Multimeter sparked

[Rotunda82]

Was looking for a fault on the heating system (turns out to be a zone valve which had melted) and was having some trouble with my multimeter.
I always thought the best way to test for voltage was L&E, L&N, N&E
I tested between L&E in the boiler (oil fired) and there was a small pop and spark which tripped the rcd.
I found and replaced the faulty zone valve and now heating is all working but when I re tested at the boiler (out of interest) the same thing happened.
Is it my basic multimeter causing the issue or could there be another fault?
When I was testing in the wiring centre it was fine. Only happened when testing at the boiler between Live and Earth

 

[ericmark]

Not all multimeter's are the same, mine is 20,000 Ω/volt so on the 250 volt range so 0.5MΩ so 0.046 mA seems very unlikely that would cause a RCD to trip. Are you sure it was on the correct range?

 

[Rotunda82]

Hi,
Thanks for the quick reply. Yes it was set to the correct range. It’s only a cheap centrebrand one but have used it many times in the past on the same settings and never had an issue.
I used it to test the fused spur and that was fine between L&E - I got 240v. Went back to the boiler and tested again and still popped.
As it’s an oil boiler the live isn’t a permanent live but a switch live if that makes any difference?
Having said that I’ve tested between switch live and earth on other boilers without problems.

 

[endecotp]

Unless you’ve accidentally set it to a current range, the meter is borked. Throw it out. (or maybe keep it for non-mains use.)

 

[JohnD]

You can't rely on it so bin it.

But first, you might like to open the case and look for a replaceable fuse or some easy fix.

 

[JohnW2]

But first, you might like to open the case and look for a replaceable fuse or some easy fix.

It sounds as if it still measures ~240V AC satisfactorily everywhere other than at his boiler!

Given the RCD trips, I have to wonder whether he is somehow managing to create shorts (most likley N-E) with his probe when measuring at the boiler.

Kind Regards, John

 

[Rotunda82]

Thanks for the replies all.
I have used the multimeter since quite a few times and all seems fine. I only use it really for testing 240 when fault finding and occasionally for continuity when testing solenoid coils.
I Definitely didn’t short it across terminals as was testing a terminal strip so just put the probes into Earth and Live.
Perhaps it will remain a mystery. However I may invest in a better quality tester.

 

[JohnW2]

I Definitely didn’t short it across terminals as was testing a terminal strip so just put the probes into Earth and Live. ... Perhaps it will remain a mystery.

It certainly sounds very mysterious. Have you had any subsequent tries, wit the same result?

Kind Regards, John

 

[Rotunda82]

It certainly sounds very mysterious. Have you had any subsequent tries, wit the same result?

Kind Regards, John

Hi John,

Yes I’ve tried it again (3 times in total on different days) and each time there is a pop and a spark.
I did read earlier that with some multimeters you should test between L&N first before testing between L&E but I’m not quite sure why. I’m just baffled as I’ve tested many boilers in the same way and never had this happen before.
I would have thought if there was an issue with the heating system it would have presented itself by way of a fault by now?

 

[JohnW2]

Hi John, Yes I’ve tried it again (3 times in total on different days) and each time there is a pop and a spark. I did read earlier that with some multimeters you should test between L&N first before testing between L&E but I’m not quite sure why.

It really makes no sense. There is no way that there should be any 'pop and spark' (and also still RCD trips?) when an (otherwise functioning satisfactorily) multimeter on a voltage range is subjected to a voltage within that range.

Does the meter have a higher AC voltage range (maybe 600V or 1000V) and, if so, have you tried that?

Kind Regards, John

 

[EFLImpudence]

The Fluke T100 etc. two-pole testers (not a multimeter) act like the OP has mentioned.

That, though, does not explain why it happens only on one appliance.

 

[JohnW2]

The Fluke T100 etc. two-pole testers (not a multimeter) act like the OP has mentioned.

That's because it has an 'internal load' (to preventing it picking up 'stray voltages'), but that should not be true of any multimeter in its normal voltage measuring configuration, when it should present an input impedance of megohms.

Having said that, some multimeters (usually only quite sophisticated/expensive ones) do have a facility (e.g. 'a button to press') to apply a load across the voltage measuring probes (again, to eliminate the pick up of induced etc. voltages).

Rotunda: can you tell us the make/model of your multimeter and/or post a piccie of it?

Kind Regards, John

 

[zebedee2001]

Are you sure you are not applying pressure to a terminal or wire that is then shorting out on the case or another earthed item? Your multimeter may be OK, the wiring inside the thing may be the issue!