A breaker closing onto an existing fault must establish current at the worst point on the voltage wave and survive the first asymmetrical peak. This making duty can impose greater mechanical force than the symmetrical rms value suggests.
Learning objectives
Connect X/R ratio and DC offset to peak current, distinguish making from breaking duty and verify the complete current path during closing.
Core engineering principles
Fault current contains AC and decaying DC components
The closing instant relative to the voltage waveform determines the initial offset. Network X/R ratio controls how slowly the DC component decays.
Peak current drives electrodynamic force
The first major peak produces strong forces in contacts, busbars, terminals and supports. Because force varies approximately with current squared, a modest peak increase can materially raise stress.
Making duty is not interruption duty
The breaker must close, latch and carry the peak before protection commands opening. A high breaking-current rating does not replace a declared making-current capability.
Pole closing spread affects phase peaks
The three poles do not make at exactly the same instant. Mechanism synchronism and prestrike influence the phase current and transient duty.
Engineering application method
- Step 1: Calculate initial symmetrical and peak current for the installation.
- Step 2: Compare peak making duty with the breaker and assembly declarations.
- Step 3: Check latching, short-time withstand and busbar support capability.
- Step 4: Verify closing time and pole spread against manufacturer limits.
Practical example
A 25 kA rms system can produce a peak well above 50 kA depending on X/R ratio and method. The breaker, removable contacts and assembly must all withstand the declared peak; comparing only 25 kA labels is incomplete.
Common mistakes
- Using rms kA as the peak value.
- Assuming making and breaking ratings are identical.
- Ignoring X/R ratio and DC decay.
- Checking the breaker but not the assembly current path.
Design and review checklist
- What peak current is calculated?
- Is the making capability declared?
- Can the breaker latch and remain closed until trip?
- Are busbars and contacts rated for the same peak?
Standards basis and official sources
- IEC 62271-100:2021 + AMD1:2024 — Alternating-current circuit-breakers.
- IEC 62271 series — High-voltage switchgear and controlgear framework.
Engineering note: Verify the contracted standard edition, amendments, manufacturer evidence and project-specific studies before applying these principles to a supplied assembly.