Period electrickery

[FamilyWiggs]

Just a quickie...

Was just replacing a ceiling rose, and just to check I'd isolated the circuit before working on it I touched each terminal with the dreaded screwdriver tester (yes I know they are dangerous and evil - but they're convenient for this type of task). This is not one of the neon-testers but has a little LCD display giving an indication of voltage.

As expected live lit up 240v, and neutral didn't. Surprisingly, earth lit up something approaching 110v.

Rechecking with a multimeter showed 240v on live and 0v on earth - which was somewhat more reassuring.

I'm assuming that it was just getting some sort of induction from the adjacent live cables to the earth? Am I right in thinking this is more likely if the earth isn't connected to earth at the other end? Do I really need to go and do some continuity testing?

Thanks,

Robin

 

[skier-hughes]

I think you need a cup of tea and a kit kat. Just finished mine and back off to dig more holes!!

 

[FamilyWiggs]

Ooh no - toasted muffin and our own fresh eggs. Much better for you - and much more protective against all this electrickery malarkery.

 

[MdB]

Just a quickie...

Was just replacing a ceiling rose, and just to check I'd isolated the circuit before working on it I touched each terminal with the dreaded screwdriver tester (yes I know they are dangerous and evil - but they're convenient for this type of task). This is not one of the neon-testers but has a little LCD display giving an indication of voltage.

As expected live lit up 240v, and neutral didn't. Surprisingly, earth lit up something approaching 110v.

Rechecking with a multimeter showed 240v on live and 0v on earth - which was somewhat more reassuring.

I'm assuming that it was just getting some sort of induction from the adjacent live cables to the earth? Am I right in thinking this is more likely if the earth isn't connected to earth at the other end? Do I really need to go and do some continuity testing?

Thanks,

Robin

Hmm. If you are getting 240V anywhere then I think you have failed in the isolation stakes...

Your screwdriver will measure the voltage between the cable and your body. Your body will be capacitively coupled to the surroundings which may or may not be at ground level. It will therefore give you not more than an indication as to the voltages around but I really wouldn't be relying on it. The 240V, 110V, 0V on Live, Earth, Neutral does suggest though that the wire is live with the earth disconnected.

The multimeter test I assume is measured relative to Neutral. The fact that you get zero on Earth doesn't tell us a great deal. Do you know what the internal impedance of your meter is when measuring AC volts? If it is an oldish meter and / or isn't battery powered, this could be low which will cause the voltage on the disconnected earth cable to discharge giving the zero reading.

To find out if your earth wire is connected or not, you really need to make a resistance measurement. I would recommend switching off all the power in the house first before doing this.

 

[Lime]

I agree with MdB.

The 240V, 110V, 0V on Live, Earth, Neutral does suggest though that the wire is live with the earth disconnected.

Exactly what I thought.
The ceiling rose is not disconnected from the mains and the earth is broken/faulty.

 

[FamilyWiggs]

The 240/110/0v readings were taken before isolation. Always test the tester before/after on a known live circuit - then you know it is a good result when you get nowt...

I'm getting nothing with the new/battery powered meter - so am assuming it was simply induction that the crappy screwdriver was picking up. I'll run through the circuit and check each of the earth connections as well as each switch, just to be sure. The circuit is not tripping the MCB, which I would have expected had there been a problem.

 

[FamilyWiggs]

To find out if your earth wire is connected or not, you really need to make a resistance measurement. I would recommend switching off all the power in the house first before doing this.

Can't I do a simple continuity test between a known good earth on the circuit and the ceiling rose earth terminal. Then if a break is found, progress along the circuit until I pass the break?

 

[Lime]

You could make a resistance test between earth and neutral at the ceiling rose.

I'm getting nothing with the new/battery powered meter - so am assuming it was simply induction that the crappy screwdriver was picking up

It won't be induction on the earth wire if it is connected to earth.
Therefore is must be broken.

Any voltage found on the broken earth wire will be produced by capacitance.

 

[Flyfisher]

I'm getting nothing with the new/battery powered meter - so am assuming it was simply induction that the crappy screwdriver was picking up. I'll run through the circuit and check each of the earth connections as well as each switch, just to be sure. The circuit is not tripping the MCB, which I would have expected had there been a problem.

I'm not sure about your 'LCD screwdriver' but if it has a very high impedance then it's quite possible to be some sort of induction artifact. Ever stuck your finger on an oscilloscope probe? You'll get a surprisingly high voltage waveform, all based on induction but with no significant power behind it so quite safe.

Or you might have a problem. I think you're wise to check the earth connections through the lighting circuit; it's possible something has come adrift in one of the junction boxes/roses.

I don't think the MCB would trip just because an earth wire had become disconnected. I can't see how that would cause the necessary over-current required to trip an MCB.

Isn't it typical how a simple little job can escalate!

 

[FamilyWiggs]

I don't think the MCB would trip just because an earth wire had become disconnected. I can't see how that would cause the necessary over-current required to trip an MCB.

Isn't it typical how a simple little job can escalate!

I was expecting the MCB to trip if there was a cross connection between live and earth?

Anyway, having now checked the circuit, the only earth is in the ceiling rose and the switch. Nothing else on the circuit is earthed. So I'm still assumng an indcution effect from adjacent live cables, and no voltage across L/E with meter. Also no continuity across L/N/E with switches off and the other ends of the cables disconnected.

This is one of the remianing circuits I haven't yet replaced. I've regaled you all previously with tales of bell cable being used on the last socket on a downstairs ring from the 1970s "modernisations" done here... so I'm no particularly surprised at this. Fortunately it is a lighting circuit only and all plastic switches.... nevertheless, replacing it has advanced up the agenda somewhat.

 

[Lime]

. The circuit is not tripping the MCB, which I would have expected had there been a problem.

A broken earth would not cause either an MCB, RCD or RCBO to trip.
However if the fittings are all plastic I would not be concerned about the possible broken earth too much.

 

[FamilyWiggs]

. The circuit is not tripping the MCB, which I would have expected had there been a problem.

A broken earth would not cause either an MCB, RCD or RCBO to trip.
However if the fittings are all plastic I would not be concerned about the possible broken earth too much.

It wasn't the broken earth I was worried about - but the potential cross-connection that ought to have tripped the MCB I would have thought.

Anyway, it seems all is OK for now - and if I hadn't replaced the fitting I'd have been as blissfully unaware of the lack of earth as I have been these past 6 years!

Thanks all.

 

[Flyfisher]

It wasn't the broken earth I was worried about - but the potential cross-connection that ought to have tripped the MCB I would have thought.

A 'potential' cross connection of live to earth is just that - potential, so until it happens the MCB won't trip (stating the obvious again!). Are you saying that there WAS a live-to-earth connection and the MCB didn't trip? That would certainly be indicative of a definite fault and a disconnected earth wire showing an induced voltage would make far more sense in such a scenario.

This is a handy little tester for mains wiring: http://www.maplin.co.uk/mains-socket-tester-9800
Intended for sockets really but it could be adapted to check lighting wiring (disclaimer: but only if you know what you're doing and take all appropriate precautions!).

I keep one in my gig bag and always like to check the mains sockets we're plugging into when the band plays anywhere. I've never yet found a missing earth but I did once find reversed live and neutral. Trust, but verify. :wink:

 

[88v8]

However if the fittings are all plastic I would not be concerned about the possible broken earth too much.

I've had mostly ye olde brass switches for the last 31 years, none of them earthed.
And some of the pendant bulb fittings are metal, also not earthed.
And we have ye olde fuse boxes.
But I'm still here.... how can that be........

Ivor

 

[Flyfisher]

But I'm still here.... how can that be........

. . . . probably because your wiring, or any connected appliance, has never developed a fault.

All the devices mentioned, including earth wires, are not necessary for the correct functioning of things. They are merely protective measures intended to limit damage if things do go wrong. If nothing ever went wrong, none of them would ever be needed, but Murphy's Law means they are generally rather useful. :wink:

 

[Penners]

All the devices mentioned, including earth wires, are not necessary for the correct functioning of things. They are merely protective measures intended to limit damage if things do go wrong. If nothing ever went wrong, none of them would ever be needed, but Murphy's Law means they are generally rather useful. :wink:

Quite so. You pay premiums on an insurance policy, and you hope that the money is completely wasted.

 

[FamilyWiggs]

All the devices mentioned, including earth wires, are not necessary for the correct functioning of things. They are merely protective measures intended to limit damage if things do go wrong. If nothing ever went wrong, none of them would ever be needed, but Murphy's Law means they are generally rather useful. :wink:

Quite so. You pay premiums on an insurance policy, and you hope that the money is completely wasted.

So is that why Mrs FW keeps upping my life insurance? :shock:

 

[JoceAndChris]

:lol:

 

[Penners]

is that why Mrs FW keeps upping my life insurance?

Unquestionably. Without doubt. Absolutely certainly. No other possible explanation.

Er... unless....

 

[LadyArowana]

is that why Mrs FW keeps upping my life insurance?

Unquestionably. Without doubt. Absolutely certainly. No other possible explanation.

Er... unless....

I'm sure it's a complete coincidence that she discouraged the installation of a modern fuse box and is always offering to save time by drying your hair while you are in the bath.