Friend asked me to check why she gets a "tingle" when touching both her stainless steel kitchen sink and an adjacent metal electrical appliance.
Inspection of her house - originally a small 2-up 2-down, extended backwards and sideways about 15 years ago - revealed many bodges. There is one lighting circuit, one radial to the boiler/CH controls and just one ring main, all basically OK. There is also a fourth circuit. This appears to have originally been a radial to a single socket in the kitchen, or possibly to a small oven (it seems to be 4 or 6mm sq at the CU end). It appears to have been extended with the house to serve 11 sockets (including washing machine, dishwasher, microwave and an electric heater) as well as various spurs to lighting circuits via FCUs. Very difficult to check the layout: most cabling is buried in the concrete floor or behind three layers of tiling or numerous fixed cabinets and half the sockets are now inaccessible (you can plug things in but not remove the faceplate). Surprisingly enough the 32A MCB never trips - seems the heavy power consumers are never on together.
CU is old 4-way Wylex now fitted with MCBs; no RCD anywhere. No main equipotential bonding of any sort in place.
What I think is the first socket on the radial reads 240v phase to neutral and the CPC has continuity back to the main earthing point at the CU. Others read between 115v and 175v phase-neutral with no load; and their CPCs between about 50v and (at worst, near the sink) 115v to the main earthing point (tested with a flying lead and multimeter). Or in the latter case to the sink itself, hence the tingle.
Connections at all accessible points OK (for what it's worth bearing in mind the circuit "design"). Haven't meggered anything yet - not sure I've found all the parts of the installation - but hardly need to to show there's a problem.
Temporarily bonding the sink to the CPC of the nearest socket solves the "tingle" problem (of course) and doesn't trip the MCB but I don't want to create a worse problem elsewhere eg in the bathroom (given the lack of main bonding).
I can install main equipotential bonding reasonable easily and will do so soon. But tracing and fixing the leakage to earth and broken CPC - i.e. replacing the broken and overloaded radial with the new ring it needs - will be disruptive and time-consuming. I won't have time to do it for ages. Friend (single mother, two kids, failing job) is smart but short of cash. I know she won't call in professionals unless I convince her her life is in danger: the problem has existed for at least 13 years and hasn't killed her (yet). Nor, amazingly, do any of the appliances on the affected circuit seem to have under-performed. Until I can do the job properly - probably several months - I plan to:
(a) install main equipotential bonding to the gas and water services
(b) extend it directly to as many accessible CPC connections on the affected circuit as necessary to restore proper earthing at all points I can test.
Yes it's a temporary bodge and no substitute for ripping out and replacing the dodgy circuit - not least because I find it hard to believe that this 50-115v potential in the CPC is not coming from some phase-to-earth fault somewhere (though as I say above, temp bonding doesn't trip the MCB). But is there any reason why in the circumstances I should not do it?