This is an extra warning. Underneath it is some warning tape, as per the wiring regulations. Underneath that is rubble. Underneath that is the electrical cable, which itself is buried in steel wire armour.
Once upon a time I was digging up a garden in Pontypridd. My spade hit a cable. Since there had been no warning tape, I assumed that it was just a dead fragment. It turned out to be the main supply to the house. Luckily there were no accidents that day.
I’m amazed that when you sell a house, there’s no obligation on you to provide plans for the electrical circuitry. Clearly, lots of people are living in houses without a clue where the live cables run. The technology exists to detect live cables – infra red cameras. Although these are prohibitively expensive, regular electricians can afford them. It should be a condition of sale that the potentially life threatening part of a house is disclosed properly. Estate agents are well aware of the risks. That’s why they always insist that they haven’t checked anything. Consequently, after I moved into my present house, I discovered the lots of the wiring was dangerous. I’d have bought it away but it might have had an impact on price. Clearly, houses with decades old wiring or shonky arrangements would be worth less than similar houses with modern properly designed circuits. Crucially, it would prevent home owners from engaging in illegal electrical DIY.