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Transformer rated values 

Transformers are equipped with a plaque containing all the information needed for transformer selection under normal circumstances.

  • Upri = nominal primary voltage, or the supply main voltage  (V)
  • Usec = nominal secondary voltage with nominal secondary current (V)
  • Sn = transformer's nominal apparent power on the secondary side (VA) or
  • Isec = nominal secondary current (A)
  • f = transformer's nominal frequency (Hz)

In atypical situations, the following are also required

  • protection class if other than class I (I = protective grounding, II = double insulation, III = extra-low voltage)
  • casing class if other than IP00
  • ta = ambient temperature if other than 25ºC
  • intermittent usage percentage if other than 100 %
  • transformer symbol according to standard

When a transformer is ordered, its mechanical requirements as well as possible short-circuit protection and overload protection must be determined.

Transformer symbols




Suojaamaton muuntaja Transformer without short-circuit protection

EN61558-2-1, EN61558-2-17

Oikosululta suojattu muuntaja  Transformer with short-circuit protection

EN61558-2-1, EN61558-2-17

 Turvallisesti vioittuva muuntaja   Fail safe transformer

EN61558-2-1, EN61558-2-17

 Suojaamaton suojaerotusmuuntaja Isolation transformer without short-circuit protection

EN61558-2-4, EN61558-2-17

 Oikosulun kestävä suojaerotusmuuntaja Isolation transformer with short-circuit protection EN61558-2-4, EN61558-2-17
 Suojajännitemuuntaja Safety voltage transformer EN61558-2-6, EN61558-2-17
 Oikosulkua vastaan suojaamatonSuojajännitemuuntaja Safety voltage transformer without short-circuit protection EN61558-2-6, EN61558-2-17
 Oikosulun kestävä suojajännitemuuntaja Safety voltage transformer with short-circuit protection EN61558-2-6, EN61558-2-17
 Turvallisesti vioittuva suojajännitemuuntaja Fail safe safety voltage transformer EN61558-2-6, EN61558-2-17
 Lääkintäsuojaerotusmuuntaja Medical isolation transformer EN-61558-2-15
Oikosululta suojaamaton ohjausmuuntaja  Control transformer without short-circuit protection EN-61558-2-2
Oikosulun kestävä ohjausmuuntaja Control transformer with short-circuit protection EN-61558-2-2
Oikosululta suojaamaton säästömuuntaja   Autotransformer without short-circuit protection EN-61558-2-13
 Oikosululun kestävä säästömuuntaja  Autotransformer with short-circuit protection  EN-61558-2-13
    Choke without overload protection EN-61558-2-20
  Choke with overload protection  EN-61558-2-20
 Säätöerotusmuuntaja Adjustable isolation transformer  IEC60044-1, IEC61558-2-14
 Säätömuuntaja piirrosmerkki  Säätömuuntaja IEC60044-1, IEC61558-2-14
    Bell transformer with short-circuit protection

Electrical device casing classes or IP-classes

IP-class is a two-digit code that describes an electrical device's external casing's ability to protect;

  • the environment from hazards inside the device (voltage, moving parts)
  • the device's sensitive parts from dirt, dust or water from the environment

The IP-class is a standardised security classification for electrical devices, and it applies to all electrical devices, such as transformers, electric motors, switch cabinets and lighting.

First identifying number

Casing class touch characteristics and foreign matter characteristics (industrial safety)


Open, unprotected structure


Penetration of min. 50 mm objects prevented, unintentional access of body parts, such as hands, prevented


Finger protection, penetration of min. 12 mm objects prevented, reaching of max. 80 mm long and max. 12 mm in diameter parts into hazardous areas prevented


Penetration of min. 2.5 mm objects, such as sticks, screws and process waste, prevented


Penetration of min. 1 mm objects, such as string, ribbon and process waste, prevented


Penetration of dust detrimental to the operation of the device prevented


Penetration of dust prevented

Second identifying number

Protection from water


Open, unprotected structure


No harm from vertical water drops


No harm from vertical water drops even case is tilted up to 15º to any direction from the normal position


No harm from water spray in max. 60° angle


No harm from water spray in any angle


No harm from water wave or strong nozzle-controlled water spray from all directions


No harm from strong nozzle-controlled water spray from all directions


Harmful amount of water will not penetrate the case when held under water for a short time


The device can be held under water according to the manufacturer's instructions without harmful amount of water penetrating the case

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