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-   -   Interesting day today, lots of smoke in the cockpit! (https://vansairforce.net/community/showthread.php?t=159638)

Walt 04-16-2018 07:13 PM

Interesting day today, lots of smoke in the cockpit!
 
So while doing a small panel upgrade for a customer I had a bit a meltdown on his aircraft. The panel mounted Lightspeed CB, with open and bare terminals was connected directly to the battery, and was located right under the xpdr rack I was replacing (within about 1/8"). Well as soon as the rack came loose it came into contact the uninsulated terminals. (of course I was unaware at the time that this situation existed with a hot bus right below the rack).

I can tell you this, there was a ton of smoke accompanied by some small flames as the wiring began to burn, disintigrate and melt. All the switches and master were off so there was no killing this thing and it just continued to burn until the wiring finally burnt thru. I was scrambling to get the cowl off so I could disconnect the battery, but by the time I got if off the wires has already burnt thru and opened up.

So whats the moral to this story, do not run 'HOT' unprotected (no fuse, CB, contactor controlled) wires into the cockpit. This situation could have easily presented itself from any number of causes (loose hardware, wires, breaker, switches getting loose etc.) If this would have happened in flight there was no way to kill power to the burning wires, and trust me with a closed canopy you would be in serious trouble fast. I was really pretty amazed at how much smoke there was, in the air this likely would have ended badly.

For your Hot battery busses (this includes battery charger pig tails) there should ALWAYS be a fuse (or some type of circuit protection) located at the battery to prevent this kind of thing from happening.

sirlegin 04-16-2018 07:18 PM

As would be said "DOH!!"

az_gila 04-16-2018 07:46 PM

I agree on the fuse bit, and just had to go and read the LSE manual.

To connect power to the ignition system, install the 15-pin input connector from your sensor harness to the ignition module. Route the single conductor shielded power lead to a pull-able breaker, 4-cyl systems use 5A and 6-cyl systems use 7.5A, and then directly to the battery plus terminal, bypassing any electrical buss or master solenoid. Refer to the Input Connector Diagram & the Electrical Requirements.

If I was installing one I think I would add a fusible link at the battery in the breaker-to-battery wire connection.

Walt 04-16-2018 08:01 PM

Quote:

Originally Posted by az_gila (Post 1253593)
I agree on the fuse bit, and just had to go and read the LSE manual.

To connect power to the ignition system, install the 15-pin input connector from your sensor harness to the ignition module. Route the single conductor shielded power lead to a pull-able breaker, 4-cyl systems use 5A and 6-cyl systems use 7.5A, and then directly to the battery plus terminal, bypassing any electrical buss or master solenoid. Refer to the Input Connector Diagram & the Electrical Requirements.

If I was installing one I think I would add a fusible link at the battery in the breaker-to-battery wire connection.

The reason they (LSE) wants it that way is to prevent noise (hence the shielded power wires), with little/no concern for aircraft safety or standard practices.
And I agree, a fusible link would also be an acceptable method of protecting the wiring.

rv7charlie 04-16-2018 08:02 PM

Of course, the *1st* lesson to be learned is that you don't start turning wrenches, especially for electrical work, without removing the negative wire from the battery terminal...

2nd lesson is, rationally examine mfgrs' install instructions.

3rd is to use a fusible links on critical circuits that nuisance trips might take off line.

Champ 04-16-2018 08:22 PM

This thread got me thinking.

My battery and master solenoid are rear mounted with a #2 wire to the engine side of the firewall where it connects to the starter solenoid and ANL fused #8 line back into the cockpit buss.

Is the master solenoid at the battery enough protection on the #2 line or should it also have an ANL fuse? It would have to be a pretty big fuse to handle starter current.

cajunwings 04-16-2018 08:23 PM

Wires
 
Thanks for posting Walt. Your efforts to increase awareness for the benefit of everyone are appreciated.

Don Broussard
RV9 Rebuild in Progress
57 Pacer

Walt 04-16-2018 08:34 PM

Quote:

Originally Posted by rv7charlie (Post 1253596)
Of course, the *1st* lesson to be learned is that you don't start turning wrenches, especially for electrical work, without removing the negative wire from the battery terminal...

2nd lesson is, rationally examine mfgrs' install instructions.

3rd is to use a fusible links on critical circuits that nuisance trips might take off line.

How about this for lesson 1, read 43-13, chapt 11:

11-47. GENERAL. All electrical wires
must be provided with some means of circuit
protection. Electrical wire should be protected
with circuit breakers or fuses located as close
as possible to the electrical power source bus.

Walt 04-16-2018 08:37 PM

Quote:

Originally Posted by Champ (Post 1253602)
This thread got me thinking.

My battery and master solenoid are rear mounted with a #2 wire to the engine side of the firewall where it connects to the starter solenoid and ANL fused #8 line back into the cockpit buss.

Is the master solenoid at the battery enough protection on the #2 line or should it also have an ANL fuse? It would have to be a pretty big fuse to handle starter current.

Yes a contactor is considered a means of wire protection, when you open it you disconnect the circuit.

rv7charlie 04-16-2018 08:44 PM

Quote:

Originally Posted by Champ (Post 1253602)
This thread got me thinking.

My battery and master solenoid are rear mounted with a #2 wire to the engine side of the firewall where it connects to the starter solenoid and ANL fused #8 line back into the cockpit buss.

Is the master solenoid at the battery enough protection on the #2 line or should it also have an ANL fuse? It would have to be a pretty big fuse to handle starter current.

The master contactor is the 'protection'. It's accepted practice in certificated a/c. With proper care in routing and installation, you shouldn't ever have an issue, but if it somehow gets its insulation cut by a/c structure (typically a bulkhead pass-through), it'll quickly burn the hole big enough to remove the short. Sounds crazy, but the short to ground is effectively self fusing. You'll likely know it happened (;-) ), and can repair when safely on the ground.

An ANL fuse big enough to handle starter current will net you the same result as what I described above, if you had a fault to ground.

Charlie


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