Puzzling Electric Issues

[Homer Shannon]

This fall I took on a project to replace my boat’s old switch and fuse electrical pane with a Bass circuit breaker panel. At the same time I cleaned up some the sloppy wiring that had been added for new electrical appliances over the years and made the electrical controls more organized and tidy. The boat is a simple 12v DC system. There is no AC shore power involved.

After completing the work I tested all of the circuits to confirm that I had rebuilt everything correctly. All of the circuits do seem to be correct, there are no shorts and all seems to be right. However, when testing I ran into two conditions that are puzzling me.

The first issue resulted from the fact that my mast loads, anchor light, steaming light and spreader light are not presently connected to the system – the mast is off the boat. I tested these circuits using a multi-meter and checked for voltage on each of the three circuits. Note that there is a common ground for all of the circuits. Although I had the lights wired to their correct circuit breaker, I found that whenever I had voltage on the powered circuit, I also had about 0.30v on each of the other two circuits – but not exactly the same amount. I tested across all three wires and had the same conditions regardless of which circuit was powered. The wires for these three circuits, and the ground, are all #12 wire and in good condition. When all three circuits are off, there is no voltage on any of the three wires. The wires are in a bundle but I cannot tell how far they are actually wrapped together and I don’t know that there are not splices in the wire run, which is about 14’, but I doubt it. Using the ohm meter in the multi-meter, I tested across the wires to see if they were passing any current between them – there was none that I could measure. Can anyone explain where the 0.30v is coming from on the off circuits?

I discovered the second issue while I was puzzling the first issue. I decided to check and see if there was any stray voltage on other circuits. Most of the other circuits would be difficult to test but the circuit I set up for the VHF radio and AIS receiver has an easy access to a hot and cold lead. I tested across the two leads and the multi-meter showed 0.00v; the circuit had been shut off for quite a while. Turning on the circuit then showed 12.6v, the voltage of the battery. However, when I turned off the circuit, the voltage quickly, but not instantly, dropped to about 0.90v then continued dropping slowly to about 0.40v, at which point it continued to drop but at a very slow rate, leaving a fraction of a volt on the wire for quite a long time. Turning other circuits on and off while testing did not change this behavior. Is this the normal behavior of a circuit breaker? My experience is mostly with plain old switches, which would not do this. Is there something in the behavior of circuit breakers that I need to learn?

Although it appears that I have rebuilt the system correctly, I am somewhat troubled by these two conditions. Can anyone suggest causes or explanations for them?

[Three Sisters]

If you are measuring current flow between two conductors using an ohm meter you may have blown the fuse in the meter. The ohms resistance between two conductors can give you an idea if they are shorted across each other, with the power switched off to the conductors. That all depends if I am comprehending your explanation properly.

[Colin A]

Was the VHF connected when you ran the test #2?

The breaker should open the circuit completely.

[deblen]

Small residual voltage on VHF & other electronic eqpt could be caused by the residual charge left in the electronic equipent's built in power line filter ckt. This ckt contains capacitors & it takes a little time for the caps to drain to zero volts.
You can verify this by applying a brief short across the + & - eqpt power wires. Voltage should now read zero.

Possibly-something similar is going on with your mast lighting wires. Difficult to tell if you have a problem without the mast light load being connected yet.
Again,with breaker off,short each light wire in turn to the common grd & see if the volts on each light wire go to zero.

Difficult to troubleshoot long distance. Be careful using ohm function & never on a live ckt.

Also,it is necessary to tell us what 2 points you are measuring between when stating voltages.

/ Len

[Paul Elliott]

Issue 1: You are measuring the voltage on a disconnected wire. Even a tiny, trivial, inconsequential amount of leakage across the breaker will cause a high-impedance DVM (Digital Volt Meter) to measure a non-zero voltage. I predict that if you connected a load from the disconnected wire to ground, you would then measure zero volts. If so, this is not a problem.

Issue 2: As Colin and Deblen suggested, with a radio or other electronics attached to the power wiring you may see a residual voltage when the breaker is opened. There are capacitors in the equipment that take a while to discharge. Also not a problem.

[SoonerSailor]

Try to find a multimeter with a low impedance, or "Low Z" input option. A very good multimeter like this is the Fluke 117, but there are plenty of others. This will mitigate the presence of deceptive stray voltages that a high impedance input voltmeter picks up, as Paul Elliott mentions.

Chip

[OzePete]

With the switch on, try removing one wire from your new switch then check to see if the low voltage disappears totally. If it does you don't really have a problem, your new switches may be a type that contains a device that suppresses arcing.
Cheers,
OzePete ozefridge

[hoolie]

You're worrying about nothing. As Paul Elliot pointed out, with a digital meter you will get small voltage readings on essentially disconnected lines.

[senormechanico]

Paul is right.
If you were to hold onto both the + and the common leads while measuring that 0.3 volts, I wouldn't be surprised if it went to 0.0 volts just from your body's resistance acting as a tiny load.

[Homer Shannon]

Thanks for all the recommendations - I'm not sure I fully understand them all though! I can easily hook up some kind of load onto one of the three mast leads and see if that eliminates the 0.30v on the other two leads. Also, the circuit to the VHF had the VHF connected but the VHF was off. The AIS receiver, however was on. Perhaps that is what is leaking voltage back to the line after the circuit breaker is shut off. I'll check that, too.

I think what I am understanding, though, is that neither of these issues is a big deal. I any event, everything is working properly, I just don't want to burn up the boat due to my own stupidity!

[senormechanico]

If you touch both + and - of a 12 volt battery, you wouldn't even feel it.
If your hands were really wet with salt water, you might feel a slight tingle.
There's no way wet hands would feel anything from 0.30 volts.
Basically what I suspect in your case is a very high resistance across a switch which is composed of a dirt film with salt and slight moisture.
Perfectly common on a boat, and nothing to worry about.

[hellosailor]

If there are idiot lights on the breakers, perhaps they are the "leak" that is allowing a small stray voltage to be seen. Disconnect the indicator lights, and shake the magic8ball again.

[Homer Shannon]

Thank you for all your thoughts. I did some further testing and I think this is resolved.

The bundle of mast wires is terminated with an old four-wire connector. I removed this and the bleeding across circuits is gone. Circuits when on measure 12.5v and 0.00 when off - as they should. I will make a new connector for the mast wires.

It would be a bit difficult to remove the radio from its circuit, but the AIS is easy to do. I tested that circuit with the AIS removed but that made no change. What did make a difference is having the radio on when the circuit is switched off. The voltage on the circuit drops to 0.00v much more quickly, though not instantaneously. I think this confirms that it is the capacitance in the radio that is causing the slow discharge of the circuit and I am not going to worry about it further.

Thanks again for your help.

[hellosailor]

Homer-
"the capacitance in the radio" Many (all?) modern radios actually don't have power switches these days. The "power" switch actually signals some internal circuit, but there are filter capacitors across the power lines (inside the radio) and the radio is actually "live" all the time. So that kind of nonsense is actually not surprising or unique.
If the 4-wire connector was a common trailer-harness connector (hey, they're always available and inexpensive and robust) that problem is also not surprising, they were intended really just for old-fashioned lighting circuits, and they do that job well enough considering their cost and intended application.
Good that you finally solved the mystery though!