About transformers

Rated Values

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
  • t= 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




Non-shielded transformer against short circuit EN61558-2-1, EN61558-2-17
Short circuit resistant transformer EN61558-2-1, EN61558-2-17
Safely crashing transformer EN61558-2-1, EN61558-2-17
Protective isolation transformer protected against short circuit EN61558-2-4, EN61558-2-17
Short circuit resistant isolation transformer EN61558-2-4, EN61558-2-17
Protective voltage transformer EN61558-2-6, EN61558-2-17
Non-protected short-circuit protection transformer EN61558-2-6, EN61558-2-17
Short-circuit proof voltage transformer EN61558-2-6, EN61558-2-17
Safely extending protective voltage transformer EN61558-2-6, EN61558-2-17
Medical Isolation Transformer EN-61558-2-15
Shortcut to unprotected control transformer EN-61558-2-2
Short circuit-proof control transformer EN-61558-2-2
Short circuit protection against unsecured savings transformer EN-61558-2-13
Short circuit-proof saving transformer EN-61558-2-13
Choke with no overload protection EN-61558-2-20
Choke with overload protection EN-61558-2-20
Adjustment Isolation Transformer IEC60044-1, IEC61558-2-14
Variable transformer IEC60044-1, IEC61558-2-14
Short-circuit proof bell transformer

Enclosures for electrical equipment, ie IP classes

An IP class is a two-digit code that describes the ability of an electrical device’s outer shell to protect;

  • environmental hazards inside the device (voltage, device moving parts)
  • the sensitive parts of the device from dirt, dust or water that penetrates the environment

The IP class is a standardized classification for the safety of electrical equipment and applies to all electrical appliances such as transformers, electric motors, electrical cabinets and lighting fixtures.

First ID number

Contact properties and barrier properties (occupational safety) of protection class

0 Open, unprotected structure
1 Entry of at least 50 mm in size blocked, including inadvertent access to body parts such as hands
2 Finger guarding, blocked at least 12 mm in length, blocking up to 80 mm in diameter and up to 12 mm in diameter for dangerous parts is prevented
3 Entry of at least 2.5 mm in size such as spikes, screws and machining waste is prevented
4 Entry of at least 1 mm in size such as threads, ribbons, and machining waste is prevented
5 The entry of dust harmful to the operation of the device is prevented
6 Dust penetration is prevented

Second identification number

Protection from water

0 Open, unprotected structure
1 Dripping water in vertical droplets does not hurt
2 Dripping water in vertical droplets does not cause harm, even though the casing tilts up to 15º in an arbitrary direction from the normal position
3 Water at an angle of up to 60 ° does not cause any damage
4 Water splashing from all directions does not cause any harm
5 A water wave or a powerful jet of water directed from all directions by the nozzle does not cause any harm
6 A powerful jet of water directed from all directions by the nozzle does not cause any harm
7 Water is not penetrated into the enclosure by water in the short term
8 The device can be kept immersed in water according to the manufacturer’s instructions, so that there is no harmful intrusion into the enclosure

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