Safe Isolation Dilemmas.

[JohnW2]

Depending on conductor size the Drummond test lamp with straight probes can pierce either paper or XLPE insulation. These are the ONLY type approved for out use

What (if anything) do you do by way of a repair after piercing the insulation?

My point was obviously that, with fine enough probes (attached to the test lamp, or whatever, of your choice) it may well be able to reach the conductor in a crimped joint without piercing or damaging any insulation, provided it is not heat-shrinked.

In a domestic environment (no spiking allowed ) proving that a cable is dead when it's fully insulated with no accesible conductors does seem to present a bit of a problem. Demonstrating that it is probably not dead might be easy (e.g. with a 'volt stick' or the like), but proving dead is a different matter. I think a lot of us probably have some ideas about what we might do in practice, but don't necessarily want to talk about that!

Kind Regards, John.

 

[JohnW2]

That's a fair point but it does give a voltage value on display, ....

That's rather intriguing - does it given even remotely reasonable voltage figures?
Kind Regards, John

Am I missing something here, or have you completely lost the plot, John?? You might be able to measure that a voltage is present, but how the hell would you take an actual reading.

Exactly! The plot is not lost. What you're 'missing' is the reason I'm 'intrigued' (euphemism for 'somewhat incredulous' ). I did, after all, previously write:

It surely can't measure volts via it's clamp - that would make no sense.

Voltage, or should I say potential difference, as the name implies, is a difference in potential between two points - How's that work with a clamp meter?? What two points is the voltage being measured between prenticeboy - when you get your reading??

If what we're being told is remotely real, I presume the two points being measured between are the conductor and earth, with capacitive coupling to both. With a high enough impedance measuring device, that could theoretically give a reasonable indication of that PD (a sort of semi-quantitative extension of the neon screwdriver concept).

Kind Regards, John.

 

[Electrifying]

presume the two points being measured between are the conductor and earth, with capacitive coupling to both. With a high enough impedance measuring device, that could theoretically give a reasonable indication of that PD (a sort of semi-quantitative extension of the neon screwdriver concept).
Kind Regards, John.

But even that couldn't give you a 'reading'.

As far as I'm aware - and back to basics here - you need two of the three to calculate the remaining one.

With a clamp-metre you don't even get one of the three - unless the cable is under load, then you get the current...........care to explain how you would then jump from that to a voltage measurement??

 

[JohnW2]

presume the two points being measured between are the conductor and earth, with capacitive coupling to both. With a high enough impedance measuring device, that could theoretically give a reasonable indication of that PD (a sort of semi-quantitative extension of the neon screwdriver concept).

But even that couldn't give you a 'reading'. As far as I'm aware - and back to basics here - you need two of the three to calculate the remaining one. With a clamp-metre you don't even get one of the three - unless the cable is under load, then you get the current...........care to explain how you would then jump from that to a voltage measurement??

When you speak of 'two of three' are you talking about Ohm's Law? If this rather 'surprising' method we're being told about works to a useful extent, we would be talking about the normal method of voltage measurement - the unknown voltage resulting in a (measured) current flowing through the (known) impedance of a voltmeter and its leads/connections. So, with the known current and known impedance, Mr Ohm tells us voltage, which is what is displayed - i.e. you have two of the three, from which you determine the third. As you say, Basic Stuff

The only difference between this and 'normal' voltage measurement is that both connections to the meter have very significant impedances (essentially capacitve reactances). The higher the internal impedance of the meter gets in comparison with the impedances of the two capacitive connections, the closer will the voltage across the meter, hence displayed voltage, get to the PD between conductor under test and earth. However, as I implied, to get readings reasonably close to that PD, one would require a very high impedance voltmeter.

Kind Regards, John

 

[Electrifying]

When you speak of 'two of three' are you talking about Ohm's Law? If this rather 'surprising' method we're being told about works to a useful extent, we would be talking about the normal method of voltage measurement - the unknown voltage resulting in a (measured) current flowing through the (known) impedance of a voltmeter and its leads/connections. So, with the known current and known impedance, Mr Ohm tells us voltage, which is what is displayed - i.e. you have two of the three, from which you determine the third. As you say, Basic Stuff

Kind Regards, John

Here you have totally lost me.

If I measure voltage to earth in the normal fashion, my probe goes on the incomming 'line' conductor and the second probe goes on my earth (say the MET).

My meter then applies a resistance between these points and, using the current that then flows through this resistance (or load)..., gives me a reading of the voltage to earth.

How on earth can you say that putting a clampmeter around a cable and measuring the current flow (by inductance), can give you current and resistance, in order to measure the voltage to earth.......resistance of what??......between what two points???.......Through the clampmeter, your body and down to earth?????

 

[Electrifying]

To make my point simpler, John.

If we had two identical cables.

One on a 50volt supply and one on a 230volt supply.

If the 50volt supplied cable was 10 metres long with a load of 500 watts, the clampmetre should read 10 amps.

If the 230volt supplied cable was 10 metres long with a load of 2300 watts, the clampmetre should read 10 amps.

Could you now explain how the clampmetre, using the 'current' readings that it has taken and it's internal impedance/resistance, can then make the leap to tell us what the voltage is on each cable?

And, just to make it more interesting, if we put the clampmetre around the neutral of either of the above examples, we should still get a reading of 10 amps - what voltage reading would the clampmetre give us then??

 

[JohnW2]

Here you have totally lost me. If I measure voltage to earth in the normal fashion, my probe goes on the incomming 'line' conductor and the second probe goes on my earth (say the MET). My meter then applies a resistance between these points and, using the current that then flows through this resistance (or load)..., gives me a reading of the voltage to earth.

Exactly.

How on earth can you say that putting a clampmeter around a cable and measuring the current flow (by inductance) ....

I said no such thing. It would be complete nonsense, not the least because there might well be no current flow to measure in the conductor under test - and, even if there were a current, it would tell one nothing about the PD between that conductor and anything else! I was suggesting that there would be a capacitive coupling between conductor and something in the clamp meter head - maybe the current sensing coil, maybe something else.

That capacitive coupling would be to one side of the ('volt') meter. The other side of the meter would have to be capacitively coupled to earth, perhaps simply from metalwork within its casing or, I would have thought more likely if such an 'unlikley' machine were to exist, by having something the operator had to touch (like with a neon screwdriver) to provide a larger capacitive coupling to earth. The meter would then indicate the PD between the conductor and earth minus the total voltage across the two capacitive couplings. The higher the impedance of the meter relative to the combined impedance of those two capacitive couplings, the closer would the voltage across the meter, hence displayed voltage, get to the PD between conductor under test and earth,

However, one would probably need a meter of almost ridiculous internal impedance for this to give useful readings. Try it with a standard multimeter and you'd probably be lucky to get much more than a volt or two displayed when 'testing' a conductor which had a PD of 230V relative to earth! That's why I question what we're being told!

Kind Regards, John.

 

[JohnW2]

To make my point simpler, John....
......
......
......

As I've just explained, you have totally misunderstood what I was saying. What I'm talking about has nothing to do with measurement of current (if any) flowing in the conductor under test. Please see my previous post - which I hope clarifies things for you.

Kind Regards, John.

 

[JohnW2]

[However, one would probably need a meter of almost ridiculous internal impedance for this to give useful readings. ....

I've just done some quick at dirty calcs. If one assumes a capacitance to earth via the body of the operater of about 100 pF (fairly typical)(31.83 MΩ at 50 Hz) and a capacitance from conductor under test to the test 'sensor' of 1 pF (probably pretty optimistic) (3183 MΩ at 50 Hz) then, to get a meter reading which was only 10% low (in relation to the true PD between conductor and earth), one would need a meter with an internal impedance of around 28.9 GΩ - which is, as I suggested, totally ridiculous!

One could, of course, give the meter a direct connection to earth, thereby eleimination of of the capacative couplings, but that would still leave the coupling of enormous impedance, hence would make virtually no difference to the situation.

If one knew what the two capacitances were, one could, of course, then use a meter of very much lower (perhaps even attainable!) impedance, and then simply multiply the (low) displayed voltage by a known correction factor - but, sadly, one will not know those capacitances!

In practical terms, the whole idea is just fanciful and plain daft.

Kind Regards, John.

 

[securespark]

Interesting reading.

I see what you're saying about the safe isolation angle, but what I was getting at was how exactly it is possible to prove safe isolation in those cases without a clamp meter.

Without the use of our old friend ASSUME, it is impossible.

With the crimped leads, you cannot prove the presence of voltage.

With the circuit already dead, how do you prove it is safe to work on?

IE, it may be dead now, but if an intermittent break in the circuit recloses, it will be live again...

And with the third scenario, similar. How can you prove which circuit the damaged (and dead) cable belongs to in order to identify and make it safe to work on?

 

[JohnW2]

I see what you're saying about the safe isolation angle, but what I was getting at was how exactly it is possible to prove safe isolation in those cases without a clamp meter.

As has been discussed, you couldn't do it even with a clamp meter.

With the crimped leads, you cannot prove the presence of voltage.

There are ways which might well (but not guaranteed) demonstrate the presence of voltage but, without gaining access to the conductors, there's no way of guaranteeing the absence of voltage.

With the circuit already dead, how do you prove it is safe to work on? IE, it may be dead now, but if an intermittent break in the circuit recloses, it will be live again...

If you don't know what circuit it belongs to, hence cannot with certainty isolate 'at source', then you obviously have a problem. Mind you, it's far from a unique problem you're talking about. Even when one does know the source of the circuit, has isolated it at source, and has tested for dead, one cannot be certain that there is not an intermittent fault that might suddenly and unexpectedly connect electricity from some other circuit to the one being worked on.

The only certain way is to completely switch off (and secure) the entire installation.

And with the third scenario, similar. How can you prove which circuit the damaged (and dead) cable belongs to in order to identify and make it safe to work on?

I suppose you could isolate the installation and then do 'dead' continuity tests back to the CU, in the hope that one of the conductors (L, N or CPC) was still intact. There will, of course, usually be clues, in that if a cable is damaged to the extent of being dead, something in the premises presumably won't be working - not to mention clues from the size and whereabouts of the cable etc..

Kind Regards, John.

 

[ban-all-sheds]

I was directing that question mainly to Ban.

Who cannot remember what he thought his plan might have been.

As far as I can see you're hosed - short of physically tracing the cable back to the supply, all you can do is best chance at having isolated (e.g. main switch), imperfect verification (e.g. voltstick as you or assistant turn it on and off a few times), followed by PPE-ing up and chopping it.

 

[ban-all-sheds]

The only certain way is to completely switch off (and secure) the entire installation.

How do you know you're looking at ... the DB/CU for it?

 

[JohnW2]

The only certain way is to completely switch off (and secure) the entire installation.

How do you know you're looking at ... the DB/CU for it?

By looking at the origin of the installation (i.e.meter) it will be possible to ascertain whether there is one or more DBs/CUs. If, in the common situation, there's only one, then you're laughing. If more than one, you find them all, and flick all their main switches. I suppose one could get caught by a submain which branches, out of sight, but that's an extremely unlikely scenario.

Of course, if there's an isolator or RCD immediately post-meter, then one really is laughing - unless some joker has been playing with wiring between different premises

Kind Regards, John.

 

[ban-all-sheds]

It's been known.