The Overhead Equipment (OHE) system supplies electrical power to locomotives through a contact wire suspended above the track. On Indian Railways, the standard traction system emphasized in the AEE syllabus is 2×25 kV AC — chosen for its ability to reduce line losses over long electrified sections compared to conventional 25 kV single-phase systems.
Why 2×25 kV AC?
- Reduces line losses by roughly a factor of 4 compared to 1×25 kV over the same distance.
- Allows wider spacing between traction sub-stations (TSS), reducing infrastructure cost.
- Uses an autotransformer (AT) feeding system with a dedicated negative feeder.
Line Loss Relationship
P_loss ∝ I² × R
Since power loss is proportional to the square of current, doubling the transmission voltage (and thus halving current for the same power) cuts losses to roughly one-fourth — the core reason 2×25kV is preferred for long electrified sections.
Foot Patrolling — What Is Actually Checked
Foot patrolling is a routine OHE maintenance activity carried out by patrolling staff walking the track to visually inspect the condition of the overhead equipment.
Key Points to Remember
- Contact wire wear (mechanical abrasion from pantograph contact)
- Insulator condition — cracks, flashover marks, contamination
- Mast plumbing and alignment (masts must remain vertical, not leaning)
- Dropper condition and dropper spacing along the catenary
- Earthing and bonding continuity at masts and structures
Exam Focus
Questions on OHE maintenance frequently test whether you can distinguish routine foot-patrolling checks from tower-wagon-based seasonal/cyclic maintenance checks — know which checks belong to which activity.
Real Exam Facts — Contact & Catenary Wire Specs
- Standard cross-section of the Contact Wire in the conventional 25kV OHE system on Indian Railways is 107 sqmm, made of Hard Drawn Copper or Cadmium Copper.
- The Catenary Wire is made of Cadmium Copper, chosen for its higher tensile strength and conductivity compared to plain copper.
- The height gauge at level crossings under 25kV OHE electrified sections is set at a maximum of 4.78m from road level, to prevent oversized vehicles from contacting the live overhead equipment.
- A plumb bob used together with a measuring tape is the standard method to check whether a traction mast has deviated from true vertical alignment.
Related Practice Questions
What is the standard traction voltage system emphasized in the syllabus?
Foot patrolling of OHE is primarily carried out to check for which of the following?