Protection in Multi Meters

[Dogmatic1983]

I would like to get some information on what protection is on a multi meter when accidentally testing a live circuit while the meter is on the ohms range?

I seem to be getting different sides to the argument.

One side says that there is no protection against this other than a audible warning on devices that you have to physically press a test button.

The other side says that you can have a fuse which will stop any internal damage.

I just don't understand how that fuse would work on the V/ohms lead connection.

 

[pc1966]

You need to look for test equipment that is rated to at least CAT-III and 300V for typical domestic circuit testing, but ideally CAT-IV and 600V if you are probing stuff around your incoming supply.

Measurement category - Wikipedia

en.wikipedia.org

This is a very important aspect because people have been killed and maimed by test gear literally exploding in their faces if they make a mistake on a high energy circuit. Even with typical professional ratings (as above), I know of one incident when the guy died from burn injuries after they tested a MV motor that was live at 2-3kV when doing continuity tests, sadly it was just so far above the safe rating of the meter that it could not protect itself using its internal fuse.

However, the usual advice and training for professional electricians, at least over here in the UK, is you don't use a multi-meter for safe isolations as there are too many ways in which it can give you a false sense of safety:

• Using DC volts and not seeing AC volts - you die
• Using current, adequate fuse - fuse goes, if you didn't notice the "pop" it reads nothing, you die
• Using current, no or adequate fuse - meter explodes in your face, you die
• Using ohms or other range - see above for DC or amps

That is why the preferred approach is to use a simple voltage indicator that does one job and does it reliably, this sort of thing:

AC/DC Voltage Tester - ET45 | Klein Tools

Klein Tools ET45 is an Electronic Voltage Tester. Designed as a simple, easy to use tester to check, verify or troubleshoot common residential electrical wiring in electrical outlets, light fixtures, doorbells and more. This tester does not require batteries, and features a pocket clip for...

www.kleintools.co.uk

As well as using a simple & safe tester, you need to know it is working as well! So normally you would use a "proving unit" that generates some voltages to verify the tester (both before and after you have proven a circuit dead) but if you can't afford one or it has gone AWOL, then test a known life circuit to check the tester responds OK and then test the one you are unsure of.

https://www.cef.co.uk/catalogue/products/1744862-proving-unit

Also be wary of the non-contact voltage detectors, they have a use but can give false results in a number of cases, both false positive (says live but just capacitive leakage) and false negative (dangerously live, but you are in a high field area so not shown).