Lighting problem

[jjm]

Hello All

Got called to a fault today on a light switch where an intermediate would not operate properly. After some investigation i found what i belive to be a link between the upstairs and downstairs lighting circuit. However when i split the link i tested the voltage on the upstairs lights i got a reading of p-n of 189 volts which is a little odd. If anyone can shed any light any why i am getting 189v i would be greatly appreciative

Many thanks

 

[RF Lighting]

Capacitive coupling I'd imagine.

Are you an electrician?

 

[jjm]

yep i am a sparks,

spent the last 5 years house bashing so having to get me head around fault finding again. Can you explain in laymen terms capacitive coupling?

 

[ban-all-sheds]

spent the last 5 years house bashing so having to get me head around fault finding again.

Don't electricians working on domestic LV installations ever have to do any fault finding?

Can you explain in laymen terms capacitive coupling?

1) Two long pieces of wire separated by PVC will act like a capacitor.

2) http://www.google.co.uk/search?q=capacitive+coupling+electrical+cables

3) It has nothing to do with a switch not working properly.

 

[securespark]

Don't be so hasty, Ban.

By house-bashing, he probably means installs, rewires etc.. He may not have done any fault-finding recently.

 

[ban-all-sheds]

I wasn't having a go.

Just curious - I hear what you say, but in 5 years.....?

 

[bernardgreen]

1) Two long pieces of wire separated by PVC will act like a capacitor.

Two short pieces of wire separated by PVC will act like a capacitor, but smaller than the one formed by longer wires.

Capacitors formed by cables are small but not small enough to be insignificant With AC they act like a high value resistor and will pass a small amount of current. This often affects switch circuits as is there is a capacitor across the switch contacts which passes some current when the switch is OFF, not enough to light a filament lamp but enough to make energy saving lamps flash and LED lamps to low dimly and to make a high impedance meter show a voltage.

 

[Space cat]

The hidden capacitor strikes again! Capacitive coupling can lead you down a lot of false trails when you're fault-finding with a high impedance meter. A dummy load in the form of a low wattage standard filament bulb will clear up the confusion. Connect it in parallel with your meter and that 189V will become millivolts.