Master power wire through firewall

szicree · #1 10-11-2007, 10:42 PM

Two questions:

1. What exactly happens if the wire connecting the two contactors shorts against the firewall?

2. What sort of grommet/pass through is best at preventing this?

gmcjetpilot · #2 10-12-2007, 01:05 AM

Quote:

Originally Posted by szicree

Two questions:

1. What exactly happens if the wire connecting the two conductors shorts against the firewall?

2. What sort of grommet/pass through is best at preventing this?

Steve you really are worrying about this wire per your other thread: LINK

1) If the battery wire shorts against the firewall it will smoke, melt, get red hot and possibly melt or cause a fire if anything is too close. However I assume the wire you are talking about is down stream of the MASTER RELAY (contactor). So if you ever smell any electrical burn the first thing to do is turn the master OFF.

You have the RV-4 so the battery is on the? (engine/cockpit side of firewall). It really does not matter but you MUST observe one of the two rules:

-FUSE/CB on the wire at the power source end (battery)
-Have a way to shut down all power to that wire (via a relay/contactor)

2) Grommets - there are many. This is the way its been done forever on planes: http://www.aircraftspruce.com/catalo...allshields.php
All this is are just stainless shields or covers to go over rubber grommets (at one time asbestos gaskets).

You could use a hard black plastic snap grommet (like what Van's supplies for the wing passthroughs) or rubber grommet you can buy anywhere. Than goop the whole deal up with permatex high temp stuff or better stuff other folks recommend. Per my suggestions in your other thread (LINK), oversize the hole & grommet to make room for a wrap around the wire'(s) with silicon fireproof tape. Make sure the wire is supported (or stabilized) on at least one side (with a clamp) so there is NO STRAIN on the wire at the grommet and firewall. You don't want to wire to be hanging off the grommet and working around. And if you don't want to get your grommets at the aviation section of the hardware store you can buy OFFICIAL "AN" rubber grommets: LINK.

TO FUSE OR NOT TO FUSE
Sometimes (often) we violate having a fuse for BIG WIRES and SHORT wire runs. The short jumper to the master relay (contactor) and jumper from there to the starter relay for example. The other exception on RV's is the MASTER WIRE from just on the switched side of the master relay to the main buss, which I think you are talking about.

I don't why more builders don't put a fuse on the "MASTER" wire, but as I said, you can justify it by saying you can turn the master off if there's a short. Regardless keep the battery to main buss wire short, protected and flammable items away from it. Personally I put a fuse on the Battery to Main Buss wire. There are all kinds of cool automotive electical fuse protection devices today. Here's a list of fuse products from Littlefuse. There are often cool fuse holders, including in-line. LINK Bussman also makes many products to protect fuses. LINK On cars EVERY wire that goes back to the battery is protected, except the starter. That is why you see on modern cars a fuse box on the battery in the engine compartment. In the old days cars used fusible links. You can still use fusible links.

Steve Sampson · #3 10-12-2007, 01:41 AM

Steve, you would have a very short circuit with very low resistance. (Battery/Contactor/Firewall - Ground/Battery)

The Odyssey type batteries have very low internal resistance, but none have much. You would certainly have hundreds of amps flowing (I=v/R) and without looking the numbers up, 1000s. This situation would not last very long. The weakest point in the system would give out.

My guess is that would be where the wire touched the firewall, since it would initially be a very poor joint/point contact. I think you would have a flash and bang while some SS and copper melted plus whatever was around it and in all probability it would all be over, with perhaps a bigger hole in the firewall.

At this stage I would want to replace Battery and contactor associated wire and fix the firewall since I think you would have to assume all were damaged.

You said 'exactly'. It is hard to be exact because the actual contact is undefined. Because we are operating at only 12v the spark would quench as soon as it had burned a gap. In big switches say 33000V ac you have to do things like blow the spark away once you open the contacts because the air ionises and the spark keeps running. In our situation though the spark would only manage a minute gap which the current will soon burn to a larger one!

Best bet is you make sure it does not happen.

Perhaps there is somone out there who has done it for real and can give their experience.

PS I am currently planning what you are doing. Are you going through the firewall or around the side and in through the cowl cheeks. I favour the later because the steel is so hard to cut neatly.

gmcjetpilot · #4 10-12-2007, 01:47 AM

Quote:

Originally Posted by Steve Sampson

Steve, you would have a very short circuit with very low resistance. (Battery/Contactor/Firewall - Ground/Battery)

You said 'exactly'. It is hard to be exact because the actual contact is undefined.

Steve I have seen it, it can make the whole wire glow red as all the insulation melts off the wire. The good news is if the battery is Firewall FWD than than the short is on the firewall side and it should eventually melt the wire. Bottom line you don't ever want a dead short in any wire, much less large gauge wires.

Kahuna · #5 10-12-2007, 03:19 AM

A paranoid person should be using one of these. Click link then search use "Product Search Item number" 47210
http://order.waytekwire.com/cgi-bin/...EBID(S0020):01

I got one on my firewall. They also make a non pass through version.

Hope the link works.
Waytek item number 47210.
Best,

sf3543 · #6 10-12-2007, 07:48 AM

I have used similar pass thru studs, like Kahuna shows, on both of my RVs and they are great. Easy to seal against fumes or flames coming through the firewall and no worry about chafing wires.
I'd recommend them for any large wires you need to take through the firewall.

Bill Dicus · #7 10-12-2007, 08:25 AM

Kahuna: The link didn't work for me. Could you try again? Thanks. Bill

RVjim · #8 10-12-2007, 08:37 AM

Quote:

Originally Posted by Kahuna

A paranoid person should be using one of these.
http://order.waytekwire.com/CGI-BIN/...B0F052+M37+ENG

I got one on my firewall. They also make a non pass through version.

Hope the link works.
Waytek item number 47210.
Best,

I must be way more paranoid than you think. :-)

From the distributors catalog:
Maximum temperature 220F (105C) and maximum current of 250 amps.

With a 2000F fire or 350 amps to the starter, it seems to fall short. (Pun intended.)

I don't have a solution for the firewall penetration for an always hot starter cable. Although I am still waiting to see what is offered.

I placed my master relay directly behind the battery. The battery is clamped down and there is a 2" cable run to the master relay terminal.
I placed my starter relay aft of the firewall.
I ran the normally cold starter cable through an eyeball firewall assy from ACS, pn 05-01380, shown on their website.
http://www.aircraftspruce.com/catalo.../eyeballfw.php
This has a steel bracket with a solid aluminum ball. (Only good to around 1000F, but fairly massive. Should fail about the same time as the aluminum rivets holding the firewall together.)
The starter cable is clamped to solid structure on each side within 8" of the firewall feedthrough.

Still looking for something better.

Regards, Jim Ayers

carguy614 · #9 10-12-2007, 09:28 AM

I think you should consider the big B+ wire to the master bus being fused with an ANL type fuse. They can take a considerable overload for a short period without opening, yet protect the circuit. The mounting block and limiter is available from aerocraftparts.com. link follows...http://www.aerocraftparts.com/Catego...8-c8fc745d61f4
This is how I wired my plane. Also consider the feed down to the alternator, as it will have to pass through as well if you use an alternator breaker on your main buss

HTH...Chris
http://www.aerocraftparts.com

carguy614 · #10 10-12-2007, 09:40 AM

Forgot to mention, I used the 80 amp fuse in my circuit with the VERY sturdy mounting block shown in the link. It will also give you a good spot to tag other circuits if you need to, although I didn't need them on my plane. If you are using Dynons, or other electronics, consider the need for a keep alive memory wire as well. Your KAM will be hot all the time, and will need to penetrate the firewall too. Generally 18 ga. fused with an inline fuseholder at the master solenoid hot side. 1 Amp should do it, but I suggest 3 amp fuse, as vibration raises heck with itty bitty glass fuse filaments. Remember fuse protects the wiring, not the device!!
HTH
(hope this helps)
Chris