Your the captain
rv72004 said:
Thanks for the reply guys, but still no one has told me if the solenoid must be protected. Yes a combination c/b switch is a good idea . Its just that I am using really nice lighted rocker switches and would like to use one of these instead.
As you know most would recommend to avoid a relay because its a failure point, but if you want to use a relay and a Rocker switch that's OK. The thing is rocker switches in general have a high rating and likely could handle the avionics buss with out a relay. Note: DC rating and AC rating are TWO different things in switches.
Here is Bob's article:
http://www.aeroelectric.com/articles/swtchrat.pdf
Rocker switches are known for taking high amps 10,15, 20 and even 30 amps DC:
(typical: small- 0.5"w x 1"h; large- .8" x 1.45"h rectangle, even round)
http://www.action-electronics.com/gcswrot.htm#Marine
http://www.steinair.com/switches.htm
The avionics you state sound reasonable. I guess you are under 10 amps max (even while transmitting/autopilot servo on). Your rocker switch capability should take this load, unless you are using a mini switch (you should use heavy duty switches of at least 10 amps). My typical avionics drain is less than 7 amps and standby initial power-up around 4 amps (not transmitting or auto pilot servo engaged).
To answer you question, yes you should provide a C/B or fuse for the relay (solenoid as you called it, also called contactor by others),
unless you don't need it.
Let me see if I can explain. (Wire protection is a little bit art, science and paranoia.)
Ok, Where should have circuit protection on the switch and or relay, if at all.
Depends on the length of the wire. Fuses/CB's/Fusible links provide wire protection of LONG lengths of wire. If two components are next to each other, essentially co-located you don't need to protect the wire between the components. Many guys set the master and avionics buss next to each other so the jump wire is short. You could squeeze a relay in between the busses and use short jumpers. If you a Rocker switch in the panel (handling the positive current) you will no doubt have a length of wire to/from the panel needing protection. So you need a fuse/CB maybe.
OK the wire run is long and not adjacent. Where to put the fuse/CB? As close to the source of power as possible, the main buss as you mentioned. Here is a crude dwg to show what I mean.
Lets say an electrical "short" of the long feed wire to the avionics buss. Assume the "short" occurs near the avionics buss, so the whole length of the feed wire will smoke and glow red, all the way back thru the relay, to the source, the main buss, which is also not protected, direct to the battery (which is common). You will have a problem. A single fuse (one close to the main buss) will save the day. With out it you are..... Since the switch (relay activation) is a ground switch it's not critical and does not need a fuse.
I would put a fuse / CB device in at least adjacent to the main buss, which you are trying to protect. Also consider a second electrical path to power the avionics buss in the event of a relay or fuse/CB failure. Depends on your mission (IFR/VFR), Examples below.
Examples:
Below is a basic (but very well done) pwr diagram showing an avionics buss with a avionics master switch from a RV-8a builder. He uses a switch with NO fuse/CB. I don't agree unless the busses and switch are close / co-located. This diagram is based on Bob N's recommend diagrams. Bob N does not use (believe in) avionic switches but does recommend an avionics buss. Bob N. likes automotive blade fuses in plastic multi fuse holders. You would use one for the main and another fuse block for the avionics items, placing them next to each other, close. The two busses are direct connect with a short wire/jumper direct or thru a diode bridge (see next example). Of course where the short jumper is, is where a switch or relay would go if you wanted an avionics master switch. Again proximity plays a part in this. If you run a long wire to a switch, relay or buss away from the main buss, protect it. May be a fusible link would be a good thing?
http://www.rv8a.com/wiring/dwg/rev6/n8wv_wire_r6.pdf
Below is a fancy (very nice) diagram that shows the avionics buss, driven by both the main and also an emergency hot batt buss. So there are two switches, main and emergency (from hot batt buss). You see he has two fuses on the main feed (and emergency feed) to the buss and a diode bridge. The diode keeps the emergency feed from back feeding the main buss from the hot batt buss when the emergency switch is on. I am not sure why he uses two fuses (four total) but he probably had room in his "Fuse Block" and used them, it works. Where he shows switch [A7] is where your relay goes, right. Emergency buss feed is not a bad idea in case the relay dies or you want to shut down the the main buss (thru the master relay). This way you can still power the avionics direct from the battery, thru the hot battery buss, even with the master off. What if the master relay fails in flight IFR? (right)
Is it worth the extra wire, CB's switch and diodes? What is the chance the master relay will fail? (small) Can you live with it if it happens in flight? What is the back up? Is a back up needed. These are the question you need to ask. There is a balance and than there is over kill. People are putting in two alternators, two batteries and all kind of switches and alternate power sources in their plane. Sounds like a lot for a little plane. Be realistic and not to paranoid. What ever floats your boat, but you have to float (fly) your airplane and WEIGHT is critical. Build is simple, straight and as light as possible. Remember most Cessna's and Piper have very simple one battery, one alternator, one or two buss wiring and have worked fine for decades.
http://www.pflanzer-aviation.com/PDF Files/Power.pdf
Wire protection and electrical systems are designed by good practice, common sense and trade-offs , safety (redundancy) vs. complication. You don't want any wire big or small, of any length, grounding or getting real hot. You have the choice of fuse, CB, fusible link or nothing. When you figure out what you want to do, post it. Make a diagram for yourself (which is a key tool) , look at builders web sites who post their wiring diagram. It will become more clear. I hope that helps. You are the Captain and you get to choose.
One thing to keep in mind, can you afford to loose ALL your avionics buss items with a single relay, single fuse. Are you VFR/IFR? Either the answer is yes you are OK without the avionics or you need to think of multipaths to assure a single fault will not leave you in the dark.
You are learning. Bob's book is good, may be over kill. Key to a good electrical system is a good central ground. I would get some more info, aviation book store, web and check back. Check Bob's web site out and look for articles, lots of free advice, diagrams and info (
http://www.aeroelectric.com/ and
http://www.aeroelectric.com/articles.html).
George
PS some other stuff to read
http://www.zenithair.com/kit-data/ra/electric1.html
http://www.zenithair.com/kit-data/ra/electric2.html
http://www.bluemountainavionics.com/pdf/aircraft_wiring_04december2004.pdf