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
Symbol |
Description |
Standard |
 |
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 |
EN61558-2-8 |
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 |