The Rail Accident Investigation Branch (RAB) has released a Safety Digest on an explosion that took place on 26 September 2019 aboard an empty train that was travelling between Bombardier Transportation’s Central Rivers maintenance depot at Barton-under-Needwood, near Burton-upon-Trent, and Birmingham New Street station.
No passengers were on the train at the time and the driver, who was the only member of staff aboard, was unaware of the explosion during the journey which probably took place shortly after the train left the depot.
Upon arrival at Birmingham New Street station, he had to transfer to the cab at the opposite end of the train. It was while he was walking through the train that he discovered the damage caused by the explosion, which had taken place within an electrical equipment case.
The damage sustained included buckling to the inter-vehicle doors, detached covers from the equipment case and damage to the ceiling. The door between the vestibule and the saloon was blown off its runners. Fragments of safety glass from the glazed area of this door were found throughout the adjacent saloon seating area.
The train had been formed of three Class 221 ‘Voyager’ diesel-electric multiple units. The explosion occurred within an equipment case located in the vestibule area of a in the rearmost of the three units, number 221132. It was caused by the failure of an electrical capacitor, a device which stores electrical charge, located in the middle section of the equipment case.
The failed capacitor generated hydrocarbon gases which mixed with air in the equipment case and then ignited. The potential for these capacitors to generate explosive gases under specific failure conditions is a known phenomenon. A previous similar accident had occurred on 29 May 2005 on another Class 221 unit.
Following the 2005 accident, Bombardier Transportation implemented modifications to both Class 220 and Class 221 units, intended to increase the air flow through the equipment case and prevent any potential build-up of explosive gases from failing capacitors.
The first stage of the modification was to fit plastic spacers in five locations on the covers for the lower section and in eight locations on the cover for the middle section of the equipment case. These would cause the covers to ‘stand-off’ slightly from the equipment case frame, improving air flow.
The second stage of the modification was intended to allow air flow between the lower and middle sections of the equipment case. This involved providing holes between the sections so that air would be moved by an existing fan in the lower equipment section. Air could then pass out from the middle section through a new louvre which was fitted to the front cover.
However, as the second stage modification proceeded, it was found that the modified trains were suffering reliability problems. This was due to water ingress affecting equipment within the middle section of the equipment case of non-driving coaches (comparable equipment cases on coaches with driving cabs have different air flow arrangements).
The second stage modification programme was stopped in 2006 to address these reliability issues. At the point when the programme was stopped, 24 units, including 221132, remained unmodified. Units which had been modified had the holes between the lower and middle sections of the equipment cases sealed but retained the louvred cover. However, the 24 unmodified units were not fitted with the louvred cover. This meant that only the stand-off spacers, fitted during the first stage modification, allowed ventilation of the middle section of the equipment cases on these units.
In May 2011 a special check was carried out to verify that the stand-off spacers on equipment case covers were correctly fitted and to ensure that the cover edges were clean. This check was instigated because of concerns that the spacers were being omitted after maintenance work, and that dirty equipment cover edges were impeding air flow.
However, inspection of the damaged cover from the middle section of the equipment case on unit 221132 following the 2019 explosion showed that only two of the eight stand-off spacers were present.
Bombardier Transportation is now modifying the spacers such that they are permanently fitted to the equipment case covers and therefore cannot be mislaid. A louvred cover has now also been fitted to the middle section of the equipment case on coaches which did not receive the modification as part of the second stage of the modification programme.
In its summary, the RAIB stated that this incident demonstrates the importance of:
- Ensuring that the risks associated with the potential failure of electrical capacitors are fully understood and mitigated;
- Ensuring that routine maintenance and inspection activities take account of changes in equipment configuration;
- Having processes in place to ensure that the intent of safety critical modification programmes is maintained when making changes during implementation.