Fuse ratings

[VW]

I need a couple of new 30mm glass fuses for my Bedford c.f.. it has a random selection of ratings in it at the moment so checked the factory manual to see what they should be. It has 12 fuses, usual 70s electrics.

The manual says all 12 fuses are 35 amp!

This seems a bit excessive.
Now I'm wondering if I should just go with it in case there is some kind of reason, or if I should use more appropriate ratings. Later cf's had 20 fuses varying from 3 amp to 10 amp.

Could there be any reason for 35 amp fuses? Or did Bedford just cone into a box of 1,000,000 35 amp fuses?

[paul99]

12 fuses! How decadent! My '71 Dolomite has a grand total of 2! Both 35A iirc, must be a '70's thing..... Did someone in BMC over order?.....  

[VW]

Jul 1, 2020 12:30:23 GMT paul99 said:

12 fuses! How decadent! My '71 Dolomite has a grand total of 2! Both 35A iirc, must be a '70's thing.....

Yeah, maybe!
Earlier cf's only had 2 fuses, not sure on rating, probably 35a. Presumably as there is more stuff running through each fuse.
Maybe moving up to 12 fuses they just stuck with the same rating.

Might try running them all at 10a and see what happens. The headlights are relayed and have separate power fuses so the original headlights fuses only power the relays.

[VW]

Just this nagging feeling that 'the book says to use 35a, maybe there is a reason

[ferny]

Keeping in mind that the fuse is there to protect the wires, if any of those circuits have wires which can't handle 35a, then those wires are now the fuse. I suspect the book is incorrect!

Could you carefully wire in an ammeter gauge and test each circuit at full chat? Most multimeters won't go past 10a without going kaput. It's that or do the calcs yourself, which you can't do unless you can find the data for everything in that circuit.

The 10a fuse test isn't reliable and I 100% definitely not (but really have) played that game. But what if it's in the middle of winter, cold and the circuit draws a tiny bit more current than it did in your summer testing?

[slater]

All circuits probably use the same cable size and therefore they have fused them all the same.

[VW]

Jul 1, 2020 16:15:23 GMT slater said:

All circuits probably use the same cable size and therefore they have fused them all the same.

Sure, but 35a!?

[VW]

Jul 1, 2020 16:12:22 GMT ferny said:

Keeping in mind that the fuse is there to protect the wires, if any of those circuits have wires which can't handle 35a, then those wires are now the fuse. I suspect the book is incorrect!

Could you carefully wire in an ammeter gauge and test each circuit at full chat? Most multimeters won't go past 10a without going kaput. It's that or do the calcs yourself, which you can't do unless you can find the data for everything in that circuit.

The 10a fuse test isn't reliable and I 100% definitely not (but really have) played that game. But what if it's in the middle of winter, cold and the circuit draws a tiny bit more current than it did in your summer testing?

I was basing 10a on later cf's having a maximum fuse rating of 10a, and that's probably on the headlights. I can check what circuits had what on the later models in the factory manual.

It says 35a for my model in numerous places, I don't think it's a mistake.

[VW]

This is what the factory manual says

[kevins]

List out what's on each fuse add up the watts and divide by 12 to give you the max current, fuse should be around a 20% above this. Anything inductive(motors) will have a higher starting current so you might need to go a bit higher still.

[VW]

Jul 2, 2020 19:16:37 GMT kevins said:

List out what's on each fuse add up the watts and divide by 12 to give you the max current, fuse should be around a 20% above this. Anything inductive(motors) will have a higher starting current so you might need to go a bit higher still.

Can't I just follow what fuses Bedford used on the later models? The electric systems are pretty much the same, they just used a few more fuses and rated them all properly.

I'm just at a bit of a loss as to why Bedford didn't do this on my model and just used 35a fuses. It seems an odd thing for them to do.

[colnerov]

Hi, Is there a difference between early and late fuses, by which I mean glass and ceramic fuses. Because ISTR that they blew at different rates ie. slow and fast blow fuses so need to be rated accordingly. Something to investigate, sorry I can't be more definite.

Colin

[VW]

Jul 2, 2020 21:17:42 GMT colnerov said:

Hi, Is there a difference between early and late fuses, by which I mean glass and ceramic fuses. Because ISTR that they blew at different rates ie. slow and fast blow fuses so need to be rated accordingly. Something to investigate, sorry I can't be more definite.

Colin

That is something I hadn't considered. I don't believe I have to worry too much about if mine are fast or slow blow etc, they are just normal 30mm glass fuses. But, the cf2, which has proper fuse ratings, uses blade fuses instead of glass.
Maybe they could be more accurate with ratings when they were using blades, but still seems a bit difference.

My current favourite plan is to put 10 amp in all of them but carry a variety of higher rated fuses to replace any that pop.
Even if 35 amp was ok when it was new, the wiring is now 40 years old and the look has some notoriously temperamental main connectors, so I'd rather the fuses blew before the van burns down.

There is an auto electrics place in town, rather than ordering fuses I'll buy them from there and run the situation by them too.

I'm leaning towards being uncomfortable with 35 amp fuses on circuits that 2 years later they decided can run happily with a 5 amp fuse.

[Phil H]

"Lucas" glass fuses are dual rated - 17A/35A for example which is 17A continuous current, but will blow at 35A (supposedly) but known as a "35A fuse". Pretty sure all other fuse types (ceramic, blade etc) are rated at their running current - something to be aware of if changing the fuse type.

[cactus]

Back in the day of 30mm glass fuses there were only 35a,10a and 5a fuses..
5a were used on radios.35a on everything else..
10a were very rare.

[VW]

That's an interesting thought I hadn't considered.

I went to an auto electrics specialist to get the fuses and ask what they thought.
They thought 35 amp was excessive too. We came to the conclusion the best bet was to put 20a in and see. So I bought enough 20 amp, and enough 35 amp, to do the lot.

They said if I wanted they could measure the current draw across each fuse if I have any problems.

Fuses cost me a bit more than buying them from eBay but they are Lucas fuses rather than Chinese fuses, and I got to talk it through with some pros so all good.

I'll see what happens.