non contact voltage detector recomendations

[crystal ball]

oooohhhh matron

 

[ericmark]

I needed a magnetic tester and would get the Martindale TEK 200

It did seem better at voltage detection than the standard volt stick but bought to test solenoids the volt test was just an extra feature.

 

[Seb101]

They are a bit hit and miss by nature and can easily be fooled by induced voltages from adjacent cables. However as long as you understand and accept the limitations, and don't attempt to use one to prove a circuit is dead, they have their uses. I would say it's worth spending a couple of extra quid for a decent one though.

How does one reliably 'prove' a circuit is dead then when you do not have confirmed access to the conductors (i.e. cutting into a cable run in a roof-space)?

I will agree you should spend money on a decent one - also understand the exact method of operation of your specific tester. I have a cheaper LAP one and it's method of operation is frankly potentially lethal. You are required to switch it on, which it does silently, position it near the cable, also looking for silence as the 'safe' result, then it silently turns itself off after 30 seconds. So in the normal mode of work, where you turn it on, test it on a known live circuit, then move to the circuit you want to verify not live, it may well have timed out and switched itself off, giving a false-positive silent result. It's very un-intuitive and I've become very wary of it.

However as noted, you should be wary of all testers.

 

[bernardgreen]

How does one reliably 'prove' a circuit is dead then when you do not have confirmed access to the conductors (i.e. cutting into a cable run in a roof-space)?

If in any doubt then you switch of the main incoming supply at the meter / consumer unit.

 

[Seb101]

If in any doubt then you switch of the main incoming supply at the meter / consumer unit.

Granted, but that does not 'prove' a circuit is dead. Some cowboy might have wired something through from another building/supply. I'd still use my tester before cutting.

 

[JohnW2]

Granted, but that does not 'prove' a circuit is dead. Some cowboy might have wired something through from another building/supply. I'd still use my tester before cutting.

Indeed so. As you say, there is a very slight chance that something may be dead even though you think that you have de-energised the entire installation. However, I think the problem you mentioned is only going to arise very rarely. You wrote .....

How does one reliably 'prove' a circuit is dead then when you do not have confirmed access to the conductors (i.e. cutting into a cable run in a roof-space)?

In practice, "cutting into a cable" is a pretty rare event. Electrical work will nearly always involve the ends of conductors, where they are terminated, so one will usually have access to those terminations (and hence can use a 'contact' 2-point testing device) before one actually touches any conductor.

On the (I would say very) rare occasions on which one is going to "cut into" a cable, then I agree that one has little choice but to do what one thinks has de-energised the entire installation and then probably supplement that with some sort of non-contact tester. However, even then, one should really avoid any contact with conductors (i.e. by using insulated tools and gloves) until the cable is cut and the conductors thus available for proper testing.

Kind Regards, John

 

[ban-all-sheds]

(i.e. by using insulated tools and gloves)

And face/eye protection

 

[Seb101]

Indeed so. As you say, there is a very slight chance that something may be dead even though you think that you have de-energised the entire installation. However, I think the problem you mentioned is only going to arise very rarely. You wrote ..... In practice, "cutting into a cable" is a pretty rare event. Electrical work will nearly always involve the ends of conductors, where they are terminated, so one will usually have access to those terminations (and hence can use a 'contact' 2-point testing device) before one actually touches any conductor.

On the (I would say very) rare occasions on which one is going to "cut into" a cable, then I agree that one has little choice but to do what one thinks has de-energised the entire installation and then probably supplement that with some sort of non-contact tester. However, even then, one should really avoid any contact with conductors (i.e. by using insulated tools and gloves) until the cable is cut and the conductors thus available for proper testing.

Kind Regards, John

Yes, well I've still managed to need to do that twice in the last year, once when adding an additional socket onto a ring by splicing in the ceiling void and running a loop down the wall. Once when adding an additional downlight into an existing lighting run. Now, in both cases I could be almost certain that I'd isolated the correct circuit, but not 100% certain because I could not visually trace the target cable all the way to a test point. In this case the non-contact provides an additional level of confidence.

Even if you did a contact test and read zero voltage differential between two conductors, that might be (however unlikely) because the supply had a transient power cut when you took the test reading and it may become live again 2 seconds later. I think the moral of the story is to not expect a single piece of test equipment to 'prove' anything and you use tests to confirm, with varying degrees of confidence (from non-contact through to direct measurement) that your assumptions are correct.

 

[JohnW2]

Yes, well I've still managed to need to do that twice in the last year, once when adding an additional socket onto a ring by splicing in the ceiling void and running a loop down the wall.

Fair enough, but I still think it's a fairly unusual thing to be done.

Even if you did a contact test and read zero voltage differential between two conductors, that might be (however unlikely) because the supply had a transient power cut when you took the test reading and it may become live again 2 seconds later.

Yep, anything is possible. The only 'foolproof' approach is to do everything as if one were 'working live', even if one believes that everything is 'switched off' and all 'tests for dead' have given satisfactory results - but that's not really practical, so one has to 'do all one can do to confirm safety', apply common sense, and accept the fact that almost nothing in this world is 100% devoid of any conceivable risks. Let's face it, deaths have been know to result from getting out of bed or putting one's socks on!

Kind Regards, John

 

[ericmark]

How does one reliably 'prove' a circuit is dead then when you do not have confirmed access to the conductors (i.e. cutting into a cable run in a roof-space)?

There is only one safe method, spike the cable first, using of course a proper spike device not just some Heath Robinson unit.

It does not matter how much you test it, you can't stop some automated system turning it on while your working on it.

However you can take precautions, it does depend on the installation, one good method is to consider it is live, remove outer insulation, then the armour, then with insulated tools cut one cable at a time.

The clamp on ammeter is also good, that does help detect borrowed neutrals, the worst I have worked on was a Robin tunnel boring machine, supply on the surface was transformed from 11 kV to 10 kV this feed a second transformer dropping it to 660 volt three phase to the main motors, then and this was the bit that caused all the problems 660 volt to 220 volt delta phase to phase, there was no earth on the 220 volt supply.

This caught many electricians out, you can't measure to earth as earth is fully floating, and proving dead was a real problem with two phases as with no voltage phase to phase how do you know if both are disconnected or just one? And with a fully floating system red phase may be down to earth when you test, and 5 minutes latter it is yellow phase down to earth.

The neon screwdriver filled with clear re-enterable compound was one tool, and a two prong LED tester needing no batteries again filled to stop iron laden water was the other, it was a shocking job, but money was good, 48,000 HK$ a month before the Chinese took over, danger money yes suppose it was, but you quickly learnt how to prove dead, and the neon screwdriver was one of the tools used. Anyone who says you should not use a neon screwdriver has never worked on an IT system.

 

[ban-all-sheds]

Yes, Eric, but we are discussing a DIYer and what's in his toolbox - time to regain some perspective, methinks.

 

[ban-all-sheds]

How does one reliably 'prove' a circuit is dead then when you do not have confirmed access to the conductors (i.e. cutting into a cable run in a roof-space)?

TBH he's far more likely to come to harm falling through the ceiling or off the ladder than he is cutting into a cable which belongs to next-door's installation.

However as noted, you should be wary of all testers.

 

[Seb101]

So... never trust them buggers.