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Default Voltage at light fitting when light switch off ?


I'm measuring 17 Vac across my bathroom light fitting when the light
switch is off. I'm also measuring 34 Vac across my landing light
fitting when the light switch is off. If I isolate the upstairs
lighting at the consumer unit then it drops to around 1.3 Vac.

What's going on?
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Default Voltage at light fitting when light switch off ?


LaserMark wrote in message
...

I'm measuring 17 Vac across my bathroom light fitting when the light
switch is off. I'm also measuring 34 Vac across my landing light
fitting when the light switch is off. If I isolate the upstairs
lighting at the consumer unit then it drops to around 1.3 Vac.

What's going on?


What are you using to measure the voltage? If you are using a modern
high impedance meter such as a Fluke, then what you are measuring is
the capacitive coupling between a live wire and an isolated one, and
it is quite normal. If on the other hand you are using an older meter
based on a moving coil movement then it is time to worry!

AWEM


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Default Voltage at light fitting when light switch off ?

On Sat, 15 Sep 2007 14:00:39 +0100, "Andrew Mawson"
wrote:


LaserMark wrote in message
.. .

I'm measuring 17 Vac across my bathroom light fitting when the light
switch is off. I'm also measuring 34 Vac across my landing light
fitting when the light switch is off. If I isolate the upstairs
lighting at the consumer unit then it drops to around 1.3 Vac.

What's going on?


What are you using to measure the voltage? If you are using a modern
high impedance meter such as a Fluke, then what you are measuring is
the capacitive coupling between a live wire and an isolated one, and
it is quite normal. If on the other hand you are using an older meter
based on a moving coil movement then it is time to worry!

AWEM


I'm using a Fluke 77. So why am I seeing 17 Vac on one fitting and 34
Vac on another fitting?
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Default Voltage at light fitting when light switch off ?

On Sat, 15 Sep 2007 14:44:56 +0100, LaserMark wrote:

On Sat, 15 Sep 2007 14:00:39 +0100, "Andrew Mawson"
wrote:


LaserMark wrote in message
. ..

I'm measuring 17 Vac across my bathroom light fitting when the light
switch is off. I'm also measuring 34 Vac across my landing light
fitting when the light switch is off. If I isolate the upstairs
lighting at the consumer unit then it drops to around 1.3 Vac.

What's going on?


What are you using to measure the voltage? If you are using a modern
high impedance meter such as a Fluke, then what you are measuring is
the capacitive coupling between a live wire and an isolated one, and
it is quite normal. If on the other hand you are using an older meter
based on a moving coil movement then it is time to worry!

AWEM


I'm using a Fluke 77. So why am I seeing 17 Vac on one fitting and 34
Vac on another fitting?


Beacause more or less that's the actual voltage of the wire, the wire is
neither earthed nor is it connected to a supply. Frankly a probe with a
230Vac pigmy bulb in it will tell you much more _relevant_ info about
mains wiring than a DVM.




--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at http://www.diyfaq.org.uk
Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html
Choosing a Boiler FAQ http://www.makewrite.demon.co.uk/BoilerChoice.html
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Default Voltage at light fitting when light switch off ?


LaserMark wrote in message
...
On Sat, 15 Sep 2007 14:00:39 +0100, "Andrew Mawson"
wrote:


LaserMark wrote in message
.. .

I'm measuring 17 Vac across my bathroom light fitting when the

light
switch is off. I'm also measuring 34 Vac across my landing light
fitting when the light switch is off. If I isolate the upstairs
lighting at the consumer unit then it drops to around 1.3 Vac.

What's going on?


What are you using to measure the voltage? If you are using a

modern
high impedance meter such as a Fluke, then what you are measuring

is
the capacitive coupling between a live wire and an isolated one,

and
it is quite normal. If on the other hand you are using an older

meter
based on a moving coil movement then it is time to worry!

AWEM


I'm using a Fluke 77. So why am I seeing 17 Vac on one fitting and

34
Vac on another fitting?


OK, you have a conducting circuit comprising your 'unconnected' wire,
the tiny capacitances to adjacent conducting wires, and the very high
impedance of the measuring instrument. Changes in the amount of
capactive coupling will vary the perceived voltage at the Fluke. So it
is probable that your 17v reading is on a wire with (about) half the
capacitive coupling than your wire associated with the 34v reading.
Neither are anything to worry about and are quite normal.

I don't wish to sound condescending or rude, but it is an example of a
little knowledge being a dangerous thing. You need to understand the
big picture and the limitations and scope of your tools.

AWEM




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Default Voltage at light fitting when light switch off ?

In message , LaserMark
writes
On Sat, 15 Sep 2007 14:00:39 +0100, "Andrew Mawson"
wrote:


LaserMark wrote in message
. ..

I'm measuring 17 Vac across my bathroom light fitting when the light
switch is off. I'm also measuring 34 Vac across my landing light
fitting when the light switch is off. If I isolate the upstairs
lighting at the consumer unit then it drops to around 1.3 Vac.

What's going on?


What are you using to measure the voltage? If you are using a modern
high impedance meter such as a Fluke, then what you are measuring is
the capacitive coupling between a live wire and an isolated one, and
it is quite normal. If on the other hand you are using an older meter
based on a moving coil movement then it is time to worry!

AWEM


I'm using a Fluke 77. So why am I seeing 17 Vac on one fitting and 34
Vac on another fitting?


Because you are just measuring pickup

--
geoff
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Default Voltage at light fitting when light switch off ?

On Sat, 15 Sep 2007 15:48:18 +0100, "Andrew Mawson"
wrote:


LaserMark wrote in message
.. .
On Sat, 15 Sep 2007 14:00:39 +0100, "Andrew Mawson"
wrote:


LaserMark wrote in message
.. .

I'm measuring 17 Vac across my bathroom light fitting when the

light
switch is off. I'm also measuring 34 Vac across my landing light
fitting when the light switch is off. If I isolate the upstairs
lighting at the consumer unit then it drops to around 1.3 Vac.

What's going on?

What are you using to measure the voltage? If you are using a

modern
high impedance meter such as a Fluke, then what you are measuring

is
the capacitive coupling between a live wire and an isolated one,

and
it is quite normal. If on the other hand you are using an older

meter
based on a moving coil movement then it is time to worry!

AWEM


I'm using a Fluke 77. So why am I seeing 17 Vac on one fitting and

34
Vac on another fitting?


OK, you have a conducting circuit comprising your 'unconnected' wire,
the tiny capacitances to adjacent conducting wires, and the very high
impedance of the measuring instrument. Changes in the amount of
capactive coupling will vary the perceived voltage at the Fluke. So it
is probable that your 17v reading is on a wire with (about) half the
capacitive coupling than your wire associated with the 34v reading.
Neither are anything to worry about and are quite normal.

I don't wish to sound condescending or rude, but it is an example of a
little knowledge being a dangerous thing. You need to understand the
big picture and the limitations and scope of your tools.

AWEM


A little knowledge would be a dangerous thing if I ignored
perceived anomolies rather than find out what they are. But anyway
thanks for your input.

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Default Voltage at light fitting when light switch off ?

In article ,
LaserMark wrote:
On Sat, 15 Sep 2007 14:00:39 +0100, "Andrew Mawson"
wrote:



LaserMark wrote in message
.. .

I'm measuring 17 Vac across my bathroom light fitting when the light
switch is off. I'm also measuring 34 Vac across my landing light
fitting when the light switch is off. If I isolate the upstairs
lighting at the consumer unit then it drops to around 1.3 Vac.

What's going on?


What are you using to measure the voltage? If you are using a modern
high impedance meter such as a Fluke, then what you are measuring is
the capacitive coupling between a live wire and an isolated one, and
it is quite normal. If on the other hand you are using an older meter
based on a moving coil movement then it is time to worry!

AWEM


I'm using a Fluke 77. So why am I seeing 17 Vac on one fitting and 34
Vac on another fitting?


I'm amazed you bought such an expensive tool without knowing what it does.

--
*I have my own little world - but it's OK...they know me here*

Dave Plowman London SW
To e-mail, change noise into sound.
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Default Voltage at light fitting when light switch off ?

In article ,
LaserMark wrote:
A little knowledge would be a dangerous thing if I ignored
perceived anomolies rather than find out what they are.


But it's basic electrical theory. If you use a very high input impedance
device you'll always get strange readings on an open circuit. Just
touching the leads will give some sort of reading. It is high impedance to
give an accurate reading under working conditions.

--
*A closed mouth gathers no feet.

Dave Plowman London SW
To e-mail, change noise into sound.
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Default Voltage at light fitting when light switch off ?

In message , LaserMark
writes
I'm measuring 17 Vac across my bathroom light fitting when the

light
switch is off. I'm also measuring 34 Vac across my landing light
fitting when the light switch is off. If I isolate the upstairs
lighting at the consumer unit then it drops to around 1.3 Vac.

What's going on?

What are you using to measure the voltage? If you are using a

modern
high impedance meter such as a Fluke, then what you are measuring

is
the capacitive coupling between a live wire and an isolated one,

and
it is quite normal. If on the other hand you are using an older

meter
based on a moving coil movement then it is time to worry!

AWEM


I'm using a Fluke 77. So why am I seeing 17 Vac on one fitting and

34
Vac on another fitting?


OK, you have a conducting circuit comprising your 'unconnected' wire,
the tiny capacitances to adjacent conducting wires, and the very high
impedance of the measuring instrument. Changes in the amount of
capactive coupling will vary the perceived voltage at the Fluke. So it
is probable that your 17v reading is on a wire with (about) half the
capacitive coupling than your wire associated with the 34v reading.
Neither are anything to worry about and are quite normal.

I don't wish to sound condescending or rude, but it is an example of a
little knowledge being a dangerous thing. You need to understand the
big picture and the limitations and scope of your tools.

AWEM


A little knowledge would be a dangerous thing if I ignored
perceived anomolies rather than find out what they are.


Don't be so precious


--
geoff


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Default Voltage at light fitting when light switch off ?

eOn Sat, 15 Sep 2007 19:21:49 +0100, "Dave Plowman (News)"
wrote:


I'm amazed you bought such an expensive tool without knowing what it does.


You shouldn't be :-). I recently watched an air conditioning
installer (clad in a boiler suit declaring under the company name
that he was an "Installation Engineer") prodding one probe of a very
expensive Fluke DMM at various contacts and saying that the reason
the device he had just fitted was not working was because the mains
was too weak (at 75VAC). When it was suggested that the second probe
belonged somewhere other than his pocket he declared he was "Part P
qualified" and knew what he was doing.

Pointing out that the destruction book said that the red wire should
go to connector A and he had connected it to connector D led to much
angst and a declaration that the instructions were not Part P and had
to be modified accordingly.

Further mentioning that Part P didn't apply to industrial
installations led to him stomping off saying he couldn't continue due
to elfandsafety. In this he was probably right as I suspect my blood
pressure was exceeding safe limits by this stage.
--
Peter Parry.
http://www.wpp.ltd.uk/
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Default Voltage at light fitting when light switch off ?

On 15 Sep, 19:21, "Dave Plowman (News)" wrote:
In article ,
LaserMark wrote:



On Sat, 15 Sep 2007 14:00:39 +0100, "Andrew Mawson"
wrote:


LaserMark wrote in message
.. .


I'm measuring 17 Vac across my bathroom light fitting when the light
switch is off. I'm also measuring 34 Vac across my landing light
fitting when the light switch is off. If I isolate the upstairs
lighting at the consumer unit then it drops to around 1.3 Vac.


What's going on?


What are you using to measure the voltage? If you are using a modern
high impedance meter such as a Fluke, then what you are measuring is
the capacitive coupling between a live wire and an isolated one, and
it is quite normal. If on the other hand you are using an older meter
based on a moving coil movement then it is time to worry!


AWEM


I'm using a Fluke 77. So why am I seeing 17 Vac on one fitting and 34
Vac on another fitting?


I'm amazed you bought such an expensive tool without knowing what it does.

--
*I have my own little world - but it's OK...they know me here*

Dave Plowman London SW
To e-mail, change noise into sound.


£203 I've just priced that meter at. When you can get a perfectly
adequate meter out of Maplin for a fiver or less, that does somewhat
beggar belief that that amount of money was spent without seemingly
understanding the uses and characteristics of the instrument.

I do quite a bit of home electronics and would love to have spare
money to buy quality instruments like that. Actually on the basis
that 'quality' in my book also includes 'value for money' I'm not sure
I would buy one.

Rob

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Default Voltage at light fitting when light switch off ?

robgraham wrote:

£203 I've just priced that meter at. When you can get a perfectly
adequate meter out of Maplin for a fiver or less, that does somewhat
beggar belief that that amount of money was spent without seemingly
understanding the uses and characteristics of the instrument.


Might explain why in every electronics lab I have worked every new batch
of fluke meters bought seems to have such an alarming rate of
disappearance! If you don't keep them in a locked drawer they grow legs!

I do quite a bit of home electronics and would love to have spare
money to buy quality instruments like that. Actually on the basis
that 'quality' in my book also includes 'value for money' I'm not sure
I would buy one.


Part of the attraction of the fluke meters is that they are nearly idiot
proof - you have to work quite hard to kill one, so they are well suited
to lab use.

--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/
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Default Voltage at light fitting when light switch off ?

On 2007-09-15 22:05:45 +0100, robgraham said:

£203 I've just priced that meter at.


Really? You must have tried very hard to find one that expensive.
They are less than half that in the U.S.



When you can get a perfectly
adequate meter out of Maplin for a fiver or less, that does somewhat
beggar belief that that amount of money was spent without seemingly
understanding the uses and characteristics of the instrument.


It depends on what you want to do.

I have a Fluke 87 that I bought in the U.S. over 10 years ago (IIRC).
This was one of the earlier version 4 true RMS ones and I think I paid
around $200 for it at the time including a bundle of accessories such
as thermocouple probe and adaptor and a current clamp.

During that time it has behaved perfectly - leads are still good,
calibration reports fine, switches are solid and so on. It's rugged
enough that if it's accidentally dropped it survives unscathed - not
that I've dropped it more than twice. Even so, it's normally kept
carefully and used as required on projects.

I've also had a succession of the £10-20 jobs that are typically used
in the workshop for rough work that doesn't need a better device.
They are OK for what they are, but accuracy is not great and there is
quite a bit of variation with temperature for some reason which I've
never investigated. It's the flimsy little leads that let them
down the most and I've found that the switch usually becomes
intermittent or causes errors. Fortunately, the errors seem to be
gross enough to be obvious.



I do quite a bit of home electronics and would love to have spare
money to buy quality instruments like that.


You can get quite a long way with these meters as long as you realise
the limitations and are prepared to throw them out when they become
troublesome.



Actually on the basis
that 'quality' in my book also includes 'value for money' I'm not sure
I would buy one.


The two are not really related that much. Quality is all about
whether the product does what it is claimed it will do and continues to
do so for a period of time represented by the price paid. It's
therefore fairly well defined.

Value for money is about the degree to which the product does what you
are looking for it to do and that varies by person and application.

Thus if you want to do a set of things that can be achieved by one of
the Maplin superspecials and you are happy to buy replacements as they
fail or become unreliable, then they are good value for money for you.
Equally, if you want to do some things for which a higher spec. meter
like the Fluke is needed or can be relied upon to do for longer without
buying a new one, then the Fluke is good value for money and the
Superspecial is not.



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Default Voltage at light fitting when light switch off ?

In article om,
robgraham wrote:
I'm amazed you bought such an expensive tool without knowing what it
does.


£203 I've just priced that meter at. When you can get a perfectly
adequate meter out of Maplin for a fiver or less, that does somewhat
beggar belief that that amount of money was spent without seemingly
understanding the uses and characteristics of the instrument.


Indeed. A cheap DVM will be more than adequate for most who will only use
it for voltage and continuity.

I do quite a bit of home electronics and would love to have spare
money to buy quality instruments like that. Actually on the basis
that 'quality' in my book also includes 'value for money' I'm not sure
I would buy one.


I did graduate to a Fluke some years ago. Deliciously tactile to hold and
use. ;-) After you pay extra for some decent flexible leads - the
originals were terrible.

--
*He who dies with the most toys is, nonetheless, dead.

Dave Plowman London SW
To e-mail, change noise into sound.


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Default Voltage at light fitting when light switch off ?

In article ,
John Rumm wrote:
Part of the attraction of the fluke meters is that they are nearly idiot
proof - you have to work quite hard to kill one, so they are well suited
to lab use.


Still easy to blow a fuse if you forget to change the leads back after
doing current measurement, though. And the fuses ain't half expensive.

--
*Lottery: A tax on people who are bad at math.

Dave Plowman London SW
To e-mail, change noise into sound.
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Default Voltage at light fitting when light switch off ?

Dave Plowman (News) wrote:

I did graduate to a Fluke some years ago. Deliciously tactile to hold and
use. ;-) After you pay extra for some decent flexible leads - the
originals were terrible.


Common to a lot of meters, I can't understand why the maker of an
expensive bit of kit cheapens the entire product by fitting leads that
probably save all of 25p off the total cost.

My antique Solartron was the same.
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Default Voltage at light fitting when light switch off ?

On 15 Sep, 23:20, Andy Hall wrote:

Thus if you want to do a set of things that can be achieved by one of
the Maplin superspecials and you are happy to buy replacements as they
fail or become unreliable, then they are good value for money for you.


At one time the Maplin ones were bogof and consequently cheaper than
the batteries inside them. I've got a stack of batteryless meters
somewhere ...

Ian

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Default Voltage at light fitting when light switch off ?

LaserMark wrote:
I'm measuring 17 Vac across my bathroom light fitting when the light
switch is off. I'm also measuring 34 Vac across my landing light
fitting when the light switch is off. If I isolate the upstairs
lighting at the consumer unit then it drops to around 1.3 Vac.

What's going on?



You are using a high impedance meter .
And measuring capacitative pickup.

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