Why do i have a varying voltage?!

[peterb20]

Ive got a small quandry - anybody out here able to help?....

Measuring voltage at the rose of my lighting circuit, the main radial circuit is fine, a steady 243v. however, when the light is off and i measure across the switch, its about 208v. I dont understand! all im measuring is the same point, only via the switch cable and the bulb. I have check the switch and it is DEFINITLY an open circuit, so there cant be any current flowing, thus no voltage drop anywhere else. Im quite good with electrics but i cant fathom it out.

Someone help please!!

 

[DESL]

Im quite good with electrics but i cant fathom it out.

OK so think about your reference points for the voltage you are measuring.

Assuming you have conventionally wired lighting cct then when you put a meter across the terminals of a light switch one terminal is connected to live feed the other is connected to the live of the lamp which in turn has a filament the other end of which is connected to neutral. Hence a volt drop across the ~80ohms of the lamp filament.

 

[Biggles..]

if your measuring across the switch then the current path is through your tester and the voltage reading is an effect of the restiance of the tester and bulb

 

[peterb20]

really? i did have an incling it was that but, if the switch is breaking the circuit and no current is flowing, how can there be a voltage drop? you only get a voltage drop if there is a current flowing dont you??

ps cheers for the speedy reply!

 

[Biggles..]

your testing across the switch so in effect its not there.if you are going between L1 and Com then your reading is a volt drop.Try going between Com and earth

 

[DESL]

True

But to measure a voltage a meter has to draw a current even if infinitessimally small.

Also a voltage is a potential difference between two points in a circuit

The two measurements you made were at different points so you would expect to get differing results.

Now if you want to take a measurement between L and E at ceiling rose and at switch you should get the same result.

 

[peterb20]

enough voltage to create an approx 35v drop? thats quite a big drawing of current (80ohm, 35v, thats close to half an amp!!)

 

[tim west]

It's not a dimmer switch is it?
without using a true RMS meter you will see a difference as a normal meter is not accurate reading a chopped waveform.

 

[peterb20]

no not a dimmer switch - just a regular one gang, one way switch.

 

[DESL]

I don't think that is what your measuring somehow.

The point I was trying to get over was that you are measuring between a live feed and a switch return which is connected through a lamp and comparing the result with a measurement made between L & N at a ceiling rose.

You implied you had a varying voltage when in fact you were measuring at two different reference points

 

[Steve]

Look, jam open the shutter on a socket and stick your probes in the live and neutral holes. THAT is your voltage reading.

 

[peterb20]

is my reading of the situation correct tho? In theory: The bulb is connected to the neutral, the switch cables are connected to the bulb and to the live, so as there is no current flowing, this should give the same voltage reading as directly to the live and neutral at the rose?

 

[peterb20]

yeah yeah i know thats the voltage, and its the same voltage on the lighting radial. Im just trying to understand WHY there is a voltage difference in my readings. Im not even trying to fix a problem, it is just curiousity i suppose!!

 

[Steve]

You have quite a large resistance across the light bulb!!

 

[peterb20]

i havent measured the resistance, im just using a 80ohms because someone mentioned it earlier in the post....