Design of solid state relays for control circuitry on railway cars

On the passenger trains of the Hong Kong Mass Transit Railway Corporation (MTRC) which were procured about twenty years ago, the conventional electro-mechanical relays are commonly used on the 110 V direct current control circuits. With the development of the power electronic components, the application of solid state relays and programmable logic controller (PLC) has been widely adopted and becomes the emerging norm in the railway industry for new generation of passenger train. However, for existing trains which have a remaining service life of about 20 years, a cost-effective method to adopt the solid state type relays on these trains has to achieve certain specific technical criteria and financial consideration