I had several new circuits and a new consumer unit installed and tested as part of an extension project last year.  I have two problems, possibly/probably unconnected.

1. I have one CFL light which when switched off is observed to flash slightly (more of a glow than full on)  and regularly.  Till happens if I change the bulb.   Does this indicate a wiring fault? The light is in the hall and is subject to two way switching.   Could such a fault have escaped the testing?  In case it helps, one of the banks of switches that controls it also controls lights on other circuits.

2. I am swapping some outlets and switches in parts of the house where new circuits were not installed for newer ones.  I switch of the MCB for the relevant circuit and my multimeter records no  voltage between earth and live or neutral.  However if I happen to touch some of the wires together the RCD trips.  Is this anything to be concerned about?  Never come across it before but as MCBs are single pole I can understand that this could happen.  Am I working safely merely to disconnect the circuit using the MCB?

Thanks for your advice.  (I am no longer in contact with the original electrician so can't ask him.

Thanks that's reassuring.  

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Hmm... I'll try to remember to come back if I ever draw a conclusion on that.  I've been swapping the bulb to try to eliminate a bulb fault so don't really have anything to compare with.... and I don't know how consisent cheaper bulbs are (are they cheaper or merely subsidised?)

As I recall swithcing lamps on annd off shortens the life.  Here's hoping that this one will last really long time 'cos its never fully off!!!

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I have been thinking about that.  Intellectually I would like to understand.  I thought that to induce a current then there must be a current flowing in the other wire.  Is that right?  I notice the problem at night when the only thing that I can think of that is drawing a current is my smoke detector (very close to the light and the wire probably follows the same path).  Indeed the smoke detector (one of several linked ones) was installed with the extension and I never had the problem before so I guess I could consider that as a possible cause but surely the currents would not be significant enough to cause that induction effect?
What voltage is needed to make a cfl flicker?  

Looking at it another way.. Do you experts see this issue reguarly?

Best regards,
Gavin

Looking at it another way, do you guys
In any event I can't do anything about it.  Indeed

Here is an example of two way switching...



The two common wires are frequently run in the same cable so they are close together. The longer the run and  the closer they are together the higher the capacitance between them.

The mains is AC and capacitors "pass" AC. The exact amount of current flowing depends many factors including the frequency and size of the capacitor and other elements in the circuit such as the light. If the mains was DC it wouldn't be an issue as capacitors "block" DC.

Capacitance can cause other interesting effects... Consider what happens if an earth wire became disconnected at the consumer unit. You would be left with a long length of copper wire parallel with a live wire in the ring main. The capacitance between the live wire and the "floating" earth wire and can cause 50V or more to appear on all "earthed" metalwork in the house. So if someone complains of mild shocks from a cooker or other metalwork it might not be a faulty live but a loose earth connection somewhere. Dangerous either way.