OK.
Lets start at the beginning...
As a starter..
Power up all the circuits.
Go round and switch on ALL the lights, then, go round again and make sure you switch them ALL off.
This way, you should be fairly sure there are no loads on the lighting (check loft for socket for booster and unplug.
Next is to go around and unplug everything, switch off spurs, cooker isolators etc.
Back to the CU. All fuses out or MCB's off, main switch off.
Next, set your tester on 250V (for now) and start with the red lead at the outgoing side of the fuse/mcb, and black on the neutral bar.
Test each circuit, leaving the probe on the neutral.
Once done, transfer to the CPC bar and repeat.
Finally, probes on neutral bar and cpc bar.
Now, if there are any circuits that are still showing flat, you need to establish what they do, disconnect them completely from the cu and test again.
If it is still showing flat, then you either still have something plugged in/switched on, or there is a damaged or trapped cable in the ciruit somewhere.
As a final point, if the CU incorporates RCD's then its unlikely you have a fault, as the RCD is not picking anything up. Not always the case tho.
Other than that, its down to a process of elimination to find the fault, and im afraid that all comes with experience!!
Just to make you feel better, im finishing some flats that were started by another sparks, and yesterday, carried out final testing, connected up CU, switched on one of the lighting circuits and 'BANG!'.
All finished and painted so i now have to start taking up chipboard flooring to trace the cables!
Wish me luck!
