Typical "induced" current in ring cpc

[Dippy]

I realise this may be a fairly obscure question for the forum but here goes anyway.

I am just coming to the end of carrying out a full periodic inspection and test on my house electrics. During testing I found a few hidden faults (including one ring incorrectly wired "through" an FCU!), one of which I can't solve: One ring has been extended and the cpc loop continuity is barely 1x the phase/neutral resistance instead of the expected 1.67x. Despite a very good search I can't find any wiring errors, although this circuit does contain a spur which for the life of me I can't trace back to the ring! Anyway, since there's nothing actually unsafe and I'm going to be doing some building work soon which will expose much of the ring, I decide to plough on.

When I tested the insulation resistance for this ring I got 12Megs and 11Megs for the phase to cpc and neutral to cpc respectively. Although these are still reasonably good, they are not as good as those for the other two rings, which were both off the scale of my meter i.e. above 200Megs. So out of interest (never having done this before), I decide that I'd see what leakage current I could actually measure in the floating cpc ring to earth with the circuit powered. I measured 2.4mA, which of course is far higher than that due to leakage alone. I decided to measure it for the other two rings and got 2.5mA and 1mA.

Hence my question. I really haven't a clue if the currents I measured are typical or not, and my search of t'internet hasn't been much help. Does anyone here know?

BTW, I don't think its relevant but I also measured the induced voltages and got 95Vac, 110Vac and 65Vac respectively.

 

[Adam_151]

A lot of it is capacitive coupling.... two conductors with an insulator in between

2.4ma at 240v is about 96ohms impedance, which is about 33nano farads of capacatance (if we forget the neutral - which is also capacitive coupled to the CPC, exists for now), I'm told that 1mm T+E has about 0.1 nano farad per m length, I've no idea what that figure would be for 2.5 off hand.

The volatages are somewhat worthless without knowing the impedance of the instrument used to measure them, the unconnected CPC will sit at 120v as the two capacitors form a voltage divider, the instrument will drag it slightly towards what it is being measured against

 

[TicklyT]

2.4ma at 240v is about 96ohms impedance....

Methhiks your decimal point has slipped a bit Adam... 2.4 ma at 230v = 96 Kohms

 

[Adam_151]

2.4ma at 240v is about 96ohms impedance....

Methhiks your decimal point has slipped a bit Adam... 2.4 ma at 230v = 96 Kohms

Well spotted, it was late when I typed that

 

[THRIPSTER]

Dippy,


What size cable are you looking at where you are expecting the CPC loop continuity resistance to be 1.67 times the phase/neutral?

 

[davelx]

2.5/1.5=1.66666666

OSG Sec 10 page 70 10.3.2 step 1

 

[Dippy]

Yes, 2.5/1.5 T&E.

Thanks for the info (just shows how much of my theory I have forgotten).

My rough estimate of the cable run is about 60m. I guess with the higher nF/m of 2.5mm2 cable, plus the inductive effects, the currents I measured seem plausible.

I still can't find a cpc short and the more I think about it the more convinced I am that the 'so-called' electrician who put in the socket outlet spur did so by tapping into the ring with a choc block buried in the wall. Looks like he did the plastering himself too. Time to start digging