When 0V Isn't Safe - Short #242

HVAC School - For Techs, By Techs

Bryan Orr

Training, Careers, Airconditioning, Self-improvement, Hvac, Business, Education, Refrigeration, Heating, Ac, Apprenticeship

4.8 • 985 Ratings

🗓️ 27 May 2025

⏱️ 8 minutes

🔗️ Recording | iTunes | RSS

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Summary

In this short podcast episode, Bryan covers some unique electrical cases: when 0v isn't safe.

When you measure voltage, you're finding out the electrical potential between two points. Just because there is 0v between one point and another doesn't mean there will be 0v between those points and other points. Non-contact voltage detectors are good tools that measure the flux from the wiring, but they have their limitations.

A common mistake is measuring voltage across a switch. When you measure 0v across the switch, that could mean there is 0 potential to ground, neutral, or the other leg, but it could also represent a closed switch where power is actively being passed. You need to measure voltage across a load or with neutral as the other point.

You will also measure 0v when one of the legs of 240v power (with a two-pole breaker) stops making contact. You'll measure 0v between the legs, but that's because one of the legs is open. There may still be a complete path on the other leg; therefore, you will want to check for voltage between each leg and ground, not leg to leg. This condition is particularly common in pool heaters.

In some cases, the meter isn't working and will read 0 when it's set to another unit setting, isn't on (if an analog), or the leads aren't fully plugged in. Check to make sure your meter is on the correct setting, is in proper working order, and has a good connection to the leads.

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Transcript

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0:00.0	Indeed, indeed. This is the HVEC School podcast. I am Brian, and this is the podcast that helps you remember some things you might have forgotten along the way as well as helps you remember some things you forgot to know in the first place. And today we're talking about when zero volts isn't safe. But before we do that, we want to thank our great sponsors.
0:23.8	Santa Fe dehumidifiers in their new five-year complete replacement coverage.
0:29.6	Find out more at HVACR school.com slash Santa Fe.
0:34.0	Refrigeration Technologies at refrigetec.com.
0:38.4	Carrier and Carrier.com.
0:40.5	I've been a carrier dealer for many years.
0:43.1	Carrier has their new green speed extreme, super high efficiency heat pumps out on the market now.
0:49.8	Find out more by going to Carrier.com.
0:53.0	The ESCO Institute with over 200 HVACR training solutions, courses, webinars, and
0:58.2	simulators, find out more by going to escowgroup.org.
1:04.1	All right, so you hear all kinds of crazy things. I remember as a up-and-coming apprentice
1:09.6	guys talking about, you know, sometimes zero volts can really shock you.
1:13.3	I measured zero volts and it still shocked the dickens out of me.
1:16.7	Well, what people are talking about are cases where maybe they measured zero volts between two points and there is no potential difference between those two points.
1:24.4	But that does not mean there's not potential difference between that
1:27.5	point and another point, which is why one of the first things that I teach to new people when they say
1:33.7	it has voltage or it doesn't have voltage, or it's got power. There was power there, is specifically how do
1:39.8	you know that? Because in a lot of cases, people are just using non-contact voltage detectors, which
1:45.2	are a very good tool, but the range at which they go off varies, and it depends on how close
1:50.5	you are and what the electrical fields are. Ultimately, that's what it's measuring is flux off of the
1:55.4	wire conductor, outlet, whatever it is. So it's not like the most reliable, but it is a good check.
2:00.7	So when we're going to
	...

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