I bought one of these the other day:

http://www.ebay.co.uk/itm/Megger-DCM330-Fork-Clamp-
Multimeter-/161750473789?hash=item25a9145c3d

I didn't have anything to measure high currents and do non-contact tests
for voltage (that was the excuse I made to myself for buying it, anyway).

I've noticed that on the no-contact checks, the thing indicates a
positive on some cables even if the switch to the (light in this case) is
turned off. Reminds me of neon testers which glow from induced current
from nearby cables when the cable of interest is not actually live. These
false positives - if that's what they are - are a PITA and I'd hoped a
meter like this would know the difference. Or is it something I'm doing
wrong?
The instruction sheet is really awful and horrendously light on detail.

On 18/08/2015 20:31, Cursitor Doom wrote:
I bought one of these the other day:

http://www.ebay.co.uk/itm/Megger-DCM330-Fork-Clamp-
Multimeter-/161750473789?hash=item25a9145c3d

I didn't have anything to measure high currents and do non-contact tests
for voltage (that was the excuse I made to myself for buying it, anyway).

I've noticed that on the no-contact checks, the thing indicates a
positive on some cables even if the switch to the (light in this case) is
turned off. Reminds me of neon testers which glow from induced current
from nearby cables when the cable of interest is not actually live. These
false positives - if that's what they are - are a PITA and I'd hoped a
meter like this would know the difference. Or is it something I'm doing
wrong?
The instruction sheet is really awful and horrendously light on detail.


How would it tell the difference between induced voltages and "real"
voltages?

On Tue, 18 Aug 2015 20:52:34 +0100, dennis@home wrote:

On 18/08/2015 20:31, Cursitor Doom wrote:
I bought one of these the other day:

http://www.ebay.co.uk/itm/Megger-DCM330-Fork-Clamp-
Multimeter-/161750473789?hash=item25a9145c3d

I didn't have anything to measure high currents and do non-contact
tests for voltage (that was the excuse I made to myself for buying it,
anyway).

I've noticed that on the no-contact checks, the thing indicates a
positive on some cables even if the switch to the (light in this case)
is turned off. Reminds me of neon testers which glow from induced
current from nearby cables when the cable of interest is not actually
live. These false positives - if that's what they are - are a PITA and
I'd hoped a meter like this would know the difference. Or is it
something I'm doing wrong?
The instruction sheet is really awful and horrendously light on detail.


How would it tell the difference between induced voltages and "real"
voltages?

I'd have thought it would put some very light loading on the detected
signal. This is one of the few advantages of old, analogue meters over
modern digital meters. The modern meters impose such a light load that
'ghost' voltages appear real.

On 18/08/2015 22:22, Cursitor Doom wrote:
On Tue, 18 Aug 2015 20:52:34 +0100, dennis@home wrote:

On 18/08/2015 20:31, Cursitor Doom wrote:
I bought one of these the other day:

http://www.ebay.co.uk/itm/Megger-DCM330-Fork-Clamp-
Multimeter-/161750473789?hash=item25a9145c3d

I didn't have anything to measure high currents and do non-contact
tests for voltage (that was the excuse I made to myself for buying it,
anyway).

I've noticed that on the no-contact checks, the thing indicates a
positive on some cables even if the switch to the (light in this case)
is turned off. Reminds me of neon testers which glow from induced
current from nearby cables when the cable of interest is not actually
live. These false positives - if that's what they are - are a PITA and
I'd hoped a meter like this would know the difference. Or is it
something I'm doing wrong?
The instruction sheet is really awful and horrendously light on detail.


How would it tell the difference between induced voltages and "real"
voltages?

I'd have thought it would put some very light loading on the detected
signal. This is one of the few advantages of old, analogue meters over
modern digital meters. The modern meters impose such a light load that
'ghost' voltages appear real.


But isn't it a no contact test? If you could apply a small load then you
may get a different answer.

On Tue, 18 Aug 2015 22:50:00 +0100, dennis@home wrote:

On 18/08/2015 22:22, Cursitor Doom wrote:
On Tue, 18 Aug 2015 20:52:34 +0100, dennis@home wrote:

On 18/08/2015 20:31, Cursitor Doom wrote:
I bought one of these the other day:

http://www.ebay.co.uk/itm/Megger-DCM330-Fork-Clamp-
Multimeter-/161750473789?hash=item25a9145c3d

I didn't have anything to measure high currents and do non-contact
tests for voltage (that was the excuse I made to myself for buying
it, anyway).

I've noticed that on the no-contact checks, the thing indicates a
positive on some cables even if the switch to the (light in this
case) is turned off. Reminds me of neon testers which glow from
induced current from nearby cables when the cable of interest is not
actually live. These false positives - if that's what they are - are
a PITA and I'd hoped a meter like this would know the difference. Or
is it something I'm doing wrong?
The instruction sheet is really awful and horrendously light on
detail.


How would it tell the difference between induced voltages and "real"
voltages?

I'd have thought it would put some very light loading on the detected
signal. This is one of the few advantages of old, analogue meters over
modern digital meters. The modern meters impose such a light load that
'ghost' voltages appear real.


But isn't it a no contact test? If you could apply a small load then you
may get a different answer.

Yes, it's a no-contact test, but that doesn't prevent a load from being
placed on it wirelessly, perhaps via the same principle as a grid dip
oscillator or some other kind of resonant sensor tuned to 50hz that
becomes an 'acceptor' shunt at that frequency. Just seems very odd to me
that there's no apparent facility for this.

On Tue, 18 Aug 2015 19:31:36 +0000, Cursitor Doom wrote:

I bought one of these the other day:

http://www.ebay.co.uk/itm/Megger-DCM330-Fork-Clamp-
Multimeter-/161750473789?hash=item25a9145c3d

I didn't have anything to measure high currents and do non-contact tests
for voltage (that was the excuse I made to myself for buying it,
anyway).

I've noticed that on the no-contact checks, the thing indicates a
positive on some cables even if the switch to the (light in this case)
is turned off. Reminds me of neon testers which glow from induced
current from nearby cables when the cable of interest is not actually
live. These false positives - if that's what they are - are a PITA and
I'd hoped a meter like this would know the difference. Or is it
something I'm doing wrong?
The instruction sheet is really awful and horrendously light on detail.

The meter is correctly detecting these 'phantom' voltages. The problem
here is over the matter of interpretation of these voltage readings. Even
a good old fashioned moving coil meter with a modest 8 or 9 kilohms per
volt sensitivity on its ac range (always just less than half of its DC
ohms per volt sensitivity - 20K ohms per volt in this example) will show
a voltage reading on the switched live when the switch is off with no
lamp fitted in the socket (or a lamp type that draws no current below a
threshold voltage such as a conventionally ballasted fluorescent light
fitting - a tungsten filament lamp will short out any such induced
voltage).

The readings won't be as high as those obtained with a DMM, contactless
or not but will, nevertheless, likely show a few tens of volts depending
on the length of 'switch drop'.

As a matter of interest, just what sort of readings are you getting of
these 'phantom' voltages?

--
Johnny B Good