Caught out by mt own lazyness.....

[Deleted account]

Anthony brags picture shows the break before any lights. If the break was half way into the circuit the down stairs lights would have a return path through neutral, so no trip.

This is provided I've understood anthonybrag, And Anthonybrag has understood doomed correctly

 

[Toneyz]

Could be a possibility that the lighting circuits were both together before and has now split up to upstairs and downstairs could also be that they switched on one of the upstairs lights first.

 

[Baddegg]

Thanks Anthony I was toying with the second option myself but convinced myself it couldn’t be that simple ha ha

 

[Doomed]

There was only one lighting circuit, the ground floor was fine, the link to upstairs had the break in the neutral and the upstairs was as per the drawing.
As I said, if I had gone upstairs and checked a light I would of found it after the DB change, but I was lazy.

 

[a-z electrics]

Interesting post.. thanks for sharing it.