How To Use A Multimeter To Test

Also, when testing power, if one clip falls off DON'T make a grab for it while the other's still connected...

Dave "50-cycle jitterbug" H. (the other one)
 
In keeping with our usual practice, I think Nelson wants us to tell him how WE use our meters, not just send him somewhere to watch videos. We're here to teach each other, and there is probably more trust in what our members tell or show him than simply refer him to someone else. If we did nothing but point out what NOT to do, preferably with pictures, as always, that would mean more to him. Besides, this is interactive instead of just watching an infomercial by Fluke.
 
At one company I worked at in the 70s we had some real "gifted" people. I watched one such brainiac take a brand-new 300$ Simpson digital voltmeter out of the box, and try to measure an 1800 volt transformer. It literally caught on fire and burned like a roman candle. Max AC rating was 700 volts I think. It was an interesting place, lots of high voltage and Van Halen!
MS
 
Sometime measuring is the easy part. Understanding what you get is the hard part. Your typical 240 volt single phase machine should be about 1/2 of the system voltage or 120 volts to ground or neutral. 3 phase is a different animal altogether. The voltage between phases should be close to system voltage. You can have 120/208, 120/240 and 277/480. The first number is the voltage you should read to ground and the second number is hot to hot. The 120/240 3 phase delta will have a high or wild leg that will be about 200 volts to ground but 240 to any other hot leg. All of these voltages can vary about 10% and still be in spec for the system.
When checking fuses go top to bottom on the fuse and a good fuse will read 0 volts with the switch on. Think of placing your leads on the same wire several inches apart. There won't be a voltage drop to read. Bad fuses will read the higher voltage of your system voltage so a 120/240 system will have 240 volts top to bottom of the fuse if the device is a 2 pole switch. A single pole switch will be 120 volts top to bottom. The same procedure can be used to check if a switch or contact is closed (on). An open switch or contact will read the voltage applied to the switch. The light switch in your wall will be 120 volts if off and 0 volts across the terminals if on.
Continuity or resistance will show if you are connected to the same wire with both probes. Typical digital meters will display OL if the probes are not on the same wire. Turn your meter to the ohm symbol and see what the meter displays when the probes aren't touching and then you'll know what to look for on the display. Touch the probes together and it should display a number close to zero. That is the indication your probes are on the same wire or that a fuse is good. Make sure when you check fuses with an ohm meter that the circuit is off ( see previous post) or pull the fuse and check it out of the circuit. Don't lay it on a metal surface when you check it or it will show good every time. Yes, I've seen "pros" make that mistake.
Read your book for the meter because some default to AC and some to DC when you turn them on. Trying to read the wrong type will give you the wrong reading and could be dangerous. Make sure you're on the proper setting before you start. Play with reading batteries and practice checking continuity on wires and cords. Check the resistance of different light bulbs. You can even check the resistance from hand to hand. Now lick your fingers and grip the probes and see how much lower the resistance goes and you'll see why standing in water while using tools isn't good.

Be careful trying to read the output of a VFD because it isn't a true sine wave and some meters can't display the voltage properly.
Hope this helps.
 
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Before doing anything, first make sure you've got good batteries in it . I always start with a continuity test before doing anything else .
Do this by setting the meter to the lowest "Ohms"setting and touch the probes together...the meter should beep or jump to zero ohms reading.
At that point you know you've got good cables and probes.

I advise you If u don't know how to use a multimeter, don't try to learn on energized live circuits. Watch a few videos as stated above, make sure you understand what you're doing lest u damage the meter , short the equipment or injure yourself.

To check for 110 VAC or 220 VAC , set it for "Volts AC" (VAC) , the meter may be labeled V~ , then set it to a range that what you're testing falls into.
Basically you don't test for 220 vac on the 0-30 VAC setting . Then hold the probes across the terminals or outlet.

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Ohms symbol, meter reades "open circuit" , infinite ohms
1Ohms.jpg
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Doing a Continuity test, meter reads almost zero ohms, little to no resistance

2Continuity.jpg
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AC volts symbol, for 110vac or 220vac

3VAC.jpg
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DC Volts symbol

4VDC.jpg
 
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Always check a known source of power like a power outlet that is working before testing a circuit that you believe is dead or de-energized. Then after using the meter, re-confirm the meter is good by checking the same known power source. If you do not get a good reading, you must investigate to find out if you measured it wrong or your meter isn't telling you the truth.

Only after proving the meter, check known grounds and prove new grounds using the meter. Only after testing a ground can you call it a known ground. If a machine isn't grounded properly, not only does it pose a serious safety (electrocution) hazard, but can pose electrical issue along with some crazy diagnostics. Prove the ground before doing any real work with a machine or doing any electrical work on it. It could save you life.
 
Also, before doing most of the above, learn the electrical standards for your area, both today if you are running a new circuit and around the time that the circuit or system that you are working on was built. Standard practices at least in the USA have changed significantly several times since around 1900.

Current practice most of the USA is that residential power at the meter is nominally 120/240 VAC single phase. This means that you can have two 120 VAC supplies and one 240 VAC supply. Going back in time you find that the "standard" or nominal voltages in the system were 117/234. 115/230 and 110/220. What you can actually have today can go as high as about 125/250 depending upon loads, time of day, time of year, etc. This is today a 4-wire system, with the four wires being named Ground, Neutral, Line 1 and Line 2.

General practice today is that all distribution cables except for wires running only to switches will be three wires for 120 circuits and four wires for 240 circuits. Color codes for the wire insulation for 120 volt circuits are either bare copper or Green for Ground, White for Neutral and Black for either Line 1 or Line 2. In 240 (only) circuits there will be bare copper or Green for Ground, Black for Line 1 and Red for Line 2. For 120/240 circuits, the same plus White for Neutral. Wires run only to a switch may be bare copper for Ground, and White and Black for the other two. AFAIK, there is no standard for which of the two colors to a switch is still hot with the switch OFF. And don't just assume (switches aside) that any white wire is safe to touch. Electricians can be as competent or incompetent as any other trade.

Local electrical codes can vary, but in most, Ground and Neutral are connected together only at the electrical service entry panel or sometimes only at the meter in jurisdictions were the code allows the entry panel and the meter to be in separate locations. Houston happens to require the entry panel to be near the meter on an exterior wall of the building. From what I've seen around the USA, that's unusual. In any case, past the entry panel the neutral and ground wires (if both are present) are generally separate and not allowed to be mechanically connected.

It is possible to get a non-zero volts reading between Ground and Neutral. This could happen for example with a large motor running on 120 VAC fed with wire that isn't quite large enough. The true voltage being applied to the motor would be that between the White and Black wires. But the voltage being applied to you if you are dumb enough to touch the black wire would be slightly higher because you would be putting yourself between Line and Ground, not Line and Neutral. And at the instant before you touched the Black wire, there would have been no current flowing in the Ground wire. The voltage differential between Neutral and Ground probably won't hurt you but play it safe and don't touch the White wire either.
 
Just to add to Robert's post. Regarding the number of wires for 120 vs 240 circuits. While in residential applications things like ranges and clothes dryers will be 4 wire 240 volt circuits. Things like water heaters and baseboard heaters are usually 240v 3 wire. At least that is the practice here.

David
 
once someone mentions "The code" on a forum , the thread has just "Jumped the Shark" and it's all downhill from there .....Lol
 
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