Switching on earth

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Ian Jones
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Switching on earth

Postby Ian Jones » Sun Jan 05, 2020 5:50 pm

This is an issue I've come across before but never managed to satisfactorily answer - maybe you can help. What is the logic behind having a circuit wired battery live, fuse, load, switch, battery earth? Everywhere else I have come across in the world is wired live, fuse, switch, load, earth. Examples are horn and wipers on our cars. I fear there is a simple explanation but I just can't see it.

Regards

Ian

oliver90owner
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Re: Switching on earth

Postby oliver90owner » Mon Jan 06, 2020 4:35 pm

Wipers generally continue to run after they are switched off. Does that answer that one?

geoberni
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Re: Switching on earth

Postby geoberni » Mon Jan 06, 2020 5:30 pm

Generally the whim of the designer in my experience.

Self parking wipers as mentioned by oliver90owner , could be a reason, but that's very much a 'could', in that on the Minor the wiper motor has 2 separate earths, one via the switch, one direct. A more modern car tends to have 2 supplies to the Wiper Motor, one via the switch, and a single earth.
But the old non self parking Minor Wipers also had the switch on the 'earth side' of the circuit.

Sometimes it can be the relative places in the vehicle that denotes the place in the circuit to place the switch.
Basil the 1955 series II

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Ian Jones
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Re: Switching on earth

Postby Ian Jones » Mon Jan 06, 2020 10:00 pm

I wondered about wipers but as has been said there are 2 earths to self park and on moderns there are 2 feeds.

I also wondered about position on vehicle and saving wire but surely not worth it.

It just seems to me that having the load and the wires feeding it live all the time, even when not in use is less safe than having the switch on the live side. There is no effect on function though.

I also wondered whether there was some link with the change from pos to neg earth and the discussions around which way electricity flows, but I can't see a link there myself.

Thanks for the thoughts. Perhaps it will remain one of life's mysteries.

Regards

Ian

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Re: Switching on earth

Postby Banned User » Mon Jan 06, 2020 10:21 pm

I've always assumed that the horn was wired like that because it's easier to make the switch in the steering wheel boss, if it was wired conventionally it'd require two wires fed through the steering wheel splines/nut. As for the wiper motor, I just though it was to do with the self parking, how were wiper motors wired before self parking came about?

Sleeper
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Re: Switching on earth

Postby Sleeper » Tue Jan 07, 2020 6:09 am

Ian

Without doing any research , I think you are closest..

" It just seems to me that having the load and the wires feeding it live all the time, even when not in use is less safe than having the switch on the live side. There is no effect on function though. "


John ;-)

Apolaustic
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Re: Switching on earth

Postby Apolaustic » Tue Jan 07, 2020 6:20 pm

We’re not the only ones to be (have been) thinking about it ...

http://www.locostbuilders.co.uk/viewthr ... ?tid=57804

:)
Apolaustic
1966 white/red convertible (1098cc with alternator, electronic ignition & brake servo)

geoberni
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Re: Switching on earth

Postby geoberni » Wed Jan 08, 2020 2:12 pm

PoolGuy wrote:
Mon Jan 06, 2020 10:21 pm
I've always assumed that the horn was wired like that because it's easier to make the switch in the steering wheel boss, if it was wired conventionally it'd require two wires fed through the steering wheel splines/nut. As for the wiper motor, I just though it was to do with the self parking, how were wiper motors wired before self parking came about?
Being a SII, Basil doesn't have self parking wipers. The circuit is Fuse > Wiper Motor > Switch > to Earth via Terminal E on the Regulator/Control Box. See N28 in the manual.
Basil the 1955 series II

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