IEC 61558-2-4:2021

Overview

IEC 61558-2-4:2021 is the International Electrotechnical Commission (IEC) safety standard for isolating transformers and power supply units (PSUs) incorporating isolating transformers for general applications. Edition 3.0 (2021) is a technical revision that must be used together with IEC 61558-1:2017. It applies to air‑cooled dry‑type transformers (stationary or portable), single‑ or polyphase, including units that incorporate electronic circuits and linear PSUs. The standard has group safety publication status (IEC Guide 104).

Key scope facts:

• Supply voltages up to ~1 100 V and frequencies up to 500 Hz.
• Rated outputs typically up to 25 kVA (single‑phase) and 40 kVA (polyphase) unless otherwise agreed.
• Output voltages range from SELV thresholds (around 50 V AC / 120 V DC) up to several hundred volts (special applications up to ~1 000 V AC).

Key topics and technical requirements

IEC 61558-2-4 specifies particular requirements and tests that supplement Part 1. Major technical areas include:

• Protection against electric shock (SELV and related requirements).
• Insulation and dielectric tests: insulation resistance, dielectric strength, leakage current.
• Creepage, clearance and distance through insulation requirements for different material groups.
• Heating and thermal performance: temperature rise tests and thermal protection considerations.
• Short‑circuit and overload protection: design and test criteria for resilience and protective devices.
• Mechanical strength and construction: including internal wiring, components, terminals and earthing provisions.
• Ingress protection and environmental resistance: dust/moisture resistance, corrosion/rusting.
• Marking, ratings and information: identification, symbols (including a new symbol for linearly regulated outputs), and user instructions.
• Special provisions for transformers incorporating electronic circuits; for switch‑mode PSUs, IEC 61558‑2‑16 applies in conjunction with this part.

Applications and who uses this standard

IEC 61558-2-4 is used by:

• Transformer and PSU manufacturers - to design safe isolating transformers and document compliance.
• Test laboratories and certification bodies - to perform conformity testing and issue safety approvals.
• Design and safety engineers - for integrating isolating transformers into equipment and ensuring SELV/control circuit safety.
• Procurement and compliance teams - to specify requirements for purchased transformers and PSUs. Practical uses include product design validation, safety testing, conformity assessment for market access, and documentation of electrical safety measures in control and supply systems.

Related standards

• IEC 61558-1:2017 (general requirements) - to be used in conjunction with this part.
• IEC 61558-2-16 - for switch‑mode power supply units (together with this part).
• IEC 61140, IEC 60364-4-41, IEC 60204-1 - referenced for protective measures such as SELV and control-circuit safety.

Keywords: IEC 61558-2-4:2021, isolating transformers, power supply units, transformer safety standard, electrical safety tests, creepage clearance, dielectric strength, SELV, IEC 61558-1.