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post #10 of Old 05-30-2011
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Originally Posted by SEMIJim View Post
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

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..

A bad day on a boat beats a good day in the office

Last edited by Classic30; 05-30-2011 at 08:36 PM.
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