldn't it be prudent to use lighter duty switches which control higher power relays for your switches such as this? Or is the consensus that a relay is another point of failure and avoided because of that?
Click to expand...
It all depends. A relay can be used for remote control and to keep heavy wires short. And like you said, to use smaller switches. But switches need labels that take up as much panel space as a heavy duty switch. And if you add up the weight of a small switch and a relay, not much is saved over using just a switch rated for the load. It is a matter of personal preference. I would use a switch for loads of 15 amps or less, and use a relay for more than 15 amps. And so the answer is: It all depends. :)
 
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