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Old 12-11-2014, 13:13   #1
RDW
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Low voltage reading on turned off circuit.

I was investigating why a water pump was not running. I got a reading of 5.5. volts. As it turned out the battery switch was turned off. That was the reason the water pump was not working.
My question is where did the 5.5. volts come from. The only thing I can think of is possible induction.
What is the most like cause?
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Old 12-11-2014, 14:11   #2
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Re: Low voltage reading on turned off circuit.

If the battery is turned off where could the "induction" be coming from?

First suspect for me is the battery switch. I would disconnect the battery and check again.

Maybe someone around you has a very bad electrical fault and you are on the receiving end = not good at 5v.
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Old 12-11-2014, 14:53   #3
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Re: Low voltage reading on turned off circuit.

My thought was that there were other wires with 12 volts or more running beside the wires from the battery cut off switch. My first thought is the wire running from the battery to the cut off switch which could be bundled tightly together.
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Old 12-11-2014, 17:19   #4
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Re: Low voltage reading on turned off circuit.

I'm confused.
If the battery is off, no wires are carrying any current and thus there can be no induction.
Maybe the bilge pump is directly wired and is running. ???
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Old 12-11-2014, 17:57   #5
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Re: Low voltage reading on turned off circuit.

There will be no induction in a DC circuit. Forget that theory.

I'm assuming the pump was not turned on when you took the voltage reading. With a modern digital volt meter, the internal resistance of the meter is high enough that the circuit is not loaded by the meter and leakage through switches can give false readings. Turn the pump on and I suspect the reading (with the battery switch off) will drop to zero.
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Old 12-11-2014, 18:04   #6
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Re: Low voltage reading on turned off circuit.

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There will be no induction in a DC circuit. Forget that theory.
Maybe only if he Tasmanian Devil helped with the wiring?
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Old 12-11-2014, 18:07   #7
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Re: Low voltage reading on turned off circuit.

Before anyone feels the need to post it, there can be an inductive pulse when the circuit is turned on and another when it is turned off. A technicality and this does not change anything as far as this circuit is concerned.
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Old 12-11-2014, 18:38   #8
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Re: Low voltage reading on turned off circuit.

You get "induction" from AC or pulsed DC currents in adjacent wire runs. You don't have induction.

You may have a ground loop, where different devices use different paths to ground, resulting in a random voltage that is not at true ground, like 5V on a 12V system. Or simply a dirty or thin connection between two circuits, like some stray strands of wire from one breaker touching another breaker.

Hook up the voltmeter to the pump circuit, then go to the breaker panel and kill EVERYTHING. Now start turning things on until the 5V comes back. That circuit, and your water pump circuit, are probably crossed someplace, with a thin or dirty or plain incorrect connection between them.
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Old 12-11-2014, 18:40   #9
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Re: Low voltage reading on turned off circuit.

Are you sure your meter was turned to the right setting? I can only see this happening if youo had the meter set to AC voltage.
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Old 13-11-2014, 08:58   #10
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Re: Low voltage reading on turned off circuit.

Quote:
Originally Posted by RDW View Post
I was investigating why a water pump was not running. I got a reading of 5.5. volts. As it turned out the battery switch was turned off. That was the reason the water pump was not working.
My question is where did the 5.5. volts come from. The only thing I can think of is possible induction.
What is the most like cause?
RDW
Digital Volt Meters change scales automatically. This has fooled me more than once. Are you sure you were not reading 5.5 mv (milivolts)?
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Old 13-11-2014, 09:47   #11
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Re: Low voltage reading on turned off circuit.

It's common for a few circuits to be connected to the battery side of the disconnect switch - bilge pumps, VHF radio, stereo yellow, and propane sniffer are a few examples. I'd disconnect the battery completely and take more readings to verify that the voltage disappears. Then use hello sailor's guide from there.
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Old 13-11-2014, 11:33   #12
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Re: Low voltage reading on turned off circuit.

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Originally Posted by Rusty123 View Post
It's common for a few circuits to be connected to the battery side of the disconnect switch - bilge pumps, VHF radio, stereo yellow, and propane sniffer are a few examples. I'd disconnect the battery completely and take more readings to verify that the voltage disappears. Then use hello sailor's guide from there.
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Old 13-11-2014, 12:38   #13
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Re: Low voltage reading on turned off circuit.

It's DC. There won't be any induction. My guess is someone has it in series with something else and your reading is coming from the neg. side. Disconnect it from the breaker and see if you still get it, just to eliminate a faulty breaker. See what is on it's neg. lead?
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Old 13-11-2014, 12:45   #14
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Re: Low voltage reading on turned off circuit.

Good points by rwidman, gbohner, siboneysail and hellosailor. I would not totally discount induction, though. Depending on the innards of the pump or what it is connected to, there could be some rectification occurring. The auto-ranging meter and being set to AC are possibilities. Could try putting a test light (having lower impedance than the meter) in place of or in parallel with the meter. I would add the possibility of a DC voltage being produced by galvanic action from a bad connection anywhere between where the meter leads are being placed.
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Old 13-11-2014, 14:19   #15
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Re: Low voltage reading on turned off circuit.

On second thought. Not that someone intentionally tying it in series with something else. A bit of corrosion under two neg. lugs put on the same terminal on a buss would give you exactly what you have.

Forget that induction junk. If I'm in left field let me know.
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