Signaling Circuits in Electrified Zones

Should not be used in electrified areas to avoid exceeding voltage limits.

Block instrument circuits are the only allowed earth-return circuits.

Must not exceed 60 volts as per CCITT standards.

Coils of some signaling devices must be isolated from external circuits (using internal battery/DC-DC converter).

All batteries and wiring inside equipment, location boxes, or cabins must be well insulated.

External and internal circuits should use separate batteries/DC-DC converters.

Permanent magnet relays should not be used in external circuits in high-voltage AC areas due to potential loss of magnetism.

Relays releasing interlocking mechanisms should be slow-acting to prevent accidental release by short-duration voltage variations.

The length of signalling circuits must be limited to prevent induced voltage from the traction system exceeding 400 volts.

Used for long signalling circuits that would exceed voltage limits, divided into sections for voltage control.

Voltage in unscreened underground cables due to catenary current; 116V/km single-line, 95V/km double-line under typical conditions.

Relays/equipment connected to signalling circuits must have an immunity rating exceeding 400 VAC.

Length of feed cables to relays must be reduced to match a specific immunity level for AC voltage.

Maximum length for DC circuits on line relays with unscreened cable is dependent on the type of relay and its AC immunity level.

Signal feed systems are typically 110V 50Hz, or an approved alternative.

Induced voltage is limited to 400V to ensure human safety and a 1.5 safety factor is considered in the calculations.

The maximum allowed distance between signal control relays and the signal in an electrified zone is set to prevent excessive voltage induction. This limit ensures that the voltage induced in the circuit won't be enough to illuminate signal lamps, even with earth faults.

The maximum length for directly feeding a signal depends on the type of cable used (screened or unscreened) and whether the track is single or double. This limit is to prevent voltage induction issues.

When the signal is located beyond the maximum direct feed distance, it needs to be fed locally by controlling relays at the signal location. Alternatively, it can be remotely fed from signal cabin using a controlling relay.

The power transformer used to feed circuits within the specified distance limit (as per para 22.6.2) must be separate from the transformer used to feed longer signal circuits.

Electric operation of points, including the detection and repeat circuits, requires AC-immunized relays. These relays are resistant to interference from the alternating current in the electrified zone.

If the distance between insulators on a rod is greater than 300 meters, additional insulators must be added to ensure a maximum distance of 300 meters between consecutive insulators.

Point, trap indicator, and lock bar operating rods must be electrically isolated from the track using insulators to prevent short circuits.

Insulators on a rod must be placed between the same sets of rollers and guides to prevent contact between insulated and un-insulated parts of rods.

Insulators must have enough clearance from the adjacent rod roller to allow for normal movement of the rod.

Operating rods of locally worked points on electrified lines must be insulated to prevent electrical connection to the point lever or point box.

The maximum permissible length of a point machine depends on its type, AC immunity level, and the number of lines it controls. Single-line operation allows for longer lengths compared to double-line operation.

The ability of a point machine to withstand induced voltage from AC electrification systems. Higher immunity levels allow for greater tolerance to electrical interference.

A safety factor of 1.5 is applied to point machine lengths, meaning the actual maximum length is 1.5 times lower than the theoretical limit.

3-phase point machines are inherently immune to induced voltage, allowing them to operate at any length without being affected by AC electrification.

Point rods in AC electrified areas are susceptible to induced voltage from the surrounding electrical system. The voltage can be transmitted to the lever frame and pose hazards.

Insulated rod joints prevent the transmission of rail voltage through the rodding system, protecting maintenance staff from electrical shocks.

Each point rod has an insulator near the cabin to prevent electrical contact between the rod and other components.

An additional insulator is placed between the last adjustable crank and the point or lock bar, preventing rail voltage from reaching the main rodding system.