Digital Meter

I got involved with someone who was wiring a external LED Floodlight into some old wiring.

The light didn't work and from a brief tracing of the cable back to the switch I suspect it has 2 lives.

Question:

Can I use my cheap digital meter to check between live and earth to prove this? Could it trip an ELCB if there is one (not checked yet)

Reply to
DerbyBorn
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Yes (on 240+ AC volts), other than it wouldn't necessarily confirm either was a real live or the load side of an incandescent lamp in series.

On the AC volts range the internal impedance of a DMM is likely to be pretty high so unlikely to provide a sufficiently high current to trip an ELCB in any usage combination (even L to E etc).

Cheers, T i m

Reply to
T i m

The input impedance on a DVM should be very high, and so it will not trip a RCD.

(In fact you can see a related problem to this sometimes with DVMs, where the impedance is so high that you see phantom lives on what are in reality just floating wires that are being loosely capacitively coupled to live).

Reply to
John Rumm

DerbyBorn submitted this idea :

Normally no, a digital meter has quite a high input resistance so they draw much less than the 30mA needed to trip an RCD - but if there are other leaks to ground bring it close to 30mA, a meter might then cause a trip.

If a neutral is open circuit, it is quite possible for both L and N to seem to be live. The only certain way to check what is what, is with something which draws a more sensible load than a digital meter. A test lamp or similar, might be more suitable.

Reply to
Harry Bloomfield

no

NT

Reply to
tabbypurr

You would expect switched live and a neutral.

It would be wiser to measure from live to neutral if at all possible. I doubt that a decent high impedance voltmeter would trip an ELCB but you have to consider the possibility that your leads are less than perfect.

I prefer a good old mains testing neon screwdriver (even though they are frowned upon these days). If the neon glows it ain't safe to touch.

Reply to
Martin Brown

It's your choice but IMHO that advice is wrong in several ways - starting with the fact that you have to touch the cable with the screwdriver and that alone can be dangerous. See

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Reply to
Robin

If you store it unwisely or hold it in your mouth then all bets are off.

The advice there to test on a known live wire either side is good.

Reply to
Martin Brown

With those, if it glows its either live or dead. If it doesn't glow then its definitely either live or dead.

You call ;-)

Reply to
John Rumm

Anyone else remember those public information films about working with HT?

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Reply to
Huge

Provided it's not on the current metering option, some students to the to see what current is coming out of the power supply. ;-{

Should be obvious I know but a studetn wants to check if 5V is coming out off the PSU and then how much current !.

Yes I saw this with my DVM when I was trying to check if the old mains wiring was really disconnected, I got volts even though the old wiring wasnlt connected to anything.

Reply to
whisky-dave

DerbyBorn has brought this to us :

Be aware that shorting neutral to earth can also trip an RCD. There is usually some voltage differential between the two, shorting them could cause more than the 30mA to flow and an RCD to trip.

Reply to
Harry Bloomfield

Robin submitted this idea :

Another reason is that the glow can be quite feeble too, improved by you yourself touching the earth.

Reply to
Harry Bloomfield

Almost none of that's good advice either. An analogue meters takes typically 50uA at FSD, digitals far less. No ELCB is going to give a damn, even an RCD wouldn't either.

The lamp advice is also bad. It also could electrocute someone else in the building under some circumstances.

This is not an electronics group. Some contributors know what they're talking about, some just think they do.

NT

Reply to
tabbypurr

snipped-for-privacy@gmail.com explained :

If it were close to the limit?

A test lamp which actually draws some current, is standard electricity board issue.

Then I would suggest you keep your advice to yourself.

Reply to
Harry Bloomfield

If you think a 1+ megohm DMM input is going to make any detectable difference to an ELCB or even a 30mA RCD, you're free to use your brain.

For people that know how to use them safely & effectively. Not for people that need to ask here how to find out what's wrong with a power feed to a light. Don't tell me you can't see how it could go wrong.

I'm sure you mean well, but you've not really thought your advice through. This what often happens with threads on a topic where most contributors simply do not have the relevant expertise.

I can see no point wasting further time.

NT

Reply to
tabbypurr

We haven't had an Electricity Board in England since 1990. Possibly technology has moved on in the intervening 27 years.

Reply to
charles

or they know the subject so well that they omit things which may be necessary for other people to know.

Reply to
charles

No, I disagree.

Its not going to trip an RCD, but the current draw on a moving coil meter is often sufficient to avoid a false live reading on a floating but capacitively coupled wire.

Since you will connect your lamp from a known good neutral to an unknown wire, then the risk posed to normal occupants of the building (i.e. not others carrying out electrical maintenance at the same time) will be vanishingly small.

and some need to climb down off their high horse...

Reply to
John Rumm

on 18/07/2017, charles supposed :

I'm sure you knew what I meant.

Reply to
Harry Bloomfield

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