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Last night on the way home from club night the headlights went out both dipped and main beam, fortunately it was close to home so I crept home on side lights.

I have the permanent live to the relays coming from the hooter supply which ofcourse is fused, and the fuse had blown. So a new fuse and all is well but to avoid this happening again I have now run the permanent live from the terminal on the starter solenoid and have a separate fuse for each relay, that way if a fuse blows I can switch to dipped or main and not be plunged into darkness 

I have a pair of small driving lights that come on with main beam I have had main beam on for half an hour with no issues apart fron the 20 ampere fuses on the relays are getting quite hot, is this normal? 

George 

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Hi George,

I believe the original wiring did not have fuses for the headlights.

Your 20 amp fuse should not be getting hot.   Have you added up the total wattage of all the bulbs in the circuit. Divide this number by 12 and this should be the current.

Roger

 

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The Works' TR4 Rally cars, all four of which were at Malvern at the recent weekend, have separate fuses for each filament of the headlights (i.e. 4 fuses) and a great many fuses for everything else - a total of 16.

With adequate fusing, a rally car could hit a wall, partially or totally short-circuiting the electrics on one side, yet still have working lights (and spotlights) on the other side.

To improve the ordinary TR's arrangement, and to avoid sudden complete blackouts, have a look at my article in TR Action 296 ((March 2017) - it's accessible to members on-line in the club's main site.

Ian Cornish

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Thank you gents, I have 18.5 amp so a 20 amp fuse is getting close. I shall disconnect the driving lamps and see if its still getting hot, I will then add a separate fuse and relay for the driving lights.

As Ian suggests it's best not to have all eggs in on basket, it gets very dark out here and I really don't want a repeat of last night's event.

George 

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Have you found out why the original fuse blew?  It could just have been old age - but just maybe something else is going on?

The heat in a fuse is proportional to the square of the current flowing through it.  Littelfuse show their 20 A blade fuses to have a typical resistance of 5milliOhms so if you are drawing 18.5A that would be a dissipation of 1.7 Watts in the fuse (by I^2R); or more if your fuse is more resistive. 

They may get a little warm to the touch with that after a while  - let's not forget that the whole point of a fuse is that it will melt if the current gets too high, which in your case is at about 1.6 times what you are putting through it now (i.e. 2.5 times the present heat).  

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Rob

I suspect that it was old age that did for the original horn fuse in the fuse box, as a replacement fuse was OK. I only noticed the fuse on the relay was hot while I was checking things  after changing the source of supply, the fuse is a modern item so should run at 20 amp, none of the wires are not getting hot so possibly it's the fuse/relay holder is not 100%.

Tomorrow I will add a fuse and relay for the driving lights and do some further checks.

As an aside I was quite surprised how hot the headlights got even the numberplate lamp was quite warm.

George 

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27 minutes ago, harlequin said:

 

As an aside I was quite surprised how hot the headlights got even the numberplate lamp was quite warm.

The headlights in particular normally have a good cooling airflow when in use. 

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Mike Jennings, published an article about fuses in TR Action not too long ago.

There is a significant difference between the way fuse ratings were expressed in the 1950s/1960s and nowadays.  In selecting an appropriate fuse, one needs to be careful to ensure what is meant by the rating(s) stated on the fuse.

As it happens, when horns were being blown in the arena on Saturday to mark Graham Robson's absence, the 20 amp fuse for my horns blew.  As I have 2 sets of air horns, I ought to have used something higher, so now there's a 50 amp fuse in there.

Ian Cornish

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Did you up-rate the wiring at the same time Ian?  

Fuses are there to protect the wiring and should be rated for a lower current than the wiring can safely carry.  A 50A fuse feeding 30A wires is a bad idea. If you have two horns each on 30 A wiring which together can draw 50A they should be individually fused at 25A not connected together to a 50A one. 

 

 

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Whilst I agree with your advice, Rob, the wiring installed in 1962 in the Comps. Dept. by the man from Lucas, will cope with 30A continuous, but horns are used (normally) only for short bursts, which is why that fuse had never blown before.

I shall see whether I have a spare 35A fuse amongst my collection, as that would be better suited - the 50A went in solely to ensure that, if necessary, I could blow the horns on the way home.

Ian Cornish

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A LUCAS tube fuse, as fitted by Triumph, will blow at the stated rating or up to 20% less (faster) for a dead short. A 20A rated fuse should over-load blow at about 20A or at about 17A for an immediate dead short.

An SAE blade type fuse is rated for the normal maximum current and will typically blow at a much higher value. A 20A rated fuse will blow at about 30-35A.

Guidance seems to be that when substituting a Blade for a Lucas fuse that a rated value of 40% less is advisable. Often hear of fuse blowing issues resolved by using blade fuses but it may be because they have inadvertantly used a higher "blow" value.

Perhaps measure the maxiumum current the toys draw (with a decent meter and at 2000rpm so the voltage is highest) and add  at least 25% - this gives and SAE blade fuse value. The final value should be no more than the wire rating.

Other oppinions are available!

Quite why Triumph decided on fusing the horns and not the head lamps is one of life's mysteries.

Mike J

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On 8/18/2021 at 9:54 AM, mikej said:

 

Quite why Triumph decided on fusing the horns and not the head lamps is one of life's mysteries.

Mike J

It's probably a safety issue, I suppose Triumph though a bit of burnt wiring is less dangerous then the headlights suddenly going out on a dark wet night. And ofcourse the cars/wiring wasn't expected to be still in use after 50 years.

George 

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On 8/18/2021 at 9:54 AM, mikej said:

 

Quite why Triumph decided on fusing the horns and not the head lamps is one of life's mysteries.

 

I believe that is exactly their thinking.   Even now you can buy dodgy fuses.

 

Roger

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10 minutes ago, harlequin said:

It's probably a safety issue, I suppose Triumph though a bit of burnt wiring is less dangerous then the headlights suddenly going out on a dark wet night. And ofcourse the cars/wiring wasn't expected to be still in use after 50 years.

George 

I’m pretty sure it is a safety issue. I have a Powerblock loom and that adopts the same approach, no fuse on the headlights. 

Rgds Ian

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  • 4 weeks later...
17 hours ago, BlueTR3A-5EKT said:

That is exactly what I now have, one each for main and dip plus a third for the driving lights that come on with main beam.

I rather like how neat the wiring becomes having the fuse built into the relay

George 

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Very neat and inexpensive at less than £11 for two - delivered!

One could afford to put a fused relay in almost every circuit in the car,

Ian Cornish

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