This chapter combines a quick-reference sheet of critical OHE numeric parameters (the kind of single-page 'cheat sheet' worth memorizing wholesale), and the official 28-point safety precaution list for anyone working on OHE.
Contact Wire Heights
| Location / Condition | Height |
|---|---|
| Normal, at support — regulated OHE (pre-sag 100mm) | 5.60 m |
| Normal, at support — regulated OHE (pre-sag 50mm) | 5.55 m |
| Normal, at support — unregulated OHE (temp. range 4°-65°C) | 5.75 m |
| Normal, at support — unregulated OHE (temp. range 15°-65°C) | 5.65 m |
| Minimum, at loco inspection pit | 5.80 m |
| Minimum, at level crossing | 5.50 m |
| Height gauge at level crossing | 4.67 m |
| Minimum, at overline structure | 4.92 m |
Contact Wire Diameter, Stagger & Slope
| Parameter | Value |
|---|---|
| New contact wire diameter | 12.24 mm |
| Condemned diameter — main line | 8.25 mm |
| Condemned diameter — loop/yard lines | 8.00 mm |
| Stagger — tangent track | ±200 mm |
| Stagger — curved track | ±300 mm |
| Stagger — catenary wire at support (single cantilever, tangent) | 0 |
| Stagger — section insulator | 0 or ±100mm max |
| Stagger — PTFE neutral section at support | 0 |
| Max permissible slope of contact wire — main line | 3 mm/metre |
| Max permissible slope of contact wire — in yards | 5 mm/metre |
Overlaps, Turnouts & Implantation
| Parameter | Value |
|---|---|
| Gap between parallel contact wires — uninsulated overlap | 200 mm |
| Gap between parallel contact wires — insulated overlap | 500 mm |
| Height of turnout OHE contact wire above main line contact wire | 5 cm (+50mm) |
| Track separation at turnout/crossover at obligatory structure | 150-700 mm |
| Minimum implantation at obligatory structure (mast supporting >1 OHE) | 3 m |
| Normal implantation (standard) | 2.50 m ±0.03 |
| Minimum implantation (critical, needs CEE approval) | 2.36 m |
| Minimum implantation on platform | 4.75 m |
| Normal encumbrance (for 72m span) | 1.4 m |
| Overlap-type turnout span | ≤54 m |
| Crossed-type equipment (facing turnouts) anchor span restriction | 54 m |
Electrical Clearances
| Clearance Type | Value |
|---|---|
| Short-time minimum vertical electrical clearance | 270 mm |
| Short-time minimum horizontal electrical clearance | 220 mm |
| Long-time minimum vertical electrical clearance | 320 mm |
| Long-time minimum horizontal electrical clearance | 320 mm |
| Minimum working clearance | 2 m |
Jumpers, Droppers & Earthing
| Parameter | Value |
|---|---|
| Earthing resistance — FOB/ROB/Girders/Bridges/Platform shed/overline structures | 10 Ω |
| 'C' jumper wire — cross-section / length | 50 mm² / 1.7 m |
| 'F' jumper wire — cross-section / length | 50 mm² / 1.6 m |
| 'G' jumper wire — cross-section / length | 105 mm² / 3.5 m |
| Distance of 'G' jumper from obligatory mast, towards overlap side | 5.6 m |
| Fixed length of dropper wire | 105 mm |
| Distance of first dropper from support | 2.25 m |
| Minimum length of dropper in a span | 150 mm |
Section Insulator Geometry
| Parameter | Value |
|---|---|
| Contact wire-to-runner distance, either side | Not less than 220 mm |
| Axial distance, catenary-to-contact wire at section insulator location | Not less than 450 mm |
| Section insulator located beyond point where track-centre distance is | ≥1.65 m |
| Swept-zone clearance of section insulator — trailing side | 1.726 m |
| Swept-zone clearance of section insulator — facing side | 1.60 m |
Tension Lengths, Spans & ATD Values
| Parameter | Value |
|---|---|
| Maximum tension length — regulated OHE | 1500 m |
| Maximum tension length — unregulated OHE | 2000 m |
| Normal sag of regulated OHE | 100 mm |
| Maximum span length on tangent track (regulated OHE) | 72 m |
| Difference in length of adjacent spans on main line — max | 18 m |
| ZY value, winch-type ATD, at half tension length (750m) at 35°C | Z = 1250, Y = 2500 (mm) |
| XY value, 3-pulley type ATD, at half tension length (750m) at 35°C | X = 750, Y = 2500 (mm) |
| Mast leaning — attend if leaning at TOP exceeds | 15 cm |
| Isolator — distance between male/female contact, open position | 500 mm ±50 |
Exam Focus — Two Different Mast-Leaning Numbers, Two Different Contexts
Don't confuse this sheet's '15cm leaning at TOP → attend' rule with the Cantilever Maintenance chapter's mast-verticality rule (≤3cm OK, ≤5cm watch with a yellow band at 1.85m, >5cm attend). These are genuinely different measurement contexts. If a question doesn't specify which context, the cantilever maintenance figures (3cm/5cm) are tied to an actual documented procedure with a physical marking (yellow band) — treat that as the primary answer unless the question clearly points to a simpler general check.
Safety Precautions While Working on OHE — Full 28-Point List
As OHE voltage is 25kV, the following precautions must be taken by staff working on OHE:
- No work shall be done ABOVE or WITHIN 2 METRES of live OHE without a 'Permit to Work'.
- Large metallic structures (fencing, structural steel of platforms running parallel to track) develop inductive effects — they must be suitably earthed for safety.
- Each working party shall be protected by AT LEAST TWO INDEPENDENT EARTHS, one on each side of the party.
- If distance between working parties exceeds 1000 METRES (1 km), an intermediate earth must be provided so no two earths are more than 1000 m (1 km) apart.
- Men shall be posted on BOTH sides of the work site to warn of approaching trains on the same and adjacent track.
- To earth the OHE: fix the earthing clamp securely to a mast AT LEAST ONE SPAN AWAY, after confirming that mast's mast-to-rail earth bond is intact.
- EARTHING SEQUENCE: always fix the earthing clamp to the traction rail or mast FIRST, then hook the top clamp to the OHE being earthed.
- REMOVAL SEQUENCE (reverse of above): first remove the hook on the OHE, THEN remove the clamp fixed to rail/mast.
- Two sections of conductors, or snapped conductor ends, should be SEPARATELY earthed at TWO points after switching off supply to both parts. Same precaution applies near sectioning points and cut-in insulators.
- Neutral sections must be TREATED AS LIVE EQUIPMENT and earthed separately at TWO points on either side of the work party before commencing work.
- Work on an isolator: BOTH sides of the isolator should be earthed at TWO points.
- No fallen wire shall be touched unless power is switched off AND the wire is suitably earthed.
- On electrified track, steel/metallic tape or tape with woven metal reinforcement must NOT be used.
- Ropes, come-along clamps, and tirfors should be TESTED ONCE A MONTH in the presence of JE-I (TRD).
- All staff should wear HELMETS to protect against dropped tools/equipment.
- Staff should wear a SAFETY BELT (rope sling) to protect against inadvertent falls.
- Ropes used with ladders must be COTTON OR JUTE — metallic ropes are PROHIBITED. One person must hold the ladder from the ground; the top end must be tied to the supporting structure/conductor to prevent sliding.
- Ladders shall never be allowed to fall on or rest against the contact wire.
- More than one person shall NOT normally be allowed on a ladder.
- Climbing a ladder with wet or slippery footwear is FORBIDDEN.
- Use a rope to pass tools/equipment to a person working on a ladder.
- No one should stand directly BELOW a ladder work spot.
- Anchoring rope strength must NOT be less than that of the cable being anchored.
- Staff requesting a power block must correctly identify/describe the exact OHE section requiring shutdown.
- If there's ANY doubt in the description, the person requesting a power block must clearly state the track and OHE structure numbers between which work is to be done.
- All shutdown/restoration/Permit-to-Work messages over telephone MUST be supported by exchange of PRIVATE NUMBERS and REPEATED TWICE.
- Do NOT tie a rope to the rail without taking a traffic block.
- It is necessary to take a TRAFFIC BLOCK when working on a tunnel or bridge.
Exam Focus — Earthing Sequence Is Reversible and Testable Both Ways
A classic sequencing question: while EARTHING, the clamp goes on rail/mast FIRST, then the top clamp hooks onto the OHE. While REMOVING earth, it's the exact REVERSE — the OHE hook comes off first, then the rail/mast clamp. The underlying safety logic: you always want the earth connection established BEFORE the OHE connection, and broken AFTER — so the OHE is never connected to a floating (non-earthed) clamp at any point in the process.