Battery cut off switch

[ib516]

I just bought one like the one in the picture below. Should I install it on the positive terminal or the negative and why?



I run two 12V group 27s in parallel.

[talonhead]

Where I work we have a number of diesel gensets. 100KW up to 1MW, they are rotated around as needed as emergency units. They all have (with the exception of the 1MW units) a welding cable quick disconnect on the NEGATIVE cable. The 1MW units have a rotary switch that cuts out all the electronics except the maintenance charger.
If you read how to disconnect/replace a battery or jump start a vehicle.. Connect POS first, NEG last. Disconnect NEG first, POS last. As was stated earlier, this is to prevent sparks. If you are concerned about forgetting to connect the battery before towing, hang a flag on the breakaway cable that says "CONNECT BATTERY BEFORE CONNECTING BREAKAWAY CABLE!" Personally, I test the breakaway before moving the rig. Just my .02.
MikeH

[DryCamper11]

Electrically, it doesn't matter where you put the switch - pos side or neg side. I once considered putting the switch between my 6 volt batteries because the mounting and placement would have made it much easier.

Safety wise, there is a great deal of sense in putting it on the negative, as it is desirable to minimize the amount of exposed metal connected to the pos side. If the installation is done properly I prefer the pos side, as that is the place most people expect it, but if I was selling a cheap switch like this, and I knew many customers would do a poor installation job, and I knew that many of them would not properly prevent inadvertent shorting out by building an insulating shield, I'd probably recommend installation in the negative side.

[Bob_Landry]

If the switch is going to be mounted outside, any marine grade component will do well. The cheapies from WalMart or Harbor Freight may be "iffy" as far as durability and resistance to the elements, but a lot of people claim to have good luck with them. I personally believe in buy quality, buy once. YMMV.

[Learjet]

My blue sea switch has worked well for several years.

[westend]

69 Avion wrote:

westend wrote:
My vote goes to Cole-Hersee M-750 because it's serviceable and has an O-ring to seal case to back plate.

It looks like a high quality switch. That is the style that I like.

Yup, it is a good piece of gear. I have three of them to distribute 12v in my rig. I have them interrupting power from two banks of batteries to the solar system, the inverter, and the converter.

[gotsmart]

I use a Hella marine battery switch on the negative terminal:

[64thunderbolt]

My breakaway & landing gear is dirct to batt. My switch does not interupt those 2 items. I have had them in both sides and can't say which is better

[69_Avion]

westend wrote:
My vote goes to Cole-Hersee M-750 because it's serviceable and has an O-ring to seal case to back plate.

It looks like a high quality switch. That is the style that I like.

[westend]

My vote goes to Cole-Hersee M-750 because it's serviceable and has an O-ring to seal case to back plate.

[mena661]

GordonThree wrote:

Thats_Ok wrote:

69 Avion wrote:
As stated above, it is a sorry excuse for a cut off switch. Get a Blue Sea switch and be done with it.
BlueSea

I agree.
Put a BlueSea m series in our fiver.
Money well spent IMO.
Ordered it from Amazon.

x3 for BlueSea ... sure it costs a bit more, but well worth it.

X4, add me in there too.

[time2roll]

ib516 wrote:
From that video, negative terminal it is, and they explain why. Thanks.

I watched the video also. The only reason said for installing on the negative is to reduce the potential short to the frame if installed on the positive. If your positive is enclosed or covered I agree but if already exposed I don't think you are adding any risk.
JMHO

[MEXICOWANDERER]

Oooooo I made lots of money repairing positive side switches that loosened the wires and managed to have the feeder wire touch a nasty negative point.

But of course Fleetwood and Bayliner have engineering teams a hell of a lot more intelligent than Caterpillar, Terex, Westinghouse, and Lockheed. Some of their PhD's can even espeak a little Eengles.

An acceptable quality cutoff switch mounted in an acceptable location can indeed safely handle switching battery positive. Wanna spend a few dozen hours chasing links I provide to see all those wonderful Blazing Dragon battery switches that can be broken with the palm of the hand? Recommending that positive be switched needs a couple of caveats to warn the unwary. I've got a lot of King Crabbers and Midwater Trawlers, never mind salmon boats floating around that are still using Cole Hersee switches installed 30 years ago. 32 volts shorted to the hull using a 4/0 cable and 8 8-volt batteries is not a pretty sight.

[Bob_Landry]

If it was a good practice to put a switch in the negative side, OEM RV manufacturers and boat builders would also put the fuse and breaker panels in the neg side. They do not. In 15 years of marine work, I've never seen anything in the negative side of any power system.

The argument about the breakaway switch not working is a valid one. Also, all appliances and devices, both 12VDC and 115VAC share a common ground. I said ground, not neutral. That would make it possible to pick up a ground from anywhere in the trailer. While you would not get a large current surge as the neg side would be disconnected, you would still have a difference of voltage potential on everything in the trailer that is fed by the 12V. That is all that is needed to draw a small arc. That arc could be strong enough to damage electrical devices, particularly those controlled by solid state devices. By breaking the hot side, you are assured that everything in the trailer is dead. Like I said, if breaking the ground was a good practice, the OEM builders would do it.

[GordonThree]

Thats_Ok wrote:

69 Avion wrote:
As stated above, it is a sorry excuse for a cut off switch. Get a Blue Sea switch and be done with it.
BlueSea

I agree.
Put a BlueSea m series in our fiver.
Money well spent IMO.
Ordered it from Amazon.

x3 for BlueSea ... sure it costs a bit more, but well worth it.