Electrical Fires and safety

[jsstevens]

OK, We've had two threads (at least) about folks with recent expereience with electrical fires. Some of the discussion of what to do raised a question in my mind:

Once you've turned off the master why would you ever turn it back on?

Now I understand about needing radios & transponders and such, but the genesis of my question is this: Turning on the master with a sizable short can weld the switch closed leaving you with no way to turn it off again. I don't know how electrical switches in planes are built, but if they are anything like switches and relays used in cars and in home & commercial wiring the arc caused by closing the switch with a dead short can weld the contacts together. This doesn't happen when breaking the connection beacause while the arc is generated the contacts are moving apart. The weld happens when the arc heats the metal to melting temp and then they are joined physically.

So, all that said, if you turn the master back on, as far as I know you risk welding the master switch (or relay) closed permanetly which is a really bad thing if you've got a short causing a fire.

Are aircraft electrical components made differeently so that can't happen?

John

 

[ScottM]

OK, We've had two threads (at least) about folks with recent expereience with electrical fires. Some of the discussion of what to do raised a question in my mind:

Once you've turned off the master why would you ever turn it back on?

Now I understand about needing radios & transponders and such, but the genesis of my question is this: Turning on the master with a sizable short can weld the switch closed leaving you with no way to turn it off again. I don't know how electrical switches in planes are built, but if they are anything like switches and relays used in cars and in home & commercial wiring the arc caused by closing the switch with a dead short can weld the contacts together. This doesn't happen when breaking the connection beacause while the arc is generated the contacts are moving apart. The weld happens when the arc heats the metal to melting temp and then they are joined physically.

So, all that said, if you turn the master back on, as far as I know you risk welding the master switch (or relay) closed permanetly which is a really bad thing if you've got a short causing a fire.

Are aircraft electrical components made differeently so that can't happen?

John

Arcs happen when the voltage potential of a gap is greater than the dielectric strength of air. It has nothing to do with the direction that the contacts are moving.

 

[jmaynard]

Arcs happen when the voltage potential of a gap is greater than the dielectric strength of air. It has nothing to do with the direction that the contacts are moving.

Right, but whether the arc dissipates harmlessly or melts the contacts together does depend on whether the contacts are moving together or apart.

In the aircraft I'm aware of, the master switch itself does not switch the main power supply to the bus; instead, it powers a contactor that does the heavy lifting. I don't know how susceptible they are to the welding phenomenon described.

 

[ScottM]

Right, but whether the arc dissipates harmlessly or melts the contacts together does depend on whether the contacts are moving together or apart.

True, I was responding to his claims that arcs happen when contacts close. Which you and I agree is not the case.

In the aircraft I'm aware of, the master switch itself does not switch the main power supply to the bus; instead, it powers a contactor that does the heavy lifting. I don't know how susceptible they are to the welding phenomenon described.

Most of the ones I have seen are as you note but I do think there are some that do not have a relay and the switch is actually doing the work.

 

[poadeleted20]

Once you've turned off the master why would you ever turn it back on?

Desperation, like in actual instrument conditions with no handheld backup units and low cigs/vis underneath you.

 

[jsstevens]

True, I was responding to his claims that arcs happen when contacts close. Which you and I agree is not the case.

Most of the ones I have seen are as you note but I do think there are some that do not have a relay and the switch is actually doing the work.

I certainly did not intend to imply that arcs only happen when it's closing (in fact I specifically said the arcs happen when opening) - only the welding phenomenon. The arc happens both ways, but if the contacts don't touch (i.e. when they're opening) when hot they don't weld. Whether it's a relay or a switch doesn't necessarily matter either. The current carrying contacts can weld closed. This is a phenomenon that I have personal experience with though, thankfully, not in an airplane!

John

 

[gismo]

OK, We've had two threads (at least) about folks with recent expereience with electrical fires. Some of the discussion of what to do raised a question in my mind:

Once you've turned off the master why would you ever turn it back on?

Now I understand about needing radios & transponders and such, but the genesis of my question is this: Turning on the master with a sizable short can weld the switch closed leaving you with no way to turn it off again. I don't know how electrical switches in planes are built, but if they are anything like switches and relays used in cars and in home & commercial wiring the arc caused by closing the switch with a dead short can weld the contacts together. This doesn't happen when breaking the connection beacause while the arc is generated the contacts are moving apart. The weld happens when the arc heats the metal to melting temp and then they are joined physically.

So, all that said, if you turn the master back on, as far as I know you risk welding the master switch (or relay) closed permanetly which is a really bad thing if you've got a short causing a fire.

Are aircraft electrical components made differeently so that can't happen?

John

The contactors used as "master relays" in most GA airplanes are capable of interrupting a very large current, on the order of a thousand amps or more. Contact welding generally occurs after the contact surfaces are pitted by corrosion and/or arcing and wear which makes for a much smaller connection than normal. That small connection forces the current to flow through a much higher resistance creating the heat necessary to weld the surfaces together, other than the pitting caused by repeated arcing over time, arcs have little to do with contact welding.

In addition, unless the fault is a direct short from the main bus to airframe ground, the current through the master contactor won't be all that high and such a direct short is more likely to vaporize the main bus conductors or if active long enough, explode the battery which isn't designed to deliver that much current.

From what I've seen and/or read, most electrical fires in GA aircraft are either internal to an "appliance" (radio, autopilot, etc), a switch or rheostat that's developed a high resistance, or a light gauge wire that's shorted out to the airframe. None of those would give a master contactor any trouble at all.

To answer your first question, the only reason to turn the master back on after turning it off resolves an electrical fire issue would be out of necessity for flight in IMC or to provide a means of navigation. The first two priorities once the fire is under control are to keep the plane right side up and land ASAP. You never can tell if the fire will start up again given enough time.

 

[SCCutler]

I had such an incident in a Cardinal RG; I switched the master back on for only long enough to extend the gear (I know, coulda pumped it down...). Did not switch on the alternator which, as it worked out, was the source of the smoke.

 

[GigG601XL]

Right, but whether the arc dissipates harmlessly or melts the contacts together does depend on whether the contacts are moving together or apart.

In the aircraft I'm aware of, the master switch itself does not switch the main power supply to the bus; instead, it powers a contactor that does the heavy lifting. I don't know how susceptible they are to the welding phenomenon described.

It the airplane that you are most aware of Jay, you see that switch way over on the right side of the panel. (I think that's where it was.) That says something along the lines of "Back-Up power" isn't connected to a contactor. At least I don't think it is.

 

[jmaynard]

It the airplane that you are most aware of Jay, you see that switch way over on the right side of the panel. (I think that's where it was.) That says something along the lines of "Back-Up power" isn't connected to a contactor. At least I don't think it is.

No, it's not...and it also only switches a few amps, since that bus only drives the primary flight instruments. (Not even the Dynon.)