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Warning Electrical fault tracing.


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We have all spent time chasing our tails trying to track down electrical faults, Yes?

Please do not discount your fuses. Check them on and off the fuse box!

I have had a batch of modern glass fuses that have fully intact fuse strips but do not or intermittently conduct. So far at least 10% have this failure and have intact fuse strips but do not conduct.

It seems as if the soldering process does not complete the circuit. The metal drum end sometime spins on the glass.

To be sure now I always carry spare fuses two of each rating and check/change the fuse as first port of call should a fault develop.

Run a continuity check while shaking the fuse vigorously also whilst holding the probes in contact with the ends and indicating flow try to turn the fuse cap ends.

This does not suggest that the fuse will last during driving on rough roads as vibration can cause a failure.

I have to presume that these are a cheapo item and as such not subject to decent QA.

Hopefully this tip will save a bit of time and heartache.

Rod

 

 

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Also found the same. Best to get them from reputable autoelectrical supply companies.

Cheers

Tim

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Whilst on the fuse issue it's worth checking the continuity of the fuse box, having changed a dodgy one on a friend's 6 I checked my 4A one and there was corrosion on the underside of contacts which is what caused problems on the 6. Probably the original but now replaced with a new one and also noticed the stronger grip of the fuse retainers.

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Hello Rod

The subject of fuse ratings came up back in 2013. This was my note to CVTR and on the forum from that time

 

On page 40 of issue of TR Action no 267 is an article on new fuse ratings and risk of setting fire to your TR's wiring loom with new fuses. In Halfords you can buy glass fuses that look similar to the original lucas type BUT the power rating of eg 25 Amp is the running ampage not the blow ampage as in the older lucas parts. This is the reply (in BOLD) I received from Halfords technical dept for my enquiry (in normal text):

Customer asks:

"Older glass fuses were usually marked with a continuous and a blow rating
eg 12 Amp continuous blow at 25 Amp. NOT MARKED IN THIS WAY NOW
Fuses to the newer SAE standard are rated by their running current ie. 25
Amp running blow at much higher power. YES ALL OUR FUSES BLOW HIGHER THAN
THE RATING.
I have read of two people with classic Triumph TR's fitting what they
thought were fuses with blow ratings of 35 Amp were in fact to the newer
specification SAE of 35 Amp running power. This lead to fires in the wiring
looms of these cars because the wires were melting before the fuses blew.
What is exactly the specification of the glass fuses (HFS 146), 25 Amp blow
or 25 Amp continuous? 25AMP CONTINUOUS THEREFORE BLOWS UP TO 200% HIGHER
You also sell a multipack, HBB 102. What is the spec for these?"
ALL FUSES THAT ARE MARKED WITH A AMPERAGE ARE CONTINUOUS AND WILL BLOW UP TO
200% OF THEIR MARKED AMPERAGE.

"The customer needs to look at what the continuous he needs and the maximum
blow, for instance if he is looking for what used to be a 17.5 amp
continuous fuse 35amp blow I would use a 15amp fuse which would blow at a
maximum of 30amps, but suggest consulting a Auto electrician for advice."
Tr's 2-4A use 25 amp blow fuses in the fuse box with a 35 amp in-line blow for the horn of the 4/4A. Not sure about other models but check handbook. You can get replacement fuses to the older spec fom Leamoco who have outlets on Althorpe St , Leamington Spa; Avenue farm Ind Estate, Stratford upon Avon; Vale Park Ind Estate, Evesham; Swan Ind Estate, Banbury. See www.leamoco.co.uk
The fuses are branded Durite www.durite.co.uk from Gordon Equipments in Harwich, Essex.
The 25 amp blow (12.5 amp continuous) are 32mm long, part number 0-374-25 and are approx £2.76 for a pack of 10.
The 35 amp blow (17.5 amp continuous) are 32 mm long, part number 0-374-35 and are the same price.
These fuses are a bit longer than the Lucas original but fit fuse holders OK.
DO NOT FIT HALFORDS 25 AMP OR 35 AMP FUSES BECAUSE THEY BLOW AT 50 AND 70 AMP! IF YOU MUST USE HALFORDS THEN USE A FUSE APPROX HALF OF RATING YOU NEED.
You have been warned!
Keith
Edited by keith1948
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Thanks Keith,

I did not know that.

Can you distinguish the old from new fuses by dimensions or color code or....?

I have a hand full of fuses, but reading your infornation makes me wonder what to do with them.

Waldi

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For anyone using the Lucas SF4 fusebox (which holds 4 active fuses and 2 spares) rather than Triumph's original fitment, be aware that a fuse longer than 30 mm will not fit between the terminal posts. Even a 30 mm fuse with solder blobs on the end can be difficult to fit into position. In transit, a 32 mm fuse in an SF4 box may fail to make good contact or may become dislodged.

Ian Cornish

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Hello Waldi

I did try to set up a Wheatstone bridge circuit I remembered from my school physics lesson days. This device can compare resistances of different fuses but I seem to remember that it wasn't that successful. You need known fuses to compare against. If you have two fuses the same then the instrument will balance at the mid point in theory. Fuses are the same dimensions and mine don't have a colour code. If I still have the Wheatstone bridge I'll have another look at this method and also see if my new multimeter can distinguish different fuse ratings.

Keith

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<anorak mode on


Its actually a bit difficult to specify the current that a fuse will blow at unless you define more parameters. Melting of the fuse element depends on the energy dissipated in it which in turn is related to the square of the current and the time it flows for. That means the relationship of overload current to the time for the fuse to blow is a reverse square-law-ish curve*, so to specify the blow current you also need to specify the time. Ordinary fuses are not very good at protecting against very small overloads because they take a long time to heat up.


A fuse rated to carry 10 Amps may take an hour to blow at 12 Amps, a couple of minutes at 15 Amps, two seconds at 50 Amps and less than a second at 80 Amps. Obviously the quicker it blows the better the protection for the wiring - luckily (!) many electrical faults on a car are major ones like short circuits which result in a very large fault current, so the fuse blows quickly. 

Specially made fast-blow fuses are available where the disconnection time is critical but those are not usually used in vehicles. 



*the shape of the curve is complicated by factors such as heat-sinking provided by the fuse holder at very low overloads and the fact that the resistance of the fuse element rises as it heats up. 


<anorak mode off


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Thanks Keith and Rob,

Think it is best to buy new fuses of the correct rating, so the number will be 1/2 of what was originally specified.

Makes me wonder:

How many people with old cars do know this?

And how often will it have resulted in burnt wiring or worse?

Regards,

Waldi

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I've never seen fuses marked with their continuous and trip ratings. Can anyone verify that?

 

When buying fuses you can just buy a rating that is sufficient for the load, i.e. the power (Wattage) of the load divided by the nominal battery voltage (12), rounded up to allow for inrush currents.

 

Pete

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For a while now many on the North American continent have been aware of this dangerous fuse situation.

I assumed that Europe just had DIN rated fuses like the original Lucas ones. But you now have SAE rated as well? Maybe some importers are bringing the wrong stuff in.

In my case after a full rewire due to a meltdown at the back of the dash my car runs just fine on North American fuses with the following ratings (rather than 35 something’s)

Red fuse 10 amp

Purple 15 amp

Green 15 amp

Spare 15 amp.

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I've never seen fuses marked with their continuous and trip ratings. Can anyone verify that?

 

When buying fuses you can just buy a rating that is sufficient for the load, i.e. the power (Wattage) of the load divided by the nominal battery voltage (12), rounded up to allow for inrush currents.

 

Pete

Hello Pete

Have a look at the Durite website in my post above (number 7) and put the part numbers in the search box and you will see continuous and blow rating for the fuses. I have several fuses with this info marked on them. Halfords fuses only have the running ampage marked on them as do most new glass fuses.

Keith

Edited by keith1948
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I've never seen fuses marked with their continuous and trip ratings. Can anyone verify that?

 

When buying fuses you can just buy a rating that is sufficient for the load, i.e. the power (Wattage) of the load divided by the nominal battery voltage (12), rounded up to allow for inrush currents.

 

Pete

I checked my fuses yesterday and found the ones in the TR6 were marked 17.5 A continuous 35 A LUCUS

 

George

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Well, it just goes to show we're never too old to learn. I've been messing about with car electrics since the early 60s, but I've never noticed fuses with two ratings on. Must have a look at all the old ones in the garage.

 

Does anybody know the conditions under which the 'blow' ratings are measured?

 

Pete

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As I said above, the 'blow' ratings are really meaningless. A 17.5 amp fuse would be subjected to only four times the normal power dissipation at 35 amps (since this is proportional to the square of the current) and will therefore not fail very quickly. Typical curves suggest it would take several tens of seconds at this 2 times overload. Reasonably quick 'blow' times are achieved only with 5 times rated current and greater.

 

It also doesn't mean the fuse can withstand lower currents than 35A without blowing - it would just take longer at even lower overloads.

 

This data is for mains fuses but the principle remains the same for automotive ones:

http://www.littelfusesales.com/public/docs/POWRGARD-fuse-characteristic-curves-charts.pdf

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Possibly in BS2950:1958 Pete - "Cartridge fuse-links for telecommunication and light electrical apparatus" , or maybe in the earlier BS 646:1935 (as B link fuses).

 

Unfortunately neither seem to be available free, and at ££ per copy I'm not inclined to buy one to find out !

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Fuses not avail via the previous mentioned link, but currently available as below

 

 

InterParts - Auto Electrical Part Supplies

 

https://www.auto-electrical-supplies.co.uk

 

Product Model Quantity Price Total

Durite 0-374-25 - 32mm Flat Ended Glass Fuse - Continuous 12.5 Amp / Blow 25 Amp Card Of 10 £1.90 + Delivery £4.95. + Vat

Edited by ChrisR-4A
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