IEC TS 62271-304 classifies the indoor environment of 1 to 52 kV switchgear by condensation (Co, CL, CH) and pollution (PL, PH), and defines three design classes: 0 (normal service), 1 and 2. Classes 1 and 2 are demonstrated with an aging test in a climatic chamber followed by a dielectric diagnostic.
Short answer
Indoor medium-voltage switchgear is designed for the normal service of IEC 62271-1. When the electrical room has frequent condensation or dust and deposits on the insulation, the technical specification IEC TS 62271-304 (2008 edition) defines design classes that demonstrate, with a test, that the switchgear withstands those conditions. It applies to indoor enclosed switchgear per IEC 62271-200 and IEC 62271-201, from 1 kV up to 52 kV. IEC 62271-1, 2017 edition, points to it in its clause 4.2.3 for indoor equipment up to 52 kV when pollution of the insulation is a concern.
What the test covers and what it does not
The specification applies to equipment in which any part of the insulation is exposed to the indoor climate. The test was designed to study the behavior of the electrical insulation and not corrosion. The performance of mechanisms, interlocks and enclosures can also be recorded, but that result is complementary. The standard notes that the procedures can also be applied to the internal insulation of outdoor equipment.
Condensation: Co, CL and CH
Condensation appears because of rapid temperature changes. Clause 3 of the specification classifies it as follows:
| Designation | Condensation | Where it applies |
|---|---|---|
| Co | Does not normally occur (no more than twice a year) | Locations with humidity or temperature control that prevents condensation; the building protects from daily variations of the outdoor climate |
| CL | Infrequent (no more than twice a month) | Locations without humidity or temperature control; the building protects from daily variations, but condensation cannot be excluded |
| CH | Frequent (more than twice a month) | Locations without temperature control; the building gives only minimal protection against daily variations, and frequent condensation may occur |
Pollution: PL and PH
- PL, light pollution: that of normal service. Reaching it in heavily polluted places may need precautions.
- PH, heavy pollution: any value higher than PL. It does not include areas with conductive dust or industrial smoke that form thick, conductive deposits. In PH there are no special precautions to reduce deposits, or the equipment is close to pollution sources.
The specification clarifies that a total absence of pollution is considered unrealistic: the starting point is at least light pollution. It also states that choosing a suitable enclosure protection degree helps reduce deposits inside the equipment.
Severity grades and design classes
Since switchgear suffers mostly from the combination of humidity and pollution, the standard defines three severity grades:
| PL | PH | |
|---|---|---|
| Co | Grade 0 | Grade 1 |
| CL | Grade 1 | Grade 2 |
| CH | Grade 2 | Grade 2 |
Three design classes of the switchgear correspond to those grades: 0, 1 and 2. Grade 0 is the normal service described in IEC 62271-1.
How each class is demonstrated
- Class 0. Requires no additional test. Standard switchgear that meets IEC 62271-200 or IEC 62271-201 is considered class 0.
- Class 1. Demonstrated with the level 1 aging test and the diagnostic procedure.
- Class 2. Demonstrated with the level 2 test, the same as level 1 but with more cycles, and the same diagnostic.
The test is done in a climatic chamber, on a complete functional unit, new and clean, with its instrument transformers. Temperature is controlled within ±3 K and varies cyclically between 30 °C and 50 °C, with relative humidity from below 80% to above 95%.
Level 1 consists of three periods of seven days. In each period, for five days, the switchgear is energized at its rated voltage and receives 60 humid-heat cycles of 2 hours. It is then stopped, inspected for the onset of conductive paths (tracking) on the insulating surfaces, and rests for two days in a climate close to the reference one. Level 2 is seven periods of seven days, and can be continued from a passed level 1 with four more periods.
The reference conditions for indoor equipment, including humidity, are in the normal service conditions for switchgear, and the cases beyond them in the special service conditions. The switchgear must also meet the IP and IK protection degrees against water and foreign objects.
The map of standards for primary equipment testing places this part among the others of IEC 62271.
Diagnostic and pass criterion
After aging, and without cleaning or drying the equipment, a dry power-frequency withstand voltage test of 1 minute is made at the rated value. A voltage test under high humidity is then repeated, phase by phase. The degree of tracking is recorded in the report. A mechanical diagnostic is optional.
The switchgear meets class 1 or 2 if there is no disruptive discharge during the cycles of the aging test or during the diagnostic. If the mechanical diagnostic is chosen, the functions must be within the manufacturer’s tolerances.
- Is there humidity or temperature control that prevents condensation?
- Yes: Is the pollution that of normal service (PL)?
- Yes: Class 0: standard switchgear, no additional test
- No: Class 1 as a minimum (Co with PH)
- No: Is the pollution that of normal service (PL)?
- Yes: Class 1 if condensation is infrequent (CL); class 2 if frequent (CH)
- No: Class 2 (CL or CH with PH)
- Yes: Is the pollution that of normal service (PL)?
What not to do
- Do not request class 2 without checking the room: it corresponds to the combinations CL with PH, CH with PL and CH with PH.
- Do not treat the design class as a substitute for conditioning the room: it demonstrates the switchgear, it does not change the environment.
- Do not mix PL with the «very light» classification of IEC 62271-1, 2017 edition. The 2008 specification uses its own terms.
- Do not take corrosion resistance as proven: the test was designed to investigate the behavior of the electrical insulation.
When to call us
At Tiza Group we review the electrical room environment against the classes of the standard and verify the switchgear insulation at commissioning. See our testing and commissioning of medium-voltage switchgear service, in Colombia or elsewhere on request. Write to us and we will quote it.
Related guides
- Normal service conditions for switchgear
- Special service conditions for switchgear
- IP and IK protection degrees of switchgear
- Medium-voltage switchgear: parts, functions and tests
- ABB UniGear ZS1, ZS2, UniGear 550 and UniSec switchgear
References
- IEC TS 62271-304, High-voltage switchgear and controlgear - Part 304: Design classes for indoor enclosed switchgear and controlgear for rated voltages above 1 kV up to and including 52 kV to be used in severe climatic conditions, 2008 edition: clauses 1, 3, 4, 5, 6, 8 and 9.
- IEC 62271-1, 2017 edition: clause 4.2.3.
Frequently asked questions
What is the difference between the Co, CL and CH conditions?
Co: condensation does not normally occur (no more than twice a year). CL: infrequent condensation (no more than twice a month). CH: frequent condensation (more than twice a month). They are defined in clause 3 of IEC TS 62271-304, 2008 edition.
Which design class does switchgear need in a room without temperature control?
It depends on the combination with pollution. With light pollution (PL), a room without temperature control and with frequent condensation (CH) corresponds to severity grade 2 and therefore to design class 2. The standard associates CH with locations without temperature control.
Does standard IEC 62271-200 switchgear already meet any of these classes?
Yes, design class 0. IEC TS 62271-304 states that standard switchgear that meets IEC 62271-200 or IEC 62271-201 is considered class 0 and requires no additional tests.