About the Circuit Breaker Rating
Circuit breaker rating selection determines the correct continuous current rating, interrupting capacity (fault current), and breaking voltage for circuit breakers in a solar plant's LT or HT switchgear — ensuring they can safely interrupt both normal load currents and short-circuit fault currents.
How to calculate it
- 1Continuous current rating: ≥ maximum load current × 1.25. For the solar plant feeder: Iload = plant kW ÷ (√3 × kV × PF).
- 2Short-circuit interrupting capacity (kA): must be ≥ maximum prospective fault current at the breaker's installation point. Calculate fault current from the supply transformer impedance.
- 3For LT MCCBs: standard ratings 10–65 kA symmetrical. For HT vacuum circuit breakers (VCBs): 16–40 kA standard.
- 4Breaking voltage: MCCB rated voltage ≥ system voltage. HT VCB: rated for system voltage × 1.05 (e.g., 12 kV breaker for 11 kV system).
- 5Standard ratings: select the next standard size above the calculated value — MCCBs in the 16, 25, 32, 40, 63, 80, 100, 125, 160, 200, 250, 400, 630 A series.
Worked example
A 500 kW inverter at 415 V, PF 0.98. Load current = 500,000 ÷ (1.732 × 415 × 0.98) = 709 A. Breaker continuous rating: 709 × 1.25 = 886 A → select 1,000 A MCCB. Fault current at 415 V LT bus (1 MVA transformer, 5% Z): Isc = 1,000,000 ÷ (1.732 × 415) × (1 ÷ 0.05) = 27.8 kA → select MCCB with 36 kA symmetrical breaking capacity.
What you can calculate
- SC rating kA
- IEC 62271
- Break time
- TRV envelope
Standards & references
Frequently asked questions
What is the difference between a circuit breaker's rated current and interrupting capacity?
Rated current (In) is the maximum continuous current the breaker can carry without tripping. Interrupting capacity (Icu or Ics) is the maximum fault current it can safely break — a much higher value. A 400 A MCCB might have a 36 kA interrupting capacity: it carries 400 A continuously but can safely break a 36,000 A fault current. Both must be adequate: a breaker with the right current rating but insufficient interrupting capacity will fail catastrophically during a fault.
What is the difference between MCCB and MCB?
MCB (Miniature Circuit Breaker) is rated up to 125 A with interrupting capacity of 6–10 kA — used for individual circuits and small loads. MCCB (Moulded Case Circuit Breaker) is rated from 16 A to 1,600 A with interrupting capacity up to 65 kA — used for main distribution boards, inverter outputs, and feeder protection. Solar plants use MCBs for auxiliary circuits and MCCBs for inverter, ACDB, and main feeder protection.
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