C3, E3, F1: what the climatic, environmental and fire classes of a cast-resin transformer mean

A cast-resin transformer has no oil to keep out moisture and no tank to hide in. Its resin has to survive cold storage, condensation, dust and – in the worst case – fire, on its own. IEC 60076-11 therefore classifies dry-type transformers in three families of classes. PNACHO-D units are rated C3 / E3 / F1, the highest of each.

Climatic class C

Class	Operation	Transport and storage
C1	not below −5 °C	down to −25 °C
C2	down to −25 °C	down to −25 °C
C3	down to −50 °C	down to −50 °C

The test: the transformer is cooled to the class temperature, held for hours, then energised and loaded – the resin must not crack. C3 is relevant not only in Scandinavia; a transformer that arrives on a construction site in January and sits unpowered for a month sees similar stress.

Environmental class E

Class	Condensation	Pollution	Typical location
E0	none	negligible	clean, air-conditioned rooms
E1	occasional	light	normal indoor substations
E2	frequent	heavy	industrial halls, outdoor enclosures
E3	frequent, heavy	heavy, conductive	coastal, chemical, cement, mining sites

Class E3 (introduced in the 2018 edition) adds a salt-fog test. It is the right choice wherever the transformer stands in an enclosure outdoors or in a dusty industrial environment.

Fire behaviour class F

• F0: no special fire precautions.
• F1: the transformer is subjected to a defined fire test (alcohol burner plus radiant panel); the resin must be self-extinguishing, flames must go out within a set time, and smoke must be of limited opacity and low toxicity. Exposure to an external fire must not cause the transformer itself to become a significant fuel source.
• F2: a special agreement between manufacturer and buyer for even stricter requirements.

F1 is what makes a cast-resin transformer acceptable inside occupied buildings without a fire-rated cell. The resin used in PNACHO-D windings contains quartz filler and aluminium hydroxide, which releases water when heated and suppresses combustion.

Other things to specify on a dry-type unit

• Insulation class F (155 °C) with a temperature rise of 100 K – standard for cast resin.
• Partial discharge ≤ 10 pC at 1.1 × Um – measured in the routine test; the best indicator of winding quality.
• Temperature monitoring: PT100 sensors in each LV winding with a monitoring relay (standard on PNACHO-D), so alarm and trip levels are supervised.
• Enclosure: IP00 (open) as standard; IP21, IP23 or IP44 enclosures on request. Every enclosure reduces cooling – specify AF fans or a rating reserve.

Every unit in the PNACHO-D range is C3 / E3 / F1 with the monitoring relay included; the class certificates are available with the technical documentation on request.