E2 C2 F1 certified cast resin transformers

Why are the cast resin transformers preferred for instalations inside the buildings all over the world?

The answer is very easy. They are considered safe and reliable, because of their fire behaviour and absence of toxic gas emission in case of fire. Cast resin ransformers are not subject to explosion of flammable liquid, oil leak or frequent maintanance works.

In order to be considered compliant with the current IEC rules they must be designed and produced meeting climatic class C2, environmental class E2 and fire behaviour class F1.

In order to prove their C2/E2/F1 capabilities a test must be carried out at an independent, certified and qualified testing laboratory.

All tests must be performed on the same transformer in the correct order, starting with C2 thermal shock test, passing to E2 condensation and humidity penetration test. At the end one complete column of the transformer is subject to the F1 fire behaviour test inside the test chamber reaching temperature of almost 800 Celsius degrees.

Only by passing all three tests and receiving an approving certification the manufacturer may declare to be compliant with IEC guaranteeing the classes E2 C2 and F1.

We invite you to pay attention, when purchasing your cast resin transformers and always ask for the official certification of an external laboratory, which confirms the tests required by the current IEC 60076-11 standard.

Make sure that the tests are actually done as prescribed by the regulations.

The two fully qualified laboratories used by the majority of resin transformer manufacturers are CESI / Italy and KEMA / Holland.

If your newly acquired transformer has a certificate from one of these laboratories for classes E2 C2 F1 you can rest assured.

In the case of other laboratories, carefully check all the declarations indicated and make sure that the laboratory is qualified and certified for these tests.

We remind you that all three tests must be performed on the same transformer and in the same laboratory or associated laboratory.

To facilitate the verification, we have briefly summarized all E/C/F classes and the procedures of all three tests mentioned above.

Environmental class (E)

E0 – There is no condensation and no pollution on the transformer Surface. Indoor installation in a dry and clean room

E1 – This is a condensation and a humidity penetration test. The humidity level of the room is higher than 93%. Water conductivity must be in range between 0.1m/s up to 0.3 m/s

E2 – This is a condensation and a humidity penetration test. The humidity level of the room is higher than 90% +/- 5%. Water conductivity must be in range between 0.5m/s up to 1.5 m/s.

Climatic class (C)

C1 – Indoor Installation. The transformer operates in the ambient temperature not lower than -5 C degrees. The shipping and installation temperature may reach -25 C degrees.

C2 – Outdoor installation. The transformer can operate, be shipped and installed at -25 C degrees.

Fire behaviour class (F)

F0 – No fire risk at the installation site. No reduced infiammability is required.

F1 – Transformers subject to fire risk. A reduced Infiammability is mandatory required.

E2- C2- F1 Test procedure (short description)


This test is a condensation & a humidity penetration test.

The transformer should be placed in a testing chamber with adjustable temperature and humidity.

The chamber capacity should be at least five times the volume of the retangular box circumscribing the transformer.

The spacing between each part of the transformer and the walls, vault and nozzles of the atomizers should be not less than the smallest phase-to-phase distance between live parts of the transformer and not less than 150 mm.

The air temperature in the testing chamber should be such as to ensure condensation on the transformer. The humidity in the chamber should be kept above 93%.

The water conductivity should be 0.5 S /m to 1.5 S /m.

The transformer should be dry before starting the humidity penetration test.

It should be in the climatic chamber for 144 hours in an unused state.

The climate chamber must be kept at a temperature (50 ± 3) ° C and relative humidity (90 ± 5).

At the end of this period, and at the latest after 3 hours in normal ambient conditions, the transformer should be tested with the supplied voltage and the test with induced voltage, but with voltages reduced to 80% of the standard values.

During the electrical strength tests no spark jump or puncture should occur, and the visual inspection should not show any serious traces.


This test has to prove that the transformer is suitable for operation, shipping and storage at very low temperatur up to -25 C degrees.

The test should be carried out on a complete transformer, but without a housing, if provided. The transformer should be placed in the test chamber.

The ambient temperature in the chamber should be measured at least in three places located 0.1 m from the external surface and halfway up the tested object. The average of the read values should be taken as the reference temperature.

The following test procedure must be performer on the transformer:

a) The temperature in the testing chamber should be gradually reduced to (-25 ± 3) ° C within 8 hours, and then kept for at least 12 hours until the steady state is reached.

b) The thermal shock is perfomed applying a current equal to twice the rated current of the tested winding (in monolithic insulation). The current should be maintained until the winding under test reaches the average temperature corresponding to the average increase in the winding temperature, plus 40 ° C (maximum ambient temperature under normal operating conditions). The average temperature reached by the windings should be determined based on the change in resistance. It is recommended to obtain thermal shock with one of the following methods:

• Test with DC supply

• Test with AC supply

• AC test, alternative method

c) After a thermal shock, the transformer should return to the temperature (25 ± 10) ° C

At least 12 hours after the end of the thermal shock test, the transformer should be tested for electrical strength according to the winding insulation level, but at voltages reduced to 80% of standard values.

In addition, in the case of transformers with monolithic insulation windings, the measurement of partial discharges must be carried out with certain considerations.

There should be no visible defects such as gaps or cracks on the windings.


This test has to prove that the transformer is auto-estingushing in case of open fire source and no toxic gas emission i sproduced during the fire process.

The test should be carried out on one complete transformer phase, consisting of MV and LV windings, core columns and insulation elements, without housing, if any. The core column can be replaced with material of similar dimensions and thermal conductivity as the original column. The yoke should not be taken into account, and the low voltage winding drains should be cut off at the upper and lower ends of the coil.

In the case of a standard transformer, the outer diameter of the cylindrical windings tested or the maximum diagonal of non-cylindrical windings should be between 400mm and 500mm.

• Sample validity:

The validity of the results of a flammability test carried out on a transformer can be extended to other transformers based on the same design assumptions, such as:

  • the same construction concept (permanent insulation windings, winding type, protection class)
  • the same winding temperature increases
  • the same main insulation materials


• Test chamber

The test chamber should be modeled on the chamber described in IEC 60332-3-10. The walls should be made of heat-resistant steel with a thickness of 1.5mm to 2mm, thermally insulated so that the heat transfer is about 0.7 W / (m2K). If possible, it is recommended to equipe the chamber with a fireproof window. The chamber dimensions are described in the IEC 60076-11 standard.

• Ignition source

The main source of heat is ethyl alcohol (caloric content 27MJ / kg) burning in a container that can be divided by concentric rings.

The initial alcohol level in the container should be (30 ± 1) mm, which corresponds to approximately 20 minutes of burning time.

The second heat source is a vertically positioned flat electric radiator, built of heating resistors with a total power of 24 kW with an adjustable power source to maintain the radiator temperature around 750 ° C.

• Temperatures

The following temperatures must be measured using thermocouples or replacement devices:

  • Air inlet
  • Air outlet
  • Upper surface of LV winding (optional)
  • Upper surface of MV winding (optional)
  • Core limbs (optional)
  • Channel between core and winding (optional)
  • Channel between LV and MV windings (optional)


  • Light transmission in the measuring section
  • Inlet air flow rate
  • Chimney gas flow rate

• Scaling

The chamber should be scaled for at least 40 min after switching on the heater to a constant power of 24 kW. Under established conditions, the inlet air flow rate shall be 0.21 m3 / s ± 15% at 20 ° C.

• Sample methods

The test object should be set in the test chamber in consideration of the following requirements:

  • The distance between the radiator and the outer surface of the winding should be approximately 175 mm.
  • The initial alcohol level in the container should be approximately 40 mm below the transformer windings
  • The semi-cylindrical metal screen should be placed opposite the radiator, concentrically with the tested object
  • At the beginning of the test, the temperature in the test chamber, inlet air and the tested object should be in the range from 15 ° C to 30 ° C.
  • Fill the container with alcohol immediately before starting the test (in practice up to 5 minutes before)

The test begins with the moment the alcohol is lit and the 24kW heater is turned on.

The heater should be turned off after 40 minutes.

The temperature and other factors should be recorded for at least 60 minutes from the start of the test or throughout its duration.

For further information do not hesitate to get in touch with our office at office@powerfullstop.com

Your Power Team