Naive electricity supply question ...

I expect the OP has been assisted already by this stage.

Clouding the issue with suggested causes for the OPs observations, with explanations that don't actually fit those observations is not (IMHO) helpful.

Hence why I did attempt to explain to you why the observed behaviour one would get from a NE short (at best a few amps of fault current) would be very different from a LN or LE short (at best 100s of amps of fault current, and possibly up into the 1000s of amps).

The damage would be limited by the let through energy permitted by the fastest operating protective device (i.e. the plug fuse or circuit MCB - the RCD would probably be way to slow to get a look in). There would be a bang and a flash (for some value of bang and flash) and the fuse or the MCB or both would open.

Perhaps there is some meaning in your statement "Not necessarily so because the speed of the action of the side cutters would just be that of a switch closure." that I failed to understand. If that is the case then accept my apologies, and please explain it better. On the face of it it seemed to be doubling down on the premise that there was a LN fault, only this time "quicker" in some way.

In message <ub5t3g$uc2p$ snipped-for-privacy@dont-email.me, John Rumm snipped-for-privacy@nowhere.null writes

'Powerfully dumb'? That's a really great term. Is it freeware, or do I have to pay you if I want to use it?

I just experimented: several different neons, touched to live. Wearing cork-soled sandals on a bath mat on tiles, a faint glow. No significant change in glow on jumping up and down (or raising one foot): I'll take that to say my earth is mostly capacitivly coupled, and the resistive path through cork soles, mat, tiles is insignicant compared to that.

Touching the water pipe caused the glow to increase very noticeably.

So yes and yes to your questions...

I'd say that capacitive coupling to earth is a lot more indeterminate than touching (say metallic) earth: ist the coupling sufficient to glow in a wooden barn, on a ladder, with just a single spur line ending three feet off to the side? I don't know, and the "powers that be" say "neon screwdriver is not to be used to prove off". Some of those rules are written in response to accidents...

It's a tool with a shortcoming, and one can work with that if you are aware of that. No matter how well it's tuned off and proved off, I do this: one hand on my back, grab the belt (prevents ground path hand-hand), touch wire with the back of my knuckles (so a live will spasm muscles away from the wire), and so deliberately test. Takes a fraction of a second, and as I try to make it a habit, it may save my neck one of these days.

I think that those readign this thread know enough to understand the workings shortcomings and dangers. *None* of the neons glow if you fail to touch the button at the back, but will an inexperienced person know that?

Thomas Prufer

My scenario is this: erroneously prove the wire to be off with a neon, and miss the faint glow. Touch the wire while leaning/touching a good ground (metal cable tray, water pipe, grounded conduit,...), get a good jolt. AIUI, the harm often is not the jolt, but falling off the ladder as a result of the jolt.

In Germany, they are "deemed insufficient to prove that a circuit is not live", and using one for that purpose is banned in industrial/trade work. (And looking up the regulations, I read that a pacemaker can misinterpret the current of the neon and shock the heart as a result -- a nasty side effect I just now heard about!)

The ones used by tradesmen have a load that is activated by two buttons, ~200 mA current. I find that useful, differentiates "real" live from capacitive-coupling "live". Also serves as a quick RCD test, though very rough & ready.

Thomas Prufer

Thomas Prufer

I think its copyrighted by the EU.

Almost certainly. They probably find it comes in handy when they describe the British.

To be fair even non contact detectors are not deemed adequate to prove a circuit is safe.

That at least is something you can not worry about with a non contact device.

My Fluke 117 True RMS meter has a very handy "Low Z" mode where it drops its input impedance significantly - making it ideal for mains proving.

That's very useful, so false positives which we can live with.

A neon screwdriver will require 90V or so to illuminate. I'm intrigued at what the Volt Stick criterion is for illumination. In the scheme of things they're not the cheapest instrument.

Available from under £5 inc delivery on Amazon. Of course you can buy more expensive ones. I have no idea though if the more expensive ones are much better.

Tim

I had one ff the Kewstick Uno, it was ok, could distinguish a live wire to either side of some T&E, but I lost it at work, so did replace with the Duo model, it has sound and you can use it in high/low sensitivity to make it less prone to false positives on e.g. parallel cable runs.