1-1- Introduction
High-voltage circuit breakers—whether live tank (interrupting chamber at line potential) or dead tank (interrupting chamber inside a grounded enclosure)—depend on healthy insulation to withstand operating stresses and switching transients. Because modern breakers may use air, oil, or SF₆ and can include multiple series interrupting chambers with grading capacitors to equalize voltage distribution, insulation testing must verify both the earth insulation capacitances (e.g., C1G, C2G or CTG) and the contact-to-contact capacitances (e.g., C12, C1T, C2T). Tests are performed per phase, even for three-phase tanks, with an applied voltage typically 10–25% above rated line-to-ground to ensure adequate dielectric margin. Correct selection of test modes (UST/GST/GSTg) confirms the integrity of internal components and exposes issues such as moisture, surface leakage, or degradation of grading capacitors—conditions that often manifest as elevated dissipation factor/power factor.
1-2- Circuit Breakers Overview
For insulation testing purposes, high voltage circuit breakers can be classified into two groups:
- Live Tank Breakers: The interrupting chamber is at high voltage potential.
- Dead Tank Breakers: The interrupting chamber is housed in a grounded metal enclosure.
Depending on the nominal line voltage, operating mechanism (spring, hydraulic), and arc-quenching medium (air, oil, sulfur hexafluoride), circuit breakers may have two or more series-connected interrupting chambers. To ensure uniform voltage distribution across the interrupting sections, grading capacitors are often installed across the chambers.
Test Voltage:
The applied test voltage should not exceed 10% to 25% above the rated operating line-to-ground voltage.
Testing Procedure:
The following sections provide two examples of circuit breaker testing procedures. For simplicity, the examples illustrate the testing procedure for one phase of the switchgear. Even if all three phases are housed in a single tank, the test procedure and result analysis can be performed on a per-phase basis.
1-2-1- Dead Tank Breaker
The test connection for a Dead Tank Breaker is outlined below:
Figure 1:Connecting CAPTAN 12 to a dead tank Breaker for measurement of C2G and C12
C12: contact insulation capacitance
C1G, C2G: earth insulation capacitance
Table 1: Test Connections for Dead Tank Circuit Breaker
| Target Capacitance | High Voltage | INPUT A | MEAS GND | Test Mode | Breaker Status |
| C1G | 1 | 2 | Tank GND | GSTg-A | open |
| C2G | 2 | 1 | Tank GND | GSTg-A | open |
| C12 | 2 | 1 | Tank GND | UST-A | open |
| C1G + C2G | 1 or 2 | – | Tank GND | GST | closed |
Note: Test line #4 can be used to cross-verify the measurement results, as it should equal the sum of test lines #1 and #2 (i.e., #4 = #1 + #2).
Higher Dissipation in GSTg-A Mode:
This may indicate excessive moisture or by-products of arced SF6 or oil that have condensed or deposited on internal insulating members.
To confirm the result, perform several make-break operations and verify that the result is reproducible.
1-2-2- Live Tank Breaker
The test procedure for a Live Tank Breaker is shown below:
Figure 2: Connecting CAPTAN 12 to a live tank Breaker
Table 2: Test Connections for Live Tank Circuit Breaker
| Target Capacitance | High Voltage | INPUT A | INPUT B | MEAS GND | Test Mode | Breaker Status |
| C1T | Tank (T) | 1 | 2 | Floor GND | UST-A | open |
| C2T | Tank (T) | 1 | 2 | Floor GND | UST-B | open |
| CTG | Tank (T) | 1 | 2 | Floor GND | GSTg-A+B | open |
| C1T + C2T | Tank (T) | 1 | 2 | Floor GND | UST-A+B | open |
Note: Test line #4 can be used to inter-check the measurement results. (#4 = #1 + #2). Additional measurements in other test modes can be executed to inter-check the measurements results.
Measurement Considerations
Pre-Insertion Resistors and Switches:
- Although pre-insertion resistors (R1, R2) and their switches are included in the sum capacitances of the interrupting chambers (C1T, C2T), their influence is negligible.
The resistors are typically very low in resistance, and the switches have minimal capacitance compared to the bushing and grading capacitors.
Higher Dissipation or Power Factor:
- For bushing/grading capacitor assemblies, higher values generally indicate degradation or contamination of the grading capacitors.
- Surface leakage on the bushings may also influence the measurement.
- High losses may result from surface leakage or moisture condensation on internal tubes and rods.
1-3- Conclusion
A disciplined insulation-test program for circuit breakers validates the dielectric performance of both interrupting gaps and paths to ground, while providing cross-checks (e.g., the sum of individual capacitances equaling a combined measurement). In dead tank units, unusually high losses in GSTg-A often indicate moisture or by-products from arcing deposited on internal members and should be confirmed by repeated make-break operations. In live tank breakers, elevated power factor typically points to grading-capacitor deterioration or surface leakage on bushings; pre-insertion resistors and their switches have negligible influence on results. By adhering to the recommended test voltages, per-phase procedures, and verification steps, asset owners can reliably detect insulation defects early, plan corrective maintenance, and preserve breaker reliability across diverse mechanisms and quenching media.
