Saturday, September 30, 2023


RVelectricity: Don’t use a Non-Contact Voltage Tester on a DC fuse….

Dear Readers,
Last week there was a reader on my RVelectricity Facebook Group who was having all sorts of electrical problems with his RV, including air conditioners tripping the circuit breaker on the pedestal, 3-way refrigerator not running, and a bunch of inside electrical outlets that didn’t have power. We eventually found that the 50-amp circuit breaker in the campground pedestal was arcing internally, causing it to overheat. But as I was assisting with troubleshooting, I found that the reader was using a few test tools incorrectly, causing much confusion and adding days to sorting out the reason for the failure. 

Rule #1: Always test your testers….

The first problem was caused by using a 12-volt circuit tester to check the fuses. Normally this would be a useful test. However, the reader didn’t realize that the bulb in the tester was burned out. So even though the fuses were good, he thought they were all bad. So he was pulling out all the fuses and swapping them around.

Of course, the possibility of all fuses burning out at the same time is pretty remote. And swapping unknown fuses around only causes more confusion and compounds the errors. So the first rule is to always confirm that your tester is actually working. In this case it’s as simple as putting this tester across your RV battery terminals. If it lights up, then you know the tester is working and the battery has at least some charge on it. If it’s not working and other lights in your RV are on, then your tester has failed, probably due to a burned out bulb in the earlier testers. Modern voltage testers generally use LED bulbs so they should never fail. However, you should still test all testers for proper operation first.

Don’t use a Non-Contact Voltage Tester on 12-volt DC circuits

Next up, my reader attempted to use a Non-Contact Voltage Tester to check the fuses in circuit. Of course, that showed no voltage anywhere, causing even more troubleshooting confusion. And that’s because an NCVT can’t detect DC voltage, it only works on AC voltage.

For review, HERE is one of my articles with a video on proper use of a Non-Contact Voltage Tester.

Since these testers work by listening for 60-Hz hum, and there’s no hum in a DC circuit, then they register nothing on a 12-volt DC fuse. So they only work on AC circuits.

Again, always check your tester first on a circuit that you know is live. So, turn on the NCVT and see that it beeps and lights up normally in active mode. Then point it close to an outlet on the pedestal that’s powered up. You may have to poke it into the hot slot on the outlet if you’re using 90 to 1,000-volt NCVT. But if your tester doesn’t work on a known-live circuit, then the tester has failed, probably due to a dead battery. And remember, these are for testing 120-volt AC circuits only, not 12-volt DC circuits.

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The most important rule for troubleshooting any sort of failure is to look for the simplest thing breaking first. In the above example, the possibility of every single fuse in a load center blowing simultaneously is very remote.

Same for the 120-volt AC side of the panel. If you have no AC power anywhere in the RV, then it’s extremely unlikely that every circuit breaker has failed at the same time. So look upstream at the Automatic Transfer Switch or twist-lock inlet or pedestal circuit breaker.

The key thing is to observe the failure and look for the simplest explanation. Multiple failures at the same time are possible, but unlikely. And always test your testers first to make sure you don’t go down the rabbit hole with incorrect data. And never swap unknown components around without a specific plan in place.

Always know how your testers work

Don’t buy a meter or tester of any sort and just throw it in the drawer for the next time you need it. Take out the manual and read up on basic operation. Always try it out on a few operating circuits so you know what to expect.

And if it requires batteries, make sure you have a backup battery in your road kit, just in case. There are few things more frustrating than trying to troubleshoot an electrical failure while camped out in the middle of nowhere and your meter or NCVT has a dead battery.

Let’s play safe out there….

Send your questions to me at my new RVelectricity forum here.

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

For information on how to support RVelectricity and No~Shock~Zone articles, seminars and videos, please click the I Like Mike Campaign.




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Gregory S
1 year ago

I taught a class in Basic Test Equipment during my Navy career.
We had a test bench with five test points, four of which were hooked to voltage sources, the fifth was a dummy with no connection.
During the section on basic voltmeters, we had the students connect to each test point and record the resulting voltage. It was amazing how many thought the meter was “broken” because it read zero volts on the last test point, despite reading voltages on the first four.
The lesson was, as Mike says, verify that your test equipment is working . . . but the rest of the lesson is to trust your test equipment once you prove it is working.

1 year ago

One thing about testing the fuses. You can probe the top of the fuse with the tester. There are small holes in the top of the fuse with metal contacts that are actually extensions of the fuse blades. No need to pierce the wires. Just check both sides for voltage.

1 year ago
Reply to  Mike Sokol

(But measuring 0-volts DC across a fuse could either mean that the fuse is okay, but could suggest that the entire circuit is dead.)

It can still be a bad fuse and the appliance it powers is cut off. Button pushers change the state of the fault, making it harder to troubleshoot.

Primo Rudy's Roadhouse
1 year ago

So obvious, thanks for the advice because in the heat of the moment, our brains get clouded

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