Multimeters, Can we use them on Random exposed cables.

[James1985]

I have been toying with multimeters on cables carrying currents I know the ratings of.

If we are out in the field, is it safe to use them on exposed cables we don't know the rating of?

 

[James]

what is the safe voltage rating of your meter?
if it allows 400v or more then you are ok unless you are in a place with HV cables.

 

[pc1966]

It depends on quite what you are doing.

1. If this is general LV cables that might include the supply to an installation then your multimeter has to be CAT-IV rated to at least the highest operating voltage you might expect to see.
2. For "proving dead" there are so many ways you can go wrong with a multimeter and end up dead yourself it is not worth it. Get a voltage tester!
3. If using a multimeter for current measurement in a power system is just asking for a blown fuse due to more than expected or switch-on surges, so really you should use a clamp meter for that rated to highest expected current (domestic and light commercial what would be 100A in most cases, but other cases you might need to kA).
4. For voltage measurement you have to allow for the ~10M impedance of a typical multimeter, so it can see capacitively coupled voltage where no real power/current is present. It is telling you the truth in a sense, but for power circuits that "truth" might be quite misleading. Again a voltage tester that draws a few mA to light a LED is more meaningful in many cases.
5. Having said all of the above, for medium resistance (few ohms to hundreds of kOhm) it covers what a Megger/MFT/IR tester lacks, as well as other aspects like very low current/voltage, diode check, and sometimes frequency/capacitance/etc so it can still be useful so long as you understand the above points.

 

[pc1966]

I would just add that if you have any MV/HV cables or circuits you need to be VERY careful about how you check stuff.

I remember reading a report from the USA from a couple of years ago on deaths and injuries from electrical accidents and it had examples where a multimeter was involved. In some cases it was down to a mistake in its use (e.g. testing for AC volts on DC range and seeing little or nothing) and in the others it was due to the meter being inadequately rated and exploding in the face of the tester (e.g. checking for volts on current range and no suitable HRC fuse in the meter or probes).

In one of those cases it was not a cheapo eBay special but a proper CAT-IV Fluke meter, but the guy was checking continuity on a 4.16kV motor that was accidentally energised and the meter was not safe at that sort of voltage & fault current.

 

[pc1966]

Found the report:

https://www.nfpa.org/-/media/Files/News-and-Research/Fire-statistics-and-reports/Electrical/RFArcFlashOccData.ashx?la=en

It makes for sobering reading, and the multimeter's rating is only one aspect of what went wrong, but Incident 5 on page 82 has the full story.

 

[timhoward]

The original question is so abstract that the answer really has to be "no". So many caveats have to be added to make it safe.
In particular, there are more suitable tools for testing exposed cables.

 

[Carolina]

Not the perfect tool for the job but it can be useful as long as its got all of the protections, I do find separate tools better though, if I want to know the voltage I just grab the voltmeter and for current the clamp or ammeter.

Few years ago I bought one of those cheap yellow multimeters to play around with, and tried measuring mains voltage, the moment I probed the live contact of the socket there was a short and the multimeter exploded, I wasn't holding it so nothing happened to me. I had it on the right scale and everything, I guess it was just a terrible unit that wasn't meant to test mains even though it claimed it could.

From that moment on I just use my multimeter to measure low voltage DC and resistors and an analogue voltmeter with a series of tiny light bulbs (load) for mains, it can measure up to 500V so enough for both single and three-phase power.