Earth/Positive switched Relays ??

[melhookv12]

Just looking at re-wiring my race car and was wondering which option is best. I'm sure when I worked at Jaguar they used some earth switched circuits.

Opinions and options. Pluses and minuses Please !!

I don't have much to power.

Electric Fans
Electric water Pump
Gauges
2x Fuel pumps
ECU and MSD ignition
ECU relates relays ( x3 )
2 lambda sensors

Thanks for reading.

[Allmycars]

Hi,
Having worked in the Electrical engineering departments at both Landrover Jaguar and Aston Martin I think I can explain why both JLR and AML used negative switched relay circuits.
Basically there is no BIG reason why neg switch became prominent. The circuit current is the same in both the positive and negative parts of the circuit, so switch wear due to contact arcing, emitted electrical noise etc are similar whether the switch is in the positive or negative.
The main reason for negative switching is for fuse protection. Normally one fuse will supply a number of relay coils. On a negative switch circuit, if there is a short circuit to ground on the switch wire; the wire from the relay to the switch, (this can be many meters long) the only effect is that the relay (and relay controlled circuit) are energised.
If positive switching was used, the effect of a short circuit to ground on the switched wire is to blow the fuse. This causes circuit failures of every function controlled (directly or indirectly) by the fuse.
So in summary a circuit failure of short circuit to ground on a negative switched circuit is generally better for the customer than in a positive switched circuit.

In the case of your race car there is no advantage between pos or neg switched. A bigger concern might be that of weight saving due to reduced wire length. Fusebox type, earthing strategy would probably dictated this. And weight saving against electrical distribution robustness might also be worth a thought.

[melhookv12]

Thanks for your reply.

[Penelope Stopit]

melhookv12 said:

Just looking at re-wiring my race car and was wondering which option is best. I'm sure when I worked at Jaguar they used some earth switched circuits.

Opinions and options. Pluses and minuses Please !!

I don't have much to power.

Electric Fans
Electric water Pump
Gauges
2x Fuel pumps
ECU and MSD ignition
ECU relates relays ( x3 )
2 lambda sensors

Thanks for reading.

Do you understand what you are asking?

There is a very big difference between earth switched circuits and earth switched relays

I explained in your duplicate topic what problems could arise from using relays to switch earths

You replied mentioning that the problems that I had pointed out didnt matter because you had a battery cut off switch

If all along you meant switching earths to operate the relays, why did you not point out my misunderstanding rather than continue to post as if you were thinking about using relays to switch the negatives

Maybe my misunderstanding

Is it just possible that you are thinking about switching earths to the relays to operate them and also use those relays to switch earths to the required circuits?

Edited by Penelope Stopit on Tuesday 15th September 13:28

[Penelope Stopit]

Allmycars said:

Hi,
Having worked in the Electrical engineering departments at both Landrover Jaguar and Aston Martin I think I can explain why both JLR and AML used negative switched relay circuits.
Basically there is no BIG reason why neg switch became prominent. The circuit current is the same in both the positive and negative parts of the circuit, so switch wear due to contact arcing, emitted electrical noise etc are similar whether the switch is in the positive or negative.
The main reason for negative switching is for fuse protection. Normally one fuse will supply a number of relay coils. On a negative switch circuit, if there is a short circuit to ground on the switch wire; the wire from the relay to the switch, (this can be many meters long) the only effect is that the relay (and relay controlled circuit) are energised.
If positive switching was used, the effect of a short circuit to ground on the switched wire is to blow the fuse. This causes circuit failures of every function controlled (directly or indirectly) by the fuse.
So in summary a circuit failure of short circuit to ground on a negative switched circuit is generally better for the customer than in a positive switched circuit.

In the case of your race car there is no advantage between pos or neg switched. A bigger concern might be that of weight saving due to reduced wire length. Fusebox type, earthing strategy would probably dictated this. And weight saving against electrical distribution robustness might also be worth a thought.

Having worked in the Electrical engineering departments at both Landrover Jaguar and Aston Martin I think I can explain why both JLR and AML used negative switched relay circuits.

I was about to post are you sure about this? I then noticed the I think I can explain

Is there any possibility that you are able to find out for sure as I and no doubt others will be interested in the reasoning