Strange issue with light switch wiring

[john2k]

I have a strange issue with a light in one room. At first the light briefly worked then it stopped working. I thought maybe the bulb, but then after changing bulb it still was not working. I decided to check the light switch itself and the two wires going to the switch (COM and L1) seem to both show a live light on my tester screw which is really odd. I thought maybe the switch itself so disconnected both wires and with both wires completely removed from the switch when I test both wires with the tester, they both seem to make the tester light up.

What could be wrong? if it was a short circuit then the circuit breaker MCB should trip but it's not tripping. What could be wrong?

Any advice would be much appreciated.

Thanks

 

[DetlefSchmitz]

Contacts in lamp socket may need bending out a bit, or could be bad connection in rose.

 

[SUNRAY]

With the power OFF have you tried joining those 2 wires together (in a rerminal block or in one of the switch terminals) then turning power back on to rule out a faulty light switch?

 

[ericmark]

Homes use AC because AC can be transformed up or down in voltage, but this also means there is some induction and capacitive linking, i.e. wires do not need to touch to transfer power, so the tester needs to present enough load to ensure a true reading, meters of yesteryear were 20,000 Ω/V or similar so we knew what load the put on the circuit, to measure no load we used a valve volt meter or a wheatstone bridge, the opamp found in today's meters was unknown.

One can often rub a no contact tester on your jumper and get the thing to light, which shows how unreliable they can be, they are really comparing the item being tested to your body, but still a good second string to your bow, so a neon screwdriver will likely alert one if you have a borrowed neutral, but as a secondary tester not the main testing method.

Electric goes in a circuit, i.e. it goes around the wires and will not simply leak out of the end like water in a hose pipe, so unless there is a return, (neutral or earth) it will do nothing.

So could be the switch, or could be no neutral, and if the switch is forming a high resistance it may well cause a bulb to flash.

As said both wires in same hole of switch will show if switch at fault.

 

[bernardgreen]

If the switched Live at the ceiling rose is not connected via the lamp to Neutral then capacitive coupling between Live and Switched Live in the cable from ceiling rose to switch will bring the potential on the Switched Live conductor close to the potential on the Live conductor when the switch is OFF.

Hence false indications from the tester.

Blown lamp, no lamp or loose terminal(s) will create the situation.

 

[JohnW2]

Electric goes in a circuit, i.e. it goes around the wires and will not simply leak out of the end like water in a hose pipe, so unless there is a return, (neutral or earth) it will do nothing.

I think that statement could be potentially misleading to some people, possibly dangerously so.

Whilst what you say is obviously correct, what it doesn't make clear that the fact that no current is flowing through a wire/conductor (because there is no 'return path', hence no circuit) does not necessarily mean that it is 'safe' (to touch) - because a human body in contact with it and neutral or earth could create a circuit through which current would flow - so that, rather than "doing nothing", the electricity could potentially kill the owner of that body!

Kind Regards, John

 

[johnny2007]

Has someone messed about with wires in the rose?
Maybe you have 2 permanent live in the switch instead of a permanent live and switch live.
Do you have a multimeter?
If so, open the rose and assess which wires are permanent live (they go on a loop), neutral (connected all together) and switch live (on its own unless serving multiple points)
To check which wires come from the switch, with electricity off at consumer unit, you could connect a 9V battery to the wires and check at the rose with the multimeter.

 

[securespark]

Broken N somewhere?