Insulators — Real Selection Criteria
Exam Focus — The Real 25mm/kV Creepage Standard
Since pollution level genuinely varies place to place, insulators used at traction substations are provided with a creepage distance of 25mm PER kV — a single, standardised ratio chosen specifically so insulators can withstand service conditions even under heavily polluted conditions, INCLUDING coastal areas. Substation design otherwise keeps insulator quantity to the MINIMUM commensurate with supply security — insulation isn't maximised blindly, it's balanced against reliability needs.
- Strung bus: standard 10" disc insulators of 7000 kgf strength used up to 132kV; for 220kV substations, 11" disc insulators of higher strength are specified.
- Rigid bus: post-type stack insulators used on the primary (HV) side; for the 25kV side, only SOLID CORE type insulators are adopted, for ALL applications.
- Due to solid-core insulator development now extending to higher system voltages, with real advantages over conventional post-type stack insulators, solid core is now used for ALL applications including busbar support insulators and isolators — a genuine technology shift, not just a 25kV-specific choice anymore.
- Insulators for circuit breakers, interruptors, PTs, CTs, and power transformer bushings are of HOLLOW PORCELAIN housing.
SF6 Gas — The Real Chemistry Behind the Breakers
This section explains the real physics/chemistry underlying the SF6 circuit breakers covered extensively in earlier chapters of this subject (maintenance schedules, gas pressure thresholds, operation-count inspections).
- SF6 gas in pure state is INERT, exhibiting exceptional thermal stability, excellent arc-quenching properties, and exceptional high insulating properties — one of the most stable compounds known: inert, non-flammable, non-toxic, and odourless.
- The gas remains a gas WITHOUT liquefaction down to -30°C at the maximum pressure of puffer-type breakers — a real, specific temperature threshold worth remembering.
- SF6 density is about 5 times that of air, and heat dissipation is also much more than air. At atmospheric pressure, dielectric strength is about 2.4 times that of air; at about 3 kg/cm², it EQUALS that of oil.
- After long periods of arcing, the gas decomposes only very slightly, with essentially no effect on dielectric strength or interrupting capability. The solid arc-product formed is METALLIC FLUORIDE, appearing as a fine grey powder — this powder has HIGH dielectric strength under the dry conditions present in the breaker.
- A good-quality absorbent is used in the apparatus to remove most gaseous decomposed by-products, keeping their level very low. Air impurities dilute the SF6, but it is explicitly NOT worth measuring this dilution in the field as long as the recommended process is followed.
Real Physical Properties of SF6 Gas
- Molecular weight: 146.07. Melting point: -50.7°C. Sublimation temperature: -63.8°C.
- Critical density: 0.730 g/cm³. Dielectric constant at 25°C, 1atm: 1.002.
- Solubility in oil: 0.297 cc/cc — noticeably higher than solubility in water (0.001 cc/cc), a roughly 300x difference.
- Density scales sharply with pressure at 20°C: 6.25 g/lit at 0 kg/cm²-g, rising to 12.3 at 1, 38.2 at 5, 75.6 at 10, and 119.0 g/lit at 15 kg/cm²-g — roughly linear growth with pressure.