RV Electricity – Just Ask Mike (J.A.M.): How to meter a fuse

7

By Mike Sokol

Welcome to my J.A.M. (Just Ask Mike) Session, a weekly column where I answer your basic electrical questions. If you’re a newbie who’s never plugged in a shore power cord (or ask – what’s a shore power cord?), or wonder why your wife’s hair dryer keeps tripping the circuit breaker, this column is for you. Send your questions to Mike Sokol at mike (at) noshockzone.org with the subject line – JAM.


Dear Mike,
Can you tell me how to measure if the fuse is good or bad? I can sometimes tell if they’re blown just by looking at them, but seeing as my eyes aren’t what they used to be it would be a good thing to be able test fuses with a meter as well. Any quick tips on how to do it? —Jason

Dear Jason,
Very good and timely question. In fact I’m working on a Quick Tips video which we’ll post in a day or so. But in the meantime, here are the basics.

First you need a basic digital meter. I really like the Southwire products because they’re quite affordable, rugged and calibrated way good enough for basic electrical testing. I’m using their 10030S for this demonstration, but there are similar manual digital meters from Klein, Amprobe and others that will do the job.

Next, you need to get the fuse out of the circuit and placed on a non-metallic surface. Most anything will do as long as it’s not a metal counter top. In a pinch, a paper or glass plate works well. There are no dangerous voltages involved for this continuity test, but having the fuse on a metal surface could mess up your readings.

Now set the meter to the lowest Ohms Scale – it’s the little Greek sign (omega) that looks like a horseshoe: Ω. Or if you have a Continuity/Diode setting that will beep, it’s even better since you’ll get an audible sign. Here’s how you do it:

To begin the test, first set the meter dial to the 200 ohms or Diode/Continuity setting, then touch the probes together to make sure that your meter is working. It should now show near zero ohms or beep at you.

Next, touch the meter probes to each end or tab of the fuse. You can use the included alligator clips, or simply touch the meter probes to the ends of the fuse. Don’t worry if your fingers touch the probes or the fuse terminals for this test, but that could certainly be dangerous if you’re testing a live 120-volt AC circuit. So now is a good time to practice your safety protocols and learn how to keep your fingers off the probe. If the meter beeps or shows close to 0 ohms (anything below 2 or 3 ohms is okay for most fuses and meters), then your fuse is good and something else must be wrong in the circuit.

However, if the meter does nothing but possibly shows something like OL (Over Limit) on the display, then the fuse is open and needs to be replaced. However, if a new fuse blows immediately then there’s something else wrong in the circuit sending too much current through the fuse and blowing it. DO NOT replace it with a larger fuse as you’ll most likely burn up something expensive.

A good basic digital meter is one of the handiest (and most important) pieces of test equipment you can keep with you since it performs all kinds of basic tests. Next time I’ll show you how to test a light bulb.

[Tip from editor: As of May 10, 2020, Amazon.com is running low on Southwire digital meters.]

OK, everyone. Remember that electricity is a useful and powerful force, so we all need to pay attention to safety precautions while using it.

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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Gary
2 months ago

People, PLEASE never think a larger value fuse is ok. Use what’s recommended only.

I used to fix guitar amps, and we’d actually see people use what we called a “Wrigley’s No-Blo”… that’s a dead fuse wrapped the foil wrapper from a stick of gum. One amp that had this stupidity was totalled, even caught on fire. Another required a new power transformer, costing $250. (The real cause of the fuse blowing would have been a $50 fix!)

If something keeps blowing fuses, there’s a reason. Find the problem, don’t use bigger fuses!

H Goff
2 months ago

i did have one very weird situation some years ago where the fuse tested “good” but wasn’t. apparently enough metal was left to make contact with an ohm meter (or perhaps just char), but not enough to conduct electricity. i finally discovered the problem by simply replacing the fuse. problem solved.

DAVE TELENKO
2 months ago

Mike it was great to to see the digital meter that depicts each reading open larger, rather than just open up & be the same size. Making them, bigger usually helps to see the fine details. Would be awesome if all your pictures would open up at least double the size. I’m not what it takes to do that, but it seems most all of the pictures in RVTravel only just open to the size that they were, no magnification!
Thanks
Snoopy

DAVE TELENKO
2 months ago
Reply to  DAVE TELENKO

Sorry Mike I forgot to mention that the information was very helpful with seeing if the fuse is still good. I usually first check them using the proper scale on the meter for the voltage that I’m testing by inserting one probe into hot (feed) side then the other side where the load is & the other probe to ground. But I agree its best to remove them & then check them! Not sure if you would recommend doing it that way but thats where I start!
Snoopy

Michael
2 months ago

What part of the fuse is the end or the tab of the fuse? A photo would be helpful.