The International Cessna^® 170 Association

Due to incorrect wiring my 1953 Cessna 170B suffered an over voltage event. All avionics and the electric turn and bank were damaged or destroyed. I wonder if I need to consider rewiring my whole electrical system. I welcome any comments or advice.
Thanks Alan

I doubt you have to rewire anything just because the wire carried a high voltage. High voltage alone won't hurt the wire. Ohms Law is what you need to know.

Voltage x Amps = Watts or W/A=V or W/V=A is Ohms Law. So if a given wire is rated at 12v and 15 amps which equals 180 Watts it is the same as running 24v at 7.5 amps equalling 180 watts.

So you could double the voltage so long as the amps remained below half that the wire is rated for and the amount of power (Watts) is the same.

Chances are the wire is actually rated for much much more than 12v to begin with.

I think you only have to correct the incorrect wiring which caused the problem. A visual inspection of the wiring for heat damage or arcing would not hurt though. I think the easiest way to look at electricity is voltage is pressure, amperage is flow and watts are force (ability to do work). If you blast your hand with a garden hose you can feel the force. I you increase the pressure in the hose you will feel a greater force. If you increase the size of the hose you will feel a greater force. So if you increase the voltage or the amperage you will increase the number of watts (ability to do work). Now this is where the wire size comes in, a given gauge of wire has a certain cross sectional area which has the ability to carry a know rate of amperage (flow) of electricity of any voltage (pressure) through the wire without heating the wire above a rated temperature, typically 105 degrees C. The conductor in the wire, what people usually call wire, does not have a limit to the voltage it can carry, but the insulation surrounding the conductor does, most aircraft wiring is rated at 600 volts. If the rated voltage (pressure) of the wire is exceeded the amperage (flow) could leak out in the form of a short or arc. Now to the point Bruce brought up, if the failure mode of your avionics or turn and bank resulted in a dead short the amount of force (watts), would increase to the point, remembering Ohms law, that the flow (amperage) carrying capacity of the wire was exceed. With a properly sized fuse or circuit breaker the fuse would blow or circuit breaker trip before the wire was damaged. I hope this explanation hasn't dragged on for to long.

Tim

Tim, that is an excellent description of all the points I was thinking about but did not adequately relate with just a Ohms Law lesson.

It is also most likely that the avionics/electronics failed causing an open circuit or the circuit protection opened in either case protecting the wire from damage.

I agree with Tim, I certainly would give the wire a once over inspection.

A friend of mine had his generator fail in a way that it blew out his radio, transponder and T&B once.
He described it as a field wire broke loose and contacted something else (I don't remember the details) inside the generator. The spike was fast enough that neither the regulator nor fuse made a difference.

The exact details of how it went wrong escape me now but an overhaul of the generator revealed the condition.

Bruce Fenstermacher wrote:I doubt you have to rewire anything just because the wire carried a high voltage. High voltage alone won't hurt the wire. Ohms Law is what you need to know.

Voltage x Amps = Watts or W/A=V or W/V=A is Ohms Law. So if a given wire is rated at 12v and 15 amps which equals 180 Watts it is the same as running 24v at 7.5 amps equalling 180 watts.

So you could double the voltage so long as the amps remained below half that the wire is rated for and the amount of power (Watts) is the same.

Chances are the wire is actually rated for much much more than 12v to begin with.

Actually, a wire that will carry 15 amps in a 12-volt system will also carry 15 amps in a 24-volt system equally as well. The majority of the power is dissipated in whatever the wire is connected to (radio, lights, etc.), not in the wire itself. The small amount of power dissipated in the wire is the current carried by the wire (15 amps in this example) times the voltage DROP in the wire, that is, the (often very small) difference between the voltage at the bus and the voltage at the driven component.

As far as the original poster is concerned, if there was adequate circuit protection (fuses and/or circuit breakers in the system when the over voltage occurred, the wiring was not damaged. Remember that fuses and circuit breakers are designed to protect the wiring, not the radios, lights, etc. connected to it.

cessna170bdriver wrote:As far as the original poster is concerned, if there was adequate circuit protection (fuses and/or circuit breakers in the system when the over voltage occurred, the wiring was not damaged. Remember that fuses and circuit breakers are designed to protect the wiring, not the radios, lights, etc. connected to it.

Doesn't the CB protect against over amperage vs. over voltage?

lowNslow wrote:

cessna170bdriver wrote:As far as the original poster is concerned, if there was adequate circuit protection (fuses and/or circuit breakers in the system when the over voltage occurred, the wiring was not damaged. Remember that fuses and circuit breakers are designed to protect the wiring, not the radios, lights, etc. connected to it.

Doesn't the CB protect against over amperage vs. over voltage?

Yes Karl, that's absolutely true, but in many cases of overvoltage there's an accompanying overcurrent, especially in resistive circuits like incandescent lighting. The overvoltage by itself is very unlikely to damage the wiring directly, but an overcurrent in unprotected wiring can.

These posts are certainly revealing.

Transient spikes are so mysterious. Why do they name them transient?

We thought we were Protecting our avionics didn't we? C/Bs everywhere.

I suspect that development of "Smart C/B s" is well under way as we speak.


What initiated the original poster's problem?

This forum has immense depth of knowledge to GA.


Regards

Guess it's a good reason to have diodes on all your solenoids, and a modern voltage regulator with field to ground protection, i.e. Zeftronics.

The Zeftronics regulator is very good at detecting overvoltage and shutting down the generator. So good, in fact, that in my Swift, which has a Delco-Remy generator, I have to leave the generator shut off until after the engine is started. If the generator is on during starting, the regulator will see spikes and shut it down. This is also why it's good practice to have avionics disconnected from the bus during starting.

jRenwick

I liked your post.

What if you just had ALL avionics shut off by the unit's off/on switch?

Still wondering What caused original poster's damage?

Why?

MoonlightVFR wrote:jRenwick

I liked your post.

What if you just had ALL avionics shut off by the unit's off/on switch?....

Thanks! You're right. By "disconnected from the bus," I meant shut off. That's what the avionics on/off switch does, if you have one. Otherwise, all avionics should be turned off individually during starting, and turned on after the generator is going.

Keep in mind, some systems have soft on/off were it's not a switch that disconnects the circuit.
The Garmin Aera and their 327 transponder for example. Turning them off does not provide the same protection as a radio with a switch that clicks.
An avionics master is a good idea and relatively easy to install.

lowNslow wrote:Guess it's a good reason to have diodes on all your solenoids, and a modern voltage regulator with field to ground protection, i.e. Zeftronics.

Can you also tell us that which company shall manufactures the best electrical spare parts?