how to measure existing shower cable?

[JohnW2]

I would want to be 100% sure I had safe isolation before using a metal caliper on an electrical cable.

I would hope that you would want to be 100% sure that you had safe isolation before coming into direct or indirect electrical contact with any live conductor, whether via a metal caliper or anything else.

A multi meter is not an 'Approved' voltage indicator .... It is difficult to know if a multi meter is working, or it could be on the wrong setting.

True. It is also difficult (actually impossible) to know for sure whether an " 'Approved voltage indicator" is working unless one 'proves' it immediately before and after use for 'testing for dead'.

I admire your concern about, and stressing of, this important safety issue. However, I sometimes worry when people may be given the impression that using an "approved 2-pole tester" is a 'foolproof' solution - since it is, in reality, no more 'foolproof' than anything else if it is not proved immediately before and after every use. It is intrinsically unsatisfactory for a test device to indicate a 'safe' situation by 'doing nothing', and the best one can do to mitigate that is to make sure that it 'does something' immediately before and after it is used in anger (and 'does nothing').

Kind Regards, John

 

[studentspark]

Indeed, but the safe isolation procedure does involves proving the device is working, either on a known source , or a proving unit, before, and after testing for isolation.

Switching off something, may indicate you have the right circuit, but it could also be coincidentally that the item in question went off, but in-fact has failed in some way, such as a lamp blowing

And of course you would want to check for isolation no matter the job, but clamping a bit of metal onto a conductor, makes me a bit more cautious. I just think its a bit dangerous to suggest on a forum, as nobody knows anybody's competence.

 

[JohnW2]

Indeed, but the safe isolation procedure does involves proving the device is working, either on a known source , or a proving unit, before, and after testing for isolation.

Exactly - but my point was that the proving procedure is as adequate for a multimeter as it is for an " 'Approved' voltage indicator ".

If the multimeter is not working, or is set to the wrong range, that will be revealed by the proving process - so I don't see why, if used properly (including 'proving') a multimeter is any less appropriate/safe than is an " 'Approved' voltage indicator " (which is what you seemed to be suggesting).

And of course you would want to check for isolation no matter the job, but clamping a bit of metal onto a conductor, makes me a bit more cautious.

Yes, and my initial gut feeling probably would be the same. However, it's not very rational .... although, in gut-feeling terms, 'clamping a bit of metal onto a conductor' sounds/feels' worse', in reality just a 'brushing against' a live conductor can theoretically be just as lethal - so I think we should be careful not to 'reassure' ourselves (or others) that it is only the most 'obvious' things (like 'clamping a bit of metal onto a conductor') that can be dangerous.

Kind Regards, John