EMU/MEMU braking is air-based, operated both electro-pneumatically (EP brake) and through direct BP reduction (auto brake). The brake pipe (BP) runs the length of the rake with all inter-coach angle cocks open for continuity; BP charging is done from the leading motor coach by turning the BIV key. Each motor coach has 8 brake cylinders, each trailer coach has 4 — cylinder pressure is set at 1.5 kg/cm² (motor coaches) and 1.8 kg/cm² (trailer coaches).
Types of Brakes
- Electro-Pneumatic (EP) brake — the primary service brake.
- Auto brake — direct BP reduction, the fallback/emergency mechanism.
- Driver's emergency brake and Guard's emergency brake — independent emergency application points.
- Dead-man brake — activates if the driver releases the master controller handle.
- Hand brake (conventional EMUs) and Parking brake (MEMUs and air-spring-fitted EMUs).
Isolating Cocks
13 distinct isolating cocks exist across the pneumatic system, each stopping air supply to a specific circuit or equipment for maintenance. When a cock's handle is parallel to the pipe line, it's open; at 90°, it's closed.
| Cock | Function |
|---|---|
| CIC | Compressor Isolating Cock — stops air from the main compressor |
| BIC | Bogie Isolating Cock — stops brake-cylinder air for one bogie (each bogie has its own) |
| EPIC | EP unit Isolating Cock — stops air to a coach's EP unit |
| ICA | Isolating Cock for Auto brake — isolates a coach's auto brake |
| MR Isolating Cock | Cuts out Main Reservoir pressure to one coach |
| BP Isolating Cock | Cuts out Brake Pipe pressure to one coach |
| Horn Isolating Cock | Stops air to the horn circuit |
| Dead-man Isolating Cock | Stops air to the dead-man valve (on the BP line, below driving cab) |
| Control circuit Cock | Stops air to the pneumatic control circuit for tap-changer/switch group |
| Panto Isolating Cock | Stops air to the pantograph circuit |
| Air Dryer Cock (A/B/C) | A & B normally open, C normally closed; reversed if isolating the dryer |
| Parking Brake Isolating Cock | Stops air to parking brake, releases parking-brake-cylinder air |
| Parking Brake MV By-pass Cock | Routes MR pressure directly to parking brake, bypassing a failed magnet valve |
Exam Focus — Parking Brake Isolation Needs a Mechanical Step Too
When the parking brake isolating cock is closed, the parking brake must ALSO be uncoupled mechanically — because parking brake applies automatically whenever there's no air in its cylinder, so simply cutting off air would leave it stuck applied, not released.
Reservoirs
Seven distinct reservoirs, each with a specific role
- Main Reservoir — on the Motor Coach underframe; stores compressed air (MR pressure) from the main compressor, fitted with drain cocks.
- Supplementary Reservoir — 2 per Trailer coach, connected in parallel to the MC's main reservoir.
- Auxiliary Reservoir — connected via the EP unit's triple valve; stays charged at 5 kg/cm² when brakes are released, feeds the brake cylinder during auto brake application.
- Equalizing Reservoir — under the driving cab, contains compressed air at 5 kg/cm².
- Horn Reservoir — under the driving cab, connected to the MR pipe.
- Control & Panto Reservoir — both in HT compartment; Control feeds the tap changer/switch groups, Panto feeds the pantograph servomotor and ABB.
Governors
| Governor | Cut-in / Cut-out | Type |
|---|---|---|
| ACP Governor | 5.2 / 6.7 kg/cm² | Normally closed |
| MCP Governor | 6 / 7 kg/cm² | Normally closed |
| ABB Governor | 5.2 / 4.5 kg/cm² | Normally open |
| Control Governor | 4.2 / 3.2 kg/cm² | Normally open |
| Equipment Governor | 4.2 / 3.2 kg/cm² | Normally open |
| Parking Brake Governor | 3 / 2 kg/cm² | — |
Exam Focus — Normally Closed vs Normally Open
ACP and MCP governors are 'normally closed' — their interlock OPENS once requisite pressure builds up (to switch OFF the compressor). ABB, Control, and Equipment governors are 'normally open' — their interlock CLOSES once requisite pressure builds up (allowing the equipment to function). Getting this backwards is a very common mistake.
Safety Valves
- ACP Safety Valve: set at 7.75 kg/cm², in HT compartment.
- MCP Safety Valve: set at 7.75 kg/cm², in the main compressor pipeline.
- Low Pressure Safety Valve: set at 3.5 kg/cm², at MCP's LP delivery.
- Brake Cylinder Safety Valve: in the EP unit — 2.7 kg/cm² for motor coaches, 3.5 kg/cm² for trailer coaches.
Air Dryer
A twin-tower heatless regenerative air dryer ensures clean, dry air supply, preventing moisture-related corrosion, lubrication washout, timing disturbance in air-brake valves, and rubber-component damage. It weighs under 100 kg and works at nominal MR pressure of 6–8 kg/cm².
- Major components: coalescing filter assembly, drain valve, desiccant housing, purge valve, humidity indicator, electronic control module, solenoid valve, pressure switch.
- The humidity indicator uses litmus-paper colour: Blue = satisfactory, Pink = approaching saturation, White = saturated, Brown = desiccant saturated with oil and needs replacement.