Why is Stray Current dangerous for buildings?

Stray current can flow through the steel reinforcement bars (rebar) inside concrete foundations. Where the current exits the rebar, it corrodes the steel. As steel rusts, it expands, causing the concrete to crack and potentially compromising the structural integrity of the building.

Do AC trains cause corrosion?

Generally, no. In AC systems, the current changes direction 50 or 60 times a second. This oscillation prevents the sustained chemical reaction required for significant electrolytic corrosion. The main issue with AC stray current is interference with electronics and safety voltages.

What is a Stray Current Drainage System?

It is a protection system where a physical cable connects a vulnerable pipe or structure directly to the substation's negative busbar. This provides a safe metallic path for the stray current to return, preventing it from exiting through the soil and causing corrosion.

The Silent Destroyer: Mastering Stray Current Control in Railways

What is Stray Current? Discover how “leaking” electricity destroys underground pipes and rebar via electrolysis, and how railways mitigate this silent threat.

December 9, 2025 9:41 pm

What is Stray Current?

Stray Current refers to the portion of the traction return current that leaks out of the intended path (the Running Rails) and flows through the ground or other metallic infrastructure. While the rails are designed to carry electricity back to the Traction Substation, they are never perfectly insulated from the earth.

Current follows the path of least resistance. If the soil or a nearby buried pipeline offers a better path than the rail, electricity will “stray” from the track, travel through the foreign metal, and jump back out to return to the source.

The Main Threat: Electrolytic Corrosion

The physical movement of stray current is not the problem; the chemical reaction it triggers is. This phenomenon is known as Electrolysis.

Entry Point (Cathode): Where current enters a pipe or rebar from the soil, the metal is generally protected (Cathodic protection).
Exit Point (Anode): The danger zone. Where the current leaves the metal to return to the substation, it takes ions of the metal with it. This causes rapid pitting corrosion, effectively eating holes in water pipes, gas lines, and building foundations.

DC vs. AC: The Corrosion Factor

Stray current behaves differently depending on the electrification system. DC systems are the primary concern for infrastructure damage.

Feature	DC Stray Current (Metro/Tram)	AC Stray Current (High Speed)
Corrosion Risk	Severe. Continuous unidirectional flow causes rapid metal loss (Electrolysis).	Low. The oscillating nature of AC prevents significant ion displacement.
Primary Hazard	Destruction of underground utilities (pipes, cables, rebar).	Electromagnetic Interference (EMI) and safety (Touch Voltage).
Mitigation Goal	Keep the rail insulated from the earth (“Floating Earth”).	Bond the rail to earth to lower impedance.

Mitigation Strategies: Controlling the Leak

Engineers use a multi-layered defense strategy, often referred to as “Source Control” and “Path Control,” to minimize damage.

Rail Insulation: Using high-quality insulated rail fasteners and pads to prevent current from touching the sleepers/ties.
Stray Current Collection Mats: Welded wire meshes placed in the concrete track bed to capture leakage and safely drain it back to the substation.
Cross Bonding: Connecting parallel tracks together to reduce the overall resistance of the return path, encouraging current to stay in the rails.
Fourth Rail: Some systems (like the London Underground) use a dedicated, fully insulated fourth rail for return current, eliminating rail-to-earth leakage entirely.