24/09/2025
Mastering Electromagnetic Interference (EMI) in Railway Systems: A Technical Deep Dive
Electromagnetic Interference (EMI) continues to be a formidable challenge in the world of electrified railways especially where high-voltage AC traction systems run alongside delicate telecommunication and signalling networks. EMI is born from the interaction between powerful traction currents and nearby conductors, threatening both operational safety and data reliability.
Unpacking EMI: The Coupling Mechanisms
At the heart of EMI in railway systems lie three primary coupling mechanisms:
1. Conductive: Direct contact allows interference to pass between systems.
2. Capacitive: Electrostatic induction from voltage differences.
3. Inductive: Magnetic fields around AC conductors induce voltages in nearby lines.
The equation that governs inductive coupling is:
V=−2πfMIL
describes induced voltage, where f is frequency, M mutual inductance, I current, and L exposure length. Fault currents over 5,000 amps can cause EMI across kilometres. Longitudinal voltage (Vl) measures voltage differences along conductors, influenced by current magnitude and duration.
Mitigation Strategies
Effective EMI control involves shielding and smart design. The ITU recommends metallic sheaths or screening conductors. Example UK systems use:
1. 150mm² or 250mm² PVC-sheathed cables.
2. Earthing every 1 km.
Booster transformers balance currents and improve power factor, while booster systems suppress EMI at the source.
Short-circuit testing simulates faults using:
1. Knife switches on masts
2. Arcs from HV lines to earth
3. High-speed recorders capturing voltage spikes within 200ms
Tests ensure Vl stays below 430V. Setups are placed ~15m from feeder stations to minimize side effects and refine EMI protection.
Measurement & Analysis: Capturing the Invisible
Precision is key when measuring EMI. Engineers use:
1. Voltmeter and chart recorders to track Vl across test pairs
2. A test circuit with wires connected to an earth point at the far end
3. High-speed playback at reduced speeds for detailed analysis
This setup reveals both peak-to-peak and RMS voltage values. But beware of unshielded circuits or poor earthing can skew results. Site-specific configurations are essential to ensure accuracy.
Future Outlook
As electrification grows, EMI risks will rise. Promising solutions include:
1. Low-resistance metallic screens
2. Real-time EMI monitoring
Students should revisit Carson and Pollaczek’s foundational field theory (1927/28). Engineers must follow ITU standards to ensure reliability.
Credit Notes
This article synthesizes insights from technical documentation and publicly available railway engineering resources, including ITU standards and historical EMI studies.
