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-   -   Battery cut-off switch (http://www.sailnet.com/forums/electrical-systems/74844-battery-cut-off-switch.html)

SothernDragon 05-28-2011 11:34 PM

Battery cut-off switch
 
I've bought a used boat that had for 20 years the cut-off batteries switch isolating the ground (negative) batt cables.
Shouldn´t be on the positive cables ?
What´s the problem if I leave it as is ? (If worked 20 years.....:) )
Thanks in advance for your replies.

mitiempo 05-29-2011 12:32 AM

Normally the switch is in the positive wire as you thought. I would change it, even though it has worked for a long time. But that is just my opinion.

Maybe someone like Maine Sail will add theirs.

captiantim68 05-29-2011 01:24 AM

you get less of an arc when you disconnect the negative first, basic electricity.

Is it a gas boat?

SEMIJim 05-29-2011 08:37 AM

Quote:

Originally Posted by captiantim68 (Post 735245)
you get less of an arc when you disconnect the negative first, basic electricity.

Which bit of basic electricity would that be?

The "disconnect the negative first" rule applies to removing the cables. (Primarily in a car.) If you're loosening the positive cable clamp with a wrench and the wrench hits some other metal object in the engine compartment, it could be welded in place and the resulting high current flow can well result in serious damage. Thus: Disconnect negative first.

Otherwise: Flowing electricity doesn't know the difference: In each case: One side of the connection has positive potential in relation to the other.

To the OP: Without giving it a lot of thought, and w/o having actually seen your wiring, I don't know as it makes any difference. I'd switch it, tho, purely because it's non-standard and unexpected.

Jim

peikenberry 05-29-2011 10:28 PM

40 or more years ago it was quite common to find boats (and cars) with positive grounds. In other words everything was reversed. But today polarity is a very important issue. In other words maintaining positive and negative are hooked up correctly. So, now all boats are negative ground and switches and fuses are in the positive wire. I would switch it, but before I did, I would make sure polarity is maintained throughout the system. In other words positive is hooked to positive throughout.

to learn about DC systems see DC Electricity New Boatbuilders Home Page - Everything Boat Building - Basic Electricity DC Page 1

Classic30 05-30-2011 01:11 AM

Quote:

Originally Posted by peikenberry (Post 735433)
40 or more years ago it was quite common to find boats (and cars) with positive grounds. In other words everything was reversed. But today polarity is a very important issue. In other words maintaining positive and negative are hooked up correctly. So, now all boats are negative ground and switches and fuses are in the positive wire. I would switch it, but before I did, I would make sure polarity is maintained throughout the system. In other words positive is hooked to positive throughout.

FWIW, the main reason cars and boats were once wired for "positive grounding" (really "negative switching") and about its only advantage was that, wired in this way, it was possible to use a cute little 12V test-bulb to test for continuity at all points in the circuit up to the point of the final switch, making fault-finding relatively easy in the days before cheap Chinese-made multimeters.

Nowadays, the disadvantages outweight the advantages. If it were me, I'd change it.

SEMIJim 05-30-2011 07:44 AM

Quote:

Originally Posted by Hartley18 (Post 735468)
FWIW, the main reason cars and boats were once wired for "positive grounding" (really "negative switching") and about its only advantage was that, wired in this way, it was possible to use a cute little 12V test-bulb to test for continuity at all points in the circuit up to the point of the final switch, making fault-finding relatively easy ...

I've only been out of bed for about (looks...) 15-20 minutes, and haven't had any coffee, yet, so maybe I'm missing something Really Obvious: But please explain why positive grounding vs. negative grounding (or "negative switched" vs. "positive switched," as you'd have it) is any easier to troubleshoot with a simple incandescent test lamp :confused:

Jim

mitiempo 05-30-2011 09:43 AM

I don't understand the troubleshooting being easier either.

btrayfors 05-30-2011 06:36 PM

He's talking about things Down Under. They work differently down there:

Toilets flush the wrong way, rivers run North, you can't see the North Star at night, shrimp is cooked "on the Barbie" (instead of Barbie cooking the shrimp as she does up our way).

And, apparently, electricity acts differently, too.

Give the guy a break :-)

Bill

Classic30 05-30-2011 08:09 PM

:p :p :)

Quote:

Originally Posted by SEMIJim (Post 735485)
I've only been out of bed for about (looks...) 15-20 minutes, and haven't had any coffee, yet, so maybe I'm missing something Really Obvious: But please explain why positive grounding vs. negative grounding (or "negative switched" vs. "positive switched," as you'd have it) is any easier to troubleshoot with a simple incandescent test lamp :confused:

Jim

Ok. Assume you are living in an age when multimeters aren't for your average layman. All you have in your test kit is:
(a) a piece of wire with bare ends.
(b) a 12VDC light bulb.

Let us take a mast light for example: It isn't working. Okay. If the switch is in the negative lead, with the switch 'off' you should have +12VDC all the way from the battery, through the light and back to the switch. Test procedure is then:

1. Connect test bulb between +ve & -ve at panel: If it lights up, we have volts.
2. Connect test bulb across switch contacts: If it lights up, the mast bulb and all wiring is good.
3. Use piece of wire across switch contacts: If mast light lights up, switch is faulty.
4. At the top of the mast, connecting your test bulb from either side of the mast light to a known ground (eg. the stays), completing the circuit the test bulb should light. If it only lights one side, the mast bulb is blown; if neither side, the feed wire is open circuit.

With the "positive switched" system we have now, there is no +12VDC available at any point in the system downstream of the switch, so the only way to test the circuit for continuity is to use a multimeter set to "ohms" (effectively a battery and an ampmeter in a box) - or hook a test bulb up to +12VDC and have it light up on everything it touches, working or not... but there is no doubt that this system is safer.

Does that make sense now?? ;)


EDIT: The above assumes the system is still negative grounded. Perhaps the confusion (I'm confusing myself here!!) is coming from the idea of changing from negative to positive grounding. IIRC, positive grounding was used to minimise arcing - not to assist with fault-finding.

My bad..


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