Dear Readers,
I’m in the middle of creating a new training video about using a multimeter for RV electrical system testing. One of the topics is all the things you can test in your RV with even a very basic digital multimeter.
Blast from the past!
HERE is the article I wrote last year about various meter kits you can purchase at big box stores and Amazon. You can get a nice kit for under $40, or just a digital multimeter for around $20.
Testing AC pedestal voltage
Use a meter to test a pedestal for proper voltage and polarity before plugging your RV into it. It’s actually pretty easy. Here’s the diagram I made years ago that shows all the voltage readings for a correctly wired pedestal.
If the voltage reads much below 105 volts or above 128 volts, then there’s a problem with the campground electrical system that could damage your RV or even create a hot-skin voltage condition. And HERE is a video I made that shows how to test a pedestal for proper voltage.
Testing DC battery voltage
Few things are more disheartening than waking up in the middle of the night with a dead house battery. Any digital multimeter will be more accurate than the simple 4-bar battery gauge in many RVs. But you will need to know what kind of batteries you have (such as flooded lead acid, AGM or lithium).
Then find a SoC (State of Charge) chart like this one that shows battery SoC at different voltages. This is also a great way to determine if your converter/charger is working. When you’re plugged into shore power you should measure between 13.5 and 14.5 volts on your battery terminals, depending on battery chemistry. If you read in the 12 volts range, then your converter isn’t charging and you’ll soon have dead batteries.
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Finding a dead fuse
All these meters have a continuity setting, which will beep when there’s a low-resistance connection across the probes. By pulling the fuse and placing the probes across the fuse contacts, if the meter “beeps” then the fuse is okay.
If the meter doesn’t “beep” then the fuse is blown. It’s also possible to check the fuse in-circuit. With the power and the load on, set the meter to 20-volts DC. If you measure 12 volts across the fuse then it’s opened up and not passing current. But if it reads close to 0 volts (maybe 0.5 volts DC) then the fuse is good and is supplying current to the load.
Lots more things to test
The list of things you can test with a basic digital multimeter is nearly endless. You can measure hot-skin voltage, find a dead GFCI, troubleshoot an open contact in a generator transfer switch, check AA and 9-volt batteries for juice, and a whole lot more. I won’t go anywhere without my trusty multimeter.
But you need to learn how your meter works when you’re not under pressure. So get a multimeter and read about how to use it HERE. And then when something in your RV stops working you can pull out your meter and start troubleshooting.
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 Amazon.com. For more info on Mike’s qualifications as an electrical expert, click here.
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Before buying a good meter, buy a cheap Harbor Freight one and get familiar with it.ive seen too many people blow up their meters on first try. Like setting on ohms scale probes on voltage. It tends to let the smoke out and causes fowl foul? Language.
Morning Mike, as I look at your diagram for pedestal voltage, I see across the 2 hots 240/208 volts. Ok I understand that, but if its 208 volts, why would the all hots be 120 volts & not 120/104 volts respectively. I must have missed something! Can you explain that please!
Thanks for your expertise
Snoopy
208 vac is 2 hot legs of a 208/120 vac 3 phase service.
240 vac is 2 hot legs of a 240/120 vac Single phase service.
How does 3phase even work on a RV 50 amp service? Perhaps you should read my question to Mike again> Its about the diagram he supplied on checking pedestal voltage!
thanks
Snoopy
You need to understand that AC voltage varies in a sine wave pattern. In a 120/240 single phase system the two legs are 180 degrees out of sync so the volts add. In a 208/120 volt three phase system the three hot legs are each 120 degrees out of sync so the volts between any two legs are adding at the point where the voltage of each leg has been reduced proportional to the sine of 120 degrees or .866. It results from the way the transformers are tapped.
Guess I need to write an article on the differences between 1-pole single-phase, 2-pole split-phase and 3-phase power. I’ve been installing and troubleshooting 3-phase industrial power since the mid ’70s so I know just about everything you can do with it. I did build my own desktop 3-phase signal generator that can create 120/208/240/277/480 volts in all flavors of AC power including 3-phase distribution in Wye, Delta, Open Leg Delta, High-Leg Delta and Grounded Leg Delta. The different voltages (240-volts vs 208 volts) has to do with the 180 degrees of phase shift in split-phase power compared to the 120 degrees of phase shift in 3-phase power. I feel a video coming on!
Hi, Mike. You get all worked up about something and “feel a video coming on!” Most of us average folks get worked up about something and “feel a migraine coming on.” Just sayin’. 😆 Later, Dude. 😀 –Diane
Here’s my desktop 3-phase signal generator. Yes, it makes lethal voltages.
Where? I don’t see anything. But, if it makes lethal voltages, I guess that could definitely give you a migraine (but not for long). 😆 –Diane