B33 electrical fuse query

[bruce.craig]

I have a 1952 BSA B33 Plunger and I am completely replacing the wiring loom and I am retaining the original 6V positive earth configuration.
Can anyone share any experience or recommendation if it is good practice or necessary to include an electrical fuse and if so where and what rating ?
Cheers for your help.
Bruce

[Derek996]

Definitely good practice to fit a fuse at the battery. The size depends on the current that it will have to carry.  Assuming a 35W headlight bulb, the lights add to about 50W including the speedo illumination. With a 6V circuit that will draw just over 8A but another 3.5A when the brake light is on. I've no idea what the horn will draw but would expect maybe 5A. So a 20A fuse should suffice. This will protect the wiring in the event of a short circuit.

[Greenfield]

There's nothing wrong and something to gain with fitting two fuses, one for the lights and the other for the horn and brakelight. Depending on the bulb wattage you'll then be dealing with 10 to 15A per fuse and you won't risk 20A flowing through your wiring. Some bikes don't have a fuse for the horn and brakelight, which I learnt to my cost when a horn shorted internally.

[scifi]

I don't think the original circuits contained a fuse.  The reason being is that they take about one volt off the supply, and when you only have 6v to start with, that is a high percentage.

[Derek996]

The voltage drop will depend on how much current is being drawn. Assuming 8A at full load of all lights on with mail beam, a 1V drop across the fuse means the fuse has a resistance of 0.125Ω. Which I think is unrealistically high.

[Rog1]

The voltage drop will depend on how much current is being drawn. Assuming 8A at full load of all lights on with mail beam, a 1V drop across the fuse means the fuse has a resistance of 0.125Ω. Which I think is unrealistically high.

Yep. I'm an electrician and I've never heard of a one volt drop across a fuse. But it is a high percentage, and as power varies according to the square of the voltage a 6 volt 36 watt bulb would only use 25 watts at 5 volts; it would hardly glow at all.

[Mike40M]

On later bikes with fuse, the horn was usually connected directly to the battery. In order to use a smaller fuse. It does not matter if the fuse is connected to positive or negative battery lead. Whichever is more practical.

[V500]

To reduce the amount of unprotected wiring, the fuse should be in the earthed line.
Fuse ratings should be as low as possible (without nuisance blowing) so a 10A should be more than sufficient. Adding up all the possible loads as a way of assessing the rating isn't advisable at all.

[KernowPete]

I'm in the process of rewiring bits of my B31 including new battery connections and inline fuse. I agree that the earthed wire should be fused as it protects the cables better. I have used a moulded inline fuse holder as the connection between the battery positive terminal and the frame. This means no joins in the cable.
I also agree on the fuse rating. I looked up the power of the bulbs in the bike, measured the horn current etc and guessed a few others based on what I'd found. I calculated around 20mm max current! To check I wired it all up with some terminal blocks and put my mutlitmeter in series. With everything on (high beam, horn blasting, brake light on etc) it was pulling just over 7 amps without the engine running so 10amp seems a good fuse size.
I decided to remake all these connections on this new to me bike as I was nervous about having everything live all the time with no fuse. As a stop gap I disconnected the battery between rides and put it on the bench. Last time I connected it up I managed to crush a neutral cable between the battery box and the frame. I must have also over tightened the strap. As I was tidying up I heard a pop and smoke started rising from behind the battery box. I had to quickly get the tools out, undo the clamp bolt, remove the strap and disconnect the battery before it went up in flames!