Just put a new kitchen in my gaff, and noticed that the old metal sink I ripped out had an electrical ground wire attached to it (yellow and green colour wire = earth in the UK). The new sink also has a little clip on it underneath that looks like it should be perfect for attaching an earth wire. Excuse my ignorance, but why do you need to electrically earth a metal sink?
Just a guess, but if a sink became electrically charged somehow and it didn’t have a ground/earth, it’d be real dangerous to the people in the kitchen. It’s inevitable that somebody’s going to touch the sink or put their hands in a basin full of water. And when they do, the best ground/earth path would be through the person down to the (possibly wet) floor. Not good.
Now how could a sink become electrically charged? Perhaps by a defective garbage disposal? If the feed or drain plumbing is metal it could come into contact with defective wires in walls.
A lot of electrical safety is belt-and-suspenders, where we install protective devices that never, ever get used in a single installation, but over the course of a year across a whole country save a few people from disaster.
My guess is that the sink was being used as the earth/ground for the nearby electrical circuits. In the days of copper waste plumbing this would have been convenient.
In the US, the ground was sometimes achieved by a wire clamped to the copper waste line under sinks.
Its called equipotential bonding.
The idea is to link all conductive parts to a common point, earth, so that in the event of a fault, every such point is at the same voltage.
It is intended that if a fault develops anywhere, that all earth points achieve the same potential.
Equipotential bonding is linking metalwork to each other and to earth, you definately would not be using your sink and its associated pipework going into the ground as you house earth point.
What happens in an earth fault condition is that there is a large current flow to earth, which is in the supply box for most domestic consumers.
The large current flow has to go down the earth wire, and this causes a volt drop along it.
The earth in the supply box is fixed (for our purposes here), which means that the earth points everywhere else the voltage must rise.
If they all rise together, you cannot get a shock from being strapped between any two of them.
If they are not linked, then one bit of metal work will rise in voltage above another, creating a possible scenario for a shock.
This is the only thing I can think of: Let’s say the sink is full of water, and an electric can opener (for example) falls into it. If the sink is not grounded the sink & water will be electrically hot (relative to earth ground). This is not good. But if the sink is grounded, and if the can opener is plugged into a GFCI outlet, then the GFCI will break the circuit as soon as it hits the water.
Of course, there’s also a risk to grounding the sink. Let’s say your sink is grounded and you’re doing dishes. If one hand is in the water, and your other hand comes in contact with 120 VAC, then you’re going to get a jolt. If the sink were not grounded, and you were otherwise isolated from earth ground, you would not get a jolt if your other hand comes in contact with 120 VAC.