I start every Troubleshooting and Meter Skills class by having my students try to find a simple open circuit. With only 20 devices per circuit, one can find a single fault on any circuit in 4-5 moves if they use divide-and-conquer.

It's not a high bar, but fewer than 5% of my students can find the fault in 60 seconds at the beginning of the first day. Nearly everyone can do it in under 30 seconds by the end of the first day. The class record is 19 seconds.

Techs repeatedly make two big mistakes.

The first big mistake is using an ohmmeter on a live circuit. The resulting measurement is practically useless and doing so creates a path for current to flow through the meter.

Several years ago, one of my employees created a ground fault while relocating a pull station in a data center. After reenabling the FM-200 and EPO, he started metering the panel. Yep, you guessed it. He put his meter in ohms mode and measured the resistance of the manual release circuit. The releasing panel tripped instantly.

The second big mistake is using an ohmmeter at all. Fire alarm systems use self-resetting fuses, so there is no problem using a voltmeter to find a short, open, or ground fault. Voltmeters are also the only way to diagnose reverse polarity.

Here's what your techs weren't taught.

1. Flip the script.

Instead of using a meter to look for the fault, look for normal. You need one end of the circuit that you can see with a meter. If you're using a voltmeter, the voltage source is your known good end. If you're using an ohmmeter, you'll need a unique resistor at one end.

The question to ask at every device is "can I see my known end?"

This is the exact opposite of what everyone was taught. We were all taught to look for the fault. Looking for the known end is faster and less susceptible to false positives.

2. Stop using your ohmmeter.

There are three big issues with using an ohmmeter. If we count in tenths of an ohm, there are 600 million measurable units that a digital ohmmeter can see. That level of precision is unnecessary for troubleshooting circuits in the field.

It's worse than unnecessary. It's distracting to the point of being confusing. What does it mean when the resistance keeps moving? Is 0.7 megaohms an open circuit or a weak short?

Ohmmeters are merely a voltage source, a voltmeter, and a calculator preprogrammed with the Ohm's Law formula – all implemented with high-precision electronics. However, the voltage source is weak. The meters I see in class tend to range from 0.5 V to 2.7 V. When looking for a weak fault, a couple of volts does not offer much potential to get through.

The third major issue with using an ohmmeter is that it shows confusing results when metering a circuit with devices or a ground fault through water.

Because the ohmmeter is also a voltage source, capacitors, regulators, coils, and other electronic components may try to turn on – even if they can't fully do so because the voltage is so low. Any amount of current draw drops the voltage, causing the math calculation to result in a lower resistance measurement. That isn't useful. When a tech sees their meter reading 300 kiloohms and climbing, they likely have no idea that they're just watching caps charge up off their meter.

The easy answer is use a voltmeter.

The voltmeter is a far better tool. For 24-volt circuits, if we count in tenths of a volt, there are 240 measurable units that we might see on our meter. That's a huge difference from the 600 million units we could see with an ohmmeter.

The 24 volts that a fire alarm system runs on is far more powerful than the 0.5-3 volts from an ohmmeter. Does the panel show a fault, but you can't see it with your meter? It's because the panel has a bigger voltage source. Use it!

When using a voltmeter, there's no chance of getting confused by devices on the circuit, either – unlike with the ohmmeter.

Another crazy thing about troubleshooting is that electrolytes and dissimilar metals dissolved in water can create a galvanic cell. Thus, we can sometimes see voltage present between the circuit and ground when there's a ground fault through water – even when disconnected from the panel. That's a problem for ohmmeters, but inconsequential when using the method I've laid out here.

I start my students out troubleshooting ground faults through water without telling them. They never know the difference until I show them. I love seeing their surprised faces when they realize they've been effortlessly finding ground faults through water all afternoon! They found it easy because they used a voltmeter.

Editor's note: This column is excerpted from Ben Adams' Troubleshooting and Meter Skills course, which can be found at https://MeterSkills.com.