Re: Kill Switch

Bayley wrote:

Lately I'm seeing a lot of teams use solid state contactors that are used in Hybrid / EV cars for their high voltage switches. With these setups, the main contactor can be located immediately next to the battery and be shut off with a simple toggle switch and low current wires. While this is the safest approach, you really can't leave the battery "on" for more than an hour or two as the contactor is always drawing current to keep itself closed.

Aircraft do this and it works fine. For service you can jumper or some have a little pin slot to hold the contractor closed (in the mechanical style)

Issue is, That's still a live circuit to the switch to actuate the contactor, That's NOT allowed.

Mistake By The Lake Racing (MBTL)
88 Thunderbird "THUNDERBIRDS ARE GO!", Ex Astris, Rubigo / Semper Fracti
A&D: 2014 Sebrings at Sebring (NSF), 2014 NJMP2 Jurassic Park (SpeedyCop), 2012 Summit Point J30 (PiNuts)
2018 Route Sucky-Suck Rally Miata, 2019 World Tour Of Texas 64 Newport

27 (edited by Guildenstern 2020-06-12 02:25 PM)

Re: Kill Switch

RSB wrote:
cheseroo wrote:

I too don't get the switched negative is much safer approach.  I don't know of a car that only has one ground conductor.  Well, my British cars tend to have few grounds because they go bad a lot but I digress.  Let's say I crash hard enough to have the car body crush into the fuel pump body or negative terminal and ground it while it's still getting 12v even though my negative ground kill switch is off.  I'd consider that to be bad.

If  you break the positive (somewhere other than right next to the battery), then you will have a length of wire that is always hot with 12V. In our car, the battery is in the trunk and  the kill switch is next to the gear lever (we use a remote pull to pass the rule about the kill switch being accessible from outside the car). So, the length of wire from the trunk to the gear lever is ALWAYS HOT. There is no way to remove voltage from that wire. In the event of a crash that pinches that wire against the chassis, there will be a dead short across the battery that can't be removed. That's a fire hazard. We insulate the hell out of that 'feeder' wire from the battery to the kill switch, I have it encased in a length of heater hose that adds a ton of  insulation/dielectric to it so its less likely to get shorted to chassis during a crash or other stupid things that we might do.

If you break the negative, then the entire length of positive wire is now dead (relative to the chassis), and it doesn't matter if its pinched against the chassis, there is no current loop and there is no short circuit.

Obvious solution, Explosive bolts for both terminals.
No way that could go wrong.

Half a joke because my street car has a Pyrotechnic positive disconnect for its trunk mounted battery specifically so that there isn't a run of hot 12V going the length of the car.

Mistake By The Lake Racing (MBTL)
88 Thunderbird "THUNDERBIRDS ARE GO!", Ex Astris, Rubigo / Semper Fracti
A&D: 2014 Sebrings at Sebring (NSF), 2014 NJMP2 Jurassic Park (SpeedyCop), 2012 Summit Point J30 (PiNuts)
2018 Route Sucky-Suck Rally Miata, 2019 World Tour Of Texas 64 Newport