What is the reduction factor for railway cables?

On electrified tracks which are operated using alternating current or under high-voltage power lines,
parallel laid railway cables are exposed at the same time to the influence of electromagnetic fields. These
electromagnetic fields induce current in the cables, which can lead to disturbances and destruction of the
equipment connected to them as well as present a hazard to life and limb. In order to reduce this influence
to a non-hazardous level, the cables are provided with a metallic shield according to their cross-section. This
shield has to be earthed on both sides of the cable.

The measure of quality used to shield cables in railway applications is referred to as the reduction factor.
The reduction factor is the ratio of induced tension with shielding to the induced tension without shielding.
A reduction factor of 1 would mean "no shielding effect". A reduction factor of 0.5, for example, would mean
a reduction of the induced tension by one half.

The effect of shielding of the materials used (copper, steel, aluminium, etc.) is dependent on the conducting
cross-section of shielding as well as the frequency of the interfering signal.

Depending upon the local circumstances, the cable design and hence the resultant reduction factor can be
optimised to best match the expected field strength along the railway track. A typical description for the
request for a cable protected against inductive interference shall include disturbing frequency and field
intensity as well as the requested reduction factor.

For example:
• Reduction factor < 0.5 at 16.7 Hz in the range of 80 – 150 V/km or
• Reduction factor < 0.3 at 50 Hz in the range of 80 – 250 V/km.

The tension induced in the cable increases with the length the cable is exposed to the electrical field. A
cable which is exposed over a length of 2000 m to the field can require a lower (better) reduction factor
than the same cable, which is only exposed to the induced field over a length of 1000 m.

The calculation of the actually required reduction factor is very complicated and depends on a multitude of
different parameters:

• Distance of the cable to be shielded from the interfering cable (overhead line...),
• Type of installation (underground, in conduits, on the ground...),
• Characteristics of the ground,
• etc.

A respective calculation of the required reduction factor can only be carried out by experts. The cable
manufacturer then develops the correct cable design based on the given factors.

As a supplier of cables for railway applications and development partners of well-known European railway
operators of long standing, Prysmian offers an extensive portfolio of different cables with reduction factor.
Upon request we shall be pleased to develop the right cable design for a customer's purposes

IEC 62153-4-17

Metallic cables and other passive components - Test methods - Part 4-17: Electromagnetic compatibility (EMC) - Reduction Factor

IEC 62153-4-17 applies to the testing of the reduction factor of multi-element metallic cables used in analogue and digital communication and control. The described method is generally applicable to all screened metallic cables.








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DIN 57472-507 VDE 0472-507:1983-04
Testing of cables, wires and flexible cords






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