China Railway Signal Dual-Power Transfer: Issues and Solutions

Why railway signal supply is a first-class load

Chinese railway design codes require two supplies for communication and signalling loads, run separately to the equipment or to the low-voltage dual-power transfer device. In current practice, one feed is taken from the station automatic block (auto-close) transformer and another from the continuous (贯通) transformer, both brought to a dual-power changeover box from which the load is fed. The transfer device is the switching point between the two supplies, so its correct selection is critical to reliable power for the signalling system.

Four transfer architectures used in China’s railway projects

Four arrangements have been used in the Chinese engineering sector for dual-power changeover:

Two-contactor type: a simple circuit of two contactors; mechanical interlocking is unreliable, with heating, contact welding and coil burnout — now near-eliminated.

Two-breaker type (CB-class): two breakers with a mechanical interlock and short-circuit/overcurrent protection; interlocking is not fully reliable, mostly used where calculated currents are large.

Excited transfer switch: an excited contactor plus a controller as one unit; interlocking is reliable and the electromagnetic coil drives the switch quickly.

Motor-driven transfer switch (PC-class): based on a load-break switch, motor-driven, smooth and fast, with an intermediate OFF (zero) position option.

The Chinese standard GB/T 14048.11 (paralleling IEC 60947-6-1) classifies transfer switch equipment as PC-class (makes and carries but does not break short-circuit currents) or CB-class (fitted with overcurrent releases whose contacts can make and break short-circuit currents).

Technical diagram shown at a readable responsive scale.

The five design debates

PC-class or CB-class for the signalling load?

Three-pole or four-pole switching?

Two-position or three-position (with zero) switching?

Can the transfer device itself act as an isolating device?

How is the transfer time determined and coordinated?

PC-class or CB-class for signal loads

CB-class carries thermal-magnetic protection: choose it only when its breaker is selected correctly — the rated breaking current must exceed the expected maximum short-circuit current, and breaker selection must coordinate with upstream and downstream feeders for selectivity. PC-class provides transfer only, with no short-circuit or overload protection; a 250 A and below unit typically withstands 5–12 kA short-time current (1 s). In practice, a CB-class unit in a distribution box needs only an isolating device upstream, while a PC-class unit needs a short-circuit protective device upstream, and outgoing MCBs must coordinate with it.

Two drawbacks of CB-class protection deserve attention: it adds a protection level that must be coordinated with adjacent levels; and because the transfer input is taken from the incoming feed, a breaker tripping on overcurrent while voltage and frequency are normal leaves the load dead without the transfer device operating. For railway signalling, where the two 10 kV feeds are relatively reliable, the transfer-only PC-class approach with upstream protection is usually the safer engineering choice; CB-class keeps an advantage in systems with very large short-circuit capacity.

Three-pole or four-pole switching

For identical supplies with a shared neutral — two public grids, two generator sets — a three-pole switch can be used. A four-pole switch is required when the supplies are of different configurations or when harmonic currents are significant, and between supplies of different capacities: their neutrals carry different zero-sequence voltages, and a closed neutral path creates circulating currents that can damage equipment. In China, the two railway signalling feeds are both 10 kV feeds from the railway traction power distribution, whose upstream is the local public grid — the same supply family — so a three-pole switch is the normal choice unless special conditions apply.

Two-position or three-position transfer

A two-position switch has only two working positions — normal and standby — and transfers faster. A three-position switch adds an intermediate OFF (zero) position, where the contacts sit idle; the zero position exists to pause during transfer for high-inductance or large-motor loads that would otherwise suffer inrush. Because the zero position adds travel, three-position switching is slower. For railway first-class loads, which demand fast transfer and have modest inrush, the two-position switch is preferred for higher reliability.

Isolation, transfer time and coordination

An isolating device must lock or make its open contacts visible, withstand 1.25 times the rated impulse withstand voltage, and keep leakage current at or below 6 mA under all conditions. A dual-power transfer device does not meet these requirements by itself, so an isolating device must be installed upstream to allow maintenance without energizing the unit.

Transfer time is the sum of the switching time and any deliberate delay, and it must exceed the arcing time so the contacts of the two feeds can never arc across and short-circuit the supplies. Two coordination rules apply: a downstream transfer switch must act more than 10 cycles (about 200 ms) later than the upstream one; and where a tie breaker connects the two feeds, the local transfer must act about 0.5 s later than the tie-breaker interlock transfer.

A specification checklist for railway signal power

Class: PC-class with upstream short-circuit protection is the safe default for relatively reliable railway feeds; verify CB-class breaker selection and coordination if used.

Poles: three-pole for same-family supplies with shared neutral; four-pole when configurations, capacity or harmonics differ.

Positions: two-position for fast transfer on first-class loads; three-position only when inrush mitigation demands the zero stop.

Isolation: always add an upstream isolating device; the transfer unit itself is not an isolator.

Time: transfer time must exceed arcing time and coordinate per the 200 ms and 0.5 s rules with adjacent and tie-breaker levels.