A circuit breaker's nameplate carries a set of ratings — voltage, current, breaking capacity, making capacity, short-time current and duty cycle. Each one answers a different question: what voltage it can 

stand, what current it can carry, what fault it can break, what fault it can close onto, and how often it must do it. This post explains every rating.

Figure 4.1: Making capacity — the first peak includes maximum DC offset — and the standard auto-reclosing duty cycle. (Schematic, not to scale.)

The ratings, one by one

• Rated voltage (V_r) and Basic Insulation Level (BIL): the highest system voltage the breaker is designed for, and the impulse voltage its insulation must withstand. The breaker must stand this voltage across its open contacts without breakdown.
• Rated normal current (I_n): the current the breaker can carry continuously without exceeding permissible temperature-rise limits — standard values include 630 A, 1250 A and 2500 A.
• Breaking capacity: the fault current the breaker can interrupt, given in two forms. Symmetrical breaking capacity S = √3 · V · I_sym (in MVA), where I_sym is the RMS symmetrical fault current. Asymmetrical breaking capacity includes the DC offset that is present at the instant of breaking: I_asym = √(I_sym² + I_dc²).
• Making capacity: the current the breaker can safely close onto a short-circuit fault. Closing onto a fault catches the first current peak, which includes maximum DC offset: I_making = 1.8 × √2 × I_sym = 2.55 × I_sym. This is the highest current the breaker ever sees, so the making capacity is always larger than the breaking capacity.
• Rated short-time current: the current the breaker can carry for a short period — 1 s or 3 s — without thermal or electrodynamic damage. This matters when the breaker is closed and a fault persists while a downstream device clears it.
• Duty cycle (auto-reclosing): the standard operating sequence O – 0.3 s – CO – 3 min – CO: Open, wait 0.3 s, Close-Open, wait 3 minutes, Close-Open. It proves the breaker can handle a fault, reclose onto it, and clear it again.

Key points to remember

• Rated voltage V_r + BIL: what the open breaker must stand. Rated normal current I_n: 630 / 1250 / 2500 A, continuous.
• Symmetrical breaking capacity S = √3·V·I_sym; asymmetrical adds the DC offset, I_asym = √(I_sym² + I_dc²).
• Making capacity I_making = 1.8·√2·I_sym = 2.55·I_sym — the first peak with maximum DC offset.
• Rated short-time current: 1 s or 3 s without damage. Duty cycle: O – 0.3 s – CO – 3 min – CO.

Based on Power System Protection & Switchgear, Complete Examination Solution, Exam 07 May 2026, Q4(a).