By TEBAOFLEX Jyen. Originally published on March 26, 2026.
Electrified railways can induce voltages in nearby metallic signalling and telecommunications circuits. Selecting a screened trackside cable is only one part of managing that interference. The return-current network, route, bonding, fault clearance and connected equipment must also be assessed.
Reduction factor is conditional data
A cable reduction factor expresses the reduction in longitudinal induced voltage under defined conditions. It is not simply the percentage of magnetic flux blocked by a jacket or braid. Frequency, field strength, metallic-layer properties, continuity and bonding arrangement affect the result.
Prysmian’s railway catalogue lists several reduction classes for AJ-2Y(L)2YDB2Y constructions at specified 16.666 Hz field conditions. This illustrates why the designation alone cannot establish a universal factor of 0.08. Use the exact supplier construction and its measured or specified curve for the intended railway system.
Read the construction drawing and network specification
Trackside designs can include solid conductors, pairs or star quads, PE insulation, a laminated moisture barrier, copper wires and steel tapes. Confirm each layer and its dimensions from the drawing; do not infer a proprietary alloy, perfect water seal or guaranteed crush resistance from a short type code.
Check the railway operator’s cable specification, conductor identification, resistance, capacitance balance, attenuation and interface requirements. A generic low-capacitance star quad does not guarantee an 8 km ETCS or axle-counter link. Signal equipment and its installation documentation determine the actual compatibility and distance.
An illustrative induction calculation
For a simplified uniform parallel exposure, an approximate induced voltage can be represented as U = 2 × pi × f × M × I × L × r_return × r_cable. The factors must represent the actual model and must not double-count return-path screening. Real routes can require a section-by-section calculation.
Assume only for arithmetic f = 50 Hz, M = 0.00048 H/km, I = 12,000 A, L = 5 km and r_return = 0.47. Before applying cable reduction, the result is 4,252.46 V. An assumed r_cable = 0.08 would give 340.20 V. Neither the mutual inductance nor either reduction factor is a verified site or TEBAOFLEX product value.
The lower result is not automatically a safety “PASS.” Identify whether the quantity is longitudinal induced voltage, touch voltage, equipment stress or another limit, then apply the appropriate duration and installation conditions. Do not compare every result with a universal 430 V or 650 V threshold.
Keep standards within their scope
ITU-T K.68 addresses management of electromagnetic interference from power and electrified-traction systems into telecommunications systems, including the conditions for its management voltages. EN 50121-4 addresses emission and immunity of signalling and telecommunications apparatus. Neither reference by itself certifies an installed cable route or replaces the railway safety assessment.
Rolling-stock cable standards and EN 45545 requirements should not be transferred automatically to trackside troughs or tunnels. Check the infrastructure owner’s applicable fire, installation and equipment requirements separately.
Bonding and commissioning need an approved design
Low-frequency reduction can depend on current flowing in the metallic screen path. Changing an end connection can therefore change performance and introduce hazardous potentials. Do not add or remove earth connections using a blanket one-end/two-end rule or a universal 2-ohm electrode target. Follow the railway earthing and bonding design, including transferred-potential and fault-current considerations.
- Confirm the actual traction frequencies, fault currents and clearing times.
- Obtain the cable reduction curve and test conditions, not just a marketing value.
- Check screen continuity, accessories and the approved bonding arrangement.
- Verify circuit transmission and insulation requirements against the equipment.
- Record commissioning tests and acceptance against the site-specific assessment; no unprovided FAT or field performance is claimed here.
Sources and further documentation
Source: knowledge.bsigroup.com
Discuss your application
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