Electrical funny (2)

[roger]

Not to complicate Adrian's thread.. here's an earthing funny that lay dormant for many years. Victorian house - Granny Flat extension added to the rear - all is well till a graphic artist takes over the flat. On arrival installs extensive computer system, and at end of day takes a shower. Two minutes into showering, adjusts the controls a 2nd or 3rd time - and gets a shock.

Shower is not an electric one, earth bonding is spot on, shower tray and outlet are plastic. Whole electrical installation seems faultless - very professionally done with separate CU in the flat on the end of long armoured cable from main board in the house. [No RCDs anywhere]

The armoured cable is neatly terminated in a proper gland at the original board.... into a box which is not earthed. So no earth connection to the armour, the earth conductor out to the flat.

The filters in the computer gear were all probably legal, but there was a lot of it and enough capacitance to bring the floating earth up to

110V on an AVO. When the water went out along the drain, presumably with its conductivity raised by some expensive toiletry - one doesn't like to enquire! - and reached actually soil-type earth via the cracks in the Victorian drains.....

I can imagine the installation was tested with the earth clip to the new box, as being the most accessible earth point: so however well- intentioned the Govt's new Regulations are, I suspect they wouldn't have made a haporth of difference in this case.

[The Natural Philosopher]

Tell me about it.

We had laptop once, that had been fitted quite normally with capacitors between live and chassis, and neutral and chassis.

And a mains cable that had no earth pin or even earth wire. In fact, AFAICR the socket in the laptop into which it plugged was a two pin affair.

Connecting the laptop to another PC via a serial cable blew the serial board in the PC.

yup. 110V on the chassis...and a minor misaligment brought the serial cable 'earth' on contact with an input pin.

[Welsh Bob]

As an occasional student of the wiring Regs, i've been wondering this for a while - when there is a remote CU in an installation like in the flat in this case, why is the connection made from the main CU to the remore one made using armoured cable if its internal to the property. Could the link be made with some hefty T&E?

[BigWallop]

It could be made with T+E but the armour gives a more secure earth bonding because there is more of it and not just a thin inner core of bare copper. Also, SWA Cable can be made more secure in any switch gear that is used to isolate the remote mains supply. It is also more protected against damage. But there is nothing stopping anyone from installing a sub main with heavy enough T+E.

[Bob Mannix]

Indeed, I have done it (yes it was inspected). There is also a separate earth to the earth rod (in my case). Mine runs through the flat roof - I guess if it was running exposed you might prefer armoured cable if it's protected at 100A - you'd get a meaty flash off that in the right circumstances.

[Frisket]

Reminds me of the one I had to look at a few months ago - Owner reported getting a shock from the running water in his bathroom sink. With past experience of customers I toddled along with the phone number of a good psychiatrist in my pocket. Went in, turned on tap, put hand in water - nothing. "Try taking your shoes off" says he. Took off shoes, turned on tap, put hand in water and nearly landed on my ar*e! Checked usual things - no fault showing. Stuck meter probe between running water and earth - best part of 100v. Pulled bullet at meter - still 100v. Pulled meter bullet for flat upstairs - voltage disappeared. Customer then said that when he first moved in he used to get a shock off the walls (it was a refurbished old building converted into upstairs / downstairs flats). Problem turned out to be a nail through the live tail insulation to the upstairs flat. It had only nicked the insulation and you couldn't see bare copper but it was obviously enough. Why the voltage appeared only in the running water is beyond me - the shock from the walls was presumably down to wet plaster and disappeared as it dried out. Richard.

[roger]

BigWallop writes

And, if the wind is in the right direction, SWA is cheaper - esp in the larger sizes.

4-core SWA, 2 cores used for N and 2 for L, used to be about as cheap as you could get for a long tough run.