Fuse location

[Tigerfeet]

I have fitted a blade fuse in the negative wire, close to the battery, in the loom for my 12volt C15 (positive Earth). This afternoon Google turned me up a post from some other forum where a chap was saying that Lucas made this mistake for years, and that the fuse should be on the Earth lead. He went onto say that Lucas did recognise this and later looms had the fuse on the Earth lead. 

I found this publication - which is interesting in its own right - and noted that the example wiring diagrams at the back, which seem to be in chronological order do show this, with the final loom having the fuse in the earth lead.   https://rexs-speedshop.com/wp-content/uploads/2018/10/Lucas-Service-Information2.pdf
I did start to get a headache thinking this all through. Does anyone have any views on the matter?

[V500]

In the Earth lead every time. It gives the most protection to the cabling.

[Catz]

Yes, i agree with V500. Here's a photo of the set up i put on my A10. It's a 12volt + earth system. Don't tell SRM i nicked their diagram. lol. Looking at it now the fuse seems to be on the - negative wire but saying that it's how my A10 is wired and it works fine. Electrics=frightening.

[Catz]

Here's the other photo from SRM pamphlet. I'm confused now. It looks like the oposite of what i first posted.

[V500]

In both diagrams the fuse is located in the non-earthed battery terminal cable.

[limeyrob]

I recently wired up my A10 as 6v neg earth and fused the live (pos) lead. There is some logic in fusing the earth because it is a single wire for everything, but fusing the live works fine too if the fuse is close to the battery.  The current is the same in both battery cables.  I wanted an in-line fuse in the live because I could unplug it and connect a charger without removing the battery.

[scifi]

From an electrical engineering perspective, you have two sources of power; The battery and the generator.
Even if the battery fuse blows, your generator can still power the sparks.  So you will continue riding across the bleak moorlands, blissfully unaware that there is a fault in the wiring.

[Novocastrian]

From an electrical engineering perspective <snip>

...and from another electrical engineering perspective, I used to be the UK Sales Manager for Littelfuse (yes, that is how it's spelt) and I would urge the greatest caution in your selection of fuse manufacturer.  Littelfuse invented the twin blade ATO fuse . If you fit the cheap unbranded shit from jumbles, £1 shops or some car accessory shops then you'll probably get what you deserve.  Look on YouTube for some of the fuse testing videos if you don't believe me.  Littelfuse, Pacific, Bussman are all trusted brands manufactured to International standards.  For many of the rest, just use a rusty nail instead 

[Rog1]

In the Earth lead every time. It gives the most protection to the cabling.

No, it does not. If there's a short circuit to earth (the most likely fault) then it will give no protection at all, as almost no current will be going through the fuse; the fuse would be bypassed.

It will protect the cabling if there's a short circuit in (say) a bulb, as the current will still be going through the fuse; this does sometimes happen. In the unlikely event of a gross overload the fuse will also protect the cabling.

[Daz]

In the Earth lead every time. It gives the most protection to the cabling.

No, it does not. If there's a short circuit to earth (the most likely fault) then it will give no protection at all, as almost no current will be going through the fuse; the fuse would be bypassed.

It will protect the cabling if there's a short circuit in (say) a bulb, as the current will still be going through the fuse; this does sometimes happen. In the unlikely event of a gross overload the fuse will also protect the cabling.

That doesn't make sense. If there is a short circuit to earth all the current will go through the fuse.

[V500]

You got there before me.

[Rog1]

No, it does not. If there's a short circuit to earth (the most likely fault) then it will give no protection at all, as almost no current will be going through the fuse; the fuse would be bypassed.

It will protect the cabling if there's a short circuit in (say) a bulb, as the current will still be going through the fuse; this does sometimes happen. In the unlikely event of a gross overload the fuse will also protect the cabling.

Sorry folks I must have had a senior moment and was thinking about old Japanese bikes with electric start and a big fat (unfused) earth lead to the engine.

[scifi]

Most cars and aeroplanes have fuses, or circuit breakers, for every component.   So by just trying to use One Fuse to cover all aspects, you will not get complete protection.   For instance, if the Brake Light develops a fault, do you really want the ignition to be disconnected as well.
.

[V500]

Fair point on a modern bike, but on a simple old bike like our BSAs too many fuses would be over-kill.
I have two on mine, one in the battery earth for general protection and other protecting the DVR2. Any other problems or issues I'll sort at home.

[limeyrob]

I've fitted an inline spade fuse in my A10. I put a 20A because its really only there to protect if a wire chafes to the frame in which case the rating doesn't much matter and I want it big enough that it will survive the odd electrical upset.
The A10 is easy because its got a mag so the fuse won't chop the ignition but on coil vehicles I don't put the ign on the fused circuit, I want to keep running until its safe to stop.
On my old Land Rover I've fitted spade fuses and separated the things that are mostly likely to fault (such as wipers and heater fan) from stuff I need such as brake lights and side lights (no ign as its diesel).  I carry a couple of spade fuse circuit breakers (very neat bits of kit) for fault finding so I don't use up all my spare fuses at the road side.