Faulty capacitor caused explosion at Guildford station

A manufacturing defect in a capacitor in the traction system of a South West Trains Class 455 electric multiple unit caused an explosion at Guildford station on July 7 2017, according to the Rail Accident Investigation Branch (RAIB) report issued on March 20.

The explosion occurred at 1437 when an underframe equipment case on 455901 exploded. The train was the 1437 Guildford-London Waterloo, also formed of 455870.

The unit involved had been fitted with a new traction system by Kiepe Electric UK, and had been released to traffic on March 10 2017. There had been no reported relevant defects on the train, nor any maintenance interventions on the new traction equipment.

The explosion occurred when the driver applied power to the train to start it. Before it moved the guard reported an explosion, and an emergency stop signal was sent to the driver. A 22kg piece of debris was found on the neighbouring Platform 3, and smaller pieces were found next to Platform 8 around 50 metres away, and 70 metres away in a Network Rail staff car park. No passengers or staff were injured.

Because the new traction system uses an Insulated Gate Bipolar Transistor (IGBT) inverter and control system, a smooth and stable electrical supply to each inverter is required. This is achieved by using a large capacitor to store electrical charge and smooth the flow of power.

RAIB’s investigation found a defect in a capacitor winding which led to the production of flammable gases. Because the traction equipment case was sealed, there was no way for the gases to escape, leading to a build-up of pressure and an eventual explosion if there is a source of ignition.

Although capacitors can self-heal many faults, when this process fails direct electric discharge can take place. This can lead to further degradation of polypropylene film and ultimately to a runaway process of damage and heating to the capacitor winding.

The investigation revealed that there had been seven previous failures of such capacitors on the Class 455 fleet, including one in service - although none as severe as the incident at Guildford. Although investigations of those failures took place, they did not identify all of the capacitor windings at risk.

There were no measures on the train to detect potentially harmful build-ups of gases and shut down the traction system - such as pressure switches or devices, or placement of the capacitors in a ventilated part of the system.

RAIB also concluded that the traction retrofit project did not adequately manage the risk of explosion caused by a capacitor with a manufacturing defect.

Since the accident, the capacitor manufacturer, Electronicon, has modified the winding machine that produced the winding which failed at Guildford, so that it is impossible for an operator to carry out unauthorised changes to the machine’s production parameters.

The operator of the Class 455, South West Trains, fitted containment straps to the traction equipment cases of the fleet. Since then, pressure switches have been fitted to the DC link capacitors to detect pressure build-ups and shut down the traction system. Kiepe (UK) has won a contract to retrofit the Class 442 fleet with new traction equipment, and capacitors used in this project will be fitted with pressure switches.

One recommendation was made in the investigation: that train operators and rolling stock suppliers should review the design of electric traction systems in their current fleets and check that there are adequate safeguards to prevent an explosion capable of causing harm in the event of a capacitor failure, and to address any shortcomings identified in those reviews.