IEC TS 63346-2-1:2026

Low-voltage auxiliary power systems - Part 2-1: Design criteria - General requirements

IEC TS 63346-2-1:2026 establishes the general principles for the design of low-voltage auxiliary power systems (APS) with nominal voltages below or equal to 1 kV AC and 1,5 kV DC, and nominal frequency up to and including 60 Hz, so as to provide safe and correct operation for the intended use.
This document applies to the design of low-voltage auxiliary systems for
- substations, which are part of an electrical system, confined to a given area, mainly including ends of transmission or distribution lines, electrical switchgear and controlgear, buildings and transformers,
- converter stations,
- hydropower stations, where the gravitational energy of water is converted into electricity, and
- thermal power stations, where the thermal energy is obtained by burning fossil fuels.

General Information

• Status: Published
• Publication Date: 28-Jun-2026
• ICS: 29.240 - Power transmission and distribution networks
• Technical Committee: PC 127 - Low-voltage auxiliary power systems for electric power stations and substations
• Drafting Committee: WG 3 - PC 127/WG 3
• Current Stage: PPUB - Publication issued
• Start Date: 29-Jun-2026
• Completion Date: 10-Jul-2026

Overview

IEC TS 63346-2-1:2026 is a technical specification developed by the International Electrotechnical Commission (IEC). It outlines the essential design criteria and general requirements for low-voltage auxiliary power systems (APS), crucial for the safe and reliable operation of substations, converter stations, hydropower, and thermal power stations. The standard applies to auxiliary power systems operating at nominal voltages up to 1 kV AC or 1.5 kV DC and frequencies up to 60 Hz.

By providing a structured framework for APS design, IEC TS 63346-2-1:2026 facilitates consistent electrical safety, operational reliability, and adaptability for future modifications across power infrastructure globally.

Key Topics

• Design Principles

  • Safety: The APS must protect personnel, equipment, and the overall system from risks such as electric shock, overvoltage, and thermal effects.
  • Reliability: The system should assure uninterrupted functioning, considering the criticality and importance of facility operations.
  • Flexibility: The design should accommodate future modifications and expansions without compromising performance or safety.

• System Configuration

  • AC Systems: Covers supply sources (transformers, generators, converters), distribution boards, load circuits, and earthing methods.
  • DC Systems: Includes energy storage (batteries), chargers, transfer switches, distribution panels, monitoring, and protection devices.

• Safety and Protection

  • Requirements for preventing electric shock, thermal hazards, overcurrents, and voltage or electromagnetic disturbances according to the IEC 60364 series.
  • Emphasis on arc-fault mitigation, explosion protection, and fire safety-including specialized considerations for battery gases.

• Load Classification

  • Differentiates critical/essential loads (e.g., control centers, emergency systems) from non-essential loads, influencing backup and redundancy arrangements.

• SCADA and Monitoring

  • Integration of measurement, digitization, remote control, and online monitoring to ensure effective management and rapid response to abnormalities.

Applications

IEC TS 63346-2-1:2026 has broad application across various segments of the power industry:

• Substations: Ensures reliable auxiliary power for operations, protection, and control systems at the nodes of transmission or distribution networks.
• Converter Stations: Provides dependable low-voltage support for power conversion, control, and monitoring equipment.
• Hydropower Stations: Supports auxiliary loads, from control gear to monitoring instrumentation, crucial for water-to-energy processes.
• Thermal Power Stations: Addresses the auxiliary power needs for fuel handling, safety interlocks, and emergency systems.

By aligning with IEC 63346-2-1, organizations can reduce downtime, safeguard personnel, and comply with international best practices, ultimately ensuring grid stability and operational excellence.

Related Standards

IEC TS 63346-2-1:2026 references and aligns with several foundational IEC standards, including:

• IEC 60038: Standard voltages for electrical systems.
• IEC 60079-0: Equipment requirements for explosive atmospheres.
• IEC 60364 series: Comprehensive guidelines for low-voltage electrical installations.
  • Part 1, 4-42, 4-43, 4-44, 5-51, 5-52, 5-53 for protection, equipment, and wiring.
• IEC 60909-0: Calculation of short-circuit currents in AC systems.
• IEC 61439-1/2: Rules for switchgear and controlgear assemblies.
• IEC TR 61641: Handling internal arc faults in low-voltage assemblies.
• IEC 61643-12: Selection and application of surge protective devices.
• IEC 61660-1: Calculation of short-circuit currents in DC auxiliary installations.
• IEC 61936-1: Power installations exceeding certain voltage thresholds.
• IEC TS 63346-1-1: Terminology for low-voltage auxiliary power systems.

Practical Value

Implementing IEC TS 63346-2-1:2026 ensures:

• Consistency and quality in APS design across global projects.
• Enhanced personnel safety and protection of critical infrastructure.
• Improved system resilience, reducing costly outages and operational risks.
• Compliance with current international best practices in low-voltage auxiliary power systems.

Organizations adopting this standard position themselves for future-proof, scalable, and robust infrastructure supporting the evolving needs of the energy sector.

Standards Content (Sample)

IEC TS 63346-2-1 ®
Edition 1.0 2026-06
TECHNICAL
SPECIFICATION
Low-voltage auxiliary power systems -
Part 2-1: Design criteria - General requirements
ICS 29.240  ISBN 978-2-8327-1303-7

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CONTENTS
FOREWORD . 2
1 Scope . 4
2 Normative references . 4
3 Terms and definitions . 5
4 Basic requirements . 5
4.1 Design principles . 5
4.1.1 Safety . 5
4.1.2 Reliability. 6
4.1.3 Future modification and expansion . 6
4.2 Determination of operational conditions and external influences for design . 6
4.2.1 Operational conditions . 6
4.2.2 External influences . 6
4.3 System configuration . 6
4.3.1 General . 6
4.3.2 AC systems . 7
4.3.3 DC systems . 8
5 Safety and protection requirements . 9
5.1 General . 9
5.2 Transient and temporary overvoltage . 9
5.3 Protection against voltage disturbances and electromagnetic disturbances . 10
5.4 Thermal effects . 10
5.5 Internal arc faults . 10
5.6 Protection against explosion . 11
6 System design . 11
6.1 AC system . 11
6.1.1 Voltage classes . 11
6.1.2 Types of system earthing . 11
6.1.3 Protection against overcurrent . 11
6.1.4 System schemes . 11
6.1.5 Loads . 12
6.2 DC system . 12
6.2.1 Voltage classes . 12
6.2.2 Types of system earthing . 12
6.2.3 Protection against overcurrent . 13
6.2.4 System schemes . 13
6.2.5 Loads . 14
7 SCADA, measurement and online monitoring . 14
7.1 Measurement and SCADA . 14
7.2 Online monitoring . 14
Bibliography . 15

Figure 1 – High-level block diagram of typical AC system configurations. 11
Figure 2 – High-level block diagram of typical DC system configurations . 13

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
Low-voltage auxiliary power systems -
Part 2-1: Design criteria - General requirements

FOREWORD
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8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
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9) IEC draws attention to the possibility that the implementation of this document may involve the use of (a)
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shall not be held responsible for identifying any or all such patent rights.
IEC TS 63346-2-1 has been prepared by IEC project committee 127: Low-voltage auxiliary
power systems for electric power stations and substations. It is a Technical Specification.
The text of this Technical Specification is based on the following documents:
Draft Report on voting
127/87/DTS 127/88/RVDTS
Full information on the voting for its approval can be found in the report on voting indicated in
the above table.
The language used for the development of this Technical Specification is English.
This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in
accordance with ISO/IEC Directives, Part 1 and ISO/IEC Directives, IEC Supplement, available
at www.iec.ch/members_experts/refdocs. The main document types developed by IEC are
described in greater detail at www.iec.ch/publications.
A list of all parts in the IEC 63346 series, published under the general title Low-voltage auxiliary
power systems, can be found on the IEC website.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under webstore.iec.ch in the data related to the
specific document. At this date, the document will be
– reconfirmed,
– withdrawn, or
– revised.
1 Scope
This part of IEC 63346 establishes the general principles for the design of low-voltage auxiliary
power systems (APS) with nominal voltages below or equal to 1 kV AC and 1,5 kV DC, and
nominal frequency up to and including 60 Hz, so as to provide safe and correct operation for
the intended use.
This document applies to the design of low-voltage auxiliary systems for
– substations, which are part of an electrical system, confined to a given area, mainly including
ends of transmission or distribution lines, electrical switchgear and controlgear, buildings
and transformers,
– converter stations,
– hydropower stations, where the gravitational energy of water is converted into electricity,
and
– thermal power stations, where the thermal energy is obtained by burning fossil fuels.
This document does not apply to the design of low-voltage auxiliary power systems of the
following:
– nuclear power stations, which have extremely high safety requirements and special loads,
such as nuclear measurement systems and protection systems;
– substations connecting a nuclear power station to the grid and its associated LV APS
integrated with the nuclear power station;
– traction substation, which have different power supply requirements, such as unbalanced
load power supply;
– offshore substations, for which it is important to take into account factors such as waves,
typhoons and salt spray, which have different requirements for power supply and equipment
selection.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies.
For undated references, the latest edition of the referenced document (including any
amendments) applies.
IEC 60038:2009, IEC standard voltages
IEC 60079-0, Explosive atmospheres - Part 0: Equipment - General requirements
IEC 60364 (all parts), Low-voltage electrical installations
IEC 60364-1:2025, Low-voltage electrical installations - Part 1: Fundamental principles,
assessment of general characteristics and definitions
IEC 60364-4-42, Low-voltage electrical installations - Part 4-42: Protection for safety -
Protection against thermal effects
IEC 60364-4-43, Low-voltage electrical installations - Part 4-43: Protection for safety -
Protection against overcurrent
IEC 60364-4-44, Low-voltage electrical installations - Part 4-44: Protection for safety -
Protection against voltage disturbances and electromagnetic disturbances
IEC 60364-5-51:2005, Electrical installations of buildings - Part 5-51: Selection and erection of
electrical equipment - Common rules
IEC 60364-5-52, Low-voltage electrical installations - Part 5-52: Selection and erection of
electrical equipment - Wiring systems
IEC 60364-5-53, Low-voltage electrical installations - Part 5-53: Selection and erection of
electrical equipment - Devices for protection for safety, isolation, switching, control and
monitoring
IEC 60909-0, Short-circuit currents in three-phase a.c. systems - Part 0: Calculation of currents
IEC 60947-1, Low-voltage switchgear and controlgear - Part 1: General rules
IEC 60947-2, Low-voltage switchgear and controlgear - Part 2: Circuit-breakers
IEC 61439-1, Low-voltage switchgear and controlgear assemblies - Part 1: General rules
IEC 61439-2, Low-voltage switchgear and controlgear assemblies - Part 2: Power switchgear
and controlgear assemblies
IEC TR 61641, Enclosed low-voltage switchgear and controlgear assemblies - Guide for testing
under conditions of arcing due to internal fault
IEC 61643-12, Low-voltage surge protective devices - Part 12: Surge protective devices
connected to low-voltage power systems - Selection and application principles
IEC 61660-1, Short-circuit currents in d.c. auxiliary installations in power plants and substations
- Part 1: Calculation of short-circuit currents
IEC 61936-1, Power installations exceeding 1 kV AC and 1,5 kV DC - Part 1: AC
IEC TS 63346-1-1, Low-voltage auxiliary power systems - Part 1-1: Terminology
3 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC TS 63346-1-1 apply.
ISO and IEC maintain terminology databases for use in standardization at the following
addresses:
– IEC Electropedia: available at https://www.electropedia.org/
– ISO Online browsing platform: available at https://www.iso.org/obp
4 Basic requirements
4.1 Design principles
4.1.1 Safety
Safety is the first principle used for the design of APS. The design shall meet safety
requirements in the following aspects: personnel safety, equipment safety and system safety.
The APS shall be designed to protect personnel from injury in the process of the operation and
the maintenance of equipment and system, as well as prevention of unsafe operation and
maintenance by personnel that can result in equipment and system damage or cascading
failures.
4.1.2 Reliability
Reliability is the most important characteristic of the operation of the APS. The reliability
requirements of APS should consider coordinating with the criticality of the facility.
4.1.3 Future modification and expansion
The installation should maintain a degree of flexibility of the design to facilitate implementation
of future modification and expansion.
4.2 Determination of operational conditions and external influences for design
4.2.1 Operational conditions
The operational conditions of APS shall be in accordance with IEC 60364-5-51:2005, 512.1,
and IEC 61936-1.
4.2.2 External influences
The external influences of APS shall be in accordance with IEC 60364-5-51:2005, 512.2, and
IEC 61936-1. The selection of the type of external influences affecting the design of the APS
shall be dependent on the specific conditions of the application and IEC 60364-5-51:2005,
Table 51A, which presents the list of classes of the external influences.
4.3 System configuration
4.3.1 General
The following requirements of the APS configuration shall be met.
a) The number and characteristics of the power sources shall meet the demand of the loads in
the APS. External supplies or temporary sources shall be configured additionally for higher
reliability requirements.
b) The selection of the types, sizes and redundancy levels of the electrical equipment shall be
in accordance with the safety and reliability of the station and IEC 61936-1.
c) APS configuration is a measure of the performance of the design, and should also consider
and maintain a degree of flexibility of the design to facilitate implementation of future
modification and expansion.
d) System earthing of the APS shall be in accordance with IEC 60364-1:2025.
e) Conductors, including earthing conductors, live conductors and neutral conductors, shall
satisfy the current-carrying capacity, voltage drop and other minimum size requirements for
the APS. The nominal cross-sectional area of conductors shall be no less than the values
given in IEC 60364-5-52. When required, flame retardant cables can be used.
f) Low-voltage switchgear and controlgear assemblies in APS shall be in accordance with
IEC 61439-2;
g) Low-voltage switchgear and controlgear in APS shall be in accordance with IEC 60947-1.
Low-voltage circuit-breake
...