23 replies to this topic

[gazza82]

I'm adding a couple of relays to reduce the load on my lighting switch.

I've fused the new main 'heavy'/coil side (30) but I was wondering if the switch/trigger side (86 or 85) should be fused too as that carries power via the switch but now to earth, rather than to the lights.

I'm erring on the 'yes' option.

TIA

[nicklouse]

Only the power circuit. No need to do the switch side as there only a low draw so low current.

your lamp side is what will draw the current.

[Carlos W]

Are both headlights on the same relay/fuse?

The issue you've got is if the fuse blows you lose all your lights.

A relay/fuse per side would be a better set up

[gazza82]

Are both headlights on the same relay/fuse?

The issue you've got is if the fuse blows you lose all your lights.

A relay/fuse per side would be a better set up

Not exactly what I asked ... well aware of one per side, in fact the design I have is one per dipped beam and one per main (four relays!).

It is more if the switch develops a fault and a short would having a fuse in the circuit make it safer. Even though it is low current across the switch, it is still a circuit. There is an example below that shows the dual fuse idea.

Attached Files

•  4-pin-flasher-unit-wiring-diagram-4-pin-flasher-unit-wiring-intended-for-relay-4-pin-wiring-diagram.jpg   33.13K   1 downloads

[Cooperman]

I do normally take my signal feed from switch to relay via a fuse, usually a 5 amp.

[GraemeC]

Personally I would NOT fuse the supply to/from the switch for a lighting circuit.

 

The relay is a small enough load that it is highly unlikely to draw too much current to damage the switch or the wiring. If the wiring is done well then there should be minimal likelihood of a dead short.

Trade this against the possibility of a fuse failing and thereby leaving you with no lights at all.  Which has the worst potential consequences? 

 

From a safety perspective, I would rather risk a fire and the loss of the car than the possibility of a sudden loss of lights on a road at 60mph and risk many lives (both worst scenarios).

 

 

Looking at the wiring diagrams - this is how Rover wired even the late model SPi and MPi cars and is probably standard industry practice based on the above and a risk managed safety approach.

Edited by GraemeC, 27 January 2021 - 09:05 AM.

[MiNiKiN]

I would fuse both - signal feed taken from fusebox AND a seperate one for the high current circuit.
I tell you why. Because it is unlikely that the fuse itself is going to fail if you do it properly - and if you have a short circuit in your supply, the light will fail with or without fuse. But with the added benefit of fire, maybe.

[Dusky]

Can’t see why anyone would advice against using a fuse.
Big load or little load doesn’t matter one bit! Only takes a rodent to chew on the cables to create a short and an under dash fire.
Fuse both lights separately though. Fuse on both supply and switched side.

[Icey]

From a safety perspective, I would rather risk a fire and the loss of the car than the possibility of a sudden loss of lights on a road at 60mph and risk many lives (both worst scenarios).

If it catches fire it's probably going to kill the lights as well, so it's not an if/or, it's an and. If that circuit fails you'll likely end up with no lights AND a car on fire.

Fit a fuse if the supply to the relay coil isn't already suitably fused further up-stream.

[Ethel]

If the fuse blows you'll lose the lights anyway.

 

The fuse will need to be upstream of whatever you're protecting, so think about the possible points of failure.

 

Can we take it the sidelights etc will be as before? If so, the headlight terminal of the lighting switch is the 1st independent +12v point in your circuit. The sidelight fuse could handle the load from the relays, but it'd introduce a stack of possibilities to blow the fuse.

 

Then you have permanent lives at all the relays with some risk of shorting through terminals 85 & 86. Best to use cable that can handle more current than will blow either fuse, at least for the earth.

 

You'd be doubly unlucky to take out wiring from 86, but it does run to probably the weakest point: the column stalk.

 

You could get a big chunk of failsafe redundancy by using normally closed relays. Power  their coils from the ignition and switch them off to turn your lights on.

[Cooperman]

In the 1960's Ford didn't use fuses in their Cortinas. I had a new Mk.1 GT in 1966 and whilst driving along the Mall one night quite late, it seemed to be getting foggy. Then I smelt the burning. A short circuit had burned ont the wiring behind the dashboard.
Ever since then I have liked to have lots of individual fuses.

[GraemeC]

Can we take it the sidelights etc will be as before? If so, the headlight terminal of the lighting switch is the 1st independent +12v point in your circuit. The sidelight fuse could handle the load from the relays, but it'd introduce a stack of possibilities to blow the fuse.

 

There is no sidelight fuse (normally).  Rover did not fuse the sidelights or headlights (at least up to '96 on the SPi) - I'm sure they had good reason......

 

 

Yes a fire would kill the lights eventually, but you'd (probably) get some warning through Lucas smoke signals.

 

 

Big load or little load - of course it matters - the larger the load the more chance of the circuit being overloaded, especially though higher resistance of aged high load components.  Short circuits aren't the only reason a fuse blows.

And I don't think rodents are selective as what wires they chew - they won't go for the fused circuits in preference for all the other ones that are already unfused 

[cal844]

Can we take it the sidelights etc will be as before? If so, the headlight terminal of the lighting switch is the 1st independent +12v point in your circuit. The sidelight fuse could handle the load from the relays, but it'd introduce a stack of possibilities to blow the fuse.

There is no sidelight fuse (normally). Rover did not fuse the sidelights or headlights (at least up to '96 on the SPi) - I'm sure they had good reason......


Yes a fire would kill the lights eventually, but you'd (probably) get some warning through Lucas smoke signals.


Big load or little load - of course it matters - the larger the load the more chance of the circuit being overloaded, especially though higher resistance of aged high load components. Short circuits aren't the only reason a fuse blows.
And I don't think rodents are selective as what wires they chew - they won't go for the fused circuits in preference for all the other ones that are already unfused

Rover did fuse the side and dipped beam from 1990, I have 2 1993 carb looms and both have 4 fuses, 2 for sidelight and 2 for dipped beam. These fuses are the inline type, fitted behind the speedo binnacle.

Hope this helps

Cal

[GraemeC]

Sorry, yes they did - but after the switch, so similar to fusing the main power feed on relays, not the switching circuit.

[cal844]

Sorry, yes they did - but after the switch, so similar to fusing the main power feed on relays, not the switching circuit.

Yes that's correct the switch is technically unfused (due to these fuses being downstream of the switch)