Overview
IEC 61558-2-2:2022 - "Safety of transformers, reactors, power supply units and combinations thereof - Part 2-2" - defines particular safety requirements and tests for control transformers and power supply units incorporating control transformers. This third edition (2022) replaces the 2007 edition, constitutes a technical revision and is published as a group safety publication (IEC Guide 104). It applies mainly to dry-type, air-cooled control transformers (including those with electronic circuits) used for control and safety extra-low-voltage (SELV) applications.
Key topics and requirements
The standard supplements IEC 61558-1:2017 and covers practical safety and test topics, including:
- Scope and classification - applicability to stationary/portable, single- or poly-phase devices; dry-type constructions; limits on voltages and frequencies.
- Rated limits - rated supply voltage ≤ 1 000 V AC and rated/internal frequency ≤ 500 Hz; no‑load or rated output voltage ≤ 1 000 V AC (or 1 415 V ripple‑free DC). Typical rated thermal output limits: 25 kVA (single-phase) and 40 kVA (polyphase), unless otherwise agreed.
- Electrical safety - protection against electric shock, insulation, creepage/clearance distances, dielectric strength, leakage current.
- Performance tests - output voltage/current under load, no-load voltage, short‑circuit voltage, short‑circuit and overload protection.
- Thermal and mechanical tests - heating, mechanical strength, resistance to fire/tracking and rusting.
- Construction and components - internal wiring, terminals, earthing provisions, components selection and marking.
- Ingress protection - protection against dust, solid objects and moisture.
- Specific symbols and markings - introduction of a new general symbol for control transformers and a new symbol for power supplies with linearly regulated outputs.
- Coordination with switch‑mode supplies - linear power supply units are covered here; switch‑mode power supplies are addressed by IEC 61558-2-16 in conjunction with this part - where requirements conflict, the more stringent applies.
Applications and who uses this standard
IEC 61558-2-2:2022 is used by:
- Transformer and power-supply manufacturers designing and marking control transformers and linear control PSUs.
- Product design and safety engineers ensuring compliance with electrical, thermal and mechanical safety requirements.
- Test laboratories and certification bodies performing safety and type tests.
- Procurement and compliance teams specifying safe control transformers for industrial control panels, machine tools, building automation, and SELV control circuits.
- Electrical installers and maintenance teams referencing safe limits and markings.
Related standards
This standard is essential for ensuring the safety, marking, testing and correct selection of control transformers and associated power supply units in industrial and control applications.