Why earthing and bonding matter: the knife rackWhy did nobody feel the live knife rack?

Start with an example, because it explains what earthing is all about. In a house, a magnetic knife rack was screwed to the kitchen wall. A cable in the wall behind it hadn't been run in a safe zone, and one screw went through the cable, touching only the line conductor.

• If the screw had touched line and CPC (half a millimetre to one side), it would have been an earth fault, and the circuit breaker, fuse or RCD would have tripped.
• If it had touched line and neutral, it would have been a short circuit, and again the protection would have operated.
• Touching line only, nothing tripped. The whole rack, and every knife on it, was live, possibly for months or years.

Nobody noticed, because it's all about potential. If your body isn't in contact with earth, touching something live just brings you up to the same voltage. Nothing moves through you, so you feel nothing. Anyone in the house could take knives off that rack every day and feel nothing.

Then one day the dishwasher door was open. Someone holding the knife rack touched the dishwasher door with her leg. The dishwasher was earthed through the CPC of its supply cable, so now there was a path to earth through her body. A shock along that path can kill.