testing between live and neutral

[jimbobz]

so I have seen youtube videos of people using multimeters to test between live and neutral on a socket.

then i read this:

If you get a short-circuit between live and neutral in a power outlet, for example, the current that will flow will be immense. Without a fuse it could easily rise to thousands of amps. Now, although this would be inconvenient, oddly enough it probably wouldn't be all that dangerous, because the cable will simply melt right through in a fraction of a second and break the circuit. There would be an enormous bang and a puff of smoke and that would be the end of the problem

so is he just talking about a short in an appliance? what happens if you test between live and neutral say on a mains cable or on the consumer unit?

 

[Iggifer]

If you test between L-N you will get ~230Vac

I think that is referring to a dead short rather than testing. Decent multimeters / testers have a fuse anyway (as do some cheap ones)

Testing between L-N and L-E is perfectly safe assuming you know what you are doing.

 

[ericmark]

He has missed out ionisation. The big danger with any electric system is although in the main air is an insulator should the air become ionised as it does with a thunder storm then the air its self conducts. Lucky does not happen that much but shorting out line and neutral can cause this to happen. Because of this I use a meter which does not in any configuration cause a short between the leads.

It is near impossible not to have an automatic means of disconnection be it a fuse (often sand filled to stop ionisation and conduct heat) or a MCB (two devices in one, with magnetic and thermal devices) or a RCBO (also has RCD built in) and with a house the DNO fuse (no more than 100A) will also blow if all else fails.

If you melt copper it does not go bang. What causes the bang is ionisation as with thunder. The resulting explosion can distribute the copper over a large area so not only is there a danger of electric shock but also molten metal and the pressure wave caused by the explosion which can propel a body quite some distance.

So he is very wrong it is very dangerous. Although technically since neutral is live connecting neutral to live will only cause a problem if the live wire is what we call line or phase.

 

[ban-all-sheds]

what happens if you test between live and neutral say on a mains cable or on the consumer unit?

Depends on what range your multimeter is set to.

 

[jimbobz]

multi meter set to 600v.

i got 250v between live and neutral. 250 live to earth. nothing neutral to earth.
continuity between neutral and earth, N to L and L to E

 

[ban-all-sheds]

Continuity testing like that is a Bad Idea.

 

[jimbobz]

well perhaps if you offer an explanation i might take your advice

 

[echoes]

Continuity testing like that is a Bad Idea.

I can attest to that. 'Measured' L-N accidentally on the ohms range on an AVO as a wet-behind-the-ears teenager. Bang. Bent the needle over the end-stop but the movement survived.

 

[ban-all-sheds]

well perhaps if you offer an explanation i might take your advice

On a resistance setting a multimeter has a very low resistance, and were you to connect it to a live circuit, N-E would trip the RCD, L-E or L-N would be spectacular and probably fatal to the meter, possibly dangerous to you.

Connecting it to a dead circuit would tell you nothing of value.

 

[ericmark]

well perhaps if you offer an explanation i might take your advice

On a resistance setting a multimeter has a very low resistance, and were you to connect it to a live circuit, N-E would trip the RCD, L-E or L-N would be spectacular and probably fatal to the meter, possibly dangerous to you.

Connecting it to a dead circuit would tell you nothing of value.

Although some meters would react that way my clamp on meter certainly does not. It was selected because on any range it will stand 250 volt so it is impossible by mistake to either damage the meter or cause ionisation of the atmosphere.

As a kid I destroyed my AVO multiminor because of wrong range selection and I even destroyed a AVO Mk 8 when testing a thyristor circuit the high back EMF took out the germanium diode even with both fuse and cut-out still not fully protected. I am sure there will be a way to destroy my clamp-on but following Health & Safety Executive Guidance Note General Series 38 one has a reasonable chance of not injuring one's self should you make a mistake.

Although one can get meters able to withstand 250 volt on ohms range when amp range is selected with a wired meter only the fuse can protect and one can still cause ionisation of the atmosphere. With this in mind I use a clamp on meter for general use.

The other point is most electricians will use a device to measure voltage which has no on/off switch or batteries so when proving dead the chances of error in either range selection of dead batteries is removed. I know we should use a proving unit of a type which tests all the voltage ranges but in real terms even when one does have a proving unit it will often only test at 500v so the tester could have a fault where at 230v the lights will not work plus of course one can have a bad connection. So I tend to use both the proper tester and a volt stick just in case I do have a bad connection. Using leads to GS38 to prove dead has the problem that the fuse may have blown or have a bad connection inside the lead.

However to test a socket I would consider a tester like
this would be far better specially if the user is not that well trained. OK I would use a proper loop impedance tester but for the DIY guy I would think the plug in tester shown is a good compromise.

 

[Monkeh]

Any serious meter will survive being put across 240VAC (hell, 1000VAC in most cases) on any setting. Obviously in current ranges you'll have a fuse to change.

Cheap toys are a different matter.

 

[acmedone]

continuity between neutral and earth, N to L and L to E

Treat a multimeter purely as device to show a fault is present. It is no good at showing that a fault is absent.

So, when measuring (power off!) between separate conductors (ie, L-E, L-N, N-E), a low reading indicates a fault; a high reading does not prove lack of fault. For example, in a test of one of my circuits, an electrician with an insulation tester found 30k N-E which showed in the meg range of a multimeter.

The best your test could achieve is to show whether a direct short exists between N-E (perhaps a nick through insulation behind a socket). A fault like this between L-E would produce a bang.

Similarly, when measuring between two points on the same conductor (ie, L-L, E-E, N-N) a high reading shows a bad connection (eg, a wire not held by a screw); a low reading is not proof of a good connection (the wire is just touching).

 

[MJN]

However to test a socket I would consider a tester like

With a caveat that they're no good for detecting neutral/earth reversal.

Mathew

 

[RF Lighting]

Or for testing TT supplies.

On and they get confused with back-feeding open circuit neutrals too