Fuses again

[keith1948]

Before I begin this post, I am aware that the newer fuse ratings are continuous and not blow. The question therefore is fuse ratings in 'old money'. The TR4/4A workshop manual states that the fuses in the fuse box should be 25A blow (12.5 continuous) but both Rimmers and Moss are selling 35A fuses for both the TR4 and 4A (I am assuming for the sake of this that these fuses are 35A blow). The only fuse that is 35A blow 17.5A continuous in the workshop manual is the horn fuse. Consequently I have been using 25A in the fuse box (with no problems). Usual reason for a fuse to blow is a short to earth and both a 25A and 35A would blow in that instance. Just wondering when it was decided by the 2 suppliers mentioned that it was ok to fit the 35A (blow) in place of the 25A (blow) mentioned in the workshop manual. Unfortunately the TR4A handbook fails to state the fuse rating. The Haynes manual follows the workshop recommendations.

 

I have also got the TR4/4A owners workshop manual by Kenneth Ball (Autopress) and that also states that the fuse carrier has 2 x 25A fuses and the horn is 35A (again blow rating).

 

Does it matter which fuse is fitted? Personally I am sticking to the 25A fuses to err a bit on the safe side. (Rather blow a fuse than cook the wires).

 

Also does anyone have a quick DIY way of telling whether a fuse marked simply 35A is blow or continuous. Presumably the 2 types have differing resistances but the resistance is too low for a standard ohmmeter to differentiate. I have tried a simple Wheatstone bridge and a simple Kelvin bridge arrangement to compare fuses with no success. I have a few of these fuses that I might end up ditching because I can't tell what they are.

 

Keith

[RogerH]

Hi Keith,

I think you would need to build a test rig that allows what you think is the continuous current and the blow current to be switch through a fuse.

 

If you have an older 35amp blow fuse then a 12V car battery and a 0.3 ohm resistor in series with the fuse will/should cause the fuse to pop - may take a few seconds.

 

For continuous running then a 1 ohm resistor will allow 12 amps thro'.

 

Obviously the resistor will need a good power rating.

 

Buy the fuses in batches and test 10% at the blow current and the rest at the continuous current.

 

Roger

[RobH]

If it only has one amperage marking you should assume this is the standard marking for continuous-carry rating Keith.

[ChrisR-4A]

Just bought these from AES

www.auto-electrical-supplies.co.uk

Durite 0-374-25 - 32mm Flat Ended Glass Fuse - Continuous 12.5 Amp/Blow 25 Amp

£1.90 for 10 plus post & vat.

Clearly marked as above on packet but not on each Fuse so don't mix with old ones.

Original Fuses were 30mm long but these at 32mm fit my standard fusebox fine.

Chris

[keith1948]

Hello Chris

I have some of the Durite ones. I think you will find they are stamped 25A at one end (on the end) and this is the blow rating as you correctly state. These are ok because they are the correct ones according to the workshop manuals for a 4A. Just wonder why Moss and Rimmers are selling 35A for the 4A fuse box? I think the 4 had 35A fuses according to a USA workshop manual I saw today.

Keith

[RogerH]

Hi Keith,

the difference between 25 & 35 amp in a very short time frame is not a great deal.

The cable would have a higher rating that will stand the extra load for a short time.

 

Extended time at the higher current may cause heating issues.

 

Roger

[DaveR]

The fuses marked with the working current rating, not blow, have been available for 80 + years as they are for electrical/electronic equipment.

 

The electrical/electronic type were always longer by a few millimetres than the original Lucas fuses. Not too sure that is still the case, but if the fuse you have is longer than the original then it probably will be the electrical/electronic type.

 

Dave