Thread: Master power switch. Electrical GURUS needed

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Originally Posted by AzDon

If there is a switch between the negative battery post and ground and it is turned off there is not a live positive wire anywhere on the vehicle that will arc if you touch it to metal because negative is disconnected from ground and therefore, ground metal is dead and cannot be used to complete a circuit.

Exactly! The first thing any manual tells you before starting most work on a vehicle is. remove the ground (negative) cable from the battery. It makes sense to me that my switch wired this way, will function as I desire it to.

Now my engine is EFI, has a computer, and electric fuel pump, so I'm thinking that in an emergency situation, IE; smoke, fire, sparks, or accident, I should turn off the ignition switch, then turn off the battery switch. Engine is off, Fuel pump is off, any shorts will stop immediately.. makes sense to me. Oh and perhaps I should add grab fire extinguisher..lol

Denny, now see what you started...

Originally Posted by Chev malibu

Exactly! The first thing any manual tells you before starting most work on a vehicle is. remove the ground (negative) cable from the battery. It makes sense to me that my switch wired this way, will function as I desire it to.
That's because of the potential spark that could ignite battery fumes if you remove the positive cable first.


Now my engine is EFI, has a computer, and electric fuel pump, so I'm thinking that in an emergency situation, IE; smoke, fire, sparks, or accident, I should turn off the ignition switch, then turn off the battery switch. Engine is off, Fuel pump is off, any shorts will stop immediately.. makes sense to me. Oh and perhaps I should add grab fire extinguisher..lol
Not necessarily so, any capacitance in the circuit could fuel the fire until it is totally discharged.
Denny, now see what you started...

And the NHRA rulebook states that a battery disconnect will interrupt the positive side per techinspector1

Originally Posted by DennyW

Just look at all the fun we just had though.

It WAS a "lively" ... discussion! Wasn't it?

It has always been an unwritten rule when working around batteries to disconnect the negative FIRST! The reason for that is, should your wrench touch the body, you won't have a spark when you touch the grounded metal and possibly cause the battery gases to explode.
(when I was in vocational high our teachers would insist that we place a folded shop towel over the positive terminal to further prevent any possible connection of positive & negative! It's a practice I still use!)

I don't believe it has anything to do with your switch manufacturer intending anyone to insert the switch into that side of the circuit.

Everyone is correct tho' in the most common and some sanctioning bodies rules is to require the Positive side to be interupted with the switch.

Originally Posted by DennyW

I think...the rules apply due to the rear mount location of the battery in the rules. That would be the closest point to the trunk mounted battery. How ever, VW, and a few other vehicles had under the seat mounted batteries...
Also in those rules for racing, they say you must also have a manual ignition off switch too, besides the disconnect switch. The disconnect switch in that case is for the track members to be able to disconnect power in case something goes way wrong... That's in case any Hot wires are pinched, causing a short, and spark if fuel is leaking.

Denny is right on the money here. As far as the old Beetles, that's the one exception that I can think of where we have to allow a battery in the driver's compartment without it being in a sealed steel, aluminum or approved poly box and vented to atmosphere if the car is stock. Once the electrical system is modified or the stock rear seat bottom cushion is removed for drag racing, it falls under the sealed box rule. Technically, with the battery still in the stock location under the rear seat cushion even though the cushion isn't there any more, a sealed box will suffice and no master cutoff is necessary on the rear of the car until the car reaches 9.99 E.T. or 135 MPH in the quarter mile.

Originally Posted by techinspector1

Technically, with the battery still in the stock location under the rear seat cushion even though the cushion isn't there any more, a sealed box will suffice and no master cutoff is necessary on the rear of the car until the car reaches 9.99 E.T. or 135 MPH in the quarter mile.

As to no master cutoff necessary, does this only apply to the VW, as to the ET/mph, or any car that runs slower then that? I'm sure there is some class, maybe street class (?)that runs without external cut off switches. Least I have never noticed them on the cars. They race, then drive home in them..If I ever take the Cobra to a strip, I hope I'd be in that class, and still drive it home after..

So would the manual "ignition off switch" that Denny mentions include the keyed ignition switch, after all that's manual?