Train Safety Circuit Functions and Fail-Safe Design

Core purpose of the safety circuit

The safety circuit provides a fail-safe emergency brake system; if the circuit is interrupted, the brakes apply automatically.

Voltage used to power the safety circuit

52 volts DC, supplied through Safety Brake MCBs A or B.

Three key circuits involved in train safety

Safety Circuit A, Safety Circuit B, and the Round Train Circuit.

Function of the emergency solenoid

To release the brakes when energized; when de-energized, it applies the brakes.

What happens when the safety circuit is broken

The emergency solenoid de-energizes and emergency brakes apply.

Why are there two safety circuits (A and B)

To allow one to serve as a backup; only one is active at a time.

What does the round train circuit ensure

That the train is complete, correctly coupled, and ready for safe operation.

What happens if the round train circuit is interrupted

The corresponding master relay de-energizes, interrupting the safety circuit and applying the brakes.

Triggers that can interrupt the round train circuit

Cab emergency buttons, PEAB activation, couplers in wrong position, TBC not stowed, or a Local Brake MCB trip.

What does the Local Brake MCB protect

It protects the brake decoder which controls the application and release solenoids.

What happens if a Local Brake MCB trips

The brake decoder fails, suspension deflates, and the round train circuit breaks, causing an emergency brake.

Function of the Remote Brake Release

It restores the round train circuit and vents air from brake cylinders to allow movement.

When can the Remote Brake Release be used

Only in 'Off & Release' or 'Motoring' positions of the TBC, not in the braking arc.

Max speed allowed after Remote Brake Release is used

32 KPH due to deflated suspension and dragging brake on the affected car.

Function of the Round Train Master Relays

They monitor the round train circuit and interrupt the safety circuit if the train is not complete.

Two key relays that will interrupt the safety circuit

Deadman relay and TBC in Emergency relay.

What happens if both Safety Circuit A and B fail

The brakes cannot be released; a push-out is required to move the train.

How is the system fail-safe by design

Brakes are energized to release—any circuit failure causes de-energization and brake application.