Voltage testers - a basic question

[TheJack]

Hi. As part of safe isolation we use a voltage tester to prove dead; I've been taught that this is done by putting the probes in the following combinations: N+L, N+E, E+L.

But why check anything apart from the Live? That's where the voltage is coming from right?

 

[westward10]

Assume every conductor is live until proved otherwise.

 

[mainline]

Hi. As part of safe isolation we use a voltage tester to prove dead; I've been taught that this is done by putting the probes in the following combinations: N+L, N+E, E+L.

But why check anything apart from the Live? That's where the voltage is coming from right?

How would you check just the L ?
How would you know that the N is disconnected
How would you know that the E isn't L

 

[TheJack]

Assume every conductor is live until proved otherwise.

Ok but still, why test between 2 conductors? Why not test each individually?

 

[mainline]

Ok but still, why test between 2 conductors? Why not test each individually?

You can't reliably test one conductor without a reference.

 

[TheJack]

You can't reliably test one conductor without a reference.

This sounds good but I don't really understand. Can you point me to a resource with a fuller explanation please? I have spent most of the evening looking online but just can't find it

 

[mainline]

This sounds good but I don't really understand. Can you point me to a resource with a fuller explanation please? I have spent most of the evening looking online but just can't find it

What is the part that you can't understand ?

This explains a bit.

Safe Isolation Procedure - A Thorough Step By Step Guide. - Kewtech

Safe isolation is a task that needs to be undertaken for the safety of persons working on electrical installations and others who may be impacted by the process.

www.kewtechcorp.com

 

[TheJack]

What is the part that you can't understand ?

This explains a bit.

Safe Isolation Procedure - A Thorough Step By Step Guide. - Kewtech

Safe isolation is a task that needs to be undertaken for the safety of persons working on electrical installations and others who may be impacted by the process.

www.kewtechcorp.com

Thanks for this, fortunately I am familiar with the process itself but what I am stuck on is if we need to check that there is no voltage potential in each conductor why a tester couldn't be designed that you simply put on the end of the conductor (like a voltage pen but I understand they are not considered reliable enough)? So in your explanation above I don't understand "without a reference".

 

[UKMeterman]

Thanks for this, fortunately I am familiar with the process itself but what I am stuck on is if we need to check that there is no voltage potential in each conductor why a tester couldn't be designed that you simply put on the end of the conductor (like a voltage pen but I understand they are not considered reliable enough)? So in your explanation above I don't understand "without a reference".

When you do a voltage test the test instrument is checking is there a "pressure" difference between that point and another point. When it is testing the pressure difference a very tiny current has to "flow" in the test instrument. To allow the test current to flow it is a two pole device and thus needs two connections. This is also why you need to be able to prove the tester works before and after any proving dead tests, this needs either a proving device or a known live that is still live such as the supply side of the main switch.

 

[timhoward]

I think there are two concepts here...
1 - the measurement of electricity - an alternative term for voltage is "potential difference" and I think it will help you to think of it like that. Any time we measure voltage we are measuring between two things. Any other method is unreliable and pretty meaningless as you can't say with confidence what it will do to you by just touching one point.

2 - how the electricity is made. the best diagram I can find at this time of night is this one which I've added to a bit...

The mass of earth is used by the supply companies as a reference voltage of 0v, and in simplified terms "neutral" is earthed to keep it as close to earth as possible, at least once at the transfomer, often more times.

Taking into account how we must measure between two things, on a single phase system we are measuring between the phase peaks and the horizontal axis of the graph, which in this simplified example is the other end of the coil.
(When you come to test 3 phase systems, you will test between phases, and are measuring from the top of one peak to the bottom of a different coloured line, and that is why you get a bigger potential difference, around 400v.)

I don't know if I'm helping or confusing you more, so I'll stop now.

 

[Marvo]

To put it in laymans terms a voltage test is always telling you the voltage difference between two points. A live wire is only live because its voltage is 230v different to the actual ground you're standing on or to the neutral that is usually connected to the ground you're standing on. In normal circumstances if you test between live and neutral or live and earth the tester will indicate or display this 230 volt difference.

The problem comes when there's a fault such as a neutral wire that's no longer actually connected to the neutral of the supply anymore. That neutral wire is no longer held or clamped at zero volts, it would be floating at any voltage that's applied to it so when you put your tester between the live and neutral there's an electrical path through your tester (allbeit a high resistance one) that will connect the 230 volt potential (voltage) of the live to the neutral that is floating causing the neutral wire to also become 230v. The difference in voltage beyween the 230v live wire and the now 230v neutral wire is zero and that's what the tester will display even though the live wire will injure or kill you if you touch it.

Likewise if the cpc (earth wire) isn't connected to anything it will also give you false readings on your tester when proving dead same as the broken neutral did. The reason you do 3 different tests is to reduce the possibility that the circuit you're testing could have a faulty conductor that will give you false reading during a safe isolation test that could kill you.

 

[littlespark]

In simplest terms, a DC battery has two connections. Out one end, to a load (light) back to the other end to creat a circuit.

AC circuits are exactly the same. Out on the live, through a load, back on the neutral. Live and neutral are just the words we use to name the two wires going back to the source. Power station etc.

We added an earth cable as a safety feature. And as mentioned, the earth and neutral are connected at several places.
If we test N-E, and there is a marked voltage present where we know there shouldn’t be, then we can assume there is a fault.

 

[timhoward]

Some very good answers now! I guess you can think of a 2 pole tester as internally saying “if I look for a source of electricity on one pole and and a return path on the other pole, is there enough to do anything with it in the middle”
It tests how the electricity will behave in real life.
A good 2 pole tester will in fact alert you to a supply and no return path but this can’t be relied upon, not least because an internal battery is needed for this test. (Even if the battery is missing or discharged, a 2 pole test will still work.)

 

[mainline]

I always do a live test before to ensure that there actually is an L N E.

 

[pc1966]

Ok but still, why test between 2 conductors? Why not test each individually?

As a few others have already said, what you measure is voltage difference. The good example from above is a battery, it will do nothing unless you have contact to both terminals.

But as with many things, you can define a point to be something, and measure relative to that to have some absolute (rather than relative) measure. For electrical power we define the Earth (i.e. our planet) as being at 0V and then we can reference anything with respect to that arbitrary point. A Martian might find our planet is at a different potential to Mars, but we don't care as all we are doing is working here on Earth.

Now the difference between a battery and the AC power system is that in almost all cases, we take one of the live conductors (i.e. those carrying current) and reference it to the Earth through some conductive path, usually an earth rod or mat at the generator or substation. The live conductor we chose to reference to E we call neutral as normally it has very little voltage on it, all of the others we call line and for the typical UK system we have a nominal 230V L-N and 400V L-L (the physical diagram of the generator in post #10 helps explain the 1.73 ratio).

When you are "proving dead" you absolutely must not assume anything about the wires in front of you. They may not have been connected/colour-coded correctly and/or they may be subject to a fault, and if you make a mistake it will be a doctor proving you are dead!

This is why you must follow the correct procedure for proving dead before any work, and you will automatically fail any practical exam if you don't. So you check from each conductor to every other conductor (and maybe to any metal work if the CPC is not already connected to it) to see if any combination could prove dangerous, not just the L you might expect. If you have a total of X conductors you have X*(X-1)/2 combinations to check, which works out as 3 for single phase (L+N+E) and 10 for most three-phase (3L+N+E) systems.

You can get no-contact voltage indicators which work by looking for electrical fields around the conductor. While these have some uses, they are not acceptable for proving dead as there can be situations when they do not reliably indicate voltage differences, and other cases when they incorrectly show a live conductor due to capacitive coupling from another cable ("phantom voltage"), a problem you also get with digital multi-meters due to their high impedance (hence very little current needed to register a voltage).