Electrical arcing at fuse

[Joker_Bones]

G15, in original ignition setup, alternator, coils, points, rectifier, zener diode, 2MC capacitor.

If I take the fuse out, which is in the + earth battery lead, and charge the battery... When I replace the fuse I get some arcing/sparks from the fuse blades to the holder as it is inserted. I have noticed this on a few occasions.

In my limited electrical understanding this suggests that I am completing a circuit... But, everything is switched off, where is the circuit?

If I remove the fuse and re-insert it I cannot replicate the arcing.

Can anyone offer an explanation as to why I get arcing when the fuse is first inserted?

[Groily]

Interesting JB. Just thinking on the keyboard, but could it be that when you make the battery circuit by refitting the fuse you are discharging the capacitor somehow? I can't remember how they're connected without going and looking it up (had one on my P11 but converted that to a mag, so it's all gone hazy now) - but I wonder . . . say you charged the battery with the capacitor disconnected until after you refitted the fuse, would there be the spark. . .?
Whether it's 'bad' though, I'm not sure! If it only does it the once (very capacitor-discharge behaviour), and the ammeter doesn't have a hissy fit, fire doesn't break out, your hair doesn't stand on end etc etc, it's probably not serious (says he!)
Probably all pure bolleaux, but just a thought!

[Joker_Bones]

Lol... Nothing calimitous has happened yet!

I wondered if it had anything to do with the capacitor, that's why I listed it as being there, but my understanding of what it's for and how it does it is pretty vague.

IMG_20240322_175741.jpg

Looking at the wiring diagramme you can follow a line from earth, through the fuse, through the battery, to the zener, to the rectifier, to the capacitor and back to earth.

The battery is charged and the fuse has been out since my earlier post. I just went and put it back in and it arced. Take it back out and reinsert it and it doesn't arc.

I would like to disconnect the capacitor and try it then but I'd have to take the seat off and the thought of getting the bolts back through the mudguard and into the captive nuts in the seat is a bit overwhelming

[Groily]

Don't know a whole lot either JB, but the capacitor is there as an alternator-fed alternative energy store so you can start and run coil ignition machines without a battery using the pulses the alternator generates to charge it up, if you want. To save weight and the risk of the battery falling off over the sand dunes or wherever!

I never used it or even tested it out when running my P11 with the twin coils and in my case a Boyer system - I just relied on a decent-sized well-charged battery until I ditched the whole kit and caboodle for a magneto (and put the parts in a box for the next owner).
The reason I did that, apart from getting rid of spaghetti and multiple potential points of failure and eliminating the need for a well-charged battery for the BB system, was to provide more juice for decent lights. The Norton Mark 1 BB system is quite power-sapping and the original alternator was on the flaky side compared to the modern 200W stators I've gone for on other machines with BB sparks and Power Boxes.

Ergo, and as you say, in some if not all circs there is a tortuous link, via ammeter, zener, ignition switch and rectifier, between the capacitor and the battery.
But, whether that is any kind of explanation for your fuse arcing, I just don't know. But can't think of much else apart from a wiring fault that would cause some of the bad stuff you're not seeing!
My MC2 was in the left hand toolbox adjacent the battery, not under the seat, so easier to get at - I know what you mean about messing with seat bolts!

[spookefoote1956]

It's all about the electrons. When you put the fuse in the electrons are being allowed through that stage. Think of it like plumbing. The water flows to the next junction until you add a new section.

[Reynard24]

I have noticed this on alternator equipped bikes that also have a combined rectifier/ regulator such as the Boyer powerbox. I assumed (not always the best policy, I know) that it was due to a capacitor somewhere in the potted gloop storing energy and that fitting the fuse caused it to discharge. It was never a massive arc/spark but was noticeable.
That said, I’ve never experienced any kind of electrical malfunction, battery draining or strange ammeter behaviour following re-fitting a fuse and always convert to 12v using the rectifier/regulator system and have not had any other issues.

[MalcW]

I can't answer your question, but my magneto/dynamo bike, running 12v with a DVR2 regulator, does the same thing. No idea why.

Malc

[Andy G]

My old physics teacher explained this once, and if I recall it was something like...
It's all about the impatience of electrons. Now these little fellows race around a completed circuit like a TT Sunday God battling the A303 on his way to a particularly promising dinner date. And that's just fine from a spectator's viewpoint, as they are all trying jolly hard to get on with their lives after the divorce, and to generally be less negative (hence more positive).
Now the problem only happens when they reach a set of temporary roadworks somewhere just past Illminster. All traffic is brought to a halt by a break in the road surface, and our hero joins so many others all trying to improve themselves, (and vicariously their chances of success in meeting the neutron of their dreams).
The Laws of Physics attempt to keep control and redirect some onto any available side roads to complete their journey, but there's just too many and a backlog is building up.
Suddenly a lovely council team declare the road repaired, spin around the Green is for Winners lollipop and the Law loses total control.
There's a brief but massive surge forward, as our Romeos all take years of their clutches in an attempt to get in front. I said brief, only the front row really succeed in securing advantage as the others tend back towards their pre blockage average speed.
And there you have it, a spark or something, or not.
Though I really should confess, my memory isnt what it was, and my Physics Teacher was as mad as a box of frogs.