UNE-EN IEC 61558-2-4:2026

Replacement

This document of the IEC 61558 series deals with the safety of isolating transformers for

general applications and power supply units incorporating isolating transformers for

general applications. Transformers incorporating electronic circuits are also covered by this

standard.

NOTE 1 Safety includes electrical, thermal and mechanical aspects.

Unless otherwise specified, from here onward, the term transformer covers isolating

transformers for general applications and power supply units incorporating isolating

transformers for general applications.

For power supply units (linear) IEC 61558-2-4 is applicable. For switch mode power

supply units, IEC 61558-2-16 is applicable.

IEC 61558-2-4 is applicable to stationary or portable, single-phase or polyphase, air-cooled

(natural or forced) independent or associated dry- type transformers. The windings may be

encapsulated or non-encapsulated.

The rated supply voltage does not exceed 1 000 V AC, and the rated supply frequency and

the internal operating frequencies do not exceed 500 Hz.

The rated output does not exceed:

25 kVA for single-phase transformers;

40 kVA for polyphase transformers.

IEC 61558-2-4 is applicable to isolating transformers without limitation of the rated output

subject to an agreement between the purchaser and the manufacturer.

NOTE 2 Transformers intended to supply distribution networks are not included in the scope.

The no-load output voltage or the rated output voltage does exceed 50 V AC or 120 V

ripple-free DC, and where applicable, does not exceed 500 V AC or 708 V ripple-free DC.

The no-load output voltage and the rated output voltage may be up to 1 000 V AC or 1 415

V ripple-free DC for special applications.

IEC 61558-2-4 is not applicable to external circuits and their components intended to be

connected to the input terminals and output terminals of the transformers.

NOTE 3 Transformers covered by IEC 61558-2-4 are used in applications where double or reinforced

insulation between circuits is required by the installation rules or by the end product standard.

Attention is drawn to the following:

- for transformers intended to be used in vehicles, on board ships, and aircraft,

additional requirements (from other applicable standards, national rules, etc.) may be

necessary;

- measures to protect the enclosure and the components inside the enclosure against

external influences such as fungus, vermin, termites, solar-radiation, and icing should also be

considered;

- the different conditions for transportation, storage, and operation of the transformers

should also be considered;

- additional requirements in accordance with other appropriate standards and national

rules may be applicable to transformers intended for use in special environments.

Future technological development of transformers may necessitate a need to increase the

upper limit of the frequencies, until then IEC 61558-2-4 may be used as a guidance

document.

This group safety publication is primarily intended to be used as a product safety standard for

the products mentioned in the scope, but shall also be used by technical committees in the

preparation of standards for products similar to those mentioned in the scope of this standard,

in accordance with the principles laid down in IEC Guide 104 and ISO/IEC Guide 51.

One of the responsibilities of a technical committee is, wherever applicable, to make use of

basic safety publications and/or group safety publications in the preparation of its

publications.