Hi L, best if you can unplug everything but usually miss something or can't get at it so,
at board disconnect lives & Neutrals for your circuit and put them together temp. then test all 4 to earth, at same time. Reading should then not be affected by anything still in circuit, also you won't hurt anything connected.
you should then get good reading, if OK test L to E. (N clear of earth contact), should be OK , then do same for N (L clear of earth contact).
If all ok try continuity of L_N on OHMS scale not M ohms if reading anything then something still connected, may be Neon on FCU?
I've tried this but am still getting very odd results that I don't understand. I am using a Fluke 1651 tester set at 500V for the insulation test. I've joined my L and N via both ends of a junction block - is this okay?
however, when I do the test 500V is not being displayed as the test voltage and I'm not getting an insulation result returned. I know the Fluke is okay as I've done the insulation test on a radial circuit okay
Hi L, i had one of those for a short while, I know it tests ok on some circuits but humour me by fitting new batteries if you havent changed them for a while,
Also just had a thought! check no voltage showing on wires as some meters won't test if more than a very small voltage level, prob with all digimeters is too high input impedance so slightest thing shuts them down.
I never use digi-meter for any fault finding, I keep a few uncalibrated but reliable moving coil meters AVO/MEGGER etc in box in van so if not happy with events get '2nd opinion' also for chasing ring faults they are much quicker & smaller to carry about. Once sorted then use calibrated meter for test sheets, good luck & let me know outcome, wish I was close by as love this kind of challenge, (prob. not what u r calling it rt. now!!)