All BG EMU stock is fitted with self-lapping electro-pneumatic (EP) brakes combined with automatic compressed-air brakes. Two makes of equipment are in real use: Westinghouse and Knorr-Bremse (Escort Knorr). Though the equipment details differ appreciably, both systems are similar in principle. This chapter describes the Westinghouse equipment fitted on ICF-built EMUs in detail.
When Automatic Brakes Are Used — 5 Real Scenarios
Electro-pneumatic braking is the normal method for service stops. Automatic (air) brakes are used under these circumstances:
- a) Automatic application on both halves of a train in the event of a parting.
- b) Application by the Guard in an emergency.
- c) Automatic application on operation of the dead-man's device.
- d) Service stops by the Driver in the event of EP-brake control-supply failure.
- e) When the Driver makes an emergency application, both EP and Auto brakes apply.
Main Components — Real List
- A main-reservoir system: a compressor in each motor coach feeding main reservoirs on motor coaches and supplementary reservoirs on trailer coaches, all inter-connected end-to-end by a main-reservoir pipe with flexible couplings between coaches. MR pressure is maintained at 5.8 to 7.0 kg/cm² by pressure governors on each compressor; all train compressors are synchronised to start/stop together.
- A brake pipe end-to-end with flexible inter-connections; maintained at 4.57 kg/cm² to keep automatic brakes released.
- A brake unit in each coach housing the control valves for both EP and Auto brakes.
- Brake cylinders on each bogie with automatic slack-adjusters and brake rigging — usually 4 per bogie on motor coaches, 2 per bogie on trailer coaches.
- A brake controller in each driving cab, made operative in use by an isolating valve switch operated with the Driver's key.
- A brake application relay in each driving cab — the one in the operative cab responds to the controller and controls supply to brake-control train wires carried with other control wires through inter-vehicular couplers.
- Five real train wires: EP supply wire, brake application (holding) train wire, brake indication wire, and EP return wire.
- A warning system indicating to the Driver whether EP brakes have applied, and warning of failure. A pilot valve + emergency valve to operate brakes if the dead-man's device is released. Isolating cocks, pressure gauges, control switches. Release valves for manual release of brakes in individual bogies.
Brake equipment in coaches uses a 'pipe-bracket' arrangement: valves bolt onto a casting with the required inter-connections cored (built-in) into the casting itself, so the only external pipe connections are from the main reservoir, the brake pipe, and atmosphere.
Brake Controller — Real 5-Position Table
| Position | Function |
|---|---|
| I | Release and Running |
| II | Full service EP |
| III | Lap (for auto application) |
| IV | Service automatic |
| V | Emergency |
Lower, self-lapped brake-cylinder pressures are obtainable at intermediate handle positions between I and II — the actual brake-cylinder pressure depends on how far the handle has moved away from position I.
EP Brake System — Real Self-Lapping Mechanism
When an EP application is made, moving the controller handle from position I toward II first energises the holding train wire, which energises holding magnet valves throughout the train, closing the exhaust from the brake cylinders. Application valves then energise via the application train wire, letting compressed air flow to the brake cylinders from the main-reservoir pipe (flow rate controlled by a choke). If the application valve stays energised, brake-cylinder pressure builds up to 3.5 kg/cm², with any rise beyond this value prevented by the 'limiting valve'.
- Between positions I and II, the self-lapping mechanism controls brake-cylinder pressure: moving the handle toward II compresses a spring in a small control cylinder (compression proportional to handle movement). This spring acts on a piston, at the back of which brake-cylinder air pressure from the leading bogie is applied.
- When brake-cylinder pressure exceeds the value corresponding to the handle position, the mechanism de-energises the application train wire, limiting the brake-cylinder pressure to the value matching that handle position. If brake-cylinder pressure drops (leakage etc.), the application wire re-energises. If the handle moves backward, the holding wire de-energises until brake-cylinder pressure drops to the required value.
- This self-lapping mechanism automatically holds brake-cylinder pressure at the exact value corresponding to handle position, enabling graduated application and release. At positions II & V, the application/holding circuits are energised via 'positive-acting' contacts that bypass the self-lapping mechanism entirely — pressure is then limited only by the limiting valve.
Automatic Brakes — Triple Valve Real Functions
With the brake controller in position I and the isolating-valve switch operative, compressed air is fed to the brake pipe from the MR pipe via a reducing valve in the brake controller, holding BP steady at 4.57 kg/cm². The vital component of the automatic brake system is the triple valve, part of the EP brake unit in every coach.
- 1. At BP = 4.57 kg/cm², it opens the brake cylinder to atmosphere (releasing brakes) while simultaneously connecting the brake pipe to an auxiliary reservoir in each coach, storing air there at 4.57 kg/cm².
- 2. When BP is reduced — voluntarily by Driver/Guard, or from train parting, or dead-man's device operation — it closes the brake-cylinder exhaust and connects the auxiliary reservoir to the brake cylinders, letting stored air flow in. The auxiliary reservoir's volume is chosen so max. brake-cylinder pressure is limited to 3.5 kg/cm². The triple valve also has a lap position letting brake-cylinder pressure be controlled below 3.5 kg/cm² by holding BP at intermediate values using controller position III.
- 3. When the brake pipe is recharged (controller back to position I), the triple valve returns to its release position — releasing brakes and storing air in the auxiliary reservoir ready for the next application. A triple-valve stabilising valve in the circuit ensures the triple valve doesn't react to small pressure pulsations.
Emergency Brakes & Other Real Equipment
- In position V (emergency), both EP and Auto brakes apply together. Rapid application is obtained by quick-venting the brake pipe to atmosphere. A safety valve in each coach's brake unit prevents excessive brake-cylinder pressure, normally set at 3.9 kg/cm². Releasing the dead-man's device causes quick BP venting via an emergency valve, resulting in full auto application.
- Each driving cab has a valve letting the Guard vent the brake pipe to atmosphere, causing brake application, with a gauge nearby showing BP to the Guard. A pressure switch (the 'control governor') connected to the brake pipe prevents traction power until brakes are 'kept ready', cutting off traction control supply if BP drops. A duplex pressure gauge shows MR and brake-pipe pressure to the Driver; another duplex gauge shows the pressure in the cylinders of the two leading bogies.
- Isolating cocks isolate a defective brake unit from MR/brake pipes, and can isolate brake cylinders in individual bogies. Brake cylinders carry automatic slack-adjusters that take up slack from brake-block wear, ensuring fairly constant piston stroke without needing manual rigging adjustment between shed visits — an essential feature for suburban services where brake-block wear is quite high due to frequent brake application.
Knorr System — Real Comparison
The Knorr (Escort Knorr) system is quite similar in principle to the Westinghouse system described above, but with one key real difference: automatic brakes become operative automatically on failure of the EP brakes. With this arrangement, the Knorr brake controller has only THREE positions — release, full-service application, and emergency — compared to the Westinghouse controller's five real positions.