Noticed this plug on my wall from when I moved into this house, and I just started wondering again about what it’s actually for. This is in the UK, if that helps. It’s on a big bulky box hanging on the wall below my desk next to two regular plug sockets.
Edit: best suggestion I’ve seen here is that it could be a fuse box for an alarm system. Makes sense since this house did have several security systems before I moved in. Also, for added context, this is in a bedroom and the wire coming out of it goes straight into the wall.
Fuse box for one line. Fuses weren’t put in as standard when electricity was introduced. Many old houses just had live wires coming in with no breaker like today. For expensive electrical items added when there was no fuse, an electrician with install it with a dedicated fuse. If the electrical system has been updated where it enters the house, it may no longer be needed. However, if it is on a different circuit, it may be. Old fuse boxes were a bunch of replaceable physical fuses. Nowadays they are breaker switches for easy resetting and less waste.
Anyway, if whatever is connected to this gets a power surge, the fuse could trip and you would need to replace it. However it is not a plug, to add a different device or appliance, but just a safety pass through for the wire coming out the other end.
Early electrical grids were fucking wild lol. In my city, there were 3 different grids, with different voltages competing for customers (one was DC!) In the early days before it was standardized.