Simple electrical problem?

A

Alcyone

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I've spent all day up the mast. The problem was that my steaming light didn't work. I ran a new cable to it a few months back and it was fine, now it won't light.

Changed the bulb. Still dead. Checked voltage 12.2V (battery was only 12.4V. Started engine, voltage 13.9 up mast. tried 4 bulbs, all known to be working, none lit.

One thing I noticed, with the bulbs in, the voltage across the terminals was 0.0V.

Suspected loose connection at fuse box. Disconnected wire at that end and connected direct to battery. Again, voltage at mast head, but light will not work. Disconnected the light and took it to cabin. Rigged a wire to battery and it was fine, with all 4 bulbs.

Up and down the mast all day.

Is it possible that there is a break in the cable, that is not bad enough to stop me getting a reading on my voltmeter, but which fails when the cable is under load (i.e. when I fit the 25W bulb)?

All I can think to do now is replace the cable, again.

/forums/images/graemlins/confused.gif
 

Twister_Ken

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Is there jointing in the cable where it passes through the deck (assuming the mast is deck-stepped)? If so, there may be corrosion or a dodgy contact at this point. Or could it be a problem at the switch?

Water can wick into electrical cable and cause corrosion and high resistance, but not usually after just a few months.
 
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Alcyone

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There was originally a joint, but I replaced it with a new length of cable with no joint. The only thing I know that has changed is my missis took down the headlining to route another cable. I'm guessing she dusturbed my lighting cable, or maybe hit it with a screw?

What has confused me is that my meter shows a good voltage when there is no bulb in the holder, as soon as I put the bulb in, the voltage shows 0.0.
 

Gumpy

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[ QUOTE ]

Is it possible that there is a break in the cable, that is not bad enough to stop me getting a reading on my voltmeter, but which fails when the cable is under load (i.e. when I fit the 25W bulb)?


[/ QUOTE ]

yes is the short answer
Most likely near either end or where the cable goes through a tight turn.
Unless of course it has been damaged elsewhere

The reason is that a meter requires very little current to get a reading however a lamp requires a decent current and the break in the cable presents a high resistance so no volts.

Had a similar thing on 120sqmm cable a few weeks ago no obvious damage to the sheath but the cable was cut through internally. Found out later it had been run over by a forklift.....
 

lw395

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A multimeter will have a very high resistance, so a resistance like a few hunderd kohms may still allow the meter to see nearly 12V. Your cabling could be only connected by a damp and salty piece of corrosion.
Check the volts at the bottom of the mast with the bulb in, this will tell you if the high res is in the mast or below decks.
 
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Alcyone

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[ QUOTE ]
[ QUOTE ]

Is it possible that there is a break in the cable, that is not bad enough to stop me getting a reading on my voltmeter, but which fails when the cable is under load (i.e. when I fit the 25W bulb)?


[/ QUOTE ]

yes is the short answer
Most likely near either end or where the cable goes through a tight turn.
Unless of course it has been damaged elsewhere

The reason is that a meter requires very little current to get a reading however a lamp requires a decent current and the break in the cable presents a high resistance so no volts.

Had a similar thing on 120sqmm cable a few weeks ago no obvious damage to the sheath but the cable was cut through internally. Found out later it had been run over by a forklift.....

[/ QUOTE ]

Short answer! Like it.

Thanks for that, it was a wild guess on my behalf.
 
A

Alcyone

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[ QUOTE ]
A multimeter will have a very high resistance, so a resistance like a few hunderd kohms may still allow the meter to see nearly 12V. Your cabling could be only connected by a damp and salty piece of corrosion.
Check the volts at the bottom of the mast with the bulb in, this will tell you if the high res is in the mast or below decks.

[/ QUOTE ]

Yes, i could understand that in an older cable with a join in it, but I replaced the cable myself, removing a cable that had those very properties.

Thanks,
 

Bodach na mara

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This thread shows that there is no such thing as a "Simple electrical problem." Although you say that you "could understand that in an older cable with a join in it", there must still be joints in the new cable at the ends where it connects to the system. You can test the cable itself by disconnecting both conductors at the battery end, twisting them together at the top end and measuring the resistance between the two disconnected lower ends with a multimeter. It should be only a few ohms. I would worry if it were more than 2 ohms, but it will be either less than that or a LOT more. If the cable is OK, then you are back to connecting up again and hoping for the best.

For what it is worth, I had a problem with my last boat that was similar. When wiring the mast, I used a bit of flex from an old lawnmower and suffered an intermitant fault. It was ultimately traced to a break in one of the conductors inside the insulating cover, caused by fatigue where the cable was compressed by a cord grip. At least I knew roughly where to start as I had repaired dozens of appliances with the same source of fault. The moral is that a bit of cable with nice intact sheath may conceal broken wires inside.
 
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Alcyone

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I agree about the simple electrical problem bit, hence the ironic question mark in the title. Two wires, a battery and a bulb should be as 'simple' as they come........

I will check the cable as you suggest, thanks, although I've bought some new anyway - I'd like to get away sailing this weekend so it might be quicker to just replace the run.

When I said no join, I meant in the middle, rather than the ends. I did bare all four ends when I tested it with a battery and not through the fuses.

I'm more of the opinion now that there is a break in the cable, as mentioned by a few of you.

As usual, your help is appreciated. Thanks.
 

Danny Jo

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Very timely thread - the MAIB report on the Ouzo tragedy has persuaded me to replace Freestyle's 21-year-old navigation lights. Finding some degradation in the insulation of the cables feeding them, I've just bought some cable to replace the wiring.

[ QUOTE ]
The moral is that a bit of cable with nice intact sheath may conceal broken wires inside.

[/ QUOTE ] The message for me is this: before threading the new cable, cut a length of wire a couple of metres longer than you estimate is required; bare the ends and check the resistance in each of the cores.

This is not a guarantee that one of the cores won't break as you are threading it. The most tricky is the one on the bow that runs up inside the pulpit: the entry to the pulpit is barely half a millimetre larger than the cable. This proved to be a good test of Freestyle's patent method (soldering each of the cores of the new cable to the old, after cleaning any tarnish off the old copper) - Scuttlebutt passim, ahem, as published in YM last summer.

So maybe the counsel of perfection is to start with the longest run and cut enough cable to feed up inside and reach back to the origin outside. Then having checked the resistance in each core, cut off the excess to use on a shorter cable run.
 

pampas

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What has confused me is that my meter shows a good voltage when there is no bulb in the holder, as soon as I put the bulb in, the voltage shows 0.0.

sounds like a high restance, will let you pass voltage at low current for the meter but not the bulb. Try renewing the fuse first and see what happens.
 

AndrewB

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As others say, sounds like high resistance somewhere in the circuit not allowing the bulb to get enough power. Can you use the resistance tester on your multimeter to check the circuit in stages? You shouldn't get readings of more than a few ohms anywhere.

Stupid question possibly, but your new cable is adequately rated for the job? You'd need something like AWG 12 grade or heavier.

Although the light worked when you took it down, it may still be the culprit. They seem to get temperamental with age.
 

matelot

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It is a simple problem and the cure is a simple logical process. Start at the bottom of the mast or between there and the battery. Use a working bulb (I recently chased a similar problem with mains lighting and the first 3 new bulbs I tested with were all duff!) and find a way of breaking into the circuit and putting the bulb across the wires. Bulb lights up - no problem between there and the battery so move on down the circuit. Continue this process until the bulb doesnt light up.

No good using a voltmeter - as you have already discovered they take almost no current. If you had long enough leads to do it, the answer would be to use the resistance scale of the meter. Thats in effect what you are doing with a bulb.

The secret is to be systematic and logical moving up from a place where the bulb lights near the battery until you find where it doesnt light.
 
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Alcyone

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Once again, thanks. I will test teh resistance when I go down this morning. I'm going to put a new run in - If I find a break in the old cable, I'll either have to replace it or join it anyway, and I don't fancy joining it.

Cheers
 
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