Multimeter shows power when circuit breaker off

[Graham22]

I am about to change over an external security light and wanted to be 100% certain the power was off before meddling around. An electric shock whilst up a ladder is not worth thinking about.
Anyway, I switched off the circuit breaker and double checked with the NCV setting on my multimeter.
Very strangely, when actually touching the external pvc casing of the cable, it buzzed to suggest power present....With the circuit breaker on I get the buzz from a couple of inches away. With it off, I have to actually touch the pvc.
But why is it giving a reading at all - is that normal? Is it an induced current or residual current, or....?
Many thanks, Graham

 

[chivers67]

Is the security light currently working or not?

 

[Echo the husky]

Test properly following the instructions here. Non contact testers are not suitable for testing if a circuit is safe to work on.

 

[ban-all-sheds]

Anyway, I switched off the circuit breaker and double checked with the NCV setting on my multimeter.

NCV?

Very strangely, when actually touching the external pvc casing of the cable, it buzzed to suggest power present....With the circuit breaker on I get the buzz from a couple of inches away. With it off, I have to actually touch the pvc.

Touching.. waving it a couple of inches away.. Are you sure this is a multimeter and not a voltstick?

But why is it giving a reading at all - is that normal?

If it is a voltstick, or if some benighted manufacturer has decided to add a magic wand feature to his multimeter, yes that is normal.

Do not trust your life to it - use a proper 2-pole tester, e.g. a multimeter on a ≥ 250VAC setting.

 

[JohnW2]

Anyway, I switched off the circuit breaker and double checked with the NCV setting on my multimeter.

NCV?

Non Contact Voltage. Some multimeters offer such a facility.

Kind Regards, John

 

[MattChan]

Fluke have NCV on their 117 model. http://www.fluke.com/fluke/uken/digital-multimeters/fluke-117.htm?pid=55996 They call it "VoltAlert". It has its uses but you need to understand its limitations.

As BAS said, just measure it using the leads rather than the NCV mode

 

[Graham22]

Thanks for the responses. Yes I have an Amprobe multimeter (http://www.amprobe.com/amprobe/usen...al-multimeter-series/amp-am-510.htm?pid=74033)
which has a non contact voltage tester. I was planning on cutting the wire in my loft to fit a junctin box so I can extend the wire and fit the light in a different position.
So it's normal that I still get a reading (though clearly not so strong as I have to touch the pvc) even with the cb off, you think? Is it induced current?

 

[EFLImpudence]

which has a non contact voltage tester...

...Is it induced current?

No.

As BAS said, just measure it using the leads rather than the NCV mode

 

[bhm1712]

Don't use a multimeter and most definitely not a non contact Voltage detector to test for dead. Use, as has been suggested, a voltage indicator or test lamp.

 

[Graham22]

Say - Yes I will. Just to be sure, I intend to access the light from a ladder, and test the cable with my multimeter, from live to neutral, live to earth (assuming it has earth I havent looked yet), and from neutral to earth.
I will then check the multimeter on another known live connection.

I am pretty certain it will all show zero reading, but agree I need to be 100% sure. Thanks for the advice. But I am intrigued about the reading from the non contact tester. If it is not an induced current, what would it be?

 

[MattChan]

Have a read: http://ecmweb.com/content/what-do-you-know-about-capacitive-voltage-sensors

 

[Graham22]

Thanks.
This is also very interesting https://www.nema.org/Technical/Documents/Bull_88_reaffirmed_12_15_11.pdf

I think it must be a phantom voltage I am seeing. The cable concerned runs all the way to the loft of our two storey house, so I imagine it must run alongside a number of other cables going upstairs.
The fact the non contact tester will only buzz if it actually touches the pvc with cb turned off, but buzzes easily without touching when it's on, must be evidence I guess. But before anyone says it, not sufficient evidence I would risk my life on!

 

[bhm1712]

That is why multimeters should not be used for checking if a circuit is live or dead. Multimeters should only be used when verifying or fault finding circuits/devices etc.

 

[ban-all-sheds]

That is why multimeters should not be used for checking if a circuit is live or dead.

No it isn't.

 

[bhm1712]

That is why multimeters should not be used for checking if a circuit is live or dead.

No it isn't.

Well apart from the HSE guidance which states you shouldn't , yes it is - they are not suitable for checking for dead for numerous reasons including the fact they can give incorrect readings whether that be false positives or negatives.