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Discuss Follow up: REALLY Low reading PFC Test in the Periodic Inspection Reporting & Certification area at



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So this is what I found:

1. At one DB found live+neutral to earth ins res was about 181Mohm... bit lower than perfect but never mind.

2. Same DB had a 30mA RCCB mounted underneath it protecting the full board, even though it was a split board, and had a RCD 30mA in it already. This is redundant as the RCCB trips first. Am I ok to change the RCCB to 300mA or even take it out?

3. A PIR was faulty, never turning on. So I replaced it, now its never turning off!

And finally (this is a good one):
4. An old Hager CU mounted on a wooden pole outside, rusted to high heaven, was used as a JB to get the supply to a toilet, wired through a 32A breaker. as I was about to start doing some test, my meter was telling me I had live and neutral polarity wrong... went back to the 3phase DB, and EVERY phase was saying wrong polarity - it was all still live! went to Main incoming DB, same again, Every Phase wrong polarity, and then 250V Between neutral and earth!!!!! Checked with 3 testers...

Then as I was about to switch the whole site off, a 63A 3phase breaker in the main DB feeding the problematic board tripped, away went the problem from the rest of civilisation, and the problem was followed through to this toilet circuit being the cause of the problem.

However, what the hell sort of fault doesn't trip either the final circuit breaker, (all of which were 6A in the toilets), or the "JB" style hager board with 32A breaker, or even the 32A breaker feeding the whole thing... how did it hang in that state for so long, long enough for me to test it 3 times plus walking (hastily) between each board, until eventually a 63A breaker goes...

and how does this not go bang??! Surely a dangerous short?!?!!?

an electrical phenomenon!

Please help me with your answers, comments and suggestions. Cheers
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Des 56

1 seems ok
2 split board with A 30m/amp Rcd
Is one half non rcd protected and are there circuits on that unprotected side requiring 30 m/amp rcd protection
3 Check connections are correct and pir is in automatic mode not on test or something simillar
4 Going for a pint will look in again


  • Thread Starter Thread Starter
  • #3
lol thanks for the reply des, no circuits on the unprotected side require RCD protection... however it is a caravan site.

is there a dip switch or something on the PIR?

and 4 was the biggest concern, and confusion.


  • Thread Starter Thread Starter
  • #4
you mentioned before you had a very low prospective fault current, maybe the thats why it didnt trip the final circuit breakers, and maybe there is a better pfc between the main db and the sub mains?

sounds like one hell of a challenge riotmaster!
Sounds to me that you have a dodgy neutral connection,possibly somewhere near the origin.
For your neutral bar to show live there must be a break somewhere,and the current between your
live neutral bar is not flowing home.
Also I'm not quite sure about your post concerning 2 earth electrodes.Why 2 of them?Maybe they should be further apart(10m is recommended) and not linked together--overlapping resistance areas should maybe be avoided.Is your incoming supply tt or just your caravan pitches?
There are quite a few regs that are specific to caravan sites to look out for if you are doing your periodic insp.such as the use of double pole circuit breakers as well as rcds.,individual rcd now needed
for each pitch site. Your rcd discrimination should also be considered with the main one having time delay if you have tt. Your toilet light tripping 63amp mcb,perhaps it was over sensitive,did you find any low readings on this circuit or how were you sure that it was this circuit that was at fault?
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