RCD tripping

[always-diying]

I recently rewired upstairs lighting circuit (in Scotland so no Part P legislation). 1 circuit for upstairs lights, 1 for bathroom/ shower lights (RCD protected).

At first the RCD tripped when I connected the new upstairs circuit. In attempt to find fault I connected a new cable straight from CU to first junction box in circuit - RCD still tripped.

However, when I connected first junction box to circuit intended for shower lights, the RCD didn't trip.

Both scenarios involve cable going directly from CU to first junction box.

I now intend to run a new circuit for shower lights but any thoughts on why RCD might have tripped?

 

[Spark123]

Why are the lights on RCD? Are you on a TT supply or something? You say the upstairs lights work OK without the shower lighting circuit connected? If so it is looking like there is some sort of leakage to earth on this leg of the circuit, wether it be from damp or damaged to an incorrectly connected cable. The easiest way to tell is to disconnect any vulnerable equipment from the circuit and perform an insulation resistance test (aka megger it).

 

[always-diying]

Lights on RCD cos 240v downlight in zone 1.

basically 2 circuits - 1 for upstairs & 1 for bathroom (not yet connected).

original upstairs circuit trips, but when connect to bathroom circuit, does not trip. Only difference is cable between first junction box and CU. Checked this cable and seems OK.

Any other explanation?

 

[BJS_Spark]

Are you certain you are putting the neutrals in the correct neutral bars?

i.e. 'RCD neutral bar' for bathroom light feed neutral / 'non-RCD neutral bar' for upstairs lights feed neutral.