top of page
  • Writer's pictureDavid Peterson

Easily Identify Open and Short Circuits

In the world of electrical troubleshooting, very few problems would be accused of being 'really simple'.

But then, on the other hand, there is almost nothing that can fall under the category of 'random'. Maybe certain computer issues, but even then, there is still some subtle reason for every failure.

The trick in troubleshooting is to shift problems from a complex category into something that is much more simple, if that's possible.

The best way to do that is to predict which problems are most likely to occur. Once we understand that, then we can look at the symptoms of a problem and see if they can be attributed to one of those most common reasons.

Two of the best candidates in this area are both common and simple. Although they can still be frustrating, it is a huge relief just to figure out the root cause of a costly system failure.

Don't let wiring problems leave you stranded. Ok, I'm done.

Those two candidates are the Open Circuit and the Short Circuit.

--- Note: for all tests, the COM lead of the meter is placed at ground or neutral ---

Open Circuits

I would like to make one point clear right here - do not attempt to identify any circuit failure using a part of your body.

No licking 9v batteries or checking for a tingling sensation to test a 120v circuit. Please.

If you are that desperate to know, go get a voltmeter and do it properly. Don't play games with electricity.

Even using your hand to test for heat can be dangerous, although if you do sense heat, it can certainly aid you in pointing out some possible factors at play.

We use voltmeters in the case of most circuits, and non-contact testers in AC circuits. Those tools can make a job like this really simple.

Several of the various nefarious testing tools, including the non-contact tester on the right.

In simple terms, an 'Open' circuit is a broken wire.

This may be on purpose like inside a light switch or button, but here we'll focus on the failure mode.

Both ends of a wire should always have the same voltage. If the wire is very long, then the size should be properly rated to keep the voltage loss to a bare minimum.

1. Use the voltmeter or the non-contact tester and measure the beginning of the circuit, possibly at the breaker/fuse. If there's no voltage, check the supply connections.

2. Move down the wire, testing each exposed connection point. In fact, with the non-contact tester, it doesn't even need to be exposed! Super easy!

3. The reading should stay at the full source voltage. If it drops suddenly, you found the open circuit.

What are you trying to find?

Well, the common culprits maybe a variety of reasons:

- They certainly can be switches left in the open position that we failed to account for. An open switch acts exactly like any open circuit failure.

- A broken wire somewhere around a pivot or flexing point - a cabinet or car door, conduit with a sharp edge where the wire flexes, something like that.

- Loose connection points like wire nuts or terminal blocks. It might help to jiggle the wires and if the voltage comes and goes, this is a sign of loose connections. The formal name of this is the 'Jiggle Test'.

- Excess heat can cause wires to burn and break. This one is tricky, because the problem may actually have been a short circuit (up next) but it turned into an open circuit. Yay, extra problems.

Regardless of the cause, the open circuit will appear as full source voltage that suddenly disappears. The open location is between those two test points.

Find the ROOT cause before fixing the symptom. Otherwise, you'll just be fixing it again as soon as you turn the power back on.

Short Circuits

This is the one that we have heard our whole lives, it's the scary problem.

There is a good reason for that fear - a short circuit might appear as one of two things: a spark, or immediate excess heat.

And in the more extreme cases when voltage is high? It could lead to an explosion. You must have proper training to work on live high voltage circuits.

In our case, we enter the scene after the failure - the circuit doesn't work and we want to know why.

Surprisingly, short circuits can't often be found with a voltmeter because as soon as short happens, a fuse or breaker pops and voltage is lost. If you try to turn the breaker back on, it turns right back off again.

Fortunately, most voltmeters also have a resistance or continuity tester. Either one will be fine, but for locating a direct short, the loud *beep* from the continuity function can make the job go faster.

For those techies out there, the continuity test is the sideways WiFi logo.

1. Put the meter into the continuity mode. We're going to be working backwards in this case, meaning we work from the end of the circuit at ground (or neutral) back to power.

2. The negative side of the load device - motor, light, speaker, whatever - should register a *beep* with no resistance.

3. Moving to the positive or line side of the load, there should be NO beep. If there is, keep working backwards towards the source at every exposed point. If you hear a beep at one point, and no beep later, the short circuit is between those readings.

Only the author will know exactly why there are bits of red wire insulation in the back of this jagged conduit box...

What are you trying to find?

- That pivot or flex point that we talked about before? If the insulation wears off of a wire, it can touch any adjacent grounded metal.

- A stranded wire in a wire nut or terminal block with one tiny little strand sticking out. It's strange how often this happens.

- Any metal flake that found its way into a control box. If you work in a shop with machine tools that send metal chips flying, watch out for this one.

- Fatigue over time can cause the insulation to wear out and crumble, then one day, the exposed metal touches grounded metal nearby.

Now, here is another important note. For electrical engineers working on circuit boards, this tool works well. For many industrial loads, the resistance of the load is low enough that you get a *beep* even if the circuit is working perfectly!

This is caused by the meter's continuity threshold. If that threshold is 10 Ohms, then any load below 10 Ohms will sound the beeper, and it's impossible to locate the short circuit.

I've seen some meters with thresholds as high as 100 Ohms, which encompasses a great deal of industrial circuits!

In these cases, use the Resistance mode. It may not have a beep, but it gives you an actual reading. A short circuit is 0 Ohms. Look for 0 Ohms to find your short circuit.

Honestly, I always use the resistance function. All the time.


Now, with these two strategies of locating failures firmly lodges in your cranial toolbox, it's time to test some circuits.

Be safe as always, but never be afraid to look deeper for problems. Electrical problems can be frustrating, but they are always consistent and systematic.

Solving the root cause of a problem changes you from a 'technician' to an 'outstanding technician' in a big hurry. And that's always a good thing.

1,121 views0 comments
bottom of page