Odd results

[kme]

Scenario:

Call out by plasterer, who`s pinking a wall. He gets blasted off his ladder, and blacks out. Calls moi.

The wall has 1 no. double socket - nowhere near where he was laying on. The socket is on a ring, which tests okay (in isolation). But here is where it gets odd.

TT (overhead) supply, straight from pole. Ra is 415 ohm. Zs (DB) is 103 ohm. With all supply disconnected, incoming Neutral to MET is 90 ohm, and phase to MET is 80 ohm.

Supply from meter to ancient ELCB (no markings - trips at 308mA). Henleys to 1 no. 9 way wylex with 80A/100mA front end (NOT serving ring circuit in question), and 1 no. 9 way Volex with 100A main switch (no other RCD provided).

The wall in question has a newly created box section (plasterboard), which apparently contains pipes & cables. I`ve tested all the heads of the plasterboard screws, AND all the skim beads - nothing!

BUT......with just the MCB for the ring off, a test between phase and CPC/exposed metalwork reads 126 ohmsheadbang. I know that the loads plugged in the ring are allowing paths back via Neutral; but WTF???

1. Where did this shock come from??

2. Why the weird readings; or am I suffering from overwork, and imbibing too much of my liquid Guinness

KME

 

[Apache]

Scenario:Call out by plasterer, who`s pinking a wall. He gets blasted off his ladder, and blacks out. Calls moi.

KME

I might have called an ambulance!

 

[ProDave]

Tell him to don a pair or rubber gloves to complete the job

But seriously to make meaningful tests, you need to disconnect everything plugged into the sockets. Then see what the IR of your phase conductors is like.

Surely he most have some vague idea of where he was skimming when he got the bang? that will show you where to look.

 

[ian3012]

Ra = 415ohms???? hmmm...

possible drywall screw through L or N... metal trowel and wet wall provide path to earth....when the belt occurs, it would probably blow the end of the screw out anyway, so you now cant find the fault...

 

[kme]

Good point, Ian. I`ve already arranged for the plasterboards to come off again, so I can see in there - We`re also fitting a twin RCD board (15 way), and dealing with the high earth rod.

Thanks gents.