Comparative Overview: GB/IEC vs IEEE Standards

In medium-voltage systems, the performance of a circuit breaker during capacitive current interruption is a decisive measure of its reliability. Under the GB/IEC 12 kV standard, the rated back-to-back capacitor switching current is generally 400 A, occasionally reaching 630 A, and in rare cases, up to 800 A. In contrast, the IEEE 15 kV standard defines a back-to-back capacitor switching current capability as high as 1640 A, reflecting a more stringent performance requirement.

This divergence stems from differing design philosophies and testing methodologies. While GB/IEC emphasizes the coordination between system voltage classes and mechanical endurance, IEEE focuses on statistical breakdown probability, re-ignition behavior, and severe capacitive switching endurance, setting higher benchmarks for North American breaker designs.

IEEE Classification of Circuit Breakers for Capacitive Current Switching

The IEEE Standards C37.04 and C37.09 (2003 revision) introduced a refined classification for circuit breakers according to their restrike (re-ignition) probability when interrupting capacitive currents. The categories are as follows:

• Class C2 – Circuit breakers with an extremely low probability of restrike.

• Class C1 – Circuit breakers with a low probability of restrike.

• Class C0 – Circuit breakers with no specified restrike probability.

These classifications were established to recognize the practical inevitability of restrike phenomena. Declaring a breaker as “restrike-free” would be scientifically untenable, as it would require infinite successful operations under capacitive switching conditions — an impossible verification.

Understanding Restrike Probability

During the interruption of a capacitive current, a restrike occurs when the arc reignites after current zero due to a breakdown of the transient recovery voltage (TRV) across the contacts. The restrike probability quantifies the likelihood of this event under controlled testing.

• C2-Class Breakers: No restrike occurs throughout the entire test series. If a single restrike is observed, the entire test series must be repeated, and no restrikes are permitted in the second sequence.

• C1-Class Breakers: A maximum of one restrike may occur during a test series. If more than one occurs, the full test series must be repeated, and only one restrike is allowed in the second sequence.

• C0-Class Breakers: A restrike is permitted in every capacitive current interruption test, indicating no restrike performance guarantee.

Statistically, a C2-class breaker has only about 10% of the restrike likelihood of a C1-class unit. Conversely, C0-class devices exhibit an effective 100% restrike probability, rendering them unsuitable for capacitor bank switching applications.

The Most Stringent Test: Back-to-Back Capacitor Switching

Among all capacitive switching tests, the back-to-back capacitor (BC) switching test is the most severe. It represents the condition where two or more capacitor banks are directly connected through the circuit breaker, producing high transient voltages and steep current gradients.

Compared to tests for line charging (LC) or cable charging (CC), which impose less stress, the BC test places extreme dielectric demands on the breaker’s interrupter and contact geometry. The tests are conducted under three-phase laboratory configurations, as opposed to single-phase tests, to simulate real-world phase coupling and transient voltage interactions.

BC1 Test Series: Low Current Verification

The BC1 test series evaluates breaker performance at 10–40% of its rated capacitive switching current. A total of 24 operations are required, consisting of either opening operations or make-and-break sequences as determined by the manufacturer.

The test mandates:

• Four opening operations distributed at approximately 15° phase intervals on each polarity.

• Six opening operations performed at the minimum arcing time per polarity.

• Additional opening operations distributed over varying contact parting instants until 24 total operations are completed.

This test primarily examines restrike immunity under mild current magnitudes and diverse contact separation timings.

BC2 Test Series: Full Current Endurance

The BC2 test series subjects the circuit breaker to its full rated capacitive switching current (100%), requiring 80 make-and-break operations under severe electrical stress.

The procedure includes:

• Four make-and-break operations per polarity spaced at 15° intervals.

• Thirty-two make-and-break operations per polarity conducted at the minimum arcing time condition.

• Additional operations with varying contact separation instants to achieve a total of 80 operations.

When combined with the BC1 sequence, the breaker undergoes at least 104 three-phase test operations to validate C2-class capability. In practice, additional repetitions are often needed to fulfill the minimum number of shortest arcing time test points — 12 for BC1 and 64 for BC2 — pushing the total beyond 104 operations.

Determining Minimum Arcing Time

Both BC1 and BC2 tests focus on identifying the minimum arcing time — the instant that presents the most severe electrical stress. This is determined by incrementally adjusting the contact parting instant in 0.3 ms intervals until the precise point of shortest arc duration is located.

This “minimum arcing” moment occurs when the contacts separate just before current zero, while the system voltage remains at its maximum. Under these conditions, the electric field strength across the small contact gap is at its peak, dramatically increasing the likelihood of a restrike.

Because this instant represents the worst-case dielectric recovery scenario, the standard mandates that:

• 80% of BC2 tests and 50% of BC1 tests must be conducted under minimum arcing time conditions.

This focus ensures that testing rigorously challenges the breaker’s insulation recovery performance rather than relying on random contact timing that might underrepresent worst-case conditions.

Test Philosophy and Rationale

By emphasizing minimum arcing time and repetitive verification, the IEEE standard ensures that C2-class circuit breakers demonstrate truly exceptional restrike immunity. These requirements simulate the statistical severity that would arise over many thousands of random switching operations in service.

Although simplified descriptions are often used in summaries, the full standard allows for additional restrike trials under controlled conditions, provided the corresponding test sequences are repeated according to prescribed criteria.

These comprehensive tests establish confidence that a C2-class breaker can safely and repeatedly interrupt capacitive currents — such as in back-to-back capacitor banks, filter banks, or long transmission lines — without causing damaging overvoltages or dielectric failure.

Conclusion

Capacitive current switching represents one of the most delicate and demanding duties for a medium-voltage circuit breaker. The C-class system introduced by IEEE has provided a precise, performance-based framework for evaluating restrike probability and dielectric strength under real-world conditions.

While GB/IEC standards generally limit back-to-back capacitor switching to 400–800 A, the IEEE standard’s 1640 A benchmark underscores the advanced endurance expectations placed on equipment in North American and global high-reliability grids.

Among the classifications, C2-class breakers stand as the most robust and reliable — engineered to withstand the harshest capacitive switching transients with negligible restrike probability. As grid networks continue to integrate capacitive elements for reactive power compensation and harmonic filtering, the role of C2-class circuit breakers will remain critical in safeguarding system stability and longevity.