why battery cut off goes on negative terminal instead of positive?

[andyt11]

I bought a battery cut off switch, primarily as protection against electrical fires.

I'm surprised to read that it is supposed to be installed on the neg terminal instead of the positive. I guess in the event of a short and things starting to melt, disconnecting the negative terminal will have the same end result as disconnecting the positive, but can someone explain the pros and cons of either approach?

[IanTBoyd]

Don't know the answer for sure but since our chassis are negatively grounded and the disconnects usually pass threw sheet metal, you don't want the disconnect insulation to fail and cause a direct short to ground.

[sc_rufctr]

When you remove a battery from a car you should ALWAYS disconnect the negative terminal first and then the positive.
The reason for this is because as soon as you disconnect the negative terminal there is no way you can accidentally short the battery when removing the positive terminal.

As far as kill switches go it really doesn't matter because either terminal will kill power from the battery.

BUT.... If you put it on the positive terminal the battery negative terminal is still connected to the chassis of the car so if anything conductive comes in contact with the positive terminal and the chassis it will short the battery.

A lot of racing cars have a kill switch that can be activated from outside of the car... The idea is if you pull the switch, the electrical system in the car immediately dies.
In a crash the body of the car can become deformed enough to contact the positive terminal or cable clamp. Or the positive cable can be crushed inside the body somewhere.

[Mitch Leland]

There's a more practical reason, the (-) negative post is more accessible and it doesn't have the extra hot wires going to the (+) positive battery clamp to deal with. So you're less likely to have the "shower of sparks" when disconnecting the switch since you're further away from the fender well sheet metal.

[John W.]

+1 to Peter, he's right on.

[regency]

I have my battery cut off switch hidden on my car. I measured the distance to each (-) terminal and had custom cables made to reach the switch. I use this switch to disable the car when it's parked too.

Steve

73 911 T MFI Coupe, Aubergine

[80-911SC]

it goes on the negative BECAUSE it does ....

If it works ... don't fix it ... don't ask why an orange is orange or would they taste different if it was called blue .... they are oranges in the end of they day ..

[Cdnone1]

Have a look at this. It works great for me
Inexpensive very easy kill switch
Steve

[HawgRyder]

I disagree with this.
If the charging system is properly grounded, and the ground of the battery is disconnected, and the engine is running, then the alternator is still supporting the ignition.
The end result could be terrible...the engine continues to run...fuel is still being pumped...oil pressure is maintained (and possibly leaking or spraying all over....and the wheels might still be driven.
The only safe system I can think of is to remove the positive in such a way as to kill the ignition.
It would require a 2 section switch....one to the main bus...and the other to the ignition circuit.
Bob

[luftgekuhlten]

The current flows from the negative terminal to the positive, so it follows that you should disconnect the outflowing terminal first to deenergize the system.

[winders]

Most race car cut offs are on the positive side. For example:

Master Battery Cutoff Switch

Look at the wiring diagram.

Scott

[Grady Clay]

Originally Posted by HawgRyder

I disagree with this.
If the charging system is properly grounded, and the ground of the battery is disconnected, and the engine is running, then the alternator is still supporting the ignition.
The end result could be terrible...the engine continues to run...fuel is still being pumped...oil pressure is maintained (and possibly leaking or spraying all over....and the wheels might still be driven.
The only safe system I can think of is to remove the positive in such a way as to kill the ignition.
It would require a 2 section switch....one to the main bus...and the other to the ignition circuit.
Bob

Bob is ‘right on’ in the result.

Our 911 application is somewhat unique.
The voltage regulator needs an attached battery as a reference.
If you read the ‘fine print’ in our Owner’s Manuals, Porsche states that you must not disconnect the battery while the engine is running.
In practical terms, if you disconnect the battery while running, the voltage will rise high enough to cause failures in many electrical devices.
Common are the alternator and the CDI ($$$).

For track applications there is a 2-circuit cut-off switch – one circuit for the battery and the other for the ignition & alternator.
This allows you to safely turn off a running engine and isolate (disconnect) the battery.

A single circuit cut-off switch (safest in the ground side of the battery) does just fine for working on the car and preventing starting – just don’t use it to turn off a running engine or ….

Of course, disconnecting the ground wire does the same thing for no $ and nothing new to fail.


This also brings up an important maintenance point.
You must keep the battery terminals tight and the wires properly connected.
I suspect some (many?) alternator failures may be traced to an intermittent connection at the battery.

Best,
Grady

[Kemo]

Electrons are negatively charged (-) and are pulled from the ground source. These electrons flow to an area that is deficient in electrons to even things out. So if you remove the source of electrons, you have no flow, ie...no juice. At least thats my understanding of it.

with that in mind, my battery cutoff switch is connected at the positive terminal and has a small resistor of sorts to drain off any excess in the event the cable is pulled. I know I shouldnt have done it, but I pulled it one day with the engine idling. I didnt notice any problems afterwards.

[Grady Clay]

Originally Posted by Kemo

I know I shouldnt have done it, but I pulled it one day with the engine idling. I didnt notice any problems afterwards.

You were lucky and the engine was at idle.

Best,
Grady

[schumicat]

there are two different kinds of switches being discussed.

one is commonly used with engine already shut off to prevent the battery draining when not in use or when you are working on electrical system. They are cheap and sold at any auto parts store. Those should be put on negative for the reasons stated (less risk of shorting battery accidentally).

The other kind is a switch used to shut off engine in an emergency (like wreck during a race). Those should be wired per the manufacturer's instructions, and may include something like a diode and additional wiring used to prevent alternator damage.

[KTL]

Definitely have to make sure your switch is wired to stall a running engine, without damaging any electrical components. I don't say this just from an additional cost of repair standpoint, after you've gone off track and balled up your car. I say this from a tech inspection standpoint too. Many racing organizations will require you to demonstrate the switch's proper operation as part of a pre-race or annual tech inspect requirement. Point being is it is a switch that is used more often than in the event of a crash. I use mine all the time to kill the power when working on the electrical system. Which seems to be quite often- tidying up routing, installing new stuff (data logger/lap timer, cool suit, new tach...) or eliminating street car wiring that no longer has a purpose in the dedicated race car.

[cgarr]

Another good idea is an Inertia Switch to cut power mainly to the fuel pump in case of a hard hit. If for some reason you are dazed the switch knocks out the power. Most new cars have them and can find them in any bone yard.

[KTL]

Originally Posted by cgarr

Another good idea is an Inertia Switch to cut power mainly to the fuel pump in case of a hard hit. If for some reason you are dazed the switch knocks out the power. Most new cars have them and can find them in any bone yard.

Yep those are a good safety measure and cheap, even if bought new

http://www.smartracingproducts.com/pdfcat/srp2009_bodyelectrical.pdf

See page 3 for the fuel cutoff and a picture of the 2 circuit battery cutoff switch is on page 4

Here's their suggestion on how to use both

http://www.smartracingproducts.com/pdfdocs/935100_fuelshutoff.pdf

http://www.smartracingproducts.com/pdfdocs/936100_battery_switch.pdf

Not trying to endorse SmartRacing necessarily, or say their way is the only way those switches should be installed. They were simply the first source that came to mind with fancy printed documentation/description information to share.

[RWebb]

Originally Posted by luftgekuhlten

The current flows from the negative terminal to the positive, so it follows that you should disconnect the outflowing terminal first to deenergize the system.

ow!!!

[andyt11]

[QUOTE=schumicat;6042907]there are two different kinds of switches being discussed.

one is commonly used with engine already shut off to prevent the battery draining when not in use or when you are working on electrical system. They are cheap and sold at any auto parts store. Those should be put on negative for the reasons stated (less risk of shorting battery accidentally).

QUOTE]

Thats what I'm up to. When something starts having a melt down I want to be able to cut the power quick. It has happened in the past on a VW.