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Old 01-13-2012, 12:09 PM   #1
Festus2
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Question Battery Cut-Off Switch (pos or neg?)

When installing a battery cut-off switch, which is the preferred way to wire it into the circuit ---- on the positive or the negative side of the battery?
There are, I believe, advantages (and disadvantages) to doing it one way versus the other and that either way is acceptable.
Just what are the pros and cons of doing it on the negative side? and the positive side? I would assume that if you used the positive side, then an inline fuse would also be necessary ????
Comments? Recommendations?
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Old 01-13-2012, 12:24 PM   #2
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I vote positive terminal

When I installed a cutoff switch on my previous travel trailer, I installed it on the negative side. That created a problem. If I forgot to turn ON the cutoff switch before towing...my breakaway switch would not work. (if the trailer separated from the truck while in motion, the trailer brakes would not activate) So I would recommend placing the cutoff switch on the positive side.
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Old 01-13-2012, 12:33 PM   #3
SteveC7010
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Quote:
Originally Posted by Festus2 View Post
When installing a battery cut-off switch, which is the preferred way to wire it into the circuit ---- on the positive or the negative side of the battery?
There are, I believe, advantages (and disadvantages) to doing it one way versus the other and that either way is acceptable.
Just what are the pros and cons of doing it on the negative side? and the positive side? I would assume that if you used the positive side, then an inline fuse would also be necessary ????
Comments? Recommendations?
I've worked with a lot of ambulances and similar vehicles that have battery cut-off switches in them. They were all pretty hefty, 300 amp or so capacity, and none of them were installed on the ground side of the battery, probably to prevent any possibility of an alternate ground making the system hot again. None of the manual switches were fused. The switches that were essentially massive relays had fusing on the control side but none on the main power itself.

If I were installing a cut-off in the Cougar, I would probably find myself one of these bad boys made by Cole-Hersee:

It's designed to handle two batteries and does not require any drilling of holes except the mounting screws. It has 4 positions:
  • All off
  • Battery 1 on
  • Battery 2 on
  • Both Batteries on
There are several different models of this switch: plain jane, pilot light connections, and a couple with alternator isolation circuitry. I'd probably buy the pilot light version just to have the option to add them later on.

They can be wired up with #00 battery cable and feed an entire car or truck with no problems. For an RV like one of ours, the same size wire as currently used will be fine.
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Old 01-13-2012, 07:36 PM   #4
Bob Landry
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ABYC says put them on the positive side. Every new OEM boat I have seen has them installed this way. One reason for doing it that way is that the AC ground and DC ground are usually tied together. If you disconnect battery ground, you could still draw current through the AC ground and it might be through a small wire size wiring harness that would not handle the current.
You do need to wire the breakaway switch directly to the battery so it will work even with the battery switch turned off.
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Old 01-14-2012, 01:17 PM   #5
smiller
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If it's the kind that mounts on the terminal itself then it really doesn't matter. If it mounts elsewhere in the supply circuit and you're not sure of how your grounds are wired then positive is probably best, for the reasons stated.
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Old 01-14-2012, 01:52 PM   #6
CarKath
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I just had a look at how my factory cutoff switch is wired on my Alpine. It is 100% on the positive side.

I'd be glad to post some pictures if you want to see how they have it wired.
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Old 01-14-2012, 02:17 PM   #7
JRTJH
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The top terminal and side terminal battery cutoff switch installation instructions say to install on the negative post of the battery. When I installed my "red key cutoff switch" I mounted it on the forward bulkhead of the front storage compartment and wired it to the negative side of the batteries. My reasoning for doing that is because there is no "spark" during disconnecting or connecting under load. As in automobile applications, if you are working on the postive terminal without disconnecting the neg terminal first, the battery is "live" and should you touch anything metal to the positive terminal and ground, it will arc. I treated several severe burns from people who were working on their cars with a wedding ring on and happened to ground out the wrench against their wedding ring while disconnecting the positive terminal first. One resulted in finger amputation due to severe burns. As Bob said, there is a possibility that the AC and DC ground circuits interconnect which could put a load on a small DC wire, but I don't think there's much potential for that in a "properly wired" RV. From a reliability vantage, installing in the Neg side of the cutoff switch will put less potential for contact arc and prolong the switch life. However, if you tow with the batteries turned off, putting the cutoff on the positive terminal and wiring the breakaway switch to the pos terminal "BEFORE" the cutoff switch will allow power to the breakaway circuit without regard to the cutoff switch position. Another possible solution to the breakaway switch is simply to wire it to the pos and neg terminals of the battery (fused wire) that way, no matter what position the cutoff switch is in, the breakaway circuit is functional.
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