Trench Effect - Background

Background

The trench effect became known because of the scientific investigation of the King's Cross fire. The fire started on an escalator (containing combustible wood) between the Piccadilly Line platforms and the ticket hall at King's Cross St. Pancras tube station. Many eyewitnesses indicated that early on, the fire in the escalator was of manageable size: officers from the Fire Brigade and British Transport Police indicated that the fire appeared no larger than a large cardboard box burning. Those present were surprised when it suddenly threw a sustained jet of flame into the ticket hall.

What seems clear is that in the early stages of the fire, the flames visible to anyone not standing on the burning escalator were a small part of the full story. Most of the flames were lying down in the escalator trench; only a few visibly protruded above the balustrade. The lack of visible flames lulled the emergency services into a false sense of security. When the treads of the escalator flashed over, the fire grew dramatically and ignited most of the ticket hall. The HSE's Health and Safety Laboratory in Buxton lit fires in 1/10-scale and 1/3 scale models of the escalator and ticket hall to prove that the trench effect was the main cause of the King's Cross fire. The sudden flashover may be attributed to the wood gas (mainly methane) emitted from the pyrolysis of the wooden escalator itself. When the concentration of gas reaches a critical value (the lower flammability limit), the gas suddenly catches fire in the presence of a flame.

An episode of TV series Extreme Evidence entitled "Flashover" detailed the King's Cross fire, along with the computer modeling and other analyses which discovered the trench effect. Outside of the United States, the show airs as an episode of Forensic Files.