TESTING FOR ABSENCE OF VOLTAGE

Ignoring regulatory advice for a moment, as I am wont to do, really what we need to show is not ' dead' in the sense of open circuit at some remote point, but rather 'safe to handle' which means the conductor to be worked on, is at more or less the same  voltage as the simultaneously accessible parts in the local envoronment. Any absolute reference to some remote 'zero' is only really of secondary interest and it ought to be possible to show no current available to the local metalwork, certainly by direct connexion, and probably by non-contact probe as well,

Mike.

Mike, whilst I think I agree with your perspective in part, the issue we have to face up to, as well, is that it's not always safe (or wise) to disconnect protective conductors.

Even in TN-S systems, diverted protective conductor currents can exist. We know they are more prevalent in TN-C-S as neutral currents can be shared also.

Even if you test for potential before starting work, the act of disconnecting a protective conductor can leave a latent voltage, as you break the circuit current was travelling through ... but the voltage that remains, even if not 230 V, may be capable of driving a few mA through the human body ... perhaps enough to be lethal.

I think there's a difference, though, on working in a very large installation, to a "basic semi" - agree with you that in the semi, unless you are unlucky enough to have diverted PME neutral currents, removing a cpc is unlikely to expose you to danger (if there's bonding or fortuitous earth connection downstream). But that's not always the case in a larger installation ... as some telecomms engineers know all too well.

Mike, whilst I think I agree with your perspective in part, the issue we have to face up to, as well, is that it's not always safe (or wise) to disconnect protective conductors.

Even in TN-S systems, diverted protective conductor currents can exist. We know they are more prevalent in TN-C-S as neutral currents can be shared also.

Even if you test for potential before starting work, the act of disconnecting a protective conductor can leave a latent voltage, as you break the circuit current was travelling through ... but the voltage that remains, even if not 230 V, may be capable of driving a few mA through the human body ... perhaps enough to be lethal.

I think there's a difference, though, on working in a very large installation, to a "basic semi" - agree with you that in the semi, unless you are unlucky enough to have diverted PME neutral currents, removing a cpc is unlikely to expose you to danger (if there's bonding or fortuitous earth connection downstream). But that's not always the case in a larger installation ... as some telecomms engineers know all too well.

Yes - actually a check for 'is current flowing' is  another useful example of a non-contact measurement to make, and a direct connection to measure current is less safe than a non -contact (clamp meter) one as for a direct connection test you have to open circuit a possibly unknown cable before you asses the current flowing.

But I agree also on the point about the scale of the installation. If you know the whole building is dead, then there is far less risk of interrupting something nasty - but not none - current may come  in on the water pipes and go out on the CPC or something.

So now we have two none contact tests - one with the E-field probe and one with a magnetic current clamp. In an ideal world one of each would precede dismantling.

M.