Electromagnetic Compatibility, Grounding, Stray Current, and Lightning Protection
Electromagnetic compatibility, grounding, stray current, and overvoltage protection are examined together because they share the return path. Questions test interference mechanisms, bonding, DC stray-current corrosion, and lightning coordination.
Core rules
- EN 50121 series sets railway EMC emission and immunity; traction converters generate harmonics and conducted/radiated noise.
- Protective and functional bonding must be coordinated. Multiple bonds can reduce touch voltage but can also create loops and stray-current paths.
- In DC traction, current returns through the running rails; imperfect rail-to-earth insulation drives stray current into buried structures, causing electrolytic corrosion. Mitigation includes insulated joints, drainage, and monitoring.
- Lightning protection uses shielding, surge arresters, and insulation coordination; lead length matters because it adds inductive voltage drop.
Worked example
A 25 kV installation has a basic impulse insulation level of 125 kV. A surge arrester clamps at 90 kV for a 10 kA impulse. The protection margin is 125 − 90 = 35 kV, about 28%. If the arrester lead adds 10 kV, the effective clamp is 100 kV and the margin falls to 25 kV (20%). That is why short, direct arrester leads are specified.
Traps
- Bonding every earth to every structure without analyzing stray current.
- Treating AC and DC touch-voltage limits as identical.
- Ignoring arrester lead inductance.
- Assuming stray current is only an AC phenomenon.