As long-term readers of this blog will know, exploring the recondite netherparts of London is something of a hobby of mine.

My latest adventure took me beneath Leicester Square tube at 2am. Tube Lines, who maintain the Jubilee, Northern and Piccadilly routes, invited me to witness the considerable refit work taking place at the station (I get all the glamourous invitations). A full account can be read elsewhere, but I wanted to mention here a simple bit of tech that’s improving safety for workers.

If you look a little ways into the tunnel, in the direction of travel, you may see a box attached to the wall with a soft blue indicator light. This is known as a permanent current rail indicator device (P-CRID), and just bagged Tube Lines a couple of awards for safety innovation.

The tube rails carry a nasty zap. Contact with the positive bar will send 630 V through your ungrateful body. Before engineers can access the tracks, a ‘Protection Master’ must clamber between the rails and use a hand-held device to confirm that the juice is off. This exposes the worker to a risk of electrocution, and being hit by a train.

The P-CRID eliminates that risk. The box measures the voltage in both positive and negative rails and calculates the difference. Because it uses the traction rail as part of the circuit, and trips to ‘fault’ condition at the slightest sign of a loose connection, it is said to be failsafe. Simple really, yet the solution was novel enough to be patentable. Most remarkably, the system was designed, built and partly installed in just 12 months. P-CRIDS are now present in 555 locations.

I put one of them to the test by awkwardly lowering myself onto the tracks at Leicester Square (after undergoing health and safety training, a passport scan, a multiple choice exam and donning full construction gear). The best stories end with a shock, but I’m happy to say that this one did not.