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Old 06-23-2013
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tips needed to chase dc short

My port side dc breaker has started tripping - usually in 30s to 2 min depending on load. My panel ammeter shows 20+ amps when this breaker is switched on, even with no load. I measure 12.6V at the port cabin lights and fan, so the high amps means very low resistance or a short somewhere. What I need is a suggestion on how to chase down the short - given that the breaker trips with nothing on, I don't think it's the appliances. And I've already tried a new breaker. So where should I look next? (Breaker panel is on starboard side). Thanks
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Old 06-23-2013
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Re: tips needed to chase dc short

The only way to find a DC short is to disconnect the positive battery terminal and start checking for the short in the wiring by clipping one jumper to the ground terminal, you can use a long extension lead on the ground to be able to test the various wires with the other terminal on your multimeter. Any reading other than an infinite or dead short reading means you are on a wire that is not shorted out. You just have to eliminate them one at a time, and even then you will have to locate where the short is once you have traced the wire on the circuit that has managed to get shorted out.

These are the wonderful joys of every boat that anyone ever buys HAHA.

I have had the same problems in boats, semi tractors, semi trailers, and heavy equipment, and sooner or later you will find it if you just keep looking.
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Old 06-23-2013
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tips needed to chase dc short

Thanks for the info, greatly appreciated
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Old 06-23-2013
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Re: tips needed to chase dc short

if you have any other questions or need help just send me a PM and I will do anything I can to help you.
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Old 06-24-2013
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Re: tips needed to chase dc short

Quote:
Originally Posted by lajimo View Post
My panel ammeter shows 20+ amps when this breaker is switched on, even with no load. I measure 12.6V at the port cabin lights and fan
If I understand you correctly, with the breaker on (before it trips) you measure 12.6 at the lights and fan, meaning they are not a dead short... This can make it difficult to measure resistance as you'll be dealing with pretty low values.

With all known loads off and the breaker on, I would measure the voltage again very carefully (to at least one one or better yet two decimal places, if your meter can do that) at *every* device on that circuit, drawing a "map" as you go. The voltage should be lower the farther you get from the breaker moving towards the heavy load or short. If there are lighter loads past the problem item, there probably won't be significantly greater voltage drop at those.

It would also help to enumerate everything that is *supposed* to be on that port breaker... and double-check. Could there be any old/unused item that only recently/accidentally got switched on? Or anything that does not work when supposed to? Is there another cabin fan with a seized/stalled motor? A light fixture with a defective socket?

My previous boat had a ceiling fixture connected to both the port and stbd light breakers (apparently from the factory) - we almost never turned them off, but in reality it put both breakers in parallel which wasn't great from a circuit-protection point of view. It also meant things were powered when we thought they were not (with only one breaker off).

If you have or can borrow a clamp-on ammeter (*wunnerful* tools, those), you can quickly check direction and magnitude of the current flowing anywhere along the wire. Hint: turn everything else off, and you can clamp around a multiple-conductor bundle and still get a reading on the one you're interested in.

Also, at 20A draw, you can try just feeling the wires wherever you can - that can tell you when you're (yes, here it comes) "getting warmer" (sorry). You can do this anywhere a fingertip can grip the wire, without needing access to the bare conductor for a voltmeter.

On the bright side, if this was a car you'd being looking for shorts to ground. Assuming your boat isn't steel or aluminum (if it is, look for them shorts), shorts are probably to the return (-12) lead or the housing/frame of some powered device - or the engine block.

Good luck.
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Old 06-24-2013
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Re: tips needed to chase dc short

I had a short finder that saved me all sorts of troubles locating shorts on tractor/trailer rigs. It was a two piece kit, a buzzing breaker that reset and an inductive amp meter. Place the buzzing breaker on the hot wire that's tripping, run the inductive amp meter along the wiring harness. The buzzing amp meter would trip and reset about every five seconds and your could hear the audible buzz as you were working. The amp meter dealy-bob would pulse back and forth with each trip as you made your way down the wiring harness. When it quit pulsing, you knew you just went by the short. The amp meter dealy-bob would pick up the current from a few inches away, so you didn't have to lay it directly on the wiring harness. It worked great.
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Re: tips needed to chase dc short

Quote:
Originally Posted by erps View Post
I had a short finder that saved me all sorts of troubles locating shorts on tractor/trailer rigs. It was a two piece kit, a buzzing breaker that reset and an inductive amp meter. Place the buzzing breaker on the hot wire that's tripping, run the inductive amp meter along the wiring harness. The buzzing amp meter would trip and reset about every five seconds and your could hear the audible buzz as you were working. The amp meter dealy-bob would pulse back and forth with each trip as you made your way down the wiring harness. When it quit pulsing, you knew you just went by the short. The amp meter dealy-bob would pick up the current from a few inches away, so you didn't have to lay it directly on the wiring harness. It worked great.
I had a similar signal injector, you clip it into the wire and it emits a tone that you find all along the circuit. I would say that not everyone has them, and depending on the unit they can get pricey. I do have an amp meter with the clamp on type sensor, also not totally cheap, but very good to have around.

The problem with boats is that as was stated earlier in most cases the boat wiring has to make a full circle circuit, meaning you have to have a long wire all the way out and back, whether it goes to a negative that is on a strip and shared or has both the positive and negative wires all the way out to the device or outlet in question. It sounds to me like he may have something that is getting a ground to the negative when and where it should not get it, and may also be something like a bad motor. A twenty amp draw is heavy enough to do some serious damage, be very dangerous, and hard on everything in the circuit including the batteries and the alternator. I would say the "you are getting warmer" method may work, just be careful not to find it the easy way by getting the crap burned out of your fingers and all of you body nicely tingled.

I can also tell you from hard earned experience that a loose connection that has leaned over to touch ground can get that done, be shorted out right at the fixture or fitting. Also look at any tight turns made into a place where the wire can abrade over time due to jostling and friction. It should provide hours of good experience hehe.
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Old 06-24-2013
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Re: tips needed to chase dc short

It's interesting that there's a delay. If this was a dead short, the breaker should trip basically right away. That suggests to me that it probably isn't something simple, like a 12V lamp, but rather that its something that has some electronics inside it. Something with an energy storage device (capacitor, inductor) that is shorting out after it starts to charge. I'd try to trace the wire and see what electronics are along that circuit, and try unplugging them.
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Old 06-26-2013
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Re: tips needed to chase dc short

As jimgo points out a dead short should cause the breaker to trip immediately. As there is a delay it would indicate to me that you have an overload situation. Regardless, one method to track down the culprit is to install a 12 volt pig tail light in series right off the breaker. Take the wire off the breaker. Put one wire of the pigtail on the breaker and join the other wire of the pigtail with the circuit wire that has just been removed. The 12V pigtail light should be illuminated.

You then start disconnecting the positive wires one at a time on the connected loads in the circuit. Start in the middle. If the light stays on the problem is between that load(but not including it) and the breaker. If the light goes off, the problem is between and including that load and the last load.

Then go to the middle of load of the problem end and repeat the process until you identify the problem connection.
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Old 06-26-2013
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Re: tips needed to chase dc short

I'm not buying the 'slow load' causing a short eventually.

I'd call this an intermittent short, very hard to find because you can test and test and if it's not shorting at the moment you won't find it. Electricians that charge by the hour love these kinds of shorts.

You're going to have to trace every wire and wiggle it while you do. Hopefully since you say it's the 'port side breaker' you are talking a couple of loads - not dozens.
You make (honestly) be better off just replacing the wiring of one load at a time, that depends on the age of the boat, your budget and your time.

On the other hand, you could just transfer one load at at time to the starboard breaker. When it trips off you know you've found the faulty load - replace every inch of that circuit and you may be done.
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