Neutral sections isolate different phases of power supply in adjoining OHE fed by adjacent substations, which are normally connected to different phases of supply. Indian Railways uses PTFE (Poly Tetra Fluoro Ethylene) type neutral sections — light-weight, composite-type contact wire insulators (a resin-bonded glass fibre core protected with wear-resistant ceramic beads, covered with PTFE). This chapter covers the three real PTFE neutral section types found in service: the Short Neutral Section Assembly, the Arthur Flury AG type, and the Balfour Beatty Power Corporation (BBPC, UK) type.
Type 1: Short Neutral Section Assembly (PTFE Type)
- Speed permitted under this neutral section assembly: 200 kmph.
- Advantages: ease of installation even on flat curved locations, and substantially reduced maintenance.
- Available lengths of PTFE type neutral sections: 9.0 m, 5.0 m, and 3.736 m.
- Governing RDSO drawings: ETI/OHE/SK/551 (arrangement of neutral section for conversion with short neutral section), ETI/OHE/SK/552 (arrangement of short neutral section at existing feeding post).
- Erected on level tangent track, flat curves, and difficult locations. Stagger of the short neutral section is ZERO at support.
- Location on track: at least 400 metres AFTER the stop signal, and 200 metres BEFORE the stop signal.
Short Neutral Section — Maintenance Checklist
- Check any cracks on the anti-torsion tube to messenger clamp; check free movement of the telescopic anti-torsion tube.
- Check the earthing lug for any flash mark and loose connections; ensure the earth wire is provided with a lug.
- Check ceramic beads for any crack; clean carbon deposits under ceramic beads by wiping with cotton cloth soaked in petrol.
- Check clearance between arcing horn and arc catcher — horizontal clearance MAXIMUM 320mm; check clearance between arc catcher and ceramic bead — vertical clearance MINIMUM 170mm.
- Check burning of arc traps and arcing horns; check complete bracket of PTFE for any abnormality.
- Check flash marks on the structure bond connected between PTFE mast and rail.
- Measure earth resistance TWICE a year — it should be LESS THAN 10 OHMS.
Type 2: Arthur Flury AG PTFE Type Neutral Section
Features & Technical Specifications
- Short installation time, easy to install and regulate. Suitable for speeds up to 200 kmph, and suitable for BOTH regulated and unregulated OHE.
- Larger air gap between arcing horns gives self-arc-extinguishing behaviour, and the arcing horn extinguishes arcs quickly.
- The insulator rod is revolvable — it can be revolved 5 times in case of wear, giving a service life of more than 4 million pantograph passes (other specifications state 2-4 million; treat 4 million as the upper/typical figure and 2 million as the lower bound).
- High breaking load: each glass fibre rod breaks above 100 kN; the section insulator's overall breaking load is 120 kN.
- Working tensile strength in each of contact and catenary wire: 10 kN to 15 kN.
- Sealed end fittings and PTFE guarantee no pollution and no ultraviolet radiation reaches the epoxy resin glass fibre bar.
| Service Condition | Value |
|---|---|
| Max. temperature of air in shade | 45°C |
| Min. temperature of air in shade | 0°C |
| Altitude above sea level | Not exceeding 1000 m |
| Max. relative humidity | 100% |
| Max. wind pressure | 200 kgf/m² |
| Average annual rainfall | 1750 to 6250 mm |
| No. of thunderstorm days/annum | 35 max. |
| No. of dust storm days/annum | 35 max. |
| No. of rainy days/annum | 120 |
| Max. temperature attainable by an object exposed to sun | 65°C |
Installation Location
- Erected on level tangent track, flat curves, and difficult locations.
- Stagger of the neutral section is ZERO or ±50mm at support (note: this is slightly different from the Short Neutral Section Assembly type above, which is strictly zero).
- Located on track at a distance of at least 400 metres after the stop signal, and 200 metres before the stop signal — the same 400m/200m rule as the Short Neutral Section type.
- The half-tension length on either side of a neutral section shall NOT EXCEED 600 metres.
Description of Parts
- Neutral Section Insulator: glass fibre rod covered by virgin PTFE for excellent electrical/mechanical properties; the PTFE cover self-cleans under rainwater with a low wear rate.
- Runner: made of copper, good conductivity property.
- Arcing Horns: made of stainless steel; specially designed to keep the arc away from insulating material and guide it quickly upward for extinguishing, without damaging material.
- Messenger Wire: conductor with resin-bonded fibre glass rod insulators covered with Teflon or equivalent, with suitable end fittings for connection to the catenary wire.
- Earth Wire: made of copper, flexible, diameter 12.0mm to 13.0mm. The far end has a lug tightened with an M16 bolt on the support mast.
- Turnbuckle: made of stainless steel, used for fine adjustment of neutral section height.
- Suspension Cable/Hanger: made of 10 mm² bronze cable, used for suspending components of the neutral section assembly from the catenary wire.
Arthur Flury AG — Quarterly Maintenance Checklist
- Check the PTFE cover of the insulating rod — clean/rotate if required.
- Check the complete bracket of the neutral section for any abnormality — looseness of screws, nuts, damages, and cracks.
- Check wear of the insulating rod. If wear is MAX 2mm, the rod should be rotated by 2 marks (72 degrees).
- Check levelling of the skid by spirit level.
- Check the length of the skid's bulb — if length is less than 2mm, the skid should be replaced IMMEDIATELY.
- Check excessive wear at the intake point of skids — readjust the skids.
- Check turnbuckles — lock with counternuts and secure with locking wire.
- Check the earth cable connection.
Type 3: BBPC (Balfour Beatty Power Corporation Ltd, UK) Type — Monthly Maintenance
- Check ceramic bead insulator — clean/replace if required.
- Check complete bracket of PTFE for any abnormality — checked for loose connections, damages, and cracks.
- Check the burning of arcing horns and arc catcher tips — attend as needed.
- Check horizontally of arc catcher — adjust by adjustable dropper if required.
- Check flexibility of the anti-torsion spring dropper — free movement checked.
- Measure air clearance between arcing horn and arc catcher tips — it should be MINIMUM 320mm.
- Check the earth jumper — check flashing marks on nut & bolt, lug, and loose connection.
- Check flashing marks on the rail, lug, and mast — check or replace nut & bolt if required.
- Check the bond between rail-to-mast and mast-to-earth electrode — check flashing marks and loose connections.
- Check any cracks on the anti-torsion tube to messenger clamp.
- Check free movement of the telescopic anti-torsion tube.
Exam Focus — Comparing the Three Types
All three types share the 400m/200m stop-signal location rule and the 320mm arcing-horn air-gap figure — safe to treat as universal PTFE neutral section facts. But watch the small, testable differences: stagger is strictly ZERO for the Short Neutral Section type, but ZERO OR ±50mm for Arthur Flury. Maintenance periodicity differs by manufacturer: BBPC is MONTHLY, Arthur Flury is QUARTERLY — despite being functionally similar PTFE assemblies. Earth resistance target (10 ohms max, checked twice yearly) is stated explicitly for the Short Neutral Section type. Arthur Flury's half-tension length limit (≤600m either side) is a figure stated only for this type, not for the other two.