What is induced voltage and immunisation?

Voltage that appears on lineside cables because they run parallel to traction power. Immunisation is the set of measures that keep it within safe limits.

Why: Too much induced voltage is a shock risk to staff and can damage equipment, so long copper and metallic cables must be designed to stay under the limits.

Where: Any long metallic cable in an electrified area, especially AC. Controls include limiting continuous length, sheath gaps, screening conductors and route position.

Where the rules are

• NR/GN/TEL/31106 cl 6.6how voltage is induced on lineside conductors
• NR/GN/TEL/31106 cl 7.2.1controlling hazardous potentials and the limits
• NR/GN/TEL/31106 cl 7.3factors affecting induced voltage
• NR/GN/TEL/31106 cl 7.4what immunisation means
• NR/GN/TEL/31106 cl 7.4.2limiting cable length (sheath gaps, sectioning)

On the drawing

Immunisation measures (sheath gaps, screening conductor earths) are shown on the copper drawings.

Also called: immunisation, longitudinal voltage, EMC from traction

Guidance in our words. Read the current issue of each standard before you design to it.