Transformer: An electrical device fitted with at least two windings. It converts electrical energy, transmitted as alternating current, from one voltage to another voltage of the same frequency.

Separating transformer: A transformer in which the primary and secondary windings are galvanically isolated by means of basic insulation.

Isolating transformer: A transformer equipped with protective separation between primary and secondary windings.

Safety isolating transformer: An isolating transformer intended for feeding circuits of safety extra-low voltage (SELV) or protective extra-low voltage (PELV) voltage.

Autotransformer: A transformer with one winding, common for primary and secondary sides. The windings are not galvanically isolated from each other.

ELV (extra low voltage): A low voltage which does not exceed an appropriate level (band I according to IEC 60449).

SELV (safety extra low voltage): A low voltage not exceeding 50 V AC or 120 V smoothed DC, measured between the conductors or between any conductors and earth. The circuit must be separated from power supply sources by means of safety transformer, taking into consideration the following notes:

  • In some cases a lower voltage than 50V AC or 120V smoothed DC may be required when a direct contact with active parts is admissible.
  • The limit voltage value should not be exceeded under any load conditions (from full load to no-load) if a safety transformer is used as the source.
  • smoothed (without ripples) DC voltage containing an AC sinusoidal component not exceeding 10% of the DC component, if the peak value of voltage does not exceed 140V for rated smoothed DC voltage of 120V, and 70V for rated smoothed DC voltage of 60V.

SELV circuit: An extra low voltage (ELV) circuit with a protective separation from other circuits and having neither parts for earthing the circuit nor uncovered conductive elements.

PELV circuit:
An extra low voltage (ELV) circuit with a protective separation from other circuits, which for the functional reasons is equipped with parts for circuit earthing and/or uncovered conductive parts.

FELV circuit: An extra low voltage (ELV) circuit which, for its proper function, does not fully meet the requirements applicable to SELV and PELV circuits.

Working insulation: The insulation of the active part of an electrical circuit, necessary to ensure proper function of the electrical device and providing simultaneously protection against electric shock.

Basic insulation: The insulation of active part of an electrical circuit, applied in order to ensure protection against electric shock.

Supplementary insulation: An independent insulation applied in addition to the basic insulation in order to ensure protection in case the basic insulation is damaged.

Double insulation: Insulation consisting of basic and supplementary insulations.

Reinforced insulation: The insulating system of active parts of an electrical circuit, which provides the same degree of protection against electric shock as double insulation.

Class I transformer: A transformer protecting against electric shock by means of basic insulation and a means of earthing exposed conductive parts. Such a solution ensures that even in the case where basic insulation fails, accessible conductive parts will not present a risk of electric shock.

Class II transformer: A transformer protecting against electric shock not only by means of basic insulation, but also by application of double or reinforced insulation. There is no means to earth provided.

Class III transformer: A transformer protecting against electric shock because it is supplied with safety extra low voltage SELV where voltages do not exceed 50 V AC. Transformers belonging to class III cannot be equipped with earth terminal.

Note: Classes I, II and III do not refer to the insulation system between primary and secondary windings.

Protective separation: The separation of the circuits by means of basic insulation and additional protection.
(By application of basic insulation and additional insulation or protective screening. Also an equivalent protective measure, e.g. reinforced insulation, may be applied).

Protective (electric) screen: A conductive layer placed between primary and secondary circuits.

Protective (electric) screening: The separation of electric circuits and/or conductors from dangerous active parts by placing between them a protective screen which is connected to the external protective earth lead.

Enclosure: The part of transformer protecting it against the influence of external factors and against direct contact from any direction.

System of transformer connections:
The various methods of winding interconnection in three-phase transformers are identified systematically with an alpha-numeric string. Letters represent the method of connection while the number represents the phase shift of the secondary voltage relative to the primary voltage, expressed in hours, where one hour is equal to an angle of 30 degrees. The first letter is a capital letter, which relates to the higher voltage winding, where Y=star, D=delta and N=neutral lead. The second letter is a small letter, which relates to the lower voltage winding, where y=star, d=delta, z=zigzag and n=neutral lead. The number is placed at the end. So Dy5 specifies a transformer with a delta primary and a star secondary, where the secondary voltage lags the primary voltage by 150 degrees.

Transformer short-circuit voltage: The voltage required on the primary winding in order to induce rated current on the shorted secondary winding. Short-circuit voltage is usually expressed as a percentage of rated supply voltage. Short-circuit voltage can range from 2 to 20 percent depending on the construction of windings. It is possible to obtain other values of the short-circuit voltage in special versions.

IP Code (International Protection, as per IEC 60529 ) is determined in the scale consisted of several degrees and it classifies the degrees of protection provided against the penetration of solid objects and water to electrical enclosures. The code describing the degree of protection consists of two digits. The first digit indicates the level of protection that the enclosure provides against the penetration of solid foreign objects as well as the degree of user protection against the direct contact with hazardous parts of device. The second digit of IP code indicates the degree of device protection against water penetration inside the enclosure. Additional letter indicates the degree of user protection against the direct contact with hazardous parts.

Types of operation – there are the following types of operation:
- continuous operation (S1) - operation for unlimited time, without breaks,
- short-time operation (S2) - operation for a determined time, started when the transformer is in a cold state,
and the breaks between the operation periods are long enough to cool the transformer down to the temperature close to the ambient temperature.
- discontinuous operation (S3) - operation with predefined breaks, where equal periods of operation are separated by equal periods of break.

Insulation class: Identifies the temperature rating of an insulating material used, where a letter indicates the maximal working temperature of the transformer. Exceeding this temperature during continuous operation would shorten the service life of transformer and the period of its failure-free operation.

Cooling: The type of cooling is identified with a letter code, depending on the cooling medium and the way it is circulated. A four letter code is defined according to EN 60076. The first letter indicates the medium used for cooling of the windings, the second letter indicates the way in which the cooling medium is put into motion, the third letter indicates an external cooling medium and the fourth letter indicates the way in which the external cooling medium is put into motion. In case of dry-type transformers without enclosure or with ventilated enclosure the code consists of just two letters.

Climatic versions: Depending on the climatic zone in which the product will be operated, it should be produced of appropriate materials. Such information may be given on rating plate in the form of alpha-numeric symbol defined in relevant standard.