Breaker Selection Criteria — Primary vs Secondary — Traction Distribution Notes

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This chapter covers a genuinely important conceptual insight that ties together the fault-current calculation and breaker-rating content from the previous chapters: WHY the deciding factor for breaker selection is completely different on the primary side versus the secondary side of a traction substation.

Real Standardised Current Series

Rated short circuit breaking current (kA): 6.0, 8.0, 10.0, 12.5, 16.0, 20.0, 25.0, 31.5, 40.0, 50.0, 63.0, 80.0, and 100 kA.
Rated normal current (Amps): 400, 630, 800, 1250, 1600, 2000, 2500, 3150, and 4000 A.

The Real 20 MVA Example — A Genuine Eye-Opener

The normal rated current of a 20 MVA transformer's primary winding, for a 132kV system, is only 151.5 Amps. Since these transformers are designed to deliver 100% overload for 5 minutes (the real overload rating from the earlier Transformer Overload Capacity chapter), primary current could reach up to 303 Amps at most. Yet the ASSIGNED normal rated current for the HV circuit breaker across the 66kV/110kV/132kV/220kV systems is 1250 Amps — roughly 4-8 TIMES the actual maximum expected current.

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Exam Focus — Why the Gap Exists, and the Reverse Rule

This large gap exists because IEC/IS/BS standards have STANDARDISED normal rated current values to MATCH the value of rated short circuit breaking current — not to match actual expected load current. This leads to the single most important conceptual takeaway: when deciding PRIMARY-side breaker ratings, the rated SHORT CIRCUIT BREAKING CURRENT is the deciding factor, and rated normal current (generally much less) is comparatively unimportant. But for SECONDARY-side breakers, the situation is the EXACT REVERSE — the deciding factor is the NORMAL RATED CURRENT, not the fault current.

Real Technical Particulars — 220kV vs 25kV Breakers

Particular	220kV (Primary)	25kV (Secondary)
No. of poles	2 or 3 as required	1
Nominal system voltage	220 kV	25 kV
Highest system voltage	245 kV	52 kV class
Rated 1-minute power frequency withstand voltage	460 kV (rms)	105 kV
Rated impulse (1.2/50 µs) withstand voltage	1050 kV (peak)	250 kV (peak)
Rated normal current	1250 A	1600 A

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Exam Focus — 25kV Rated Current Is HIGHER Than 220kV's

A genuinely counter-intuitive real fact worth remembering: the 25kV (secondary/low-voltage) breaker's rated normal current (1600A) is actually HIGHER than the 220kV (primary/high-voltage) breaker's (1250A) — the opposite of what intuition about 'high voltage = bigger equipment' might suggest. This makes sense once you recall Power = Voltage × Current: for roughly similar power levels, the LOWER-voltage side genuinely carries MORE current.