Channel 4 will have to explain why it failed to provide subtitles to viewers for several weeks last year, after key parts of its output were knocked off air in September following a fire alarm that inadvertently destroyed its computer equipment. The media regulator Ofcom has launched an investigation into the incident at the London offices of the media company Red Bee, which provides many leading broadcasters with the equipment that gets television broadcasts on air. The activation of a fire suppression system inadvertently destroyed many of the hard drives in the servers housed in the building and required two months of round-the-clock work to rebuild Channel 4’s systems. Although other broadcasters also use Red Bee’s services, Channel 4 was particularly badly affected because its back-up broadcast system also failed. In addition to breaking the subtitling system, the incident knocked out many audio description and signing services, took some of Channel 4’s channels off air completely, and – in a move that infuriated fans of the programme – led to Channel 4 accidentally replacing the finale of Married at First Sight Australia with an old episode. Although Channel 4 still met its statutory requirement to have 90% of programming on all services with subtitles, Ofcom said it would investigate why the broadcaster was unable to provide service used “by millions, including deaf, hard-of-hearing, blind and partially sighted people, to watch and listen to television”. Subtitles are increasingly popular among viewers, not just for the hard of hearing but also for general audiences who find it easier to follow programmes with them on screen. Ofcom will also undertake a review of the transmission arrangements and backup facilities that all of the affected broadcasters had in place at the time of the outage.