Master relay operation

Goodmorning friends!
I have a request for some education. First let me explain an embarassing event yesterday...I had hooked the battery leads up inreverse!! There I said it! Why? Not paying attention. I have the battery on a cart next to firewall battery tray and I have the ground wire off firewall hanging free for ground hook up and the pos lead attached to master relay. I simply use two leads with aligator clips to quicky connect the battery and then work on Dynon set up etc. After many times of hooking in, I reversed by mistake and the wire from master switch to master relay burned up! Very scary and I dont ever want to smell or see that again!! I have since removed that wire completely as well as the ground wire from switch. I untied all wires in the bundle and inspected each for any breaks in tefzel coating or signs of melting. and reinstall new wiresing as required.
I plan to buy a new master relay and master switch and diode just for good measure.
Question-how does the master relay function? Im confused as to why the small wire from center stud on master relay is the one that burned up. I guess it fused and shorted the ground wire from the master switch as well.
Im replacing the relay because I have no idea if its damaged or not. Is there a way to check?
Ive pretty much decided to replace just for peace of mind but I am looking for instruction and commects and advice. Thanks in advance!
BTW, this almost made me quit as I dont know much about wiring but Im trying to fix and push on. After puitting new wires, I tested the function of radios and such by hooking into main buss (carefully) and bypassing the relay. This verified that radios and panel items ok. No smoke either.
Thanks!

Couple of possibilities

1) It was the smallest wire in the current path

2) Most heavy coils (relays and contactors) have a reverse biased diode connected across the coils. When the coil voltage is reversed, this diode becomes forward biased creating a low resistance current path!

So the diode, wire,and switch should be replaced (diode is most likely burned out). Master relay is probably okay.

Im a bit confused as to what happens inside the master relay when the master switch is turned on. Does a switch allow current to flow thru from the battery cable to the out going lug to starter relay?
When the cables are hooked up in reverse, Does that mean power was flowing first thru the ground and then back to relay and thru it backwards too? Sorry for confusion but just trying to understand better. I have already ordered new relay and diode and switch. wires already replaced.

Normal operation: + battery side flows current to the relay coil. Current flows thru the coil and out at the small terminal; down the small wire to your master switch, thru the switch, to ground (- side of battery). You switch the ground side. There is a reverse diode across the coil. Because it is wired in reverse, it conducts no current. Its function is to reduce sparking across your switch when you open (turn off) the switch. The current flow is strongly limited by the coil's resistance.
Backwards operation: current flow from (-) terminal to coil. Current says, why go thru the coil when I can go thru this (now forward, conducting) diode with almost no resistance? So a huge current flows thru the diode (and the usual current thru the coil), down the wire, thru the switch to ground which is now (+).

Diode, wire, switch, wire from switch to ground are all turned into toaster wire. Coil should be okay, wire from battery to coil is #2 gauge and can handle that load, at least briefly.

Kato's 8 said:

Im a bit confused as to what happens inside the master relay when the master switch is turned on. Does a switch allow current to flow thru from the battery cable to the out going lug to starter relay?

Click to expand...

Yes, there are a large sized set of contacts inside the relay for the high current load.

There is an electromagnet coil which is activated by the master switch on the panel, and the electromagnet pulls an iron/steel slug toward it. The slug is on a shaft that also has the movable portion of the high current switch.

The following image is pretty close, but the external wiring is a bit different in a typical aircraft

Our friends at Vertical Power have some very good docs on how the master relay and starter relay function and how to hook them up properly. See the doc below:

http://verticalpower.com/_documents/_pdf/contactor_wiring.pdf

Thankyou thankyou!
I understand much better now. I didnt realize that the 18awg wire was carring current normal operation to the switch from battey input side. And that flipping the swiotch was essentially grounding the solenoid and allowing power thru exit side of solenoid.
The description of "toaster wire" is perfect as my ground wire looked like toaster wire for a short length and the other wire did too before it burned in half. The diode is completely covered in shrink type material and I didnt bother to inspect it inside but I did note some scorching about the diode part.
Thanks for the link to vertical powers write up too!

Since you're going to replace, why not cut it open and see what the actual parts looks like and how they're connected? Diagrams are good, but nothing beats seeing the actual components

Good idea. I believe I will cut it open and make a tutorial of it. Better than just junking it. Good plan!

Two comments. First I always use color coded wire. It's a occupational habit. Black is ground and Red is positive for DC voltages. You are very luckily you didn't damage more than you did. Second I'm not sure if it was explained why a relay is used. The whole point of the relay is to allow you to switch larger current by using a lower current control (switch in this case).

I once worked at a place where a technician made the same mistake that you did and wired an entire power supply backward. He had his head above the piece of equipment turned on the power switch when he melted a bundle of #14 wire into carbon. He will never make that mistake again. Lol. Scared the **** out of him. He was very cautious after that and I suspect you will be too.

And now a note on behalf of TC's everywhere....

The homebuilder's basic reference book stack should include a copy of Bob Nuckolls' The AeroElectric Connection, currently 12th edition. It explains, in plain language, everything you could want to know about electrical functions in a light airplane.

http://www.aeroelectric.com/Catalog/pub/pub.html