Trapped-key interlocks from Castell are guaranteeing the safety of maintenance workers at the UK's first underground liquid hydrogen bus refuelling station. Located near Hornchurch, Essex, the depot is used by three fuel cell buses serving London's RV1 route, which runs between Covent Garden and Tower Gateway.

To combat any fire risk, the site's three-level, 55,000-litre storage vessel is protected by a CO2 deluge system. In addition to the existing electrical safety measure, BOC - which is responsible for the project on behalf of BP - wanted to install a mechanical safeguard to ensure that no-one could gain access to the storage chamber while the CO2 supply was still active.

John Roach, Principal Design Engineer at BOC, explains why the search for such a device led him to Castell: Whenever we've needed critical safety systems on previous projects, we've always used Castell. The name has just become synonymous with safety interlocking.

When entry to the chamber is required, a three-way MBV ball valve interlock must first be set to discharge to atmosphere. The interlock then releases a coded key, which is inserted into the lock fitted to the chamber door, allowing the door to be opened. The key is now trapped in the lock, while a second key is released. By retaining this key, any worker entering the chamber ensures that the CO2 supply cannot be re-established.