Bus Bars & Lightning Arresters — Traction Distribution Notes

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Bus Bars — Strung vs Rigid Type

Two types of busbar are used at traction substations: STRUNG type on the HV side, and RIGID type on the LV (25kV) side.

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Exam Focus — The Real Conductor Upgrade Story

Earlier electrification schemes used All-Aluminium 'spider' conductor (size 19/3, 99mm) for the strung bus, adequate only for fault currents above 17kA. As grid fault levels rose beyond 25kA, this became inadequate — ongoing schemes now use 'Zebra' ACSR conductor (size 61/3.18, dia 28.62mm), capable of withstanding fault currents up to 31.5kA. The strung bus tension is kept between 500-900 kgf, and minimum conductor/jumper diameter must be at least 28mm for a 220kV system (to keep corona losses within limits).

Similarly, the rigid bus in earlier schemes was Aluminium Alloy of 36mm O.D. (36/28mm), capable of carrying 960 Amps normal rated current. With 20 MVA transformers now common, the required rated current increased to 1500 Amps (for short duration), so rigid busbar size increased to 50mm O.D. (50/39mm), capable of carrying 1530 Amps CONTINUOUS current. The rigid bus is supported on insulators at intervals not exceeding 3m, and the minimum height of the 25kV busbar is kept at 3800mm.

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Memory Aid — Rigid Bus Is Costlier but More Reliable

Plainly stated: chances of rigid bus failure are very remote, but its installation is costlier than the strung bus — a genuine real-world cost-vs-reliability trade-off, which is exactly why rigid bus is reserved for the 25kV side (fewer, more critical connections) rather than used throughout.

Lightning Arresters — Real Selection & Types

Lightning arresters (also called surge diverters) protect against over-voltages from lightning strokes and switching surges by absorbing energy. An ideal arrester draws negligible current at operating voltage (very high impedance) but negligible impedance during actual surge current flow. LA breakdown voltage must be kept much LOWER than other substation equipment — but not so low that normal supply-voltage variation or switching surges cause a power-frequency flashover.

5 real selection factors: maximum line-to-line voltage, rated discharge current, power frequency flashover voltage, impulse flashover voltage, and residual discharge voltage.
4 real types: rod gap, expulsion, conventional valve, and zinc oxide gapless. The rod gap and expulsion types do NOT provide the required level of protection; conventional valve types have become complicated (large component count — ionising system, trigger systems, grading network, interrupting system) and are STRETCHING (being phased out).

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Exam Focus — Zinc Oxide Gapless Is the Real Modern Standard

Zinc oxide gapless LAs are the new generation, with 8 real advantages over older types: matching/controlled protective level, faster response (from eliminating the series gap), reduced over-voltages (energy is absorbed), light/rugged construction, improved thermal characteristics, better performance under polluted conditions, better pressure-relief performance, and better sealing. That's genuinely 8 distinct, listable advantages — a real exam could ask for any subset.

Real Installation Rules

The LA must be installed very close to the apparatus it protects — a practical rule specifies the distance should be NO MORE THAN 10 metres.
For BIG transformers specifically, LA is installed immediately after the bushing.
If the incoming transmission line is MORE than 4km long, LAs must ALSO be provided at the entry point of the substation — a second layer of protection, not a replacement for the equipment-side LA.
Connections must be solid and direct. Earth connections need ample cross-section to carry the rated discharge current, and the arrester's earthing terminal must be connected together with all other electrical equipment's earthing terminal to the MAIN earthing bus — never a separate, isolated earth.