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post #1 of 15 Old 09-29-2017 Thread Starter
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Weird Electrical Problem

Back story: My foredeck/steaming light crapped out. It had been behaving oddly in that when I turned the steaming light on the foredeck light would glow somewhat.
Thinking it was the bulb I went up the mast. Checked the bulb w/ the multimeter and it was fine. Started testing all the contacts on both lights. Had good voltage for each but when I just happened to test the positive terminal for both lights I got 11+ volts even though only one breaker was on. Hmmmmm.
Longstory short I disconnected that wiring from the breaker panel and started testing my panel. Here is where it starts getting weird.
Putting the multimeter probe on the POS terminal of the panel and touching any NEG bus I have good voltage (12.6). If I touch the POS and the load side of any breaker I get 11some volts. If I CLOSE that breaker I get 0.0 volts This happens no matter what breaker I test. I know what I'm measuring is voltage DROP but where in the hell is it leaking to ground? I went through and disconnected every breaker one at a time both POS on the breaker and NEG on the terminal block and could not find any change in this behavior. For the time being I have just left the foredeck light disconnected and the steaming light and everything else work perfectly. I just want to find out what is going on.
So, since I have several days before I go back to the boat I thought I would turn to the experts! What say you?

CS 36M DIANTHUS
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Re: Weird Electrical Problem

With the breaker open and reading voyage drop across the breaker you are reading the voltage thur the light bulb to ground. to do a voltage drop test on the wiring the load must be turned on

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post #3 of 15 Old 09-29-2017
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Re: Weird Electrical Problem

Hi,

Firstly if you put the positive multimeter lead on the 12v feeding a switch then put the negative lead on the load side of the switch a reading of 11v is not unusual, what is happening here is the multimeter is grounded through the load resistance of the device, this is normal, closing the switch will effectively short out the switch and give 0 volts measured on the multimeter. With the switch open however there is no current in the circuit and the device will not operate.

You mention that if you disconnect the foredeck and steaming light all is well, if the fuse is not blowing a short circuit is unlikely.

The most likely cause is a loose connection between the battery and the control panel either the positive or the negative.

Try putting the multimeter on Volts across the battery terminals and see what happens when you active your deck lights.
If the voltage drops significantly on the battery when the lights are on then the cause may be the battery if the voltage on the battery terminals drops there is an issue with the contact between the battery and the terminals.
If all is well at the battery (i.e. the voltage changes little) try again on the supply to your control panel, again just testing Voltage on the positive and the negative, if there is a Voltage drop here this would indicate the wiring from the battery to the terminal needs to be checked.

A very handy item to have is a long wire with crocodile clips on either end, you can then test the resistance of each wire, you would connect the long wire onto on terminal on the battery then onto on probe on the multimeter, then with the other probe touch the point where it connects to the bus bar. With the multimeter set to the lowest Ohms reading you can get an idea of the resistance of the cable, couple of things to note if you are doing this firstly disconnect one terminal on your battery and also test the resistance of your test wire, you can then deduct the resistance of your test wire when measuring your boat's wiring. the lower the resistance the better, it should be way less than 1 ohm.

The wire can also be used to test if there is a bad connection by connecting to the positive on the battery and the positive on the bus or control panel, also duplicating the procedure on the negative run. Effectively making a parallel connection. If doing this be very careful not to connect the positive to the negative.
Just do this very briefly to confirm a problem connection.

Boats are shocking places for batteries and electrical wiring, hope this gives you some pointers.
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Re: Weird Electrical Problem

"when I turned the steaming light on the foredeck light would glow somewhat."
Sounds like the problem is a (relatively) high impedance short between the two lights. Could be a stray wire strand from the steaming light is contacting the supply wire for the foredeck light, maybe where both wires go past a piece of metal that has chafed through them and created a high resistance short.
If it is typical Old Boat Wiring...it could be time to replace the wire runs in the mast, and at the same time make sure the new wiring is adequate for less voltage drop.
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post #5 of 15 Old 09-29-2017 Thread Starter
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Re: Weird Electrical Problem

It's NOT the device. I'm getting these readings at the panel with the wiring disconnected at the base of the mast. And it happens regardless of which breaker I test. SOMEWHERE there is something grounding the system.

CS 36M DIANTHUS
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Re: Weird Electrical Problem

let's leave the panel out, what you are testing is showing how the panel is supposed to work, next we can cross off the battery to panel connections, as you state everything else works so you should have a good connection from the battery to the panel....

So where the problem lies is between the panel and the lights themselves, I like to start at the end where things aren't working, but you can start at either end....

Ok so back up the mast you go.... this time bring the tools to remove the fixture and test the wires themselves now test for voltage and a short to ground on the wires not the fixture (disconnect the fixture) Now either it will be ok in that event it's the fixture or you will find a short or open circuit. either of the 2 most likely will mean time to tape some new wire and have your helper pull it down the mast and reconnect the fixture.

So in review it's either the fixture or the supply wires....

Bob
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Re: Weird Electrical Problem

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Originally Posted by sailordave View Post
It's NOT the device. I'm getting these readings at the panel with the wiring disconnected at the base of the mast. And it happens regardless of which breaker I test. SOMEWHERE there is something grounding the system.
Are you measuring across the breaker, + input terminal to + output terminal, and getting 0V or are you measuring + terminal, input or output to ground, (return) and getting 0 v.

If you are measuring + Vdc in to breaker to + Vdc out of the breaker and when open read 12vdc and 0vdc when closed then your breaker is fine nothing is hokey. You just put a wire across a voltage drop when open, i.e. You have 12 volts on the battery side, and an open on the load side which your meter will read as somewhere near 0 v so the meter will read around 12 volts. When you close the breaker you now have equal potential from the battery source to the end of the wire, by placing both leads any where on this wire you should read 0 volts, as in no difference in potential in the wire, when you open the breaker this reverses the setup and you have 12vdc on the battery side and nothing on the load side, resulting in a reading of 12vdc difference. Your "phantom ground" may be provided by your test set up since you removed the streaming light as a load it is now floating with nothing to reference it, when you use your meter, while meter due have high impeadence to avoid affecting circuits it is not a true open, so it does provide a path for current resulting in your voltage display when the circuit is open, and 9 once you close the breaker and the entire run is at the same potential.

That said, with out pictures of what you are reading and how it is hard to tell, assuming it is a fault in the circuit, low voltage and a load usually is the result of excessive resistance, so check the usual suspects, check contacts and inspect them, greenish tint for copper, white or grey ashy for aluminium indicate corrosion.
Also check their condition, do they look good and uniform are the wires damaged going into the connection? do they have enough play so not to be over stressed? Does the insulation cover the entire wire? or does it gap right at the end? If all these are good, then you need to inspect the entire wire run is the insulation in good condition and free of abrasion and cracking? Any tight bends, a tight bend is one that is less than 5 times the diameter is the general rule. Look for errant screws that could have abraded the wire and fix the issue and the wire, i.e repair if necessary and move the wire away from the screw and secure it so it cannot happen again.


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Re: Weird Electrical Problem

figured I would spend a couple more minutes to show the dc panel.

if you probe between the negative buss and the positive buss side of the breaker you will get 12v that's simple.
but now if you probe between the positive buss and the load side of the breaker you will even with the switch open/off will get a voltage reading, follow the line from the load side of the breaker to the led to ground on the photo, your meter is making the circuit.

hope this helps you.....

Bob
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post #9 of 15 Old 09-30-2017
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Re: Weird Electrical Problem

Quote:
Originally Posted by sailordave View Post
It's NOT the device. I'm getting these readings at the panel with the wiring disconnected at the base of the mast. And it happens regardless of which breaker I test. SOMEWHERE there is something grounding the system.
I just read the responses to your question, they are all very good answers.

Something you must understand:

Every device on the boat is effectively grounded when measuring with respect to the positive connection.

I think you may be mistaken that something is grounding the system. note that when a load is placed on a battery the internal resistance of the device determines the current, if there were a problem with a short to ground the fuse would blow on the relevant circuit.

Consider a light bulb, this is just a coil of wire (which has an internal resistance/impedence, don't worry about the term impedence it's not really applicable in your DC system) with one side to the negative and one to the positive through the switch.

If you place your multimeter on Volts with one probe on the positive from the battery and the other probe on the feed to the light you will measure a voltage because the coil of wire gives the continuity back to the battery negative.

If you move the black lead to the Current terminal on the multimeter, then switch the meter to current the meter will measure the current through the device this will then allow the current to flow through the meter and the device will operate.

Last edited by TonyCross; 09-30-2017 at 05:23 AM.
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post #10 of 15 Old 10-01-2017 Thread Starter
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Re: Weird Electrical Problem

Quote:
Originally Posted by snokid View Post
let's leave the panel out, what you are testing is showing how the panel is supposed to work, next we can cross off the battery to panel connections, as you state everything else works so you should have a good connection from the battery to the panel....

So where the problem lies is between the panel and the lights themselves, I like to start at the end where things aren't working, but you can start at either end....

Ok so back up the mast you go.... this time bring the tools to remove the fixture and test the wires themselves now test for voltage and a short to ground on the wires not the fixture (disconnect the fixture) Now either it will be ok in that event it's the fixture or you will find a short or open circuit. either of the 2 most likely will mean time to tape some new wire and have your helper pull it down the mast and reconnect the fixture.

So in review it's either the fixture or the supply wires....

Bob

So tell me why when I put my probe on the POS bus coming into the panel and then touch the NEG probe to any one of the breakers on the LOAD side, with the breaker turned off, I'm getting 11.something volts. There should not be ANY voltage. This is happening at every breaker that is wired up! This has nothing to do with the original problem of the steaming/deck light. It's not even hooked up now.

CS 36M DIANTHUS
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