should unswitched live should as live on a neon screwdriver?

[dave L]

I've got three tools and I've been using them to test my lamp pendants. I've got the faithful neon screwdriver, a digital multimeter (set to

500V) and a 'digital circuit tester' from Blackspur which I bought for a quid. This last tool has two functions: (a) it shows if the terminal in contact is live; (b) what the voltage is. It has a reading of 12-36-55-110-220v and illuminates the value below what the actual voltage is. It works by the user holding the non-contact end to complete the circuit a la the neon screwdriver.

When the pendant is switched off, the phase terminal shows (in order of the tool above):

1. LED lit quite brightly
2. (null reading)
3. between 12V and 36v (36v is slightly illuminated).

When the pendant is switched on, the phase terminal shows (in order of the tool above):

1. LED lit very brightly
2. 246-250v (depending upon pendant)
3. 220V.

This situation is present on each pendant I have tested. None of the pendants appear to faulty in day-to-day use.

I'm a little intrigued that the two cheaper tools are showing the the phase terminal is live when there should be no current. My real concern is that these tools infer there is current flowing to the terminal with the pendant is swicthed off and that there's a possibility of getting electrocuted when changing a bulb.

Are all my pendants wired incorrectly?

Thanks

[Dave Plowman]

What you're seeing is inductive coupling between the 'live' wire and the switched one running next to it. Put the bulb back in and you'll see zero, as the switched line will be connected to neutral via the much lower impedance of the bulb.

Things like neon screwdrivers - and DVMs - have a very high input impedance so will read very low currents that the body wouldn't feel.

It's a good idea to use a low value shunt which draws about 0.1 amp when using a high impedance test device on mains - or alternately make up a test lamp using a small mains bulb.

[Dave Liquorice]

s/LED/neon/ I think you mean. B-)

I think you need to be more careful about your descriptions and where you are actually measuring. "phase" I would take to mean the permenant live loop through in the ceiling rose and this figures with the Subject: "unswitched live". But this should be at full mains potential at all times, if it went on/off with a wall switch then all the lights in the house would go on and off with that switch (unless you have some *really* weird lighting wiring).

Just to clarify, do you really mean the above or the switched live that connects to the bulb?

s/LED/neon/ I think you mean. B-)

Assuming you mean the switched live, they aren't. You need to understand the tools you are using and how they interact with what they are measuring. All of those tools are high impedance and are voltage measuring devices. What you are seeing is induction from the permenant live wiring into the open circuit switched live wire. With high impedance voltage measuring tools it is normal to see this induction.

[Chris Vowles]

I am glad you posted this as I have one of these cheap tools and it exhibits the same characteristics, checking some metal light switches (previous owner installed) it did at least make me realise that it wasn't earthed so it has paid for itself and I shall be fixing that this afternoon ....

C

[mike ring]

Dave Plowman wrote in news: [e-mail]:

Be a damn good idea if a 5 or 10K shunt was a plugin accessoery for these things,

mike r

[dave L]

Sorry about the ambiguity. What I was measuring was the 'live' terminal of the pendant, ie, that which is connected to the output of the switch, the switched live.

Thanks (and to previous respondents); that makes things a bit clearer. I've been a bit wary of the pendants as I thought they had voltage when they were switched 'off'. But I couldn't figure where the voltage was coming from or why the voltage didn't cause current to flow when I put a bulb in. Very puzzling. Thanks again.

As a matter of interest do people who know more about electrics than me (that'll be all of you, then) use these neon screwdrivers? I read a wiring book the other day which was quite sneery about them, without being specific about why. I accept your comment that I didn't understand what it was measuring. As they're sold as a current-detecting device (ie, to show if a components is live and likely to shock you) I think they're perhaps mis-sold.

Or is that -more likely, esp in my case- mis-bought?

[mike ring]

[e-mail] (dave L) wrote in news: [e-mail]:

The trouble with neons, and also to some extent digital meters is that they are fearfully sensitive, and

It's volts wot jolts, but mills wot kills.

Your switched live has travelled close alongside a genuine live inside a twin and earth all the way from a rose, or junction box.

So 240 v has radiated into it, but only a tiny amount; but enough to light a neon, which prolly has half a million ohms in series limiting the current to about half a milliamp, like blackpool front, but not enough for you to feel.

HTH

mike r

If this wire was connected even very faintly to earth, all these volts would dribble straight down, but as it is open at bothe ends ( the switch is off and the bulb which is out - I infer from your last sentence).

But as it isn't quite high voltages may appear, these are completely harmless, like the static a good old brinylon shirt will generate.

A good oldfashioned meter with a needle is prolly better to work with, or as Dave said, try a shunt resistor across a DVM.

(sometimes I think they're more trouble than they are worth, it's truee they will detect a .1 volt change, or check your battery to within about a microvolt, but they can cause unnecessary alarm and despondency.

mike r

[Dave Plowman]

They're really made for electronics use where a high input impedance is near essential.

There could easily be a case made for selling a much lower impedance device for mains work.

[Dave Plowman]

Think of them more as a voltage detecting device as they draw so little current.

And as the old saying goes, it's better to be safe than sorry - if you switch off at the main when it lights up *anywhere*, you'll be doing the safe thing.

[Dave Liquorice]

I have one and use it occasionally when I can't find my prefered tester a "Volt Stick". The big plus about the volt stick is that is non-contact, you can see if a cable is live without needing to be at an end or cut into it.

Voltage detecting device, you could touch it against a cable carrying a thousand amps at a low voltage and it wouldn't light.

Cheap modern ones seem to be much more prone to this "false" indication than an old (>40years) one that my father has. I have never known that give a false indiaction and I've never felt a tingle from it either. Both of which I have had from a cheap modern one.

[ARWadsworth]

There are times when a neon can be more useful than the volt stick e.g. when testing inside a central heating control centre. The volt stick picks up too many signals from adjacent cables.

-- Adam

[e-mail]

[dave L]

Can you explain how to use a shunt resistor in this context ? I've had a search and most the hits are quite advanced enginnering or electronics references.

However what I thought they meant was that I get a 0.1 ohm resistor and wire it up between the live and neutral terminals of the pendant and then measure the current that flows across the resistor. By my maths (I=V/R = 240/0.1) this would measure (um) 2400A. I'm *know* that's not what I want to do.

One post

-which Dave Plowman contributed to in '01- mentions adapting the multimeter. Is this the only way I can use the stunt ? That's going too far for me, I think, as I want to master (ahem) wiring before I get into electronics.

Whatever the method, if it does inolve measuring the current then I need to head to Maplin as my multimeter doesn't measure AC current (Gunson Pocketmeter2).

Thanks for your thoughts.

[mike ring]

Dave Plowman wrote in news: [e-mail]:

Absolutely agree; a gorilla's version for Sparks ;-), and a separate shunt accessory for hi impedance jobbies would do nicely.

Of course as this is ukd-i-y why am I whining instead of making one?

mike r

[Tony Williams]

How about a Radiospares 259-533? A 300Vac panel meter, with about a 20k resistance. £13-67 each, but no other components needed.

[Dave Plowman]

That would be a shunt for measuring current using a sensitive meter - you 'shunt' the majority of the current across the resistor and the meter reads the rest in proportion. These sort of shunts will be fitted to the amps range of any meter, but are rather specialised to make since the ohmic value can be extremely low and needs to be accurate. They're often simply a strip of metal.

However, for increasing the current drawn while measuring mains, you need wire the resistor in parallel with the meter in the same way, but of course for measuring voltage the combination is wired in parallel with the mains rather than in series as for current measurement.

If you want to experiment, I'd use a 22000 ohm (22k0) 3 watt resistor from Maplin (W22K) at 0.37 gbp. This will draw about 10 milliamps and is within its 3 watt rating.

Obviously, there are safety implications since you'll be measuring mains, so don't attempt this if you're unsure about how to do it safely. I'd buy a second set of test leads and incorporate it into these, enclosing it in a box suitable for the fact that it will also get quite hot with prolonged use.

[Dave Liquorice]

Certainly. B-)

You need to choose a value that won't drawn exceesive current if you connect it across the full supply by accident but will draw enough to generate better readings on open circuit cables.

Generally you rarely need to measure current. Most times you are only interested in the voltage.

[usenet]

Capacitive more likely.

This is one of the reasons that I use my 'cheap' analogue meter when checking for mains, it's much more likely to give me a useful reading than a digital meter and is also easier to quickly glance at and check.

[Zymurgy]

I saw these floats on unterminated earth wires whilst doing my electrical installations City & Guilds.

But it took completion of my HND in Electrical and Electronic Engineering to understand why ;-) [1]

I too reckon it's inductive coupling, not capacitive. Although i'm not saying that metres and metres of dilectric wouldn't hold "some" charge.

Cheers,

Paul.

[1] Hell, I could probably calculate it !

[Dave Plowman]

I'm not even guessing which it actually is, but if you lay an audio cable alongside a mains cable - especially one fed from with a dimmer - the resultant buzz is always called induction.

Perhaps I should have said induced which is suitably vague. Even for this group. ;-)

[Andy Wade]

Definitly capacitive, but the terminology can be confusing because the effect that we call capacitance comes about because of induced charge (called "electrostatic induction" in days gone by).

That is inductive coupling since it's the current flowing in the mains cable, rich in harmonics due to the dimmer, that is causing a small EMF to be induced in series with your audio circuit.

There'd still have been a debate.

Stockbroker: someone who keeps investing your money until it's all gone [Woody Allen].