Making Test Introduction for 24kV Vacuum Circuit Breaker

Applicable Standards: GB 1984-2014 (IEC 62271-100); Rated voltage Ur=24kV. Test Purpose: To verify the capability of the circuit breaker to close onto a pre-existing short-circuit fault. The breaker shall withstand the peak short-circuit making current (rated short-circuit making current = 2.5 × peak value of rated short-circuit breaking current), electrodynamic forces and pre-strike arcing during closing. No contact welding, mechanism jamming or vacuum interrupter damage shall occur.

⚠️ Distinction: Short-circuit making test ≠ No-load mechanical closing test. The making test is closing onto a short-circuit under high voltage; mechanical closing characteristics test is performed without voltage.

I. Preconditions Before Test (Mandatory for Type Test)

1. Test Sample Status

  1. The circuit breaker shall be fully assembled (solid-sealed pole, spring/permanent magnet operating mechanism, interlock and anti-pumping device installed consistent with delivery status).
  2. The circuit breaker is in open position and the operating mechanism is fully charged.
  3. Pre-tests shall be completed prior to formal test: mechanical characteristics test, contact resistance measurement and power frequency withstand voltage test to confirm basic performance compliance.
  4. Operating power supply: The making test shall be carried out under three operating conditions respectively: maximum operating voltage (110% Uc), rated operating voltage Uc, minimum closing voltage (80% Uc).

2. Test Circuit Selection

  • Direct test: High-capacity impulse generator test station (three-phase test preferred).
  • Synthetic test: Adopted when laboratory capacity is insufficient (in accordance with GB/T 4473). Direct test is commonly used for 24kV circuit breakers. Circuit composition: Test power supply → Short-circuit transformer → Adjustable reactance/resistor → Tested 24kV circuit breaker → Earth. Equipped with voltage divider, current transformer and high-speed oscillograph.

3. Key Test Parameters (General for 24kV products)

  • Applied voltage: ≥24kV for three-phase test (0.9~1.1Ur); For single-pole first-pole test, the first-pole factor of 1.3 times phase voltage shall be adopted.
  • Peak rated short-circuit making current Ipeak
  • Frequency: 50Hz
  • Required test operations: 3 valid making operations (three poles tested simultaneously for three-phase circuit breaker)
  • Pre-strike is permitted: Pre-strike breakdown and arcing during closing are normal phenomena recognized by standards.

II. Complete Standard Test Procedure

Stage 1: Test Preparation & Safety Verification

  1. Isolate the test area with fencing, warning signs and reliable earthing. All non-test metallic enclosures shall be earthed.
  2. Wiring verification:
    • Primary circuit: Test power supply connected to short-circuit circuit and breaker incoming terminal; breaker outgoing side short-circuited and earthed to simulate short-circuit fault.
    • Secondary circuit: Closing control power supply connected to breaker closing circuit; oscilloscope channels connected to signals of interrupter voltage and phase currents.
  3. Instrument commissioning: High-speed oscillograph (sampling rate ≥1MHz) to record interrupter voltage, phase currents and closing trigger timing.
  4. No-load timing pre-adjustment: Perform multiple closing operations without high voltage to verify closing timing and trigger synchronization, confirm no mechanism jamming.

Stage 2: Half-current Pre-test (Circuit Calibration, not counted as formal test operations)

  1. Reduce test power supply, set 50% prospective short-circuit current and rated test voltage.
  2. Trigger circuit breaker closing; record voltage and current waveforms.
  3. Verify short-circuit current amplitude, phase angle, circuit impedance and trigger delay.
  4. Fine-tune reactance and closing trigger phase angle to find the most onerous closing phase (near voltage peak, causing maximum pre-strike and making current).

Key point: The making test shall be performed at the most unfavourable phase angle to simulate worst site operating conditions.

Stage 3: Formal Short-circuit Making Test (Core Procedure)

Standard sequence: Tests shall be implemented respectively under maximum operating voltage, rated operating voltage and minimum closing voltage. Procedure for each operating condition:

  1. Restore circuit parameters to rated test voltage and rated short-circuit making current; reset breaker to open position and recharge mechanism.
  2. Precise synchronization by control system; send closing command at preset worst-case phase angle.
  3. Circuit breaker closes onto pre-existing short-circuit; oscillograph continuously captures waveforms: ✅ Pre-strike moment ✅ Peak making current and DC component ✅ Three-phase closing asynchronous time
  4. Maintain closing status for ≥0.1s to ensure full application of short-circuit current; perform remote opening to interrupt short-circuit current.
  5. Allow sufficient cooling interval for test sample (normally ≥3 minutes per manufacturer’s requirement to avoid cumulative overheating).
  6. Preliminary visual inspection: Check for obvious arc ejection, abnormal noise or smoke; measure contact resistance and compare with initial value before test.
  7. Repeat operations to complete 3 consecutive valid making operations under identical conditions.

For Type Test T100s test duty: Symmetrical short-circuit making test shall be completed independently; subsequent CO operations verify asymmetrical making capability.

Stage 4: Post-test Inspection (Key Criteria for Acceptance)

Inspection after each making operation and upon completion of all tests:

  1. Visual inspection:
    • No damage or obvious gas leakage of vacuum interrupter;
    • No severe contact welding or extensive ablation;
    • No mechanism deformation, jamming or loose fasteners.
  2. Functional test: Perform 5 consecutive no-load closing-opening operations; smooth operation without sticking.
  3. Conductive loop resistance: Resistance after test shall not exceed 1.2 times initial value (limit specified by manufacturer).
  4. Power frequency withstand voltage test across open contacts: Retest in accordance with standard withstand voltage value for 24kV equipment; no breakdown or flashover.
  5. Waveform analysis based on oscillograph records:
    • Making current reaches specified peak value;
    • No permanent contact adhesion preventing opening;
    • Pre-strike arc shall not cause permanent equipment damage.

III. Acceptance Criteria (GB 1984)

The test is deemed passed only if all requirements below are satisfied:

  1. All 3 making operations are successful; no contact welding and adhesion, the circuit breaker can open reliably.
  2. Mechanism operates normally after test without permanent mechanical damage.
  3. Loop resistance and insulation performance meet technical specifications.
  4. Forbidden phenomena: Vacuum interrupter rupture, insulation breakdown, permanent mechanism deformation.
  5. Permitted phenomena: Minor contact ablation and pre-strike arcing during closing.

IV. Common Test Abnormalities & Explanation

  1. Excessive closing bounce: Bounce superimposed with short-circuit electrodynamic force easily leads to contact welding and test failure. Closing bounce of 24kV vacuum circuit breaker is generally required ≤2ms.
  2. Excessively long pre-strike duration: High arc energy causes severe contact ablation. Frequent long-duration pre-strike requires inspection of vacuum interrupter and closing speed.
  3. Excessive DC component: Caused by improper circuit parameter setting, leading to high asymmetry of making current.
  4. Concept distinction:
    • Making current: Closing onto short-circuit; mandatory item for type test.
    • Ordinary closing test: No-load without high voltage, only for mechanical characteristics measurement, cannot replace short-circuit making test.

V. Supplementary Information: Routine Test vs Type Test

  • Type Test (Third-party Laboratory): Complete short-circuit making test with high-voltage short-circuit circuit.
  • Factory Routine Test: No high voltage short-circuit making test, only no-load mechanical closing test, operating voltage test and loop resistance measurement. Short-circuit making test is only performed for type test.