Light on separate circuit stopped working

[Alan-Carter-Electrical]

Hi everyone. I’m a newly qualified electrician. Just had a confusing issue. I put up a new light in a clock room for a customer. It’s a microwave sensor light. So I left out the switch live and terminated feed in feed out lives and neutrals.

Now, an upstairs light stopped working. I can’t understand why. So I dropped the new fitting and double checked the cables I had were right. They are. So I connectored them and left the fitting off. Same issue. The light that has stopped working has permanent live at the pendant in loop but no voltage on switch.

like I said it’s a different circuit which is just baffling me. Also a separate led fitting on the downstairs circuit has gone very very dim. Again is this a coincidence or part of a bigger problem.

Would really appreciate some help as I’m at a loss. My background is new build housing and I’ve only recently qualified. I’m massively aware of my limitations but this seemed like a very straight forward job.
Another thing. The main switch seems to crackle when I turn it on.

thanks everyone

 

[GBDamo]

Do you have 230V PL to earth but not PL to neutral at the upstairs light.

Could there be a borrowed neutral from the downstairs light that has been misidentified?

My golden rule with this kind of scenario is to leave anything that I have not touched and concentrate my attentions on what I could have changed.

 

[ipf]

Have you broken an existing neutral, as well as live, at the switch? ( ins and outs, you say)

 

[SJD]

Are you 100% sure said upstairs light was actually working before you started? For example, could putting the Xmas decorations away in the loft have pulled a cable loose, and nobody has tried using the light since?