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Voltage at light fitting when light switch off ?



 
 
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  #1  
Old September 15th 07, 01:44 PM posted to uk.d-i-y
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Posts: 7
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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  #2  
Old September 15th 07, 02:00 PM posted to uk.d-i-y
external usenet poster
 
Posts: 1,160
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


  #3  
Old September 15th 07, 02:44 PM posted to uk.d-i-y
external usenet poster
 
Posts: 7
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?
  #4  
Old September 15th 07, 03:17 PM posted to uk.d-i-y
external usenet poster
 
Posts: 2,231
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
  #5  
Old September 15th 07, 03:48 PM posted to uk.d-i-y
external usenet poster
 
Posts: 1,160
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


  #6  
Old September 15th 07, 03:59 PM posted to uk.d-i-y
external usenet poster
 
Posts: 2,466
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
  #7  
Old September 15th 07, 04:03 PM posted to uk.d-i-y
external usenet poster
 
Posts: 7
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.

  #8  
Old September 15th 07, 07:21 PM posted to uk.d-i-y
external usenet poster
 
Posts: 19,626
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.
  #9  
Old September 15th 07, 07:25 PM posted to uk.d-i-y
external usenet poster
 
Posts: 19,626
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.
  #10  
Old September 15th 07, 08:56 PM posted to uk.d-i-y
external usenet poster
 
Posts: 3,864
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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