Every rating on an equipment nameplate is valid under stated conditions. A switchboard’s rated current assumes a certain air temperature around it. A switchgear panel’s insulation level assumes a certain air density. A transformer’s temperature-rise limits assume a certain cooling-air temperature, day and year. When the real site is hotter, higher, wetter, dustier or more corrosive than those assumptions, a product that is technically sound can become unsuitable for that location, sometimes without anyone noticing until it is in service.
This article answers the question: which site and service conditions must be defined for electrical equipment, how does each one affect selection and rating, and how should they be stated so that manufacturers design for the real site?
Key takeaways
- Each IEC equipment standard defines its own normal service conditions. They are similar in places but not identical: for example, the normal altitude limit is 1 000 m for MV switchgear and liquid-immersed transformers, and 2 000 m for LV assemblies.
- If a site falls outside the normal service conditions of the relevant standard, it is a special service condition. It must be stated in the specification, so the manufacturer can derate, correct or choose a suitable design.
- Temperature and altitude mainly affect thermal ratings and insulation. Humidity, condensation and pollution mainly affect insulation surfaces and corrosion. Seismic and vibration data affect the mechanical design.
- Enclosure protection (IP code, IEC 60529) and ventilation work in opposite directions: closing an enclosure against dust and water also restricts cooling.
- Operating duty and maintenance access are service conditions too. A product sized for normal continuous duty may not suit frequent switching or cyclic overloads, and a design that needs rear access is unsuitable where there is no rear clearance.
- State conditions as measured or project-agreed values with their source, not as “to suit site”.
Why site conditions change ratings
Electrical equipment is designed and tested to carry current without overheating, to withstand voltage without flashover, and to keep working mechanically for its service life. The site affects all three:
- Heat. Rated currents are limited by temperature rise. If the surrounding air is hotter, less temperature rise is available before materials reach their limits, so the equipment can carry less current, or needs better cooling.
- Insulation. Air insulates less well when its density is lower (at altitude) or when insulating surfaces are wet or contaminated. Clearances and creepage distances that are adequate at sea level in clean air may not be adequate elsewhere.
- Materials and mechanics. Humidity, salt, chemicals and dust attack metals, coatings and seals. Low temperatures affect lubricants, mechanisms and electronics. Earthquakes and vibration load the structure and its fixings.
Standards make this manageable by defining a set of normal conditions under which the declared ratings apply. The specification then only needs to state where the site differs, but it must state that clearly.
Normal service conditions: defined per standard, not universal
There is no single “standard operating condition” for all electrical equipment. Each equipment standard sets its own, and the differences matter when one site uses equipment from several families.
| Equipment family | Standard | Normal ambient temperature | Normal altitude | Other conditions defined |
|---|---|---|---|---|
| MV switchgear and controlgear, indoor | IEC 62271-1 | Maximum 40 °C; 24 h average not above 35 °C; minimum −5 °C | Up to 1 000 m | Average relative humidity over 24 h not above 95% and over one month not above 90%; under these conditions condensation may occasionally occur; air not significantly polluted |
| MV switchgear and controlgear, outdoor | IEC 62271-1 | Maximum 40 °C; minimum −25 °C | Up to 1 000 m | Additional outdoor conditions such as solar radiation, wind and ice are defined in the standard |
| LV switchgear and controlgear assemblies, indoor | IEC 61439-1 | Maximum 40 °C; 24 h average not above 35 °C; minimum −5 °C | Up to 2 000 m | Relative humidity not above 50% at 40 °C (higher at lower temperatures, for example 90% at 20 °C); pollution degree 3 for industrial use unless otherwise stated |
| Liquid-immersed power transformers | IEC 60076-1 | Cooling air not above 40 °C at any time, not above 30 °C as the hottest-month average and not above 20 °C as the yearly average | Up to 1 000 m | Three separate temperature limits; each must be checked |
| Reciprocating generating sets | ISO 8528-1 | Rating reference conditions: 25 °C, 100 kPa, 30% relative humidity | (expressed through barometric pressure) | These are reference conditions for declaring power, not site limits: the manufacturer declares the output for the actual site conditions |
Summary for orientation only. The equipment standard and edition applied to the project govern; read the full service-condition clauses there.
Two points stand out:
- The same site can be “normal” for one item and “special” for another. A site at 1 500 m is within the normal altitude for an LV assembly but above it for MV switchgear and a liquid-immersed transformer.
- Averages matter as much as maxima. A transformer site with a 38 °C maximum may still exceed the 30 °C hottest-month average, and switchgear and LV assemblies both have a 24 h average limit. Specifications that give only the maximum temperature leave this unanswered.
The conditions to define, and what each one changes
Ambient temperature
State the maximum, the averages the relevant standards use (24 h average; hottest-month and yearly averages for transformers) and the minimum. For indoor equipment, the relevant temperature is the air in the room or enclosure around the equipment, not the outdoor weather. That air is often hotter than outdoors, because of solar gain on the building and the heat lost by the equipment itself.
What it changes:
- Current ratings. Above the normal ambient, rated currents must be reduced or the equipment designed with more thermal margin. For LV assemblies, the circuit ratings declared by the assembly manufacturer depend on the temperature inside the enclosure.
- Transformers. For liquid-immersed transformers, IEC 60076-2 reduces the permitted temperature rises when the cooling-air temperature exceeds the normal values, so the transformer is designed for the site rather than relabelled afterwards.
- Low temperature. Very low minimum temperatures affect mechanisms, lubricants, insulating materials, liquids and electronics. State the minimum, including during storage and transport if it is lower.
Altitude
State the site altitude above sea level.
What it changes:
- Insulation. Air density falls with altitude, and so does the dielectric strength of air. For MV switchgear above 1 000 m, IEC 62271-1 applies altitude correction to the insulation level. In practice this can mean a higher insulation level, larger clearances, or a different design.
- Cooling. Thinner air removes heat less effectively. Transformer temperature-rise limits are corrected for altitude above 1 000 m under IEC 60076-2.
- Engines. Generating-set output at altitude and high temperature is declared by the manufacturer for the site conditions, as described in Sizing a Diesel Generator.
Humidity and condensation
State typical and maximum relative humidity, and whether rapid temperature changes occur, for example cool nights after hot days, or air conditioning that cycles on and off.
What it changes:
- Condensation on insulating surfaces, especially when combined with dust or salt, leads to surface tracking and partial discharge. It also corrodes metal parts.
- Typical measures are anti-condensation heaters, controlled ventilation, sealed or conditioned rooms, and designs intended for high humidity. For indoor MV switchgear, IEC TS 62271-304 defines design classes for use under severe condensation and pollution. For dry-type transformers, IEC 60076-11 defines environmental classes that reflect condensation and pollution levels, alongside climatic and fire-behaviour classes.
Pollution and corrosion
Describe the source and type of pollution: coastal salt, industrial emissions, desert dust, chemical vapours, agricultural dust, or conductive dust from processes such as mining or metal working.
What it changes:
- Outdoor insulation. For outdoor high-voltage insulators, IEC TS 60815-1 describes how to assess site pollution severity and assigns SPS classes from “very light” to “very heavy”, with a further “extremely heavy” class added in the current edition. The class determines the creepage distance required.
- LV assemblies. IEC 61439-1 uses pollution degrees for clearances and creepage inside assemblies. Pollution degree 3 is the normal assumption for industrial use.
- Coatings and materials. Atmospheric corrosivity categories (ISO 9223, and ISO 12944-2 for protective paint systems) help define enclosure materials and coating systems. The category and the expected durability should be agreed explicitly.
Enclosure protection: IP and IK codes
State the degree of protection needed for each location, with the reason: dust, water, splashing, cleaning with water jets, or access by unskilled persons.
- The IP code (IEC 60529) describes protection against access to hazardous parts, against solid foreign objects and against water, as verified by the standard’s tests.
- The IK code (IEC 62262) describes protection against external mechanical impact.
What it changes: a higher IP code usually means a more closed enclosure, which reduces natural ventilation. The equipment may then need thermal derating, larger enclosures or forced cooling. Specify the IP code the location actually needs, not the highest available.
Installation environment and ventilation
State whether the equipment is indoor, outdoor, or inside a package enclosure, and describe the room or enclosure: size, ventilation method, air conditioning, exposure to direct sun, and other heat sources nearby.
What it changes:
- The heat lost by transformers, switchboards, drives and UPS units must be removed from the room. If ventilation is not designed for it, the room temperature rises above the value the equipment was rated for.
- In a compact / package substation, the transformer works inside an enclosure that adds to its temperature rise. IEC 62271-202 addresses this through the enclosure’s declared class, and the transformer and enclosure ratings must be considered together. See Compact / Package Substations.
Seismic and other mechanical conditions
State the seismic requirement from the project’s structural or seismic design basis, and any vibration or shock from nearby machinery or the mounting structure.
What it changes: the structure, fixings and internal supports of the equipment, and in some cases the choice of design. For metal-enclosed and solid-insulation MV switchgear, seismic qualification is covered by IEC 62271-207 (its current edition includes MV equipment previously covered by IEC TS 62271-210). For other equipment, the qualification method should be agreed with the manufacturer. The seismic level itself must come from the project’s design data, not from the equipment supplier.
Operating duty
Service conditions include how the equipment will be used:
- Load profile: continuous, cyclic, or with planned overloads.
- Switching duty: how often circuits are switched, for example capacitor banks, motors or frequent transfer operations.
- Supply quality: harmonic content, voltage unbalance, expected voltage variation.
- Role: normal supply or standby. For generating sets, the rating category in ISO 8528-1 (for example emergency standby or prime power) describes the duty, as explained in Sizing a Diesel Generator.
What it changes: thermal sizing, mechanical and electrical endurance class, harmonic losses in transformers, and the choice of switching device.
Maintenance access
Describe the space available (front only, or front and rear), lifting and handling routes, and the maintenance policy: who will maintain the equipment, how often, and with which tools and skills.
What it changes: withdrawable versus fixed designs, front-access versus rear-access assemblies, the need for special tools, and the recommended spare parts. A design that needs rear access is unsuitable for a room without rear clearance, however good its ratings are.
Transport and storage
If equipment will be stored before installation, or will travel through hot, humid or cold regions, state the storage period and conditions and any known transport conditions. IEC 60721-3-1 (storage) and IEC 60721-3-2 (transportation and handling) offer a common vocabulary for these conditions. Packing and preservation are chosen to suit them. See International Electrical Equipment Sourcing.
Describing conditions with a common vocabulary
Two IEC frameworks help describe conditions consistently:
- IEC 60721-3 classifies environmental conditions into groups of parameters and severities for different situations, for example storage, transportation, stationary use at weather-protected locations (IEC 60721-3-3) and stationary use at non-weather-protected locations (IEC 60721-3-4). It is useful for describing a site systematically.
- IEC 60364-5-51 classifies external influences on LV installations with codes, for example AA for ambient temperature, AC for altitude, AD for water, AE for solid foreign bodies and AF for corrosive or polluting substances. Installation designers use it to select equipment for each location.
These frameworks complement, but do not replace, the service-condition clauses of each equipment standard. The equipment standard defines the conditions under which the ratings apply. The classification describes the site. The specification links the two.
Practical examples: suitable products in the wrong conditions
The following scenarios are illustrative. They show how a technically sound product can become unsuitable when site conditions are ignored. No product data are used, and the outcome in each case depends on the manufacturer’s assessment.
1. An LV switchboard in an unventilated room. A switchboard is selected with circuit ratings declared for the normal service conditions of IEC 61439-1. It is installed in a small, prefabricated electrical room in a hot climate, with no ventilation design. In summer the air inside reaches well above 40 °C. The circuits can no longer carry their declared currents without exceeding temperature limits. What should have been stated: the expected room temperature, or the ventilation design, so that the assembly manufacturer could declare circuit ratings for that temperature or the room could be designed to keep within the normal range.
2. MV switchgear at high altitude. Switchgear with a standard insulation level for its rated voltage is ordered for a mine site at an altitude well above 1 000 m. The altitude is not mentioned in the specification. At that altitude, the insulation needs correction under IEC 62271-1. What should have been stated: the site altitude, so the manufacturer could confirm the corrected insulation level or offer a suitable design.
3. A transformer at a hot inland site. A liquid-immersed transformer is specified as “to IEC 60076-1” with the maximum site temperature given as 40 °C. The hottest-month average at the site is above 30 °C and the yearly average is above 20 °C. The transformer, designed for normal conditions, ages faster than expected. What should have been stated: all three temperature values, so the temperature-rise limits could be adjusted under IEC 60076-2 at the design stage. Sizing is covered in Sizing a Distribution Transformer.
4. Indoor switchgear near the coast. A ring main unit designed for normal indoor conditions is installed in a coastal substation building with daily condensation and salt-laden air entering through the ventilation openings. Corrosion and surface tracking follow. What should have been stated: the condensation and pollution conditions, so that a suitable design class, heaters, filtered ventilation or a conditioned room could be considered.
5. A sealed enclosure for a dusty site. To keep out desert dust, an outdoor LV enclosure is specified with a high IP code. Natural ventilation is reduced, and the internal temperature rises. What should have been stated: the dust condition and the ambient temperature together, so that the enclosure design, derating or cooling could be balanced against ingress protection.
6. A generating set rated at reference conditions. A generating set is selected from its output at the ISO 8528-1 reference conditions and installed at a hot site at altitude. The available output is lower than expected. What should have been stated: site temperature, altitude and humidity, so that the manufacturer could declare output at site conditions.
Framework: a site and service conditions schedule
A simple schedule, attached to every equipment specification for the project, keeps conditions consistent across all packages.
| Condition | What to state | Typical source of data | Mainly affects |
|---|---|---|---|
| Ambient temperature | Maximum, 24 h average, monthly and yearly averages where relevant, minimum; indoor room temperature where different | Meteorological data for the site; building services design | Current ratings, transformer temperature rise, low-temperature performance |
| Altitude | Metres above sea level | Site survey | Insulation level, cooling, engine output |
| Humidity and condensation | Typical and maximum relative humidity; occurrence of condensation | Meteorological data; experience from existing installations | Insulation surfaces, corrosion, need for heaters or conditioning |
| Pollution | Type and source; severity class where assessed | Site assessment; nearby industry or coast; existing installations | Creepage distances, enclosure design, coatings |
| Corrosivity | Corrosivity category where assessed; required coating durability | Site assessment | Enclosure materials and coating systems |
| Solar radiation, wind, ice, rain | Exposure for outdoor equipment | Meteorological data | Outdoor enclosures, temperature rise |
| Degree of protection | IP and, where relevant, IK code per location, with the reason | Project design | Enclosure design and thermal performance |
| Installation environment | Indoor / outdoor / package enclosure; room ventilation and cooling | Building and substation design | Effective ambient for the equipment |
| Seismic and vibration | Seismic requirement and qualification method; vibration sources | Project structural and seismic design basis | Structure, fixings, qualification evidence |
| Operating duty | Load profile, switching frequency, harmonics, standby or continuous role | Electrical design and operating philosophy | Thermal sizing, endurance, device choice |
| Maintenance access | Front / rear access; lifting routes; maintenance policy | Layout drawings; operator | Construction type, spares, tools |
| Storage and transport | Expected storage period and conditions; transport route conditions | Project programme; logistics plan | Packing and preservation |
Where a condition is unknown, say so and state the assumption made. An honest “assumed: normal service conditions to IEC 62271-1, to be confirmed” is more useful than a blank, because it can be reviewed and corrected.
Relevant standards
| Standard | What it covers | Why it matters |
|---|---|---|
| IEC 62271-1 | Common specifications for AC high-voltage switchgear and controlgear, including normal and special service conditions and altitude correction | Defines when MV switchgear ratings apply and how altitude affects insulation |
| IEC TS 62271-304 | Design classes for indoor enclosed MV switchgear used under severe condensation and pollution | A way to specify switchgear for humid or polluted indoor locations |
| IEC 62271-207 | Seismic qualification of gas-insulated, metal-enclosed and solid-insulation enclosed switchgear | Qualification route for MV switchgear at seismic sites |
| IEC 62271-202 | High-voltage / low-voltage prefabricated substations | Relates transformer performance to the enclosure in package substations |
| IEC 61439-1 | LV switchgear and controlgear assemblies: general rules, including service conditions and pollution degree | Defines when LV assembly ratings apply |
| IEC 60076-1 | Power transformers: general requirements, including normal service conditions | Defines the temperature and altitude limits for transformer ratings |
| IEC 60076-2 | Temperature rise of liquid-immersed transformers | Explains how temperature-rise limits are adjusted for high ambient and altitude |
| IEC 60076-11 | Dry-type transformers, including environmental, climatic and fire-behaviour classes | The way to specify dry-type transformers for humid, polluted or cold locations |
| ISO 8528-1 | Reciprocating internal combustion engine driven generating sets: application, ratings and performance | Defines rating reference conditions and duty categories |
| IEC 60529 | Degrees of protection provided by enclosures (IP code) | Specifies protection against access, solid objects and water |
| IEC 62262 | Degrees of protection against external mechanical impacts (IK code) | Specifies impact protection for exposed enclosures |
| IEC TS 60815-1 | Selection and dimensioning of high-voltage insulators for polluted conditions | Site pollution severity classes for outdoor insulation |
| IEC 60721-3 series | Classification of environmental conditions for storage, transport and use | A common vocabulary for describing site, storage and transport conditions |
| IEC 60364-5-51 | Selection and erection of electrical equipment: common rules, including the classification of external influences | Installation-level classification of external influences |
| ISO 9223 and ISO 12944-2 | Atmospheric corrosivity and its classification for protective paint systems | A basis for agreeing materials and coatings |
National rules and project-specific seismic or climatic design data apply in addition, for the destination concerned.
Common mistakes
- Writing “to suit site conditions” without giving the conditions.
- Giving only the maximum temperature, without the averages that switchgear, LV assemblies and transformers each use.
- Assuming the same normal conditions apply to every equipment family on the site.
- Using outdoor weather data for indoor equipment without allowing for room temperature and heat from the equipment.
- Forgetting altitude, especially when it is between 1 000 m and 2 000 m and therefore “normal” for some items only.
- Specifying the highest IP code everywhere, without considering ventilation and derating.
- Omitting condensation, because the average humidity looks moderate.
- Leaving the seismic level for the supplier to choose instead of stating the project’s design basis.
- Ignoring storage conditions when equipment will wait on site before installation.
Conclusion
Site and service conditions are part of the equipment’s rating, not background information. Each IEC equipment standard defines the normal conditions under which its ratings apply, and those conditions differ between standards. A clear specification states the real site values (temperature with its averages, altitude, humidity and condensation, pollution and corrosivity, enclosure protection, installation environment and ventilation, seismic data, operating duty, maintenance access, storage and transport) so that each manufacturer can confirm whether its design is suitable, or how it must be adapted. A shared conditions schedule keeps every package on the same basis, and an honestly stated assumption is always better than a blank.
Related AdepaTrade solutions
AdepaTrade sources MV switchgear, transformers, compact / package substations and LV equipment to project specifications, including site and service conditions, from Türkiye-based and international manufacturers. Compact / Package Substations · Transformers
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