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How good are cheap multimeters?

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Proud Mary

Sun May 15 2016, 12:00PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

A very grave lesson indeed.

Similar disasters can happen when TV/CRT servicing HV probes (the 1000:1 divider type) are used to measure sources where significant current can be drawn without pulling down the voltage. It is said that minor surface tracking can very quickly turn into an arc or flash-over under these conditions.

Bjørn

Sun May 15 2016, 02:37PM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

I have a UNI-T UT61D multimeter that is marked Category IV and it uses the same tiny fuses as the â‚¬4 multimeters. It would probably go off like a bomb instead of breaking the current so the rating is not always to be trusted.

ABCs of multimeter safety:
http://support.fluke.com/find-sales/download/asset/1263690_6116_eng_h_w.pdf

radiotech

Mon May 16 2016, 05:08AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

Truly sad commercial accident. Condolences to the Brother's family.

An electromagnetic voltage tester, is called a WIGGY. It is so called because the original was
made by the Wiggington Company. As an electrician for 50 years, that is how I detect the presence
of voltage, up to 600 volts AC or DC.

Dr. Slack

Mon May 16 2016, 08:44AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Yikes. All the stuff in that ABCs of multimeter safety is obvious (sort of), but I hadn't really thought hard about the safety implications of the possible cascade of events before properly. I am guilty of using whatever cheapo meter comes to hand to check the existence of mains voltage, even right at my distribution panel tails.

What I should do is invest immediately in a catIII or preferably catIV meter. What I will probably do is raid my box of HV parts and find some resistors I bought for Marx use, good for 10kV pulse, and build a mains probe that will protect even a catI meter from transients. I don't sell my electrical engineering services, so that's a viable solution. (it turns out that a fairly cheap distributor brand meter I have is alleged to be cat2/1000 cat3/600)

Donning PPE (personal protective equipment) before making a measurement? I am gradually learning to put gloves on before taking a chunk out of my hand with a sharp tool, rather than after, but for making mains measurements? I'll try, honestly, maybe. Fortunately, with an astigmatism, I wear glasses 24/7, so the bare minimum of hardened eyes is met (there has to be an up side to wearing glasses).

Proud Mary

Mon May 16 2016, 11:19AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

I received a small but frightening shock from the handle of an HV probe (the 1000:1 type used to service TV CRT EHT supplies) which I had bought on ebay. Inspection of the handle showed that one of four small nylon countersunk screws holding down an inspection plate had been replaced with a metal screw. This metal screw was not in contact with the circuit in the handle - the measuring resistor - and can only have become live as part of a line of least resistance between my hand and the wires within the handle.

The measurand was about 25kV, the maximum safe working voltage of the probe, according to a sticker on the red insulator.

It seems to me that the danger to which I was exposed here was not limited to the small but disagreeable shock from the handle to my hand, but the possibility of a secondary calamity, for example the possibility of my being thrown by muscle contraction onto some other HV equipment etc. by the force of the shock.

Following this event, I obtained from a Polish company a pair of Class 3 insulating gloves certified to 26.5 kV and now wear one of these on the odd occasion when I use an HV probe, with and without metal screws in the handle!

rikkitikkitavi

Mon May 16 2016, 12:50PM

Registered Member #57278 Joined: Wed Sept 16 2015, 11:33AM
Location:
Posts: 8

Wolfram wrote ...

The only major caveat with cheap meters is that they often have very poor input protection, replacing expensive HRC fuses with cheap glass fuses. This can be very dangerous in situations where high fault current is available. This is mainly a concern when measuring supply-side mains wiring and capacitor banks. See this link

for an account of the failure of a cheap multimeter leading to two deaths.

For mostly this reason I use my cheap < 10 E meters only at my labbench (isolated 230VAC supply through a not to low impedance variac) and low voltage DC of course.

For household work I use my "real" meters, ie known brand with certified CAT rating.

radiotech

Mon May 16 2016, 04:56PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

In a nutshell. Your little meter is an enclosed air space. An electric arc in that space will heat the air
and everything else too, so quickly that an explosion will take place. Rate of rise of temperature.

Your safety categories are useless once the arc starts. Arcs shoot out highly ionized
gasses, that can short out adjacent conductor pairs of opposite polarity.

You may have noted this phrase on switches and relays. Same polarity only.

The energy that drives the arc, in joules, comes from what you have connected the wires to.

hen918

Mon May 16 2016, 05:11PM

Registered Member #11591 Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556

I've put enough current through my triggered spark gap to know I don't want that happening in an electrical panel near me. I think an email to my manager is in order: My work multimeter is definitely not up to scratch.

radiotech: I think the idea is to not end up with an arc in the first place. Cat IV probes have built in fuses in the handle that can interrupt more current than the resistance of the test leads allow and multimeters should have large creepage distances, so very high transients don't breakdown into arcs.

Cheap multimeter's traces seem to have a greater current interrupting capability than their fuses in my experience :)

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