RVelectricity: Finding short circuits in 12-volt systems, Part 2B


By Mike Sokol

Dear Readers,
2B or not 2B…. (You knew I was going to do that, didn’t you?)

Just like the Star Wars Trilogy I’m renumbering these articles so that 1 is 1, 2 is 2A, 3 is 2B, etc. Hey, I didn’t realize just how many parts I was going to have to divide the topic of short circuits into. If you missed them or simply want a refresher, please (re)read Parts 1 and 2A before you continue.

In this article we’re going to delve into one of the most handy pieces of test gear you can have to assist in finding short circuits in both AC and DC electrical systems. It’s the clamp-ammeter. But not just ANY clamp-ammeter: You need one that the clamp jaws will read DC current as well as AC current.

Many of the early clamp meters used a current transformer in the jaws as a detector, and transformers only work with Alternating Current. So AC/DC clamp meters use something called a Hall-Effects sensor which is capable of measuring magnetic flux from both AC and DC currents. My favorite general purpose AC/DC clamp ammeter is the Southwire 21050T, or its updated version, the 21550T.

After reading the first two articles in the series you should know that electrical current flows around in a “complete circuit,” and a “short circuit” will interrupt that flow making it shorter than it should be. And just like a dam in a river can divert the flow of water, a short circuit in an electrical system can divert the flow of electrons from its intended path. You’ll see in the first diagram that a direct short circuit has diverted the flow of electrons away from the light bulb, not only stopping the bulb from lighting up, but also allowing a huge amount of current to flow through the wiring. Up to 100 amps of current is easily available from an RV house battery, which can send a small wire up in smoke within a few seconds – which is what the fuse is there to prevent.

So let’s take a look at my next diagram, where I’ve created a low-current situation that won’t damage the wires, but which will allow enough current flow to troubleshoot where the short circuit is. In this diagram I’ve replaced the fuse with an old-school 12-volt trouble light which has its own tungsten bulb that draws maybe 1 ampere of current with 12 volts across it.

Now we take out our trusty AC/DC clamp ammeter and go exploring. By placing the clamp ammeter around just one of the wires (not the return or negative wire) we can find where the current is flowing, and where it is not. In this diagram you’ll see that when the clamp ammeter is placed in the left position between the battery and the short circuit, that current is flowing in the wire and the ammeter will indicate around 1 amp of current. The exact amount of current isn’t important, as long as there’s something flowing to measure.

But you should see that the ammeter on the right that was placed outside of the short(er) current path will indicate 0 amps of current flow. That’s because just like diverting water to another path, the short circuit is diverting electron current flow away from the light bulb. And not only that, it’s letting a LOT more current flow than the 1 ampere or so that the resistance of the bulb filament will allow. And that’s what blows fuses.

Once we make the short circuit go away, perhaps by twisting the wire so it loses contact with the sharp piece of metal it was rubbing against, the electron current is free to continue its complete path through the bulb. However, now that we have the resistance of the bulb limiting the current flow, as well as the resistance in the trouble light, the amount of current flow will be reduced to perhaps 1/2 of what it was. Simple, isn’t it?

The key is to create a low-current situation that won’t blow the fuse (with the trouble light), then measure the wire in question in a variety of places starting nearest the battery. When you find the spot where the current drops to zero, even though the trouble light is still on, you’ll know that you’re outside of the actual shorted current path.

Once you have the short circuit narrowed down to a small section of wire, it’s much simpler to inspect it inch-by-inch until you find the culprit, which in many cases can be as dumb as a nail through the wall piercing the wire and shorting out the conductors.

There’s one more thing that these AC/DC clamp ammeters are good for, and that’s determining how much current you’re charging your RV’s house battery with. Since this is a non-contact tool, you don’t need to disconnect any wires at all.

Simply clamp the ammeter jaws around the main positive battery wire and read out the amperage. Depending on how you orient it, the amp meter may indicate plus or minus current, but don’t let that bother your measurements.

While properly charging from your converter the meter should indicate anywhere from a few amperes of current up to 80 amps or so, depending on the state of the battery’s charge and the capability of your converter or charger. And by measuring the voltage across the battery you should be seeing something close to 14.3 volts if your charging system if working properly. If you’re only seeing negative amperage flow that changes up and down as you turn more appliances on and off, and the battery voltage is hovering around 12.6 volts, then your inverter/converter/charger is definitely not working and needs further testing.

How is everyone doing so far? Getting the hang of it? Good, because next week we take off the training wheels and begin troubleshooting 120-volt AC short circuits. See you then.

Until then, let’s play safe out there….



Mike Sokol is an electrical and professional sound expert with 50+ years in the industry. His excellent book RV Electrical Safety is available at Amazon.com. For more info on Mike’s qualifications as an electrical expert, click here.

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Robert E Staples
7 months ago

Great technique! Thanks Mike.