Master cutoff switch - cut the positive or the negative? (or both!)

[anthonym]

Subject: Master cutoff switch - cut the positive or the negative? (or both!)

rj and revilla have been discussing this and I'm curious if anyone else out here has views / experience?

It is clear (I think!) that from a race safety perspective to cut the negative is the safer option; principally I think because cutting the neg right by the battery disables all possible fire sources which derive from all the unfused uncut wire section(s) (like the battery side run to the master switch).

However, that's not the popular option; presumably because it's not what CC deliver.

Also involved in this is using a physical cut like we are used to:

https://www.rallynuts.com/battery-isolation/autolec-fia-battery-master-switch.html

or using an electronic cut using one of these:  http://www.brise.co.uk/Battery-Isolator-EV200-H.html   (the farnel version is now about the same price.) Wiring seems to get complicated if ecu/alternator protection is desired.

The conversation flows around complexity, ballast resistors and whether destroying alternator and ecu is acceptable (which in extremis it is), but I don't want that option.

"I" am about to rewire my engine loom etc, so this has my attention.

Anthony

 

 

 

[anthonym]

and in their/your own words: 

from revilla

Not sure I'm convinced that cutting the -ve is inherently safer that cutting the +ve - I think cutting the -ve right next to the battery leaves less chance of a short than cutting the +ve on the other side of the car, but I don't see why cutting the +ve right next to the battery would be much different to cutting the -ve. But cutting either next to the battery means a remote relay solution.
Not much safer, but safer it is as you have half the "dangerous" points (one rather than two)

from rj 

Using the zener and a resistor is not much more complex than the resistor and long wires. You could bolt the zener directly to the engine block near the alternator and strap the resistor to the LHS engine mount.

If the remote (scuttle mounted) switch is to used it's much safer to cut the -ve

[rj]

Anthony,

"bear the alternator" - should have said "near the alternator" - Could you edit that for me :-p

[AdrianC]

So my experience is KISS led.

Cut a hole through passenger bulkhead and fit one of these in the negative side

https://www.ebay.co.uk/itm/Battery-Isolator-Switch-with-spare-key-100a-Continuous-12v-500a-cut-off-kill-/121365927711

Has worked faultlessly for years, act as a basic anti theft device as well (FIA switch externally is a bit obvious for this purpose) 

Adrian

[rj]

You need to be able to reach it strapped in and upside down.

[anthonym]

And I prefer not to be simple.

[john milner]

A clever or simple solution may depend on your reasons for having it. I have a four pole switch with an alternator resistor fitted inside underneath on the passenger side of the scuttle. My main reason for having it is to prevent the immobiliser running the battery down. My others are as a secondary anti-theft device, an easy way to disconnect the battery when working on the car and very much at the bottom of the list is as a safety device to cut off the battery in an emergency. I could probably get the same features with a two pole switch.

Before this thread I had not even considered fitting it on the negative side and I can see why it might be worth thinking about. As an emergency safety device the negative side may be slightly better as most of the positive side will already have fuses and switches that may break a circuit whereas the negative side has nothing but at the end of the day a big switch that cuts either leg is much the same. I am struggling to think of a scenario where a negative switch would be advantageous. Electron flow is negative to positive but no electrons flow in an open circuit regardless of where the switch is.

[rj]

If you cut the negative you don't have a live feed to the switch that is unfused. In case of a short circuit to the chassis of the feed wire to the switch it's just an earth and the worst that can happen is that the master switch can't cut power whereas if it's a positive then you have a short circuit - and this is the battery you short. Who said fireworks?

[elie boone]

Cut both by using a jump plug.

[Jonathan Kay]

A clever or simple solution may depend on your reasons for having it.

 

Agreed. That's why I asked...

 

Jonathan

[anthonym]

Interim conclusion: in my context of replacing the engine loom as a whole I have decided to replicate with new wires the existing red side master cut off. Alternative wiring diagrams and wire specifications will be very welcome - this is for an "FIA" switch, in case there are choices I might like to make. It worked fine until multiple roadside fixes mostly direct to the battery, so it's probably ok as it is, when the fixes are fixed. 

At the same time I will acquire the kit to install a dark side relay solution. 

As such staying simple by staying the same, allowing for change in due course if I wish. 

May the force .. ;-)

Anthony

[anthonym]

it's taken me a while to realise I could add the (relay type) negative cutout while retaining the positive. 

I have the (not cheap) relay

http://www.brise.co.uk/Battery-Isolator-EV200-H.html 

on the shelf and the additional security aspect is attractive.

Anthony

 

must google "jump plug".

[SM25T]

How much !!! My simple cut off switch was about £7

[anthonym]

Should be said it is a horse of a different colour.
 

Placed beside the battery it reduces the length of uncut cable from several feet, a bit less if in front of the passenger's knees like yours, to a mere few inches, and of course it also reduces the cutout live side to zero as well. Hide the relay switch and it's another security tweak, and if it needs the master key inserted first that's another one. 

[Roger Ford]

Back to the original question: if you cut the negative, but the shunt causes any part of the vehicle to make contact with the negative terminal of the battery, then every circuit in the car is live again. Conversely, if you cut the positive but the car makes contact with the positive terminal, then you have a fairly localised short circuit. It's not going to be pretty, and the battery might even explode, but at least it's confined to the engine bay. You're not going to see any fireworks near the fuel tank.

That, I assume, is why they always cut the positive on race cars.

There's an additional point that the positive terminal is required to be shrouded, but the negative terminal isn't, but of course that's easily rectified (and should be, if you're going to cut the negative).

[anthonym]

That is fascinating Roger and explains so much I have wondered about during this thread. 
Presumably cutting both is belt and braces.

Anthony

[jbcollier]

If you cut the positive then you need to make sure that there is a mech to shut down the alternator.  Otherwise, depending on how it is wired, the car may well stay running as if nothing has happened.

 

It is common to have a pull cable working a remote cut-off switch.  This allows you to keep short battery cable runs and have two shut off locations (one internal, one external as required by many race organizations.  Some switches can be used with a solid cable to make it an on/off switch even though it is remote operated.

 

It can function as an anti-theft device but this only works with casual thieves.  The knowledgeable thief will not be slowed very long by a battery cut off.

 

I have mine installed mainly in case of a shunt/fire.  Naturally, I have yet to use it for that reason -- knock on wood -- but that's its primary function and it is located accordingly.

[David_Stamp]

This is the solution that I have fitted - a negative cut-off switch right next to the battery: https://jackwebbmotorsport.co.uk/Caterham-Battery-Cut-Off-Switch-Metric-Chassis-p505353190

The car also has a Caterham FIA style external cut-off switch (cutting off the positive). My reason for cutting off the negative within the engine bay is to avoid all possibilities of the battery being drained by the immobiliser (when garaged in a garage with alarm fitted), plus to have a secondary security device to avoid car theft. That being said, a Caterham is light enough to be stolen by other means anyway ...