What Standard Governs Disconnector Testing in China?
Disconnector testing in China is governed by GB/T 1985-2023 High-Voltage Alternating Current Disconnectors and Earthing Switches (replacing GB/T 1985-2014), which is modified-adopted from IEC 62271-102:2018. The standard covers disconnectors and earthing switches rated for 3 kV and above at 50 Hz, installed indoors or outdoors. It defines the rated values, design requirements, and the complete type-test and routine-test panel.
The companion standard is GB/T 11022-2020 Common Specifications for High-Voltage Switchgear and Controlgear (= IEC 62271-1:2017), which provides the shared technical requirements — insulation levels, temperature rise, main-circuit resistance, short-time withstand current, EMC — that GB/T 1985 invokes. When the two standards overlap, GB/T 1985's disconnector-specific requirements take precedence over GB/T 11022's general requirements.
For GIS (gas-insulated switchgear) disconnectors, GB 7674 (= IEC 62271-203) applies as the GIS product standard, with the disconnector-specific clauses of GB/T 1985 overlaid. For the Very Fast Transient Overvoltage (VFTO) testing unique to GIS disconnectors, IEC 62271-102 Test Duty 1 (TD1) defines the type-test setup that proves the disconnector's dielectric design under switching-generated VFTO.
What Are the Dielectric (Insulation) Tests?
The dielectric tests verify that the disconnector's insulation — across the open contacts, between phases, and to earth — can withstand the voltages it will see in service, including lightning surges and switching surges.
Power-frequency withstand voltage: a defined AC voltage (50/60 Hz) is applied for a defined duration (typically 1 minute for type tests) between the main terminals (with contacts closed) and earth, and across the open contacts. The test voltage is selected from the standard's insulation-level table by rated voltage. For a 40.5 kV disconnector, the power-frequency withstand is typically 95 kV (phase-to-earth) and 118 kV (across open contacts). No flashover or breakdown may occur. For outdoor installations, the test may be performed under wet conditions (artificial rain) to verify the insulation under rain.
Lightning impulse withstand voltage (LIWV): a standard 1.2/50 µs impulse voltage wave is applied between the main terminals and earth, and across the open contacts, in both polarities. The LIWV for a 40.5 kV disconnector is typically 185 kV (phase-to-earth) and 215 kV (across open contacts). For 1100 kV UHV GIS disconnectors (the Chinese UHV pilot project), the LIWV is 2400 kV. No flashover may occur across 15 impulses per polarity.
Dielectric test on auxiliary and control circuits: a power-frequency test at 1 kV or 2 kV for 1 second on the low-voltage auxiliary and control wiring verifies the insulation of the operating mechanism's electrical circuits.
The altitude correction factor is applied to the test voltages when the disconnector is specified for installation at altitudes above 1000 m — the thinner air reduces its dielectric strength, requiring higher external insulation dimensions.
What Is the Main-Circuit Resistance Measurement?
The main-circuit resistance test measures the DC resistance of the current-carrying path through the disconnector's closed contacts — the property that determines how much heat the disconnector generates at rated current. High contact resistance means excessive power loss and temperature rise, which degrades the contact surfaces over time.
Method: a DC current (typically 100 A or more, to ensure adequate signal-to-noise ratio) is passed through the closed disconnector, and the voltage drop across the contact path is measured. The resistance is calculated by Ohm's law (R = V/I). The measurement is taken on each pole of a three-phase disconnector.
Acceptance: for routine tests, the measured resistance shall not exceed 1.2 × Ru, where Ru is the resistance measured at the type-test reference. For type tests, the resistance is measured before and after the mechanical endurance test and temperature-rise test — the post-test resistance shall not vary by more than 20 % from the pre-test value. A resistance that increases beyond 20 % after mechanical cycling indicates contact wear, oxidation, or loosening of the contact mechanism.
The main-circuit resistance measurement is the fastest diagnostic for contact quality — it catches loose contacts, contaminated contact surfaces, and misalignment that would produce excessive heating in service.
What Is the Temperature-Rise Test?
The temperature-rise test verifies that the disconnector does not overheat at rated current. The disconnector is energised at its rated continuous current (e.g. 1250 A, 2500 A, 4000 A) in a draught-free enclosure at defined ambient temperature, and the temperatures of the contacts, terminals, and accessible parts are measured by thermocouples until thermal equilibrium.
Acceptance: the temperature rise of each component must not exceed the limits defined in GB/T 11022 for its insulation class and material. For silver-plated contacts in air, the typical limit is 65 K rise above ambient; for bolted connections in air with copper busbars, 50 K or 75 K depending on surface treatment. The main-circuit resistance is measured before and after the temperature-rise test to confirm that the contacts have not degraded.
The temperature-rise test is the one that catches the "invisible" failure mode — a disconnector that has correct insulation, correct mechanical operation, and correct resistance on the bench, but whose contacts overheat under sustained rated current because the contact force is inadequate or the contact material is wrong. The thermal signature (which component overheats and by how much) is diagnostic: contact overtemperature points to contact-force or contact-surface issues; terminal overtemperature points to busbar-joint design.
What Is the Mechanical Operation and Endurance Test?
The mechanical operation test verifies that the disconnector opens and closes correctly within its specified operating voltage range and that it survives the specified number of operating cycles without mechanical degradation.
Routine mechanical operation test: the disconnector is operated (open-close) at 85 %, 100 %, and 110 % of the rated supply voltage of the operating mechanism. At each voltage, 10 close-open cycles are performed (50 cycles at nominal voltage). The operating time, motor current (for motor-operated mechanisms), and output-shaft torque are recorded. The auxiliary contacts and position indicators are verified. After the test, no parts may be damaged, and the main-circuit resistance shall not vary by more than 20 %.
Type-test mechanical endurance: the disconnector is subjected to the rated number of mechanical operating cycles (defined in GB/T 1985 clause 4.106, typically 1000 to 2000 cycles for classes M1 and M2 — class M2 is the higher endurance). This test verifies the mechanism's durability — the springs, linkages, bearings, and contact system must survive the specified cycles without failure or excessive wear. After the endurance test, the disconnector must still meet the dielectric and main-circuit-resistance requirements.
Interlock verification: disconnectors have mechanical and electrical interlocks that prevent unsafe operation (e.g. operating a disconnector under load, or opening an earthing switch on a live circuit). The interlock verification test confirms that the interlocks function correctly at each step of the operation sequence — this is a safety-critical test, because interlock failure can lead to catastrophic fault.
What Is the Short-Time Withstand Current Test?
The short-time withstand current test verifies that the disconnector can carry the rated short-circuit current for the rated duration without mechanical or thermal damage. The test applies the rated short-time withstand current (e.g. 31.5 kA, 40 kA, 50 kA) for the rated duration (typically 3 seconds) with the disconnector closed.
Pass criteria: no mechanical damage (no deformation of the contacts or insulators, no parts ejected), and the main-circuit resistance measured after the test shall not vary by more than 20 % from the pre-test value. The short-time withstand current is what the disconnector must survive when a fault occurs on the system and the protective relay clears it — the disconnector does not interrupt the fault current (that is the circuit breaker's job), but it must carry it without damage until the breaker opens.
For earthing switches, an additional test is the rated peak withstand current (the asymmetrical peak of the fault current, typically 2.5× the RMS short-time current) and, for fast-acting earthing switches, the rated making current (the ability to close into a live fault without welding or exploding).
How Is the VFTO Test Applied to GIS Disconnectors?
For GIS disconnectors, the switching operation generates Very Fast Transient Overvoltages (VFTO) — steep-fronted voltage surges caused by the nanosecond-timescale breakdown of SF₆ gas between the opening contacts. At UHV voltage levels (1100 kV), VFTO can approach the insulation-withstand level of the GIS and become a dimensioning factor for the design.
Test Duty 1 (TD1) per IEC 62271-102: this is the mandatory type test that proves the disconnector's dielectric design. The disconnector is set up with a defined GIS busbar layout — a source-side busbar of 5.5–14 m, a load-side busbar of 3–5 m, and a defined capacitance. The source voltage is set to 1.1 p.u. of the rated voltage. During the test, the disconnector is opened and closed multiple times, generating VFTO through pre-strikes and re-strikes. The peak VFTO must not exceed the specified withstand level (for 1100 kV GIS, the VFTO withstand level is 2087 kV = LIWV/1.15). No flashover to the grounded enclosure may occur.
The Trapped Charge Voltage (TCV) — the DC voltage remaining on the load side after the disconnector opens — is the key parameter that determines the maximum VFTO during the subsequent close operation. The conservative assumption is TCV = −1.0 p.u. (worst case), but measurements and simulations show that the actual TCV for a slow-moving disconnector (contact speed ~0.4 m/s) is typically only 0.3–0.4 p.u., which reduces the VFTO significantly and allows a more optimised (lower-cost) design without a closing resistor.
How Does the GB Framework Map to International Standards?
| Scope | China (GB/T) | International (IEC) |
|---|---|---|
| Disconnector product | GB/T 1985-2023 | IEC 62271-102:2018 |
| Common specifications | GB/T 11022-2020 | IEC 62271-1:2017 |
| GIS product | GB 7674 | IEC 62271-203 |
| High-voltage test techniques | GB/T 16927.1 | IEC 60060-1 |
| Circuit breaker (reference) | GB/T 1984 | IEC 62271-100 |
The test methods and acceptance criteria in GB/T 1985 are closely aligned with IEC 62271-102 because GB/T 1985 is modified-adopted from it. The differences are in scope details and in the adoption timing (GB/T 1985-2023 aligns with IEC 62271-102:2018). A disconnector tested to GB/T 1985 will generally satisfy IEC 62271-102, and the report can be cross-referenced for international acceptance via the IECEE CB Scheme.
Our Testing Capabilities
Beijing ZKGX Research provides disconnector testing at both the type-test and routine-test levels, against GB/T 1985-2023 and GB/T 11022-2020.
Dielectric tests:
- Power-frequency withstand voltage (dry and wet, per GB/T 16927.1)
- Lightning impulse withstand voltage (1.2/50 µs, both polarities)
- Dielectric on auxiliary and control circuits (1 kV / 2 kV, 1 s)
Main circuit:
- DC resistance measurement (per pole, acceptance ≤ 1.2 × Ru for routine)
- Temperature-rise test at rated current (thermocouple, thermal equilibrium)
- Short-time withstand current (rated kA, 3 s)
Mechanical:
- Routine mechanical operation (85/100/110 % supply voltage, 10+50+10 cycles)
- Type-test mechanical endurance (1000–2000 cycles, class M1/M2)
- Interlock verification (mechanical and electrical)
Earthing switch specific:
- Rated peak withstand current
- Rated making current (for fast-acting earthing switches)
GIS disconnector VFTO:
- Test Duty 1 (TD1) per IEC 62271-102, with VFTO measurement and TCV analysis
Voltage scope: 3.6 kV to 1100 kV; indoor and outdoor; air-insulated and gas-insulated; motor-operated and manual mechanisms.
If you need a GB/T 1985 type-test report for a disconnector product release, a routine-test verification for a production batch, a temperature-rise qualification, a mechanical-endurance certification, an earthing-switch making-current test, or a GIS-disconnector VFTO type test — contact our laboratory with the disconnector type (air-break / GIS / pantograph), rated voltage, rated current, short-time withstand current, and applicable standard, and we will scope the test plan.
FAQ
What is the difference between a disconnector and a circuit breaker?
A disconnector provides visible isolation — it opens the circuit so that maintenance can be performed safely downstream, but it cannot interrupt load current or fault current. A circuit breaker interrupts both load and fault currents. The disconnector is operated only after the circuit breaker has opened (zero current), and the interlock prevents operating the disconnector under load. The disconnector's contacts are designed for carrying current and surviving short-circuit, not for interrupting arcs.
Why is the main-circuit resistance measured before and after tests?
Because contact resistance is the most sensitive indicator of contact degradation. The temperature-rise test heats the contacts (which can oxidise or anneal the surface), the mechanical endurance test cycles the contacts (which can wear the silver plating), and the short-time withstand test passes fault current through the contacts (which can produce electromagnetic repulsion forces). A resistance that increases beyond 20 % after any of these tests indicates that the contacts have degraded — the disconnector may still carry rated current today but will fail in service.
What is Test Duty 1 (TD1) and why is it only for GIS disconnectors?
TD1 is the VFTO type test defined in IEC 62271-102. It is only applicable to GIS disconnectors because VFTO is generated by the breakdown of SF₆ gas in the enclosed busbar — air-insulated disconnectors do not produce VFTO of comparable magnitude because the air breakdown is slower and the surge propagation is different. TD1 proves that the disconnector can switch without producing flashover to the grounded enclosure, using a defined busbar layout that produces the maximum VFTO for the design.
Can a disconnector interrupt current?
No — a disconnector is designed for isolation only, not for interruption. Operating a disconnector under load produces an arc that the disconnector is not designed to extinguish, which can cause a flashover, a phase-to-phase fault, or injury to the operator. The interlock system prevents this. The exception is the earthing switch with making capacity, which is designed to close into a live (but de-energised-by-the-breaker) circuit — it can make (close) into a fault, but it still cannot break (interrupt) one.
How does the GB/T 1985-2023 differ from the 2014 edition?
The 2023 revision aligns with IEC 62271-102:2018, with updates to the earthing-switch switching-induced-current parameters, design requirements, and structure. The key changes include revised requirements for the earthing switch's short-circuit making-current verification and updated test methods for induced-current switching. A laboratory using the 2014 edition after the transition is reporting against a withdrawn standard.