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Almost daily I receive calls and questions about the avionics master switch, regarding who needs it and which type should be used. Many say the aircraft didn't have a master switch when originally built so why should one be required now. In the early years few aircraft had an avionics master switch and for good reasons. One reason was that few aircraft had any avionics to protect. I remember years ago when an aircraft that had a Mark 12A and a Genave marker beacon was considered an aircraft with massive means of navigation and advanced electronics. Of course this is not the case today. Another reason an avionics master was not required in early years is because tube radios weren't as subject to damage from spikes from the charging system when the aircraft was started with the radios turned on. Try starting the engine with the avionics on today and there's a good chance you will damage your avionics. The damage may not show up immediately, but nevertheless, the damage has been done.
There are several type of avionics master switches in use today. One of Cessna's shots at an avionics master was a relay that opened an avionics relay when you started the engine. The problem with this set up is you don't know if the circuit is working correctly or not until things start to fail. Of course you could leave the avionics on and see if they quit when you started the aircraft but I don't recommend this. Another disadvantage of this system is you can't use external power to check out the radios, you must use the aircraft battery. Of course without using an external power source the avionics technician has a limited amount of time to troubleshoot problems until the battery is depleted. The main reason I don't like the Cessna setup using a avionics drop out relay is because if the battery voltage is low and you try starting the aircraft, the relay will drop in and out letting the voltage spikes visit your nice expensive avionics. Anytime we install new avionics in a Cessna that has this type of avionics protection, I yank out the drop out relay system and install "real" avionics protection. Avionics protection is the best money you can spend when you upgrade your radios.
An avionics install can run anywhere from a few hundred dollars to well over one hundred thousand dollars. To invest this kind of money and not have proper protection would not make economic sense much less common sense. You may think that you could just turn on and off the radios before and after starting the aircraft and wouldn't have a need for an avionics master switch. True, you may be able to turn on and off the navigation and communications equipment but how about the ICS, fuel computer, glideslope receiver, marker beacon, encoder, well the list goes on. Much of your modern avionics does not have a avionics master switch. They assume the installing agency knows what they are doing and will provide spike protection. No doubt today few avionics shops would install avionics without proper protection for the spikes generated when you start or even stop the aircraft. Normally avionics that has been subjected to voltage spikes will not show a problem right then but within time will fail. Our equipment is very sensitive to voltage spikes and P-Static. When we work on modern avionics in the shop we have to protect the avionics from static charges from our body, that's just how sensitive this new equipment is. On the other hand, if your avionics is protected and the aircraft is bonded, you should have years of trouble-free use of the equipment. In fact, most problems related with modern avionics are caused by starts/stops with the avionics turned on, heat related or P-Static discharges.
In the mid-seventies Cessna started mounting an avionics master switch on the left side of the aircraft near the pilots left knee. You probably will notice most of the 210 series have the avionics master switch there. By the way, if this switch is getting hard to turn on or off, replace it now! When it fails and it soon will, you will lose all your avionics. Not a good position to be in. This switch provides current to the avionics buss located in the same general area. This particular type of avionics switch does not incorporate a circuit breaker because the avionics buss is only an inch away from the switch. The avionics buss is normally a copper strip that all the individual avionics circuit breakers attach to and is supplied power via the avionics master switch. Avionics master switches located more than an inch or two from the main avionics buss should be the circuit breaker type. The reason for this is if the wire from the avionics master switch to the avionics buss shorts, the avionics master switch which is also a circuit breaker would "pop" instead of the wiring burning and causing all sorts of in-flight problems. We recommend the avionics master switch (circuit breaker type) be rated about 25% greater than the maximum load the avionics could draw. This allows plenty of protection should the wire from the avionics master switch to the avionics buss short, plus would let you add a few small items in the future to the avionics buss without changing the avionics master switch. Keep in mind that each individual piece of avionics is protected by a smaller circuit breaker or fuse so that circuit breaker should "pop", not the avionics master switch should a problem arise. Again, the main purpose of the avionics master switch is a central point to turn on/off your sensitive avionics and buss shorting protection is a secondary issue.
Some have asked, should they install two avionics master switches just in case one fails, after all, all the current for the avionics passes through it. I've never seen a avionics master switch fail that was a protection circuit breaker also. But in all honesty, I've talked to folks that have had a failure of this type of switch. I'd say the odds are pretty low though. I have seen the avionics masters fail that Cessna installed on the little panel on the left side of the aircraft. If you are uncomfortable with only one avionics master then by all means have the shop install another. If there's room in the panel, the price shouldn't be too expensive. The average avionics master switch with a built-in circuit breaker sells for around $60.00 plus installation.
In summary, if you have modern avionics such as KX-155s, Loran, DMEs, GPS, fuel flows and sorts, then in my opinion an avionics master is a must. The few dollars you try to save by not installing an avionics master will return to haunt you in repair bills, guaranteed.