why did my RCD trip?

[ExArmy]

was replacing a light fitting in my own house and the upstairs light cct was turned off at the MCB in the DB, i tested for voltage using a non contact voltage indicator at the light fitting to be replaced, it did not light up so i proceeded to do the job.

near the end of the job whilst trying to get 3 neutrals into one terminal I touched one of them onto the metal casing of the light fitting and the RCD tripped. i'm not really sure why this happened, does it mean theres a circuit in my house that shares a neutral with the upstairs lighting circuit? and if so, why was there no voltage present?

 

[Rockingit]

It is common for RCD's to trip when you accidentally short N-E to a dead circuit and nothing to worry about. Caused by RCD's being very sensitive and various things to do with eddy currents.

 

[cbw]

All normal....Dont worry!

 

[cbw]

Beat me to it

 

[MarkieSparkie]

This event has nothing to do with eddy currents.
When the neutral touched the protective conductor or other earthy item, this created a parallel path for a proportion of any neutral current flowing in the other, un-isolated, circuits. This current bypassed the current balance core of the RCD, causing the RCD to trip.
Therefore, it is not a 'nuisance' tripping in the normal sense of an accumulation of earth leakage current or a transient voltage spike. The only way to avoid the tripping problem is to work with the circuit isolated by a double-pole device, such as the main switch of the installation, or with the neutral of the final circuit disconnected at the consumer unit.
The issue described above is a problem particularly in TN systems. However, it could also occur in a TT system. If the earth electrode resistance is low enough to allow the tripping current of the RCD (perhaps as low as about 20 mA) to flow. This is the result of the small voltage that typically exists on the consumer unit neutral bus-bar, that is due to voltage drop in the neutral conductor of the supply network. Remember that the isolation requirement for a TT system is always that both the line and neutral conductors are isolated (537.2.1.1).

 

[Knobhead]

Caused by RCD's being very sensitive and various things to do with eddy currents.

Excuse the pun, but where on earth did you get that idea from?

 

[Rockingit]

Excuse the pun, but where on earth did you get that idea from?

:75:

Can always rely on you two to keep us on our toes!!!

I was just trying to give a shorter one-liner than the full explanation to justify my assertion that it is a 'normal occurence'. I could have also used 'stray voltage' or some other reference that's not quite technically correct. It was late, after all.

 

[Knobhead]

If you look closely in to the physics of a RCD core balance CT your not all that wrong! I was just being pedantic.

 

[Rockingit]

If you look closely in to the physics of a RCD core balance CT your not all that wrong! I was just being pedantic.

I thought I had bought all the Rights up on that one?! At least, if you ask Mrs Rockingit then I have.