/ Help with electricity/multimeter

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Ben Sharp - on 08 Nov 2012
I've just put two screws into the base of a broken wooden lamp (to mend it), I'm sure they're no where near the wire but I'd be interested to know how to test it. I've glued it as well and will take the screws out once it's set but it occurred to me I wouldn't really know how to test it if I needed to so would be interested to find out.

Testing the resistance from a screw to each of the bulb pins and each of the plug pins in turn reads infinity (i.e. putting one needle on the screw and then alternately touching the parts mentioned, then repeating for the other screw). Does that mean there's no connection between the screw and the wire and is that enough to mean it's safe?
The voltage across the screws is 0 but then changes to 0.005V if it's plugged in and on. Why is this?

Thanks

Ben
teflonpete - on 08 Nov 2012
In reply to Ben Sharp:
> I've just put two screws into the base of a broken wooden lamp (to mend it), I'm sure they're no where near the wire but I'd be interested to know how to test it. I've glued it as well and will take the screws out once it's set but it occurred to me I wouldn't really know how to test it if I needed to so would be interested to find out.
>
> Testing the resistance from a screw to each of the bulb pins and each of the plug pins in turn reads infinity (i.e. putting one needle on the screw and then alternately touching the parts mentioned, then repeating for the other screw). Does that mean there's no connection between the screw and the wire and is that enough to mean it's safe?
> The voltage across the screws is 0 but then changes to 0.005V if it's plugged in and on. Why is this?
>
> Thanks
>
> Ben

0.005V might be a little bit of induction in the screws when the nearby cable has current running through it, effectively the screw acting as one side of a transformer. You'd probably find similar in lots of other applications. Either that or it may be just the meter fluctuating between its lowest possible reading and 0, and within the accuracy range of the meter.
Ideally, you should test between the screws and plug / lamp pins with a Megger to prove no circuit at operational voltages rather than using a multi meter. If in doubt, get it checked by an electrician who does PAT. Personally, I'd be happy with the test you've done but can't advise you that it is OK.
EeeByGum - on 08 Nov 2012
In reply to Ben Sharp: Do you have an electrician's screw driver? Very useful for checking if something you are working on is live. I don't think 0.005V is anything to worry about though.
Ben Sharp - on 08 Nov 2012
In reply to teflonpete: Cheers for the info, out of interest what is the advantage of using a Meggger over a MM, or rather what are the limitations of the reading you get with a MM?
The glue should hold it ok so I'll probably take the screws out as a precaution anyway. The other option of course would be to turn it on and lick them, but I'm not sure that's standard electrical testing procedure!

Ben
elsewhere on 08 Nov 2012
In reply to Ben Sharp:
The multimeter tests with a small voltage and it is posible to have insulation that is good at low voltages but might fail/arc at mains voltages.

The Megger tests with a large voltage so it will detect if the insulation will fail at mains voltages.
DaveN - on 08 Nov 2012
In reply to Ben Sharp: megger tests the insulation under a voltage load, so tests if its breaking down in use. Your test seems reasonable,but any doubts speak to an electrician.
Ben Sharp - on 08 Nov 2012
In reply to Ben Sharp: cheers guys
jkarran - on 08 Nov 2012
In reply to Ben Sharp:

> Cheers for the info, out of interest what is the advantage of using a Meggger over a MM, or rather what are the limitations of the reading you get with a MM?

The multimeter applies a low voltage and measures the resultant current when testing resistance, some resistive/insulating materials only work up to a certain voltage then they break down (think about a spark gap, the air is insulating until enough voltage is applied). Mains working voltage is still reasonably low (<340V) but there can be kV+ transients superimposed on it hence testing/specifying insulation at at least 340V, usually more like 2kV.

> ...The other option of course would be to turn it on and lick them, but I'm not sure that's standard electrical testing procedure!

You'd think not but sometimes I wonder!
jk
Wulfrunian - on 08 Nov 2012
In reply to Ben Sharp:

What you actually need is an Insulation Resistance Tester. This will measure the resistance between plug/bulb pins and screw at a voltage of (usually) 500V dc. Megger make 'em, as do a host of other people. Realistically, you aren't going to buy one and would be calling a local spark to carry out the test for you if you can be bothered with the expense and hassle.

Sounds like you meter is picking up an induced voltage. 0.005v is nowt to worry about.

And a quick note about 'electrician's screwdrivers' - best place for this potentially lethal piece of shite is in the bin.
richyfenn on 08 Nov 2012
In reply to Ben Sharp:

Or...to throw in another option for a high voltage test :)

Plug in and turn on the lamp (don't touch those screws now!), then set the multimeter to test Voltage (AC). Shove one probe into the top hole of a mains socket (or touch it onto some bare metal pipework) and touch the other probe onto each of the screws. If the MM shows 240V then carefully unplug the lamp and know that you have managed to screw through the cable. If you're reading less than 1V you'll live.
vark - on 08 Nov 2012
In reply to Ben Sharp:
Is it you that the meter is detecting? 5mV is not far off want you could record from an ECG. Were you touching anything
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Ben Sharp - on 09 Nov 2012
In reply to Ben Sharp: Cheers everyone, I've taken the screws out this morning and the glues holding it ok.

Before I took the screws out I measured the V across the screws while it was on again. It was reading a solid 0.005V yesterday but today I noticed that the direction the probes are in makes a difference, one way round it fluctuates around 0.02V and with the probes swapped over it fluctuates around 0.3V. Reading a clear 0.0V while the switch is off.

I also measured the voltage between the screw and an earth while the lamp was on which reads about 6.6V for each screw.

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