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Hi Group,

Working on my electrical schematic for my RV8 with the battery in the rear location. I am planning to mount the battery contactor near the battery. This means that the 2AWG cable will run through the fuselage to the starter contactor on the firewall. Looking at other people's schematics I notice there is no circuit protection being used to prevent burning the cable in case the shielding fails and the cable contacts the fuselage.
I know I could open the battery contactor (if it doesn't burn out) in case this happens, but would a more 'automatic' (fuse) option be wise?

Your thoughts on this?

Kind regards,
Duncan
PH-CYA

 

[airguy]

I'm interested in hearing the group-think on this, I've got my battery in the rear of my 9A and the cable running forward through the fuse does not sit well with me.

What is the max surge draw on the starter, would a 200-amp fuse be useful?

 

[chaskuss]

How I did it

Duncan,
I had considered this same issue. Bob Nuckholls view on this is that if the 2 AWG wire were to ground out on the airframe, it would simply arc weld the offending sheet aluminum away in a few seconds. Therefore, the short circuit would be of very short duration. While that is most likely true, it would be a pain to make the required sheet metal repairs, "after the fact". [Not to mention your soiled underwear! ]
Late model Ford trucks have a large "Mega" fuse on the battery main cable. Littelfuse [no I did not misspell it] makes the fuse and holder that Ford uses. I installed one of these on the rear of my 8A battery tray. I designed it so that if it became a problem or did not work as intended, I could simply delete it. See photo below.

I tinned and insulated that bus bar, after this photo was taken. I cleaned the bar, tinned it with solder and applied 3M "triple thick" heat shrink tubing. The heat shrink tubing is available from Waytek Wire. Waytek has lots of good stuff. www.waytekwire.com

While I like Bob N's suggestion to use an AMP ANL fuse for the alternator "crowbar" over-voltage circuit, I was not wild about how exposed the fuse holder for it was. With a slight slotting of the hole in the ANL fuse, it will easily fit in the fully insulated Littelfuse Megafuse holder. See below.

The covers for these fuse holders allow you to "punch out" the cover so that the cables can enter from any direction. The open nature of the bottom of the Littelfuse holders made me fabricate phenolic resin base plates as additional electrical insulation. See below

Megafuses are available from 40 amps to 250 amp ratings. These are "slow blow" fuses, so will resist "popping" during initial inrush currents.

http://www.littelfuse.com/searchresults.html?Ntt=Mega+fuse

http://www.littelfuse.com/searchresults.html?Ntt=Mega+fuse+holder

You could also buy them from your local Ford dealer's parts department. 1998 Ford Expedition is one truck that has them. You won't get anywhere with a parts man, without year, make and model I think I have the Ford part numbers stashed away somewhere. I'll try to dig them up. I bought mine directly from Littelfuse's web site.

They also have Minifuses, Midifuses [great for protecting cable from main contactor to your main fuse block] & Maxifuses

I hope this helps
Charlie

 

[Walt]

In the certified world starter circuits are excluded from circuit protection. If you install the cable with due dilegence the chances of a short are extremely unlikely.

 

[chaskuss]

In the certified world starter circuits are excluded from circuit protection. If you install the cable with due diligence the chances of a short are extremely unlikely.

Walt,
That is the main relay, not the starter relay you see in that photo. When the main contactor is closed, that 2 AWG wire is hot. I agree that fusing the starter circuit is not necessary, as it's a momentary circuit.
Each builder can choose to run a separate 8 AWG or 6 AWG wire back to the main contactor to supply the main electrical bus. I did not.
Charlie
PS I am not sure if your comments were directed towards me, Greg or both of us.

 

[Walt]

Charlie,
My comments were not directed at anyone, just general info/observation. My only point is that there are always going to be unprotected cables, the next "logical" step would be to protect the cable from the battery to the MC as well?

I agree with you not running another "protected" cable for the main bus when you already have the the power up front from the "starter" cable. So I guess my point is that if you do a good install the chances of a cable short will be practically zero, and the power is still cut off by the MC so from a safety standpoint I don't see the problem. If you smell smoke and a fuse hasn't blown I would assume most folks would know to shut off the master and land.

Personally my system has no "fuse" protecting the main bus feed from the contactor, why... because I don't want nuisance circuit protection shutting down a perfectly good system. If that 6 ga wire shorts (which it won't because it was carefully installed to protect it) then it's time to shut off the master and revert to the "back-up" battery feed (ESS bus).

If having protection on every wire makes someone feel better by all means go for it, I'm just not in that same camp is all.

 

[chaskuss]

Walt,
I prefaced my first post mentioning how Bob N feels that what I did really was not necessary. Like you say, I suppose it just makes me feel better. If/when it ever gives me trouble, I'll simply remove it and reconnect the battery cable directly to the main contactor.
Probably the only time that fuse would come into play, would be during a crash landing scenario. Like you, I was very careful of how I installed that big cable.
Charlie

 

[RV8TOR]

Another option is to locate both the master and battery contractors on the battery tray. This option only energizes the large cable during starting and the smaller #8 wire to the bus is hot all the time which is much easier to protect and draws minimal amperage.

 

[Walt]

Charlie,
The only trouble it's likely to cause you is when you are on the way home, the weather is cold, you're tired, the wife is cranky, you don't have any tools, and the Mega fuse blows when you hit the starter

PS: if the cable shorts, fuse or no fuse, you will be doing sheet metal repairs to the melted area.

 

[Mike S]

Another option is to locate both the master and battery contractors on the battery tray.
.

I believe you meant to say "both the master, and starter contactors"

As far as I am concerned, dont try to fix a problem that aint broke.

The 10 is designed with a rear battery, and no protection on the main feed line, and I trust Vans to design it correctly.

$0.02

 

[Walt]

Another option is to locate both the master and battery contractors on the battery tray. This option only energizes the large cable during starting and the smaller #8 wire to the bus is hot all the time which is much easier to protect and draws minimal amperage.

Thats true, but you are carrying around another 10-12 ft of 6 or 8 ga. cable which you don't really need (my opinion only). Probably only weights a pound or so, so not really that big a deal either way I guess.

 

[chaskuss]

No tools??

Charlie,
The only trouble it's likely to cause you is when you are on the way home, the weather is cold, you're tired, the wife is cranky, you don't have any tools, and the Mega fuse blows when you hit the starter

PS: if the cable shorts, fuse or no fuse, you will be doing sheet metal repairs to the melted area.

Walt,
No wife to get cranky. I think that you are simply trying to scare me! The thought of no tools, indeed. You'll give me nightmares. You know that guy Murphy too?

Charlie

 

[Bill.Peyton]

I thought I already read this, but if you fuse it with a 250 amp fuse, you will still toast the sheet metal. The fuse will truly provide zero protection and is just another electrical item to fail either mechanically or electrically. Route the wire and attach it properly and it should never have an issue.
My $ .03

 

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Hi Group,

Thank you all for your thoughts on this, I will decide on what to do at a later stage. Charlie, thank you for your view on the subject and for posting your specs and pictures.

Kind regards,
Duncan

 

[Andy Hill]

My take on it (similarity to some above):

1. It takes a lot of effort to correctly solder / crimp Starter Cable terminals and they are a "failure point", either absolute, or poor connection at 200A+
2. The current requirement for the starter is fairly vague e.g. Skytec LS is "185A-285A" - given that you need a 300A+ fuse
3. If your concern is the starter cable chafing through, to get 300A+ to flow, you are going to need to clean the airframe part it chafes onto, then bolt/clamp a large exposed surface of starter cable to it
4. By adding a fuse you add 2 starter cable connections, and the connections / mechanics of the fuse / fuse holder
5. The most frequent 'starter issue" is too much V drop between battery and starter motor. Every connection in the line just potentially increases that.

So ask the question - what are you actually trying to protect against (fault e.g. chafe), the (adverse) consequence (e.g. fire / structure melting), and how reliable will your protection be (e.g. fuse).

IMHO if you build it badly enough for chafe to be a concern, then the fire / damage is a potential consequence. The chances of 300A+ flowing is almost nil, and so the fuse will not help - you will generate quite an impressive fire at 20A? 30A? 50A?

The more likely consequence is poor starting, either from the outset, or downstream after X years/hours of vibration / corrosion / movement / maintenance at the added components / connection points.

In the certified world starter circuits are excluded from circuit protection

tells a story - why second guess them

 

[Wayne Gillispie]

I have 40' of unprotected 0000 ga...

running into my house with 243 VAC. Many of you do too. I sleep just fine with 9 smoke alarms interconnected with battery backups(yep, crawl space and attic too).

Just build the plane per plans, so you can join us someday soon. Then plan for any emergencies before and during the flight.

 

[airguy]

running into my house with 243 VAC. Many of you do too. I sleep just fine with 9 smoke alarms interconnected with battery backups(yep, crawl space and attic too).

Just build the plane per plans, so you can join us someday soon. Then plan for any emergencies before and during the flight.

Valid point. We are building very fun tractors, not swiss watches.