Using a digital multimeter to test a fuse
Q: Hi Mike. Please forgive me for being a newbie. I went to my local auto store and looked at digital voltmeters. I found one for $20 but the max fuse amperage is 10 amp only and my RV has up to 20 amp fuses. I asked the employees but nobody could give me a straight answer. Someone told me to go to Lowe’s and get one with a higher amperage capacity. But before I spend more money, do I need a meter with a higher amp capacity or should the $20 10-amp max meter work? Please advise me what to get for what I need to do. Thanks a lot for your help and advice. I really appreciate it very much. —Robert
A:Â Hey Robert. There’s a lot of confusion in your question mainly because the guys at the auto parts store have no training on how digital meters work. That’s a shame, especially since it’s something I could teach if AutoZone or Advanced Auto Parts or anyone else would come up with a budget for me to create a class for them. But more on that later.
Here’s your answer, which is going to take an extended article and a bunch more pictures to show you the details. But the main point is the 10-amp scale on a digital meter has NOTHING to do with how big of a fuse it can measure. In fact, you don’t use the ampere scale at all when testing fuses. Here’s a diagram (on the left) I just made showing the basic functions of a manual ranging meter from Southwire which only costs around $20 from Lowe’s, and which is my favorite low-cost meter for working on RVs. To read why the amperage settings aren’t used for fuse testing, and to see how to do this correctly, read the rest of the article below. But any meter you buy that has a resistance or continuity setting (and they all do) will do the trick.
Meter basics – It’s all Greek to me…
You’ll see on the meter picture above has a bunch of different settings with foreign-looking words. Don’t worry, this is actually English for Engineers. I’ll help translate. (As usual, click on the picture to enlarge it.)
Let’s start at the top right of the meter. The AC voltage settings are for exactly that – testing AC voltage. Note that there are 200 and 600 settings, each of which represents the maximum voltage the meter can read before the display shows OL, for Over-Load. For most AC testing in an RV you’ll want to use the 600-volt setting.
Next down on the settings you’ll see what’s called micro-amp and milli-amp settings. Don’t worry about those settings at this time, since you’ll probably never use them except for advanced testing. We’ll skip those for now.
Same for the setting that says 10A for the 10-ampere current testing when the black test lead is plugged into the special 10A socket on the lower left of the meter. You may use them for advanced testing, but not right now so we’ll skip it.
Down on the lower right of the dial you’ll see something that says Battery 1.5V and 9V. This is for providing the proper load to test 1.5-volt and 9-volt batteries. Should be obvious what this is for, but more on that later.
On the lower left of the dial you’ll see settings for 200 ohms, 2,000 ohms, 20k ohms (which is 20,000 ohms), 200k ohms, and 2,000k ohms (which is 2 million ohms). The only one we’ll worry about for basic RV continuity testing is the 200-ohm setting. Ignore the rest for now.
On the upper left part of the dial you’ll see the DC voltage settings for 200 mV (200 milli-volts or 0.200 volts), 2,000mV, 20 volts, 200 volts and 600 volts. We’ll likely use the 20-volt setting someday for reading the charge on 12-volt batteries, but not right now.
Let the testing begin…
Here’s how to test any fuse for being open (blown) or continuity (good). All good fuses should measure very close to a dead short resistance (around 1 ohm or less). All blown fuses should measure very close to infinity (many millions of ohms), which will show up on this particular meter with an OL reading on the face for Over-Load.
Set the meter to the Diode/Beep setting and place the test leads on both sides of the fuse which has been removed from the circuit. NEVER test a fuse in the circuit especially if power is applied since you can blow the meter.
I’m showing a little cartridge fuse in the picture, but this works for any fuse. And because the resistance you’re looking for is so low, you can touch the fuse contacts with your fingers and it won’t affect your readings, nor can it shock you. If the beeper sounds, and the reading on the meter face is around 1 ohm, then all is well with the fuse. Note that this same setting works for 1-amp, 10-amp, 100-amp or even 1,000-amp fuses. The 10-amp setting on the meter has nothing to do with resistance testing of fuses.
You can also use the 200-ohm setting on the meter if you like, or if your own meter doesn’t have a continuity/beep setting. In this case the same basic rules apply. If the meter reads less than 1 ohm or so, then the fuse has continuity and is good. But if the meter reads OL, for Over-Load, then the fuse has infinite resistance and is blown. Again, never test a fuse using the resistance or continuity settings while the fuse is in the circuit, especially if power is applied. You’ll either get confusing meter readings, or you might just blow up your meter.
Digital voltmeters are great tools, but only if you understand how to use them. And always be extra careful when measuring 120- or 240-volt circuits. Yes, that voltage is really going into your meter, and if you make a mistake like putting the test leads on the energized circuit first, and then turn the selector knob to the desired setting, you’ll probably spin though the milli-amp or resistance settings on the way there. A good meter will just blow the fuse, but a cheap meter may just die. So don’t go there.
Get your own meter…Â
By the way, Southwire sells this exact 10030S Multimeter as part of an inexpensive package that also includes a basic 3-light outlet checker and a Non-Contact Voltage Tester for around $25. It’s a great troubleshooting and test kit that everyone should have for their RV adventures. You can find it in your local Lowe’s store or online here.
Let’s play safe out there….
Email me at mike (at) noshockzone.org with your questions.
Mike Sokol is an electrical and professional sound expert with 40+ years in the industry. Visit NoShockZone.org for more electrical safety tips. His excellent book RV Electrical Safety is available at Amazon.com. For more info on Mike’s qualifications as an electrical expert, click here.
Mike, thanks for recommending a great inexpensive meter. One of its best features that you did not comment on was the class III safety rating. Many inexpensive meters do not have a safety rating at all. As we all know, it is important to keep safety foremost in our minds when working with electricity.
Mike, if the gentleman just wants to test for an open fuse after removing it, wouldn’t a simple continuity tester be more apt for him? Doesn’t sound like he intended to use it for anything else. No settings to worry about.
That’s true, and companies such as Sperry make a nice $6 tester you can purchase at most any Lowe’s or Home Depot store. However, I try not to get one-trick ponies, unless that trick is a great one that can’t be done any other way easily. I still think a basic $20 digital meter with a continuity function is something everyone can grow into. So RT, I’m curious if you have any electrical meters or test gear? If so, what brands and models? I’m not being judgmental (really, I’m not). I just want to know what everyone uses. I should do that as a poll in my next RV Electricity newsletter.
When I was working as an electronic tech, we used to test fuses in circuit with power on. Set the meter to the proper voltage range for the circuit and measure across the fuse. If the fuse is good, there will be no voltage across it. If it is bad, you will have full circuit voltage. Easy to do on glass fuses, but a little harder on the AT type fuses. If you look at the top of the AT fuse, you will see ends of the spades recessed slightly. Use the pointed probes to contact these. Safe to do on 12 volt circuits, but be careful on higher voltages.
Most of the fuses in an RV are low voltage and the 110/220 is normally protected by breakers.
Bob, that’s exactly what I plan to teach in an advanced meter testing article. But as I’m sure you know, for that type of test you can’t use the continuity or amperage modes, you need to set your meter to the appropriate predicted AC or DC open-circuit voltage. Plus you need to take all safety precautions in case you accidentally contact the meter probes with your fingers while the fuse is in the circuit. So baby steps for now, and I’ll show more advanced test procedures later.
Hi Mike, I too liked your article today. lest hare from Linda and Kevin. Ok I have a Cen-Teck Digital Multimeter and I think I followed you, I would to fond out do I use the 2000k or 20k ? if you have the same meter would you go through it. I have only used to check my battery’s and house (RV) voltage, or should I upgrade my volt meter?
Sorry, I don’t have a Cen-Teck meter, only ones from Southwire, Fluke, Amprobe, Klein and Greenlee. Yikes, that’s a lotta meters. To test a fuse you’ll want to use the lowest ohm scale on your meter, or a continuity/diode test function that beeps. Using a 2,000K ohm setting will give you strange results if you touch the meter points with your fingers. I highly recommend the little Southwire kit mentioned in the article because it also includes a 3-light outlet checker and a Non-Contact Voltage Tester, all of which you’ll need for real RV electrical troubleshooting.
Hi Mike, I really liked all your interesting articles today. I do have a question & its about testing amperage draw. In particular are the way you’ve installed the leads on the meter, you have the black lead going to the 10A socket, I’ll assume that the red lead is still in The only time I’ve had to use that Amp setting was to find out what was sucking the life out of my car battery. I believe I connected the red (+) lead to the 10A socket. After disconnecting the ground lead from my battery I tested in-between the battery post & the battery clamp connector. I got a reading of .2 Amps (200Ma?) anyway that little amount would drain my battery down in 2 weeks to 12 VDC.
Actually this was only a question about what color lead you were using & where you put it for the amperage test!
Dave
I’ll do a video next week on how to use the 10-amp setting. Stay tuned….
Dave,
To measure DC amperage greater than 200mA (0.200 amperes) you keep black test lead in the black/middle socket and move the red test lead to the leftmost socket that says 10A (for 10 amperes).
Thanks Mike; I always learn something every time I read one of your articles! You make it easy to understand and execute!
I enjoy your technical articles very much, I’ve been using test meters for years and never knew OL stands for over load, guess I never took the time to read the owners manual. I worked as a machine repairman for GM alongside electricians for 30 yrs and have a decent understanding of electricity but I always seem to understand something a little better after reading your articles. Thank you
You’re very welcome. Electricity is one of those things that most people don’t understand at all. Hey, I even worked with a few mechanical engineers early in my career who I considered to be a genius level when it came to mechanical design. But these same guys would admit zero understanding of how I designed the control and power circuits for their fabulous machines. I started as a mechanical engineer and moved into electrical engineering, so luckily I could still speak their language. If you meet me in a bar and offer to buy me a beer I can still wax poetic on topics such as Modulus of Elasticity as well as discuss using Karnaugh maps to simplify logic control circuits. Yeah, I know…. super geek stuff.
” If you meet me in a bar and offer to buy me a beer I can still wax poetic on topics such as Modulus of Elasticity as well as discuss using Karnaugh maps to simplify logic control circuits. ”
Mike, if I met you in a bar, offered to buy you a beer, and you started “waxing poetic” on these topics, I would excuse myself as if going to the bathroom, and RUN like heck down the street.
Yeah, I know. I generally don’t tell people what I do because it quickly turns into a geek fest. You should meet my identical twin boys sometime. My head hurts listening to their stereo barrage on technical topics. Yikes!!!