Earth is live on neon screw driver but don\\\'t understand w

I\\\'ve come across something weird and wondered if any of you guys could give me a bit of advice.
Recently rewired some lighting upstairs as found there was no earth circuit for it.
As no power in loft until I finished (upstairs lights isolated), I wired all new junctions up carefully and switched the power back on.
At this point the new earth was not connected to the house earth circuit but was essentially as is before including new earth connections - lights worked fine but when I tested the earth circuit (as yet unearthed) I found that my live detector lit up!

I began testing each bit of new wiring I had put in and all showed the same but I could find no fault - I had satisfactorily sleeved up all earth wires etc.

I became so miffed and flumuxed that I decided to test a complete new piece of wire:
- I folded the earth back on one end of the 1.00 mm twin and earth and insulated it.
On the other end I folded back the live and neutral and insulated them.
- So in summary I had a single short length of wire connected to live and neutral at one end butonly an exposed earth at the other (connected to nothing).
... and ... you guessed it! ... it showed live (dimly).

eh? what is causing this - is this just an induced current or do we have a real problem???
will the problem go away when the earth wire is connected to the earth circuit?
would a proper volt meter show me if whatever voltage is there is acceptable? what voltage would be acceptable?

neon tester screwdrivers are marvelous ..........as screwdrivers, apart from that throw it away, they are unrelaible, give "false readings" and are useless

a multimeter should show zero volts

cheers

but I'm intrigued now! presumably something is causing it to light up ... what? ... or is it an x file?!!!

There were some phase tester screwdrivers which came out a few years ago which showed continuity also (ie hold both ends of the driver and it lights up). If this is the case you'll see it light if you're earthed and you touch it off an earthed part. Apart from that all phase testers provide unreliable info and for about £5 you can get a cheap and nasty meter. For £20 you'll get something more useful.

ok - so I've invested in a multi meter (just a tenner but I'm assured that it's up to my needs) but want to make sure I use it correctly

so if I want to check whether there is earth leakage I stick red on live and black on earth right? and it should read 0V??? or will doing this actually show me the potential difference between the two points - please advise

What you need to do is test between neutral and earth. Between live and earth there is, obviously, 240v.

It may not be EXACTLY zero volts, but no worry so long as the voltage is negligable (i.e. not 240).

I wont bother explaining why it may not be 0v, but dont worry either way.

One thing to remember is that unless that Earth is connected to Earth, there is nothing to prevent induced voltages showing on it as the cable is not at the same potential as the fundermental mass of Earth.

There are things, we all know, that will produce small voltages and currents on cpc's, when these are at earth potential there is no indication of them. However when the cpc is "floating", there will be a potential difference between the Neutral and the cpc, and the cpc and true earth for that matter.

If your curious about the voltage on this cpc, simply leave the cpc disconnected in the board, then measure the voltage between this and the earth of your installation. You will likely find it is something like 0.5V to 12V, neons will operate at these low voltages.

Measure the potential difference between the cpc and the neutral bar in your board, that will likely show a similar difference.

... also test between neutral and earth

Then let us know what your readings are
Live - Neutral =
Live - Earth =
Neutral - Earth =

Also - have you an RCD protecting this circuit (or the main board)?, and do you have an earth spike supply to your house?

ok - but what's a cpc?

earth

A CPC is a "Circuit Protective Conductor"

pa if you are amazed by a neon lighting up like that then try the old "rubbing a flourescent tube in a dark room" and you'll be even more amazed that it lights up in your hand without any connections to it!
Just watch what you are actually rubbing as it's dark

Try standing under a pylon in the dark holding two 8 foots - you can play Star Wars!! (NOT RECOMMENDED _ DON'T TRY THIS AT HOME!!)

Hi Guys. There are two different streams of correspondence on this topic, so I am including this in both in the hope that one or other will be picked up. Apologies if you end up with both. In summary, I have had the same problem. Using a neon screwdriver, I am detecting current in a ‘floating’ earth wire in a lighting circuit. I am off to get a decent voltmeter to check just how much voltage there is in the earth wire. I want to make sure that I have understood the advice. So here goes. I check the voltage between the neutral and the earth. If this is below (what?) 30V this is ‘normal’. I should then connect the earth wire to a good earth. The current should then run off down the earth without tripping the circuit breaker at the switchboard. Have I got this right?. Thanks. IanC.

p_a_johnson said:

ok - but what's a cpc?

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The earth wire is sandwiched between one wire at 240V and one at 0V. The capacitace and inductance between the cables would induce a voltage on the middle wire physically half way between the other two of half the voltage difference between them. The current available from this induced voltage is very small, so connecting it properly to earth should short it out completely, and still have negligiblle current flowing down the earth conductor.

If you leave the earth wire disconnected and instead short it to N or true E using a voltmeter, then the voltage you measure will depend upon the resistance of the voltmeter. Cheap analog meters might have a resistance of a few K ohm, a digital meter might have a resistance of M ohm. This means different meters may give different results.

A neon tester is slightly different. It wil give no light until the applied voltage is greater then the breakdown voltage of the gas inside it. Then it conducts current, which tends to reduce the voltage on the thing you attached it to.

Neons were once used as voltage regulators for DC electronics as they will always have a fixed voltage across the terminals of the lamp bit. Though remember there is a resistor inside too, which may have a much bigger voltage across it.

A modern electrical installation does not include anything which tests or limits the amount of current through the earth conductor. maximum L is limited by mcb or fuses. Any DIFFERENCE between L and N is detected by a RCD. The idea is a RCD will switch off if any current is going anywhere else. Say, to non earth bonded water pipe instead.