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MG MGA - MG MGA - Battery cut-off switch

There is a, now closed, thread that I have read on the subject of fitting a battery cut-off switch. There is some misinformation in there that needs correcting.

Comments suggest that it does not matter which lead the switch is inserted into, but this is false and can lead to big problems and/or fire.

The switch should 'always' be inserted into the 'Earthed' lead (this follows for both Pos and Neg earthed cars), for the following reason:

The starting motor can easily draw upwards of 400amps and a cut-off switch is usually around 100amps so you can guess that putting 400amps through a 100amp switch while starting will lead to problems of the switch burning out or worse, a fire.

P Bennellick

Do you think that starter current only flows through one half of the circuit - i.e. from the battery to the starter?

Dave O'Neill 2

Of course not, but you don't place a weak link in the side of a circuit that is supplying the current and taking the full load. The return is very low compared with the supply.
P Bennellick

The cables on both sides of the battery carry the same load. The only reason for putting a cut-off switch in the "earthened" cable is that the exposed terminals on the switch will not arc if touched by something that is also touching the body or frame.
Ed Bell

I agree that putting 400A through a device designed for a max of 100A is likely to cause problems - however, I must tell you that as far as physics is concerned, current will be the same in both "input" and "output" wires - that is current is not "used up" on its journey. The switch is placed in the live side so that the rest of the circuit is not live when it is disconnected and therefore cannot be shorted out to earth.
Cam Cunningham

Like the others have said, the current in the ground wire is exactly the same as the current in the hot wire.

Also, that 100amp rating is for continuous current. The short term overload of the starter would most likely have no effect on a 100amp rated switch. I have seen pages that rate them for 1000 amps of surge current.

Jeff Schultz

Sometimes you need to worry.
Who gave you that incorrect advice Mr P Bennellick?

My master switch is in the total current draw and so will isolate the starter and everything on the car.

This means that the starter current always passes through the isolator switch

The switch however is NOT USED to break the current when the starter is engaged. To disconnect 100amps plus of DC current is very difficult and could arc badly at the switch but when the switch is made then 100 amps plus is no problem at all Do you understand the difference?

And it matters nothing whether that is placed into the battery neg or the battery pos.
Robert (Bob) Midget Turbo

I can see the advantages of placing the isolator switch CLOSE to the battery.
Fitting it close on either the positive or the negative side will remove ANY possibility of a short circuit.
If you are a "belt and braces" type of person then I suppose you could fit a switch on each side of the battery for double security but this is probably a bit over the top.

My car has a battery isolator switch fitted underneath the dashboard which is a relic of it being a race car earlier in its life. This means that there is approx 8 feet of constantly live battery cable between the switch and the battery which isnt the safest set up.
But its convenient location has already proved its worth in preventing a fire when I had a total short circuit on the lighting circuit.
I intend fitting another isolator nearer the battery ( probably on the earth side ) to properly isolate it.

Colyn
Colyn Firth

Colyn I wish I had a pound for each MGA I have known that has burnt to death when the UNFUSED lighting circuit (and others) has shorted out

I wonder how long it will be before these concours lads realise the cost is too high to keep out dated technolegy alive? LOL get a fuse box in it.

Robert (Bob) Midget Turbo

My switch is on the earth wire close to the + battery terminal so that it can be switched off from behind the passenger seat. In the off position surely everything is open circuit and isolated (ie no danger of a short circuit). I also put it on the earth side so that as Ed Bell says there's no danger with shorting out to the frame. Anyone a problem with this? I do agree that fusing the lighting circuit is a good thing but it seems a lot of work now the cars completed and I was fairly low on the learning curve when I did it.
J H Cole

Sounds good to me JH, with your set up, with the isolator switch turned off, the only part of the battery cable you could possibly have a short circuit from on the entire car is from the very short stretch between the battery and the switch!
Exactly the same applies if you put the switch close to the live side.
For safety, the key word is "CLOSE"!

Bob, I learned my lesson, my lighting circuit now has its own fuse protection

Colyn
Colyn Firth

I have the same as JH and others - close to battery on earth side.
One project this winter was to fuse the lights as Robert(Bob)Midget(& MGA) sensibly suggests(do you have an identity crisis Bob?). Does anyone have any guidance, suggestions, etc. - thanks
Graham M V

Graham & JH,
For lighting protection, I simply put an in-line fuse in the feed to the light switch rated to cater for the total load of all lights plus headlights at full beam.
regards
Colin
C Manley

Cm, what fuse rating did you use and what type of fuse?
J H Cole

Bob, I believe that you'd only have about 10 pounds or so, but that's beside the point. I did notice that your fuse panel has what appears to be 16 gauge wiring, if that going to each fuse from the power side. Really, you are still at risk of fire if you haven't enlarged the power supply line. You still appear to draw all power from the weak sized lines. BTW, from appearances, the engine bay looks clean and nice.
Mike Parker

Colin,
That seems a simple solution and one I was certainly considering. But maybe there should also be a line fuse (of higher rating) on the feed to ignition switch - i.e. on output from control box, to protect the fuel pump, coil, etc. What do you think?
The only other thing that bothers me is that if any one bulb shorts, then all the lights will go out.
Graham M V

quick observation Mike. I used most of the original loom to wire my multi fuse box. If the wire diameter is too small now then it would have been more so on the original fuse or not.

I added an extra supply which is now used for ignition circuits, and kept the original simply for non ignition
Robert (Bob) Midget Turbo

OK
Mike Parker

Graham & JH,
Photo (sorry, out of focus) of glass in-line fuse below (Dashboard was removed at the time). Yes, a bulb short will cause the fuse to blow and all lights to go out, but that's better than a dead short and a fire. I can't recall the Amp rating but i just totted up the total load. No reason why you could not install other in line fuses.

regards
Colin


C Manley

That is OK Colin but why place it in such a poor position fitting a quality fuse box under the bonnet is a far better solution IMO.
Bob Turbo Midget England

Bob.
I wouldn't disagree, but the fuse is actually quite easily reached in the event of needing to change the fuse, plus my underbonnet view is unchanged from original.
regards
Colin
C Manley

:)
Bob Turbo Midget England

Fuses won't stop a lighting circuit from starting a fire. If you have enough current to make tungsten glow white-hot, you have enough to melt, char, and ignite insulation/oil/natural fiber.

People who add fuses seldom see their cars ignite because they are careful. Adding fuses is simply an extension of this behavior. Note that both "improvement" pictures show new looms, shiny contacts, and intact bulkhead grommets.

I'm not against fuses. I wouldn't doubt that a blown fuse has stopped a fire. On the other hand, an inline fuse holder is just as likely to cause a fire by popping open than it is to prevent one. Aftermarket electronics use them to protect the device, not the wiring.

Main value of battery disconnects are theft prevention, fire after collision, and long term storage. For long term storage, you are protecting the battery, not the car as long as you avoid rodent-infested outbuildings ;-)

To close the logic loop- if you are sufficiently conscientius to install a battery cutoff and have the discipline to use it, you probably don't need one. Main value is peace of mind. In my businees we call that placebo effect.

Bill

PS- I just spent two months on "special assignment" reviewing customer complaints. I'm a bit "owly"

Bill Eastman

Interesting thoughts Bill. That will get them all going! For my part, I don't have the additional fuses although I do run a battery isolator (on the live side) for the reasons you mention - primarily safety, when working on the car; also, when I used to own a house with an integrated garage, it was comforting to know that the car was 'dead' when I was tucked up in bed at night.

Steve
Steve Gyles

Steve, your comment about "knowing the car was dead in the integral garage when you were tucked up in bed at night" reminded me of when I burnt out the front loom a couple of years back.
(Only partially my fault as a DPO had wired up the Rt front sidelight a using black cable for the live feed instead of the correct red cable, so I assumed that the black was the earth! I know, I should have checked it first! AND disconnected the battery before I started)

Well, the result was that the front loom was pretty well melted down and the ignition was permanently on,
even without the key!

So when Bob West said to park my car outside his workshop overnight I was, at first, a bit concerned that it should really have been parked away a little more securely.

However, it occurred to me later on that he was just safeguarding all the other cars in his workshop (and the buildings as well) in case my car decided to spontaneously combust!

Colyn
Colyn Firth

Bill, I am not sure what you mean. If you choose a fuse that will blow at an amperage that is less than the amperage the loom can carry why is it of no use?

Mick
M F Anderson

Mick

I think the same and I wonder if this is some kind of US rest of the world thing.

In the UK domestic and industrial we always use the fuse to protect the cable supplying the equipment circuit? We do have equipment with fuses inside the gear but these are specific.

I would be very worried if a fuse did not blow before the cable became so hot as to cause damage.

Is this not the case in the USA?

If it is the case in the USA then as asked above what is it for?
Bob Turbo Midget England

Hmm. Rather controversial me thinks. From time to time we must all have shorted a circuit and blown a fuse - how often has the fuse held out and the loom burnt instead?
And is the logical cloncusion that I can get rid of that ugly fuse box I always thought protected my house electrics?
Graham M V

Fuses may be a little off topic, but I guess we are talking about shorts and safety here. Following on Graham's questions:

I once had a shorted lamp for rear license plate in my MGA. That circuit of course has no fuse. The feed wire got warm to the touch but did not burn or melt insulation. I presume it was because that is a long run of small gauge wire (18 ga), and maybe the wire resistance would keep the current low enough to prevent burning the wire. There might otherwise have been another high resistance connection somewhere in that circuit.

The horn circuit uses a 25/50 fuse for one dedicated larger wire, which is a reasonable match up. The other fuse is 17/35 amp feeding a number of circuits with 18-ga wire. I can imagine a few circumstances where a faulty device might draw close to 20 amps continuously and might eventually build enough heat in a wire to melt the insulation without blowing the fuse. This is most likely to happen with a loose connector and high resistance joint making the connector hot. It could also happen with an old switch with high resistance switch contacts. Either of these things would have to happen in conjunction with a down stream short to ground.

If you want to protect a wire you need a fuse with slightly lower current capacity than the wire. If you want to protect an individual end device you need a fuse for that device only with slightly higher current capacity than normal expected current draw of that device. As such it may at first seem reasonable to use a large fuse at supply end of a circuit (main fuse) and smaller fuse at each end device on the same wire (branch circuits). In the real world it is bad practice to use more than one fuse in the same circuit, as each fuse represents added electrical connection points and increased probability of some failure occurring. Better solution is to use more primary wires and fewer branch circuits with smaller individual dedicated fuses (one to a circuit). Robert (Bob) shows a fuse block going the right way in this respect.

The secret to reliability with any electrical connection is a good solid connection with little or no contact resistance. Spring type fuse clips go awry when the retaining fingers get corroded or lose spring strength. Periodic cleaning of the fuse clips helps. In-line fuses go awry with vibration or any mechanical load that would pull on the wires to break the contact at ends of the fuse inside the holder. This requires good strain relief for the wires very near the fuse holder,as the wires are the structural support for the assembly (which is why I HATE in-line fuses). Plug-in spade fuses in a fixed fuse block are better in all respects, not vintage in appearance to the old MGs, but adding any fuse is non-original anyway, so you might as well use the better parts.

Push-on Lucar connectors on a fuse block (or any electrical device) are another source of decreased reliability, getting worse with increased current load. Switches and relays and fuse holders with screw terminals are very good for the wire connections. Plug-in socket contacts for relays are usually good but can degrade over 10 years of simple aging. The 3-pole headlamp connector is a good example of a push connector that can degrade with time, so plenty of other spade connectors may be conducive to the same long term problems.

Every "optional" device or wire you add to the car will increase complexity and reduce reliability. Adding half a dozen electrical devices, some with relays and related switches and fuses can easily double the number of electrical connections and contacts in the entire car. If you like a little reality check, compare the late model MGB wiring diagram with the MGA wiring diagram. Personally I prefer the KISS principal, combined with conscientious maintenance.

Back on subject, I put my battery cut-off switch on the rear bulkhead behind the driver's seat (LHD). Initially it is connected VERY CLOSE to the original earthing connection for the battery with shortest cables possible. Mine as the battery ground cable connected to the switch, and a copy of the braided engine grounding strap between switch and frame.

Reason for switch connection close by in the grounding cable is same as why the shop manuals tell you to connect the battery ground cable last, and disconnect the battery ground cable first. If you short a wrench between the battery ground post and frame, nothing happens. Once the battery ground connection is disconnected, shorting a wrench between the battery hot terminal and frame does nothing.

I use the switch mostly for maintenance convenience, making no representation to theft deterrent. I view this switch also as a safety device, being able to disconnect the battery in a flash without having to dig out and open the battery cover.

I would NEVER disconnect the battery with engine running (except in dire emergency), as it is ill advised to disconnect the battery from a running generator or alternator. If this is the intention (as with some racing rules) then you should use a three terminal battery cut-off switch where the third terminal is used to short the ignition coil (points wire) to ground to assure the engine will actually stop, and to preserve the generator or alternator.

If you chose to connect the battery cut-off switch in the hot cable (or may be compelled to do that by racing rules), by all means keep the switch as close as possible to the battery. When the battery is still connected to ground on the chassis, any inadvertent shorting contact between the hot cable and the chassis (or intermediate cable with twin 6's) will cause a short across the battery posts resulting in extremely high current capable of melting a wrench or possibly exploding a battery. If you are compelled to mount the switch in an outside accessible position for race rules, particularly some distance from the battery, then it may be good to install a second switch in the grounding cable between battery and frame for all of the previous mentioned reasons.
Barney Gaylord

What Barney said.

Forums such as these are classic examples of "Tribal knowledge." We sit around the campfire swapping stories. We perpetuate solutions to things can't be "figured out." We short circuit natural selection by warning against truly bad ideas (i.e. the grizzly bear saddle)

A classic MGA BBS example is cylindr head studs- unmarked bad, marked OK, ARP great but overkill. In this case, adding a fuse box looks similar to using ARP cylinder head studs- great but probably overkill.

Fusebox tribal knowledge:
1. Fuses limit maximum current and, in theory reduce the risk of electrical fires.
2. Blown fuses are often preceded by the unmistakable smell of smoldering insulation. This smell also precedes electrical fires.
3. I've never seen or heard of a car with a properly-installed fusebox upgrade catching fire.
4. Owners of spontaneus combusting MGA's lament that, if they'd added more fuses, their car would not have burned.

A compelling case for sure. Unfortunately, as Barney describes, the real solution is proper installation and maintanance. People who properly install accessory fuses also inspect/maintain their wiring and components.

Finally, for fans of campy Kung Fu philosophy- If fuses protect from electrical fires, you'd never see a scorched fuse box.

Have fun,
Bill
Bill Eastman

???

I obviously do not understand the language spoken in the USA

:)
Robert (Bob) Midget Turbo

Well I am just a simple guy and looking for a simple solution (and I dont have the slightest idea what a grizly bear saddle, etc is). My concern is I might well be sitting round a fire, but not a camp-fire!
My question is simple, if I want to help protect my loom - particularly the unfused section, should I install a fuse box like Robs, which to me looks excellent. Barney appears to endorse that. I accept good connections etc are very important, but assuming all such things are equal, surely I am better with the fuse box than without?
Graham M V

Well I think you are right Graham and I too thought Barney agreed?

Barney makes a good point that good quality fuse box and connections are essential.

A correctly specified fuse will ALWAYS blow before the wire melts, if it does not then it was engineered and specified incorrectly.

The fusebox I have on my MGA is OK but if I do it again I will use the "blade" type box which is the one I have fitted into my Midget.

The beauty of multi fuses is that circuits are split and fault finding becomes very easy and a fault in one circuit does not interfere with another,

I have a rule of thumb that I try to stick too.


2.5mmSQ = 20 amps
4.0mmSQ = 25 amps
6.0mmSQ = 40 amps

A lot of things affect this calculation but IMO if you stick to fusing as above you will be fine
Robert (Bob) Midget Turbo

Out of all of this the following observations and comments come to mind:

1. Now I am retired I will only drive in daylight; then the fuse problem goes away!!

2. I do not run additional fuses and, so far have had no problems. Nevertheless, it looks like a winter project for me. Who has the best wiring diagram to plan the task - Barney? Not had a look at his pages yet.

3. I have the isolator on the live side. I will have a look to see if I can swap over. Barney's comment about the melted spanner is valid. On the rare occasions I delve into that area I also remove the negative terminal first. That said, even when the terminal is disconnected a spanner or other metal object can still short circuit the 2 terminal posts. For that reason I always retain the post insulators that come with new batteries and put them in place after disconnecting the cable and before doing any other work down there.

3. I keep a good quality fire extinguisher within arms reach of the driver's sitting position. How many of you keep one in the driving compartment?

4. Finally, I am always amused at the number of people who post an opening thread and then never get properly involved in the subsequent discussion. This thread is one of those. It's like a bomber pilot dropping a bomb. then disappearing into the sunset, not wanting to witness the carnage.

Cheers

Steve
Steve Gyles

I agree with Steve in that I think that this "hit and run" thread has made for great reading so I for one am glad that P in Devon started it.

We have even been graced with Barney's great (as usual) input.

I have been amused by the concept of a Grizzly Bear Saddle and I am forced to agree that this would be a really BAD IDEA. So, thanks for that Bill in Minnesota.

Is there some Werewolf blood in you Steve so that you darent venture out in the moonlight?
Maybe its safer for all of us (and your car) if you stay indoors after dark!

We could always convert to gas lighting and a magneto to get rid of the electrical system entirely!

Maybe then we would be able to open a thread on the threat of gas leaks and explosions!!

Colyn

PS I thought we would have heard from Lindsay on the fusebox subject by now.
Colyn Firth

Bob, I like your "rule of thumb" but can you tell me how I can easily tell what the ratings of each cable is likely to be as I have no way of measuring - eg in the standard loom would the lighting circuit be 2.5mm sq? I see that blade fuseboxes are in ample supply on ebay etc for just £3 or £4.
Graham M V

Good question Graham and I can not give you an easy answer.

I have been using wiring all my life, (instrumentation and electrical engineer) and know instantly what size a particular bit of wire is but for you my advice would be to pop down to a local DIY electrical stare and get a sample of each wire size. Then use comparison
Robert (Bob) Midget Turbo

Thanks - will visit halfords for the education!
Graham M V

I added extra fuses to my car so that everything switchable is protected. The only unprotected circuits being the starter motor, the charging circuit, the thick black cable from the batteries and the brown feed wire from the live side of the starter switch to the control box. I have used a 4 way lucas fusebox and glass fuses simply because they are quaint and fit in with the rest of the car! These fuses are in addition to the existing two under the bonnet, and are mounted under the dash on the dashboard stiffening bars near the heater controls (see picture. This is dead simple and involves no modification to the standard wiring loom, just an addition to it.
I have since added relays for the headlamps and ignition circuit to reduce the current flow through the ageing switches, this gets a little more involved and adds a further two fuses to bring the total number to 8, but again, does not involve cutting the standard wiring loom about. Like many others, I also have a battery cut-off switch in the live side of the battery and mounted on the board behind the driver's seat (RH-drive), but were I to fit it again, I would put it on the earth side, as that way, the switch can't be accidently 'shorted to earth' when working under the car. I mainly use the isolator as an anti-theft device and to guard against 'little fingers' playing with the horn and light switches when the car is parked without the tonneau in a public place!

Lindsay Sampford

And here is the basic (non relay) additional fuse diagram with wire colour numbers to tie-in with the standard 1500 wiring diagram.

Lindsay Sampford

A small correction to my first posting; the wire '36' is, of course, also not protected, but that is nothing compared with what is not protected on the standard wiring set-up!
Lindsay Sampford

And just another thing I forgot to mention; the fuse box is shown with the cover removed. Without the cover it might present new and improved ways of starting a fire if one of those bare control cables were to get out of position!
Lindsay Sampford

Nice job Lindsay

But I think once you overcome the need for originality and enjoy simply owning and using the car then modifying the loom and adding a fusebox in a very accessable location (underbonnet) is far better than making it inaccessable under the dash.

Do you kneel outside and poke your head under the dash? not my idea of fun on a cool night down a French lane in the middle of the Loire valley :)

I have also installed a lamp under the bonnet for such an occassion :)
Robert (Bob) Midget Turbo

I was not concerned with originality, and did consider using modern 'blade type' fuses, but the Lucas fuse holder was more convenient, and meant that all of the fuses on the car were the same type. As for the positioning, I wanted to keep the un-protected wiring as short as possible without having to cut the harness about; running long, un-protected wires to a 'remote' fuse box introduces another chance of something earthing somewhere, so under the dash was the logical place to put it. Accessability is easy, many modern cars have fuse panels in far worse places, I've been there! Now that I have added the relays for the headlamp and ignition circuits (under the bonnet), the power source that they switch comes from the A1 terminal connected via about 2 inches of wire to another 2 way Lucas fuse holder right next to the regulator, again keeping the un-protected wiring as short as possible. The most important thing is to have all wiring and especially insulation and as has already been mentioned, grommets in good condition.
I posted my fuse diagram because suggestions were asked for, not for it to be judged, I know that my loom will not melt because a headlamp 'pigtail' has earthed where a grommet has worn through, or the live connector inside the number plate lamp vibrates loose, and that is good enough for me.
Many wiring fires happen in the garage when we are fiddling about with things, especially behind the dashboard, so the isolator switch is a most useful device whether you have fuses or not. Look in any car workshop manual and see how many procedures start with the words "Disconnect the battery"!
Lindsay Sampford

No problems mate

My blooming MGF has a fuse box under the dash and I can not put into words the feeling I have trying to see what each fuse does and its condition!

I too have an isolator and would never part with it, great anti theft device.
Bob Turbo Midget England

The MGF fuse box might be bad Bob, but what about the access to that engine (I've been there too!). It's like trying to re-decorate your hall-way through the letterbox!
Lindsay Sampford

Thanks for the diagram Lindsay. I will put our originality banter on the back burner for the time being! I am presently running the original regulator box as a convenient terminal post block with my alternator so I need to think this one through. May be I can install the fuses in a stripped out regulator box casing. Any thoughts?

Steve
Steve Gyles

Not everyone is as careful as the average MGA owner when it comes to resolving battery issues.........See attached.

Steve

Steve Gyles

Hold mah beer whal I tighten this clamp... wow.
Del Rawlins

That's a real gem Steve, thank you!
Lindsay Sampford

This thread was discussed between 08/10/2010 and 19/10/2010

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