What is this fuse-protected electrical panel with on-off switch and indicator light? It bears the label MAINS WATER SUPPLY.

I have tried leaving it switched on or off for long periods of time (weeks or months) and can detect no effect either way.

The switch is in my UK kitchen on the 8th floor of a tower block. The building dates from 1965 but apparently had extensive renovation in the 90's. The kitchen certainly does not look out of date.

I have looked under the sink and at the back of cupboards but can find nothing electrical that it might connect to. The switch is in plain view above one of the work surfaces.

I asked a plumber - he had never seen one before.

I am mystified, can you help?

EDIT

I discovered that, on a quiet day, I can hear a faint humming sound when the switch is turned on. It sounds like an electric motor. However I can detect no difference in water flow.

[![enter image description here](https://i.stack.imgur.com/e6x5v.jpg)](https://i.stack.imgur.com/e6x5v.jpg)