Fuses & Fusing

[PEU186F]

I have recently seen a suggestion that, at the time the majority of our cars were built, it was Lucas's practice to rate their glass fuses by the rupture current, i.e. the current at which the fuse would "blow".

Whereas current practise is to rate fuses (including apparently identical glass fuses) by the rated current of the protected circuits, which might be aprroximately half the rupture current

This implies that the replacement of original 35 Amp fuse, designed to rupture at 35 Amps, with its modern equivalent, bought over the counter, would mean that one would be fitting a fuse that would only rupture at approx 70 Amps, which I frankly find a bit frightening.

I would be most interested in anybody else's views on this subject, as I would be happy to be told I've got the wrong end of the stick.

Roger Morrall

[sv8predator]

The standard glass fuse that was fitted in the day (if we are talking about the 35 amp version) was rated 35 amp (blow) and running current 17 amps.

Thus if you are saying that "current" glass fuses are rated at 35 amp running current, then plainly they are the wrong specification to replace an original 35/17 fuse.

[David Olivant]

The rating of a fuse is current and time. Modern fuses are more accurate ( or should be).
With the lack of fuses in our older cars, the number of circuits protected by one fuse is too many. I split my circuits up, fitting about 12 from memory. I also fed all the circuits that I wanted only operating with the ignition on through a 100Amp Durite relay. A battery isolator finished it off.

[John Simmons]

To perhaps clarify a bit, regarding, as you said, that "Whereas current practise is to rate fuses (including apparently identical glass fuses) by the rated current of the protected circuits, which might be approximately half the rupture current." If the rupture current is to be 35 amps, then you would want a 17 amp fuse (approx half the rupture current).

[Kenneth Andrén]

Thanks for the important info! Any idea of when e.g. Lucas changed their rating method?

[Clive Foster]

I don't think Lucas ever did change their rating. The wikipedia article at https://en.wikipedia.org/wiki/Fuse_(automotive) seems to have the correct information.

Confusion stems from the various same size package but different inside versions made to other specifications such as AG et al. This page from Littlefuse Glass Fuses | Automotive Glass Fuses - Littelfuse gives some idea of the variety in glass tube types. Several are made to same size as the automotive fuses but the ratings are as per specific specifications. That page looks to be anything but comprehensive.

Lucas ratings are quite conservative. Half as much again current as the rated one is almost invariably safe. Unlike UK mains in-plug fuses there is no great concern over protecting the wiring harness as, for nearly all loads, wires are well oversize.

Realistically if it has the correct colour code card inside it should be to Lucas specs. If it doesn't have a card its anybodies guess what's actually inside. Something as, apparently, simple as wire tension affects the relationship between actual instant blow and continuous blow currents.

Clive

[sv8predator]

Originally Posted by Clive Foster

Half as much again current as the rated one is almost invariably safe. Unlike UK mains in-plug fuses there is no great concern over protecting the wiring harness as, for nearly all loads, wires are well oversize.

Sorry, I don't like any of that.

Running too much current through the fuse (although not enough to blow the fuse), will overheat the fuse. This in itself leads to higher resistance through the fuse which leads to further overheating. And the wiring certainly can be affected by this.

So can the plastic fuse blocks; they melt. Once it get to this stage, there can be a real fire danger. Do don't take the risk by running more current through a fuse that it's rated for.

[PEU186F]

Many thanks for your inputs on this, which have helped clarify things for me.

It appears that we need to be very sure that we are indeed buying glass fuses to the original spec, if originality is the thing. I seem to vaguely recall that fuses were once marked eg 17/35, but I haven't seen that recently.

For myself I think I'm going to follow David Olivants lead in fitting multiple modern blade type fuses (or circuit breakers?) and a high current relay to take the load of the circuits that are switched off the ignition switch. I have three things in my mind

1) I have had quite a lot of trouble with glass fuses failing, without apparently blowing, in that one of the end caps fall off, but the fuse link is not "blown". Very odd and it has usually happened at the most inconvenient moment, bringing eg indicators, cooling fan, wipers and fuel gauge to a simultaneous halt. Tedious, especially as it's not immediately obvious that the fuse has blown.

2) On my 410 the handbook specifies a 60 Amp fuse to "protect", amongst other things, the feed to the interior light. Really.......?. I don't fit anything over 35 Amps, reckoning that it's unlikely that I will light a cigar while flashing the headlights, but even so......

3) I know, from bitter experience, that the ignition switch really isn't up to the total load that it's being asked to switch. It would seem sensible to let a contactor take the load.

David, if you have any further record of what you did I'd be very glad to have a copy as there's little point in reinventing the whole thing.

My regards and thanks again