REGISTER

KHOO HEAN SIANG B.Eng (Hons)., C.Eng MIEE. MIRSE, P-Eng.

Adv. Dip. in Mkt. Ag Assistant Manager, System

Our railway track is divided into track circuits such that there is a braking distance from 40 km/h to zero over one block and three blocks from 78 km/h. The coded electrical signal is fed into the rails by means of a transmitting device and a receiver at the other end to detect and evaluate the in-coming signal. When a track circuit is occupied, the axles of the train form a low impedance across the rails. This reduces the signal reaching the receiver and in turn indicates the track is occupied.

The signalling information is communicated to the train ATP system in a fail-safe manner. The trainborne ATP system constantly monitors the maximum allowable safe speed to' ensure that the train is not in an unsafe condition of overspeeding. A train will normally be free running on a 78/77 code. On approach to a train in front, it will receive a restrictive code i.e. 77/62 on the fourth track circuit from the obstruction.

The trainborne ATP consists of two sub-systems i.e. the vital system and non-vital system. The two systems perform the same safety functions independently of each other and in a dissimilar manner, thus providing effective protection against common made failures.

The'two independent system receive ATP signals from the trackside equipment via separate ATP antennae and the train speed signals from two separate tachogenerators. If a safe condition is present, the ATP system keeps emergency brake relays energised, and via train control circuit, hold off the energency brakes and allow the train to proceed.