Jump to content


Photo
- - - - -

Wiring In A Relay: Fuses?


Best Answer Dusky , 29 January 2021 - 12:58 PM

I had an electrical issue with my 1964 car a while ago. It was blowing one of the main 35 amp fuses. I managed to
isolate it to the wiper/fuel gauge circuit and stripped the speedo housing out. There was a bare connection on that circuit.
As the wipers are what might be considered a safety item, I re-wired them with their own fuse which is much safer.
My fuel pumps and headlights are relayed and all the circuits are fused independently.

Both my rally cars have all beams, spots, both fuel pumps, brake lights, horn, washers, wiper, and reversing light, relayed with individual fuses. The MGB also has relays on the overdrive and Kenlowe fan.
That's what I'm intending to do but the question was are both "powered" circuits fused?? The main feed to power the relay is probably obviously yes as this takes the load, but it's whether the feed to the "switched" side is also fused. That is the 12v feed from the wiper/horn/etc switches?? Would you fuse the feed to the brake light switch circuit for example?

That should be fused as well. ( 2 fuses per relay, 1 on the power and one on the switch side). Go to the full post


  • Please log in to reply
23 replies to this topic

#1 gazza82

gazza82

    Up Into Fourth

  • Members
  • PipPipPipPipPipPipPipPipPip
  • 2,495 posts
  • Location: Bucks
  • Local Club: TMF+

Posted 26 January 2021 - 09:31 PM

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

#2 nicklouse

nicklouse

    Moved Into The Garage

  • Members
  • PipPipPipPipPipPipPipPipPipPipPip
  • 15,041 posts
  • Location: Not Yorkshire
  • Local Club: Anonyme Miniholiker

Posted 26 January 2021 - 09:38 PM

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.



#3 Carlos W

Carlos W

    Mine is purple, but I have been told that's normal

  • TMF Team
  • PipPipPipPipPipPipPipPipPipPipPip
  • 10,068 posts
  • Location: Sittingbourne, Kent

Posted 26 January 2021 - 09:59 PM

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

#4 gazza82

gazza82

    Up Into Fourth

  • Members
  • PipPipPipPipPipPipPipPipPip
  • 2,495 posts
  • Location: Bucks
  • Local Club: TMF+

Posted 26 January 2021 - 11:33 PM

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.

#5 Cooperman

Cooperman

    Uncle Cooperman, Voted Mr TMF 2011

  • TMF+ Member
  • PipPipPipPipPipPipPipPipPipPipPip
  • 22,189 posts
  • Location: Cambs.
  • Local Club: MCR, HAMOC, Chelmsford M.C.

Posted 26 January 2021 - 11:56 PM

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



#6 GraemeC

GraemeC

    Crazy About Mini's

  • TMF+ Member
  • PipPipPipPipPipPipPipPipPipPip
  • 6,407 posts
  • Location: Carnforth

Posted 27 January 2021 - 08:59 AM

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.


#7 MiNiKiN

MiNiKiN

    Mini Mad

  • TMF+ Member
  • PipPipPipPip
  • 118 posts
  • Location: Graz

Posted 27 January 2021 - 09:47 AM

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.



#8 Dusky

Dusky

    Crazy About Mini's

  • Members
  • PipPipPipPipPipPipPipPipPipPip
  • 5,199 posts
  • Location: Belgium

Posted 27 January 2021 - 01:09 PM

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.

#9 Icey

Icey

    One Carb Or Two?

  • Traders
  • PipPipPipPipPipPipPip
  • 1,236 posts
  • Location: Wiltshire

Posted 27 January 2021 - 02:36 PM

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.

#10 Ethel

Ethel

    ..is NOT a girl!

  • TMF Team
  • PipPipPipPipPipPipPipPipPipPipPip
  • 23,878 posts
  • Local Club: none

Posted 27 January 2021 - 02:44 PM

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.



#11 Cooperman

Cooperman

    Uncle Cooperman, Voted Mr TMF 2011

  • TMF+ Member
  • PipPipPipPipPipPipPipPipPipPipPip
  • 22,189 posts
  • Location: Cambs.
  • Local Club: MCR, HAMOC, Chelmsford M.C.

Posted 27 January 2021 - 03:47 PM

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.

#12 GraemeC

GraemeC

    Crazy About Mini's

  • TMF+ Member
  • PipPipPipPipPipPipPipPipPipPip
  • 6,407 posts
  • Location: Carnforth

Posted 27 January 2021 - 03:59 PM

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  :rolleyes:



#13 cal844

cal844

    Crazy About Mini's

  • Members
  • PipPipPipPipPipPipPipPipPipPip
  • 8,243 posts
  • Location: Ballingry, Fife
  • Local Club: TFMOC

Posted 27 January 2021 - 04:24 PM


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 :rolleyes:

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

#14 GraemeC

GraemeC

    Crazy About Mini's

  • TMF+ Member
  • PipPipPipPipPipPipPipPipPipPip
  • 6,407 posts
  • Location: Carnforth

Posted 27 January 2021 - 04:27 PM

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



#15 cal844

cal844

    Crazy About Mini's

  • Members
  • PipPipPipPipPipPipPipPipPipPip
  • 8,243 posts
  • Location: Ballingry, Fife
  • Local Club: TFMOC

Posted 27 January 2021 - 04:33 PM

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)




0 user(s) are reading this topic

0 members, 0 guests, 0 anonymous users

Mini Spares