IEC 62271-1 classifies switchgear with gas or vacuum into three pressure systems: controlled (refills itself), closed (topped up manually; for SF6 up to 0.5% per year) and sealed (no processing during its service life, minimum 20 years). Each has its own tightness criterion and its own type and routine test.
Short answer
Switchgear that uses a gas other than ambient air, or vacuum, as an insulating, interrupting or operating medium must declare what type of pressure system it has. IEC 62271-1 (2017 edition, definitions 3.6.6 and requirements 6.16) distinguishes three: controlled, closed and sealed. The permitted leakage rate, the expected maintenance and the tightness test depend on that type.
The three systems
| System | Definition in the standard | Example in the standard |
|---|---|---|
| Controlled | Volume that is automatically replenished from an external source of compressed gas or an internal source | Air-blast circuit breakers, pneumatic mechanisms |
| Closed | Volume that is replenished when necessary by manual connection to an external gas source | Single-pressure SF6 circuit breakers |
| Sealed | Volume that requires no further processing of liquid, gas or vacuum during its expected operating life | Vacuum interrupters and some SF6 circuit breakers |
A sealed system is assembled and tested complete at the factory. Its expected life starts when the device is sealed.
How tightness is measured in each one
Clause 6.16 requires the absolute leakage rate F not to exceed the permissible rate Fp at 20 °C. At extreme temperatures a higher leakage is allowed, provided it returns to a value no higher than Fp at 20 °C.
- Controlled (6.16.2). Specified by the number of refills per day (N) or by the pressure drop per day (Δp).
- Closed (6.16.3). Specified by the relative leakage rate Frel of each compartment: 0.5% per year for SF6 and its mixtures, and 1% per year for other gases, at 20 °C. The manufacturer must declare the time between refills in normal service, which must be at least 10 years for maintenance planning. In addition, the equipment must allow the gas to be topped up while the equipment is in service, that is, energized.
- Sealed (6.16.4). Specified by its expected operating life: at least 20 years, with 30 and 40 years as preferred values. The minimum functional pressure must not be reached before the end of the expected life, and the manufacturer declares a permissible leakage. The standard considers that a sealed SF6 device loses less than 0.1% per year during its life.
- Vacuum. No leakage rate is specified: the vacuum level and the expected operating life are stated.
Pressures the manufacturer declares
Clause 6.22 requires the manufacturer to assign the filling level (pressure or density, referred to 20 °C) and, where applicable, these values:
- Alarm pressure for insulation or interruption, and another for operation.
- Minimum functional pressure for insulation or interruption, and another for operation.
The minimum functional pressure is the one from which the rated characteristics are maintained. For operation, the energy storage must also be replenished, which is why it is often called the lockout pressure. The alarm is the pressure at which a supervision signal can be given. This explains the logic of the SF6 low-pressure signals and of the density monitor of a circuit breaker.
How it is verified
Type test (7.8). It is demonstrated that F does not exceed Fp at 20 °C; the acceptable test temperature ranges from 15 °C to 30 °C. The same fluid and the same pressure (or density) as in service are used, and the method must have sufficient sensitivity. If the equipment has a mechanical switching device, the test is made with it open and closed, unless the leakage does not depend on the position. For closed pressure systems, the preferred method is the cumulative one, and it is accepted if the measured leakage does not exceed the permissible one within +10% (7.8.3).
Equipment with vacuum interrupters (7.8.4). No specific tightness test is required for the vacuum interrupter: its tightness is verified in manufacturing, and it is considered zero leakage during its life. Where a standard asks for a tightness test as a condition check, the vacuum integrity can be checked with the dielectric condition check test (7.2.12).
Routine test (8.5). It is done at ambient temperature, with the parts at or above the minimum functional pressure for insulation. For closed pressure systems, the cumulative method is preferred at the factory and a «sniffer» type detector can be used; if it detects a leak, the test fails or the leak must be quantified. For routine tests on site, the preferred method is a detector with a sensitivity of at least 10⁻⁸ Pa·m³/s.
- Is the volume refilled automatically from a gas source?
- Yes: Controlled: measured by refills or pressure drop per day
- No: Is gas topped up by manual connection when needed?
- Yes: Closed: relative annual leakage and time between refills
- No: Sealed: expected life, at least 20 years
The design pressure and the enclosure partitions of a GIS are covered in pressures and compartments of a GIS. The dew point when filling the equipment is explained in switchgear installation.
To see how this fits in the rest of the IEC 62271 series, see the standards for primary equipment testing.
What not to do
- Do not apply the 0.5% per year of a closed system to a sealed system: the latter is specified by years of operation, not by annual leakage.
- Do not treat a recurring low-pressure alarm in a sealed system as normal: by definition it needs no gas refill during its expected life.
- Do not top up gas in equipment without following the manufacturer’s instructions; the standard refers to them and to the user’s practices.
- Do not compare leakage measured at extreme temperatures with the permissible rate at 20 °C without taking into account the higher temporary leakage the standard allows.
When to call us
At Tiza Group we check the condition of the gas systems of your switchgear and the SF6 analysis at commissioning and during maintenance. See our testing and commissioning service, in Colombia or elsewhere on request. Write to us and we will quote it.
Related guides
- GIS switchgear: pressures and compartments
- Switchgear installation: final inspection and input data
- SF6 low-pressure alarm: causes and what to do
- SF6 density monitor: alarm and lockout stages
- SF6 handling: recovery, transfer and filling units
References
- IEC 62271-1, High-voltage switchgear and controlgear - Part 1: Common specifications for alternating current switchgear and controlgear, 2017 edition: clauses 3.6.5, 3.6.6, 6.16.1 to 6.16.4, 6.22, 7.8.1 to 7.8.4 and 8.5.1 to 8.5.4.
Frequently asked questions
What maximum annual leakage does a closed pressure system with SF6 allow?
Up to 0.5% per year per compartment, at 20 °C. For other gases, 1% per year (IEC 62271-1, 2017 edition, clause 6.16.3). The standard adds that some local regulations may require less, for example 0.1% per year.
How long must a sealed pressure system last?
The manufacturer must specify an expected operating life of at least 20 years. The other preferred values are 30 and 40 years (clause 6.16.4). Vacuum interrupters are examples of sealed pressure systems.
Can SF6 be used in a controlled pressure system?
No. Clause 6.16.2 states that SF6 and its mixtures are not applicable to controlled pressure systems, which mainly use compressed air, such as pneumatic operating mechanisms.