With the contacts open and voltage applied, a vacuum interrupter can emit X-rays. IEC 62271-1 (2017) sets the limit at 5 µSv/h at 1 m at rated voltage and requires it to be measured on a test bench with the contacts locked at the minimum distance. It is not the vacuum integrity test.
Short answer
A vacuum interrupter with open contacts and a high voltage across them can emit X-rays. IEC 62271-1, 2017 edition, deals with that emission in two clauses: 6.20 requires interrupters to be designed to meet the acceptance criteria, and 7.11 defines the test. The central criterion is short: no more than 5 µSv/h at 1 m at rated voltage.
Do not confuse this test with the vacuum integrity test. That one verifies that the interrupter is still sealed, and this one measures the radiation emitted by a new interrupter under voltage.
Who it is required of and when
Clause 7.11.1.1 asks for the test on new vacuum interrupters. There is no requirement to test the complete switchgear when the interrupter type has already been successfully tested as a component. In practice, the interrupter report is the one that backs several switchgear and circuit-breaker manufacturers.
How the test is prepared
The interrupter is mounted on a test bench designed to set the open contact gap to the minimum distance it will have in the switchgear. Interrupters that operate in an insulating medium other than air, such as oil or SF₆, may be tested in that medium if needed to withstand the voltage. The vessel of the medium must be made of an insulating material whose radiation attenuation is not greater than that of 9.5 mm of polymethyl methacrylate. Between the interrupter and the instrument there must be only the minimum medium needed for insulation (clause 7.11.1.2).
The measuring instrument
A radiation measuring instrument shielded against radio frequency is used, with these minimum characteristics (7.11.1.3):
- Measuring range from 5 µSv/h to 150 µSv/h, with an accuracy of ±25% over the whole range.
- Response time not exceeding 15 s.
- Energy response of at least 25 keV to 0.5 MeV.
The sensing element is placed in the plane of the separable contacts and pointing at them. The preferred distance between the instrument and the wall of the interrupter is 1 m. Any distance up to 15 m is also accepted, in which case the reading is adjusted with the inverse square law of distance (7.11.1.4).
Procedure and criterion
With the interrupter on the bench, the contacts locked open at the minimum distance and the instrument in place, a voltage equal to the rated voltage Ur of the switchgear is applied across the contacts. After at least 15 s the radiation reading is taken. Then the voltage is raised to the short-duration power-frequency withstand voltage Ud of the insulation level tables and, after another 15 s at least, the reading is recorded in the report (clause 7.11.2).
- New interrupter on the bench. Contacts locked at the minimum distance
- Instrument at 1 m. In the plane of the contacts, pointing at them
- Rated voltage Ur. At least 15 s; reading up to 5 µSv/h at 1 m
- Withstand voltage Ud. At least 15 s; reading recorded in the report
- Report. Declared value if above 150 µSv/h
| Switchgear condition | Applied voltage | Limit at 1 m |
|---|---|---|
| Any | Rated voltage Ur | 5 µSv/h |
| Ud up to and including 160 kV | Withstand voltage Ud | 150 µSv/h |
| Ud greater than 160 kV | Withstand voltage Ud | Measured; if above 150 µSv/h, the manufacturer declares the actual value |
The 2017 edition changed this criterion from the previous one to recognize interrupters of higher rated voltage: the acceptance criteria were modified for them, according to the list of changes of the standard itself.
What it means for those who test in the field
The note to clause 7.11.3 gives a practical hint. The value declared by the manufacturer can be used to arrange a safe working environment, according to local regulations. It applies when a power-frequency withstand voltage test is made across the open contacts of an interrupter. And the note to clause 6.22 reminds that, when testing switchgear with vacuum devices, precautions are taken so that the possible emission during the high-voltage test stays within safe limits, and that national standards may influence those measures.
This is not for improvising. Before applying voltage across open contacts:
- Ask the manufacturer for the emission information of the interrupter model.
- Define a distance and a staff dwell time consistent with that information and with local regulations.
- Apply the voltage only to the interrupter under test, with the rest of the equipment cleared.
To learn how the field test is done, see the guide on the vacuum integrity test, and for the dielectric verification of the circuit breaker, the on-site withstand voltage test.
The X-ray test is a type test, and the complete map of these tests is in type tests of switchgear. The tightness check made in the factory is summarized in the routine tests of switchgear. Further detail on the electromagnetic compatibility tests for switchgear: electromagnetic compatibility of switchgear.
To see how this fits with the rest of the IEC 62271 series, see the standards for primary equipment testing.
Relation to vacuum integrity
The standard does not ask for specific tightness tests for interrupters, because their hermeticity is verified during manufacture and they are considered to have zero leakage during their life. When a product standard asks for a tightness test as a condition check (for example, after a mechanical test), vacuum integrity is checked with the condition-check dielectric test of clause 7.2.12 (clause 7.8.4 b). That test is made at 80% of the rated power-frequency withstand voltage for equipment up to 245 kV. It has another purpose: here the aim is to know whether the interrupter keeps its vacuum, not how much radiation it emits.
What not to do
- Do not use the vacuum integrity report as an X-ray test, or the other way round.
- Do not apply voltage across open contacts without knowing the declared emission of the interrupter.
- Do not measure at a distance other than 1 m without correcting the reading.
- Do not take a switchgear assembly as proven by an interrupter of another model: the test is per interrupter type.
When to call us
At Tiza Group we test vacuum circuit breakers on site, with the values and precautions of the manufacturer of each interrupter, and we review the type test reports that back your equipment. This field work is done in Colombia, or elsewhere on request. See our circuit breaker testing service and write to us for a quote.
Related guides
- Type tests of switchgear: what is tested
- Routine tests of switchgear in the factory
- Vacuum integrity test: what it detects in circuit breakers
- Medium-voltage vacuum circuit breaker: function and tests
- Circuit breaker interrupting chamber: vacuum, SF6 and air
- Electromagnetic Compatibility of Switchgear (IEC 62271-1)
References
- IEC 62271-1, High-voltage switchgear and controlgear - Part 1: Common specifications for alternating current switchgear and controlgear, 2017 edition, clauses 6.20, 6.22, 7.2.12, 7.8.4 and 7.11 (7.11.1 to 7.11.3).
Frequently asked questions
What X-ray limit does the standard set for a vacuum interrupter?
The emission must not exceed 5 µSv/h at a distance of 1 m at rated voltage Ur. At the power-frequency withstand voltage Ud, the limit is 150 µSv/h at 1 m for switchgear with Ud up to and including 160 kV. Above 160 kV it is measured, and if it exceeds 150 µSv/h, the manufacturer declares the actual value (IEC 62271-1, 2017 edition, clause 7.11.3).
Is the X-ray test made on the finished switchgear?
It is not necessary. The test is made on new interrupters, on a bench, and the standard does not require testing the switchgear if the interrupter type has already passed the test as a component (clause 7.11.1.1).
What should I do when applying voltage to an interrupter in the field?
Keep in mind that it may emit X-rays. The standard states that the value declared by the manufacturer serves to arrange a safe working environment according to local regulations when making a withstand voltage test across open contacts (note to clause 7.11.3). Follow the manufacturer's instructions and the radiation protection rules of your country.