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Old 10-22-2016, 06:19 AM   #1
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Voltage Mystery

The AC volt meter on our electrical panel shows 50 - 55 volts both on shore power and when the generator is running. Measuring voltage with a meter at AC outlets shows proper voltage and all AC equipment functions properly.

Split buss, ships service and AC. There are two ammeters tthat appear to be switchable with a toggle switch and only one works.

I'm finally getting down the list of more critical items (boat became fully operational again just yesterday) and will be looking at the panel soon.

Any suggestions about probable cause appreciated.
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Old 10-22-2016, 06:54 AM   #2
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Why don't you measure the voltage at the panel volt meter with your portable voltmeter and see if they agree?
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Old 10-22-2016, 07:49 AM   #3
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From a fire safety standpoint getting this sorted out should be a priority. Suggest getting A good marine electrician on board to check things out.
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Old 10-22-2016, 08:10 AM   #4
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Well if the outlets are measuring 110 or so and the meter is showing 55, then the meter is bad (unlikely) or the connections to the meter are bad. Measure the voltage at the meter and if low, follow it back to the source until it is good. The bad connection is somewhere in between.


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Old 10-22-2016, 08:24 AM   #5
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The mystery is that it is reading exactly half voltage. That seems an unlikely coincidence. If it was a bad connection, I would expect something other than exactly half. Strange that it is reading this way both on shorepower and generator.
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Old 10-22-2016, 08:43 AM   #6
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Begin by figuring out whether the meter is right -- hook up a voltmeter to the same points being read by your panel meter. If the panel meter is off, it isn't necessarily bad, but may only require calibration. For that purpose, you will find a small screw at the base of the needle. I doubt that your voltage is actually 55. That isn't easy to produce, without a transformer, even if you were trying, so unlikely to be the result of wiring gone bad, etc.
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Old 10-22-2016, 08:45 AM   #7
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I had that on one outlet...found that the AC and DC wires were taped together feeding the refrigerator....noticed getting shocks from the refrigerator frame.

Had to completely isolate that AC circuit to keep from getting shocks.

From reading, sounded like a short in the neutral into the DC wiring.....giving half voltage at the outlet and leaking into the refrigerator frame and cold plate.

Not sure if one of those cheap outlet testers will tell you anything.
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Old 10-22-2016, 09:24 AM   #8
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............... If the panel meter is off, it isn't necessarily bad, but may only require calibration. For that purpose, you will find a small screw at the base of the needle. ...........
No, no, no! That screw is to set the needle at "0" with no voltage applied.

Until the voltage is measured (where this meter is connected) with a known accurate meter, anything else is nothing but guessing. Measuring the voltage with a known accurate meter is the first logical troubleshooting step.
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Old 10-22-2016, 09:31 AM   #9
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Measure the voltage coming into and out of the meter. If the voltage is the same and at the correct levels, get a new meter.
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Old 10-22-2016, 11:24 AM   #10
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Check for an open neutral,you are getting a feedback on the ground
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Old 10-22-2016, 11:33 AM   #11
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The RMS value of a half wave AC 120V waveform is right about 55 V. IIRC AC voltmeters rectify the waveform then read the RMS value. If half the rectifier is open circuit the meter would read the RMS value of a half wave or 54V.


So without referencing some math I suspect the meter internals. Check the voltage at the meter connections and I suspect that they will be 120VAC. meaning a new meter is needed.

Of course the meter could be connected to the wrong side of a diode somewhere.

Now we wait for the answer []
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Old 10-22-2016, 12:52 PM   #12
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No, no, no! That screw is to set the needle at "0" with no voltage applied.

Until the voltage is measured (where this meter is connected) with a known accurate meter, anything else is nothing but guessing. Measuring the voltage with a known accurate meter is the first logical troubleshooting step.
Agree. That screw is the meter zero adjustment only
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Old 10-22-2016, 02:47 PM   #13
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And the screw should only change needle position slightly.

Lots of crappy meters on boats. Even more of a PITA is ammeters. This voltmeter can be sorted with a simple volt check at it's terminals.
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Old 10-23-2016, 04:56 AM   #14
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The RMS value of a half wave AC 120V waveform is right about 55 V. IIRC AC voltmeters rectify the waveform then read the RMS value. If half the rectifier is open circuit the meter would read the RMS value of a half wave or 54V.
That's the answer I was looking for! I checked yesterday and 54 volts is exactly what it was reading, at least as close as I can tell on the small scale. It's good to know there is an internal meter issue that could cause thisl

Getting to the meter will require a lot of disassembly. There are no other indications of AC problems and everything functions correctly, even the small Inverter I use to run my laptop. Those are notoriously sensitive to AC issues.

It will require some disassembly around the panel to get at the meter on this boat. The marina closes shortly and getting her from the Hudson to the Chesapeake is my first priority.
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Old 10-23-2016, 08:35 AM   #15
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Like Click and Clack would do: put some tape over it. You can always check for 110v at an outlet. Get to the Bay, and replace it later.

How often do you check the voltage in your house? (I actually have had problems in my house that had me going around with a meter; the problem turned out to be a bad splice in my service entry.)
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Old 10-23-2016, 03:20 PM   #16
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The volt meter might be for 220 volts which is common in other parts of the world. Many foreign boats are wired for 220 volts 3 wire so since its reading 50%. Take a volt meter and read at the terminals of the meter.

220 volts ac and 24 volts dc are commen in many parts of the world.
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Old 10-23-2016, 03:58 PM   #17
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OOC, how long has this been happening? What was the last thing you did/changed/installed/ uninstalled concerning the 110v system?

Other than bypassing the built in meter, to remove it or its wiring being suspect, I would look for last 'messed with' connections being suspect.
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Old 10-24-2016, 10:42 AM   #18
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The volt meter might be for 220 volts which is common in other parts of the world. .
Seriously?

"Volts" are the same no matter where you come from or where the boat was built. If you apply a certain voltage to a meter, the meter should read that voltage no matter what.

50 volts will read 50 volts on any volt meter (provided it is rated at or above 50 volts).
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Old 10-24-2016, 10:45 AM   #19
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No, no, no! That screw is to set the needle at "0" with no voltage applied.
Looks like I learned something, but please explain the thinking behind this. If my meter is reading, say 117 volts, and if I know (by measuring with a more reliable voltmeter) that the voltage is actually 120, if I use the screw to adjust the meter to 120, then the meter will be measuring correctly. If, later, the meter measures a different voltage, that would indicate that my voltage had changed. What am I missing? (I really want to know since I paid an electrician whose solution was to adjust my needles in this way.)
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Old 10-24-2016, 10:47 AM   #20
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Seriously?

"Volts" are the same no matter where you come from or where the boat was built. If you apply a certain voltage to a meter, the meter should read that voltage no matter what.

50 volts will read 50 volts on any volt meter (provided it is rated at or above 50 volts).
I agree with your point, but with AC voltage, there is a difference between RMS volts and peak volts -- in that case, volts are not volts, and RMS isn't as easy to measure as peak / instantaneous / DC volts.
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