I committed a sin today. I removed an old consumer unit and installed a new all R.C.B.O. consumer unit. Afterwards I found that the two lighting circuits had a shared neutral somewhere. When I turned on the lounge light the two lighting circuits' R.C.B.O.s tripped off.
I then just connected the two lighting circuits up to just one of the R.C.B.O.s to get the lights on.
Sometimes you just can't be asked.
Will I go to hell?
Z.