[szicree]

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]

It's going to get HOT!

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/catalog/appages/firewallshields.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]

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]

I agree OUCH

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]

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/lansaweb?procfun+wordpr01+webfunc+m37+FUNCPARMS+WEBCMP(S0020):01+WEBID(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]

Use the pass thru

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]

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

 

[RVjim]

A paranoid person should be using one of these.
http://order.waytekwire.com/CGI-BIN/LANSAWEB?WEBEVENT+L0D842A401F5102008B0F052+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/catalog/appages/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]

anl recommended

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/Categories.aspx?Category=44d447d6-d21f-40ee-9b58-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]

anl more info

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

 

[w1curtis]

I must be way more paranoid than you think.

Hmm, maybe.

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.)

Are you building a space shuttle?

You do realize that a 2 AWG wire is only rated for about 180 Amps continuous right? That means, unless you have a larger wire, these connectors are rated for more continuous power than the wire to which they are attached.

As for the temperature, there are many pieces of hardware forward of the firewall that fall way below your (2000F!) standard.

Sometimes we spend so much time finding solutions for problems that if we think about it are in the forth digit to the right of the decimal in percentage probability. The purpose of a firewall is not to allow you to continue on behind a fire until the temperature passes 1000 degrees, but to allow you the time to put the plane down and get away from it. If you allow a fire to get to 2000 degree F, then you just haven't been paying attention. But, whatever makes you feel comfortable.

 

[RVjim]

Are you building a space shuttle?

(Stuff Cut).

It seems like we got off topic a little bit.
I though we were discussing a firewall passthrough of an "always hot" starter cable.

My RV-6A came with an "always hot" battery cable directly to the starter relay. No master relay in the system. The battery was behind the firewall and the started relay was in front of the firewall.

The firewall feedthrough was a 1" diameter hole with a 3" long piece of PVC jammed half way through the firewall. The battery cable ran through the PVC with lots of room to spare. This extra room was filled at the engine side with red RTV.

So I guess I am a little paranoid about firewall penetrations.

BTW, my first engineering job was as a design engineer on Orbit 101 and 102. How did you know?

Regards,
Jim Ayers

 

[f1rocket]

Everyone should be paranoid about drilling through their firewall.

I think the point about the temperature is that the temperature on the engine side of the firewall is routinely above 220 dF. If the plastic in this fitting breaks down above 220 dF, then you're going to have a problem. Maybe not immediately but possibly over time.

I still think the best solution for wires passing through the firewall is to use one of those tubular fittings that are made of stainless and are covered with firesleeve and stuffed with fireblock. They come in three sizes and create a simple, effective, safe, and inexpensive technique for passing just about anything through your firewall.

 

[gmcjetpilot]

Good (enough), better and Best

I am a little fatalistic about a massive fuel / oil fire in the engine compartment in flight. The flames will melt the cowling off; than the flames will lick around the firewall and melt the windscreen, worse case. If you are in the air any appreciable altitude you are in a heap of hurt, unless you have a chute and hit the silk.

See my link I mentioned before, including the bulkhead terminals (plastic non fire resistant) and the better method, if you are serious about fire. LINK

Make it fire resistant, but when a thin piece of sheet stainless is 3/32 riveted to sheet aluminum is the firewall, and the engine is wrapped in a flammable fiberglass cowl........ I'm not impressed with the my chances, thus I don't worry about making every penetration 2000F proof. However I would not criticize anyone making the effort. The key is make sure you have the BEST hoses & equipment FWF. However the rare catastrophic engine failure where a connecting rod punches the case and breaks all heck with all the fuel & oil is a worst case. A good firewall penetrations will not matter to much.

 

[szicree]

I've returned with some follow up questions. My choices are like this:

1. Send the wire forward through the firewall and then make a u-turn to connect to the firewall-mounted starter relay. Everything fits well and the only downside is that I'll have to find an acceptable pass-through.

2. Use one of those pass-through stud gadgets. One nice feature of this method is that the location of the stud is only about 4 inches from the starter relay, which would allow me to use a copper bar to connect them instead of wire (no connectors, crimps, etc.)

I'd love to hear others' opinions on this. Thanks.

 

[Mike S]

One nice feature of this method is that the location of the stud is only about 4 inches from the starter relay, which would allow me to use a copper bar to connect them instead of wire (no connectors, crimps, etc.)

I'd love to hear others' opinions on this. Thanks.

Copper bar, fine, but be sure you protect it with some kind of insulation, as this sounds like it will be on the hot side of the starter relay. You dont want anything conductive to hit it, and with all the vibration, and air movement under the cowl in flight who knows what will happen.

As to the wiring passing through the firewall, unless you are using some kind of remote control device, there is no way of getting around the fact that some wiring must go through the firewall. This is not really a big deal, just follow the advice already given in this thread.

Pay attention to what George has listed, his advice is sound. The biggest problem in passing wiring through the firewall is allowing the vibration to chafe the insulation/grommet/bushing and resulting in a short.

Good luck,