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Old 16-04-2009, 20:00   #1
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DC Circuitry Question

I notice that several switched circuits on my boat (some lights, macerator, etc.) have the switch on the negative side of the load. On the surface, this would appear to increase the possibility of a short. Is there any advantage to having a switch on the negative side of the load?

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Old 16-04-2009, 20:23   #2
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I think it is 6 one way half a dozen the other. If the boat is negative ground it would be more open to shorts as in bypassing the off position of the switch but less exposed to the boat incinerating kind of short. I would suspect the switch has been put on the negative side most probably because either the positive side for the load(s) comes off a common bus supply or it was simply more convenient for the installer. Regardless of the side of the circuit the switch is installed on, it will always have -ve on one pole and +ve on the other when the switch is in the off position - only the polarity side the load is on will change.

More importantly, the fuses / breakers should all be located on the same side of the circuit (polarity) throughout the boat otherwise bad things could indeed happen if one circuit were to short to another.

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Old 17-04-2009, 02:55   #3
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Your circuits should all be above ground, ie nothing earthed to the boat. What often happens in reality is that the engine and hence prop shaft and prop are connected to the negative. That means that if any of your devices are leaking to earth you may get a current path causing electrolysis. To be on the safe isde, I would switch everything in the positive lead ( Its better to have a coherent system, anyway). We also isolate the engine when it is not running, and switch off both sides of the circuit when we are not on the boat.(Important for a metal boat) Regards, Richard.
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Old 17-04-2009, 03:28   #4
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My boat came with a key type switch to disconnect the neg buss and all returns not connected directly to the battery.

I haven't seen this on other boats nor heard mention of this, what is the main use etc.

Any thoughts?
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Old 17-04-2009, 04:50   #5
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If single pole switches are used in DC branch circuits, they shall be installed only in the positive conductor of the circuit.
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Old 17-04-2009, 08:47   #6
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I concur with Gord. Having the switch in the negative leg means that the equipment is always powered; i.e., is kept at B+ potential. Any breakdown in insulation in the equipment could cause unwanted current flow.
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Old 17-04-2009, 09:36   #7
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I agree with Gord & CharlieJ. I think there's good reasons to maintaining standards for all the various systems on boats, Safety is a big one. Simplification as changes are made in the boat its easier to know that not only are the positive sides are switched but the color of wire on the positive side always the same.
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Old 17-04-2009, 09:43   #8
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Another Reason

Another reason to never switch the negative (open) on vessel wiring is that it is more difficult to troubleshoot a problem when something does not work. One takes a DVM with one lead on negative and begins tracing the source of power back towards the load looking for an open. With the negative switched off power seems to appear everywhere.

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