IEC 61225:2025

Nuclear power plants - Instrumentation, control and electrical power systems - Requirements for static uninterruptible DC and AC power supply systems

IEC 61225:2025 specifies the performance and the functional characteristics of the low voltage static uninterruptible power supply (SUPS) systems in a nuclear power plant (NPP) and, when applicable, in nuclear facilities in general. An uninterruptible power supply (UPS) is an electrical equipment which draws electrical energy from a source, stores it, and maintains the supply in a specified form by means inside the equipment to output terminals. A SUPS has no rotating parts to perform its functions. The specific design requirements for the components of the power supply system are covered by IEC standards and other standards listed in the normative references. Otherwise, specific component-level design requirements are outside the scope of this document.

Centrales nucléaire de puissance - Systèmes d'instrumentation, de contrôle-commande et d'alimentation électrique - Exigences pour les systèmes d'alimentation en courant alternatif et en courant continu statiques sans interruption

General Information

• Status: Published
• Publication Date: 07-Jul-2025
• ICS: 27.120.20 - Nuclear power plants. Safety
• Technical Committee: SC 45A - Instrumentation, control and electrical power systems of nuclear facilities
• Drafting Committee: WG 11 - TC 45/SC 45A/WG 11
• Current Stage: PPUB - Publication issued
• Start Date: 08-Jul-2025
• Completion Date: 27-Jun-2025

Relations

• Revises: IEC 61225:2019 - Nuclear power plants - Instrumentation, control and electrical power systems - Requirements for static uninterruptible DC and AC power supply systems
• Effective Date: 05-Sep-2023

Overview - What IEC 61225:2025 covers

IEC 61225:2025 (Edition 4.0) defines performance and functional characteristics for low‑voltage static uninterruptible power supply (SUPS) systems used in nuclear power plants (NPPs) and nuclear facilities. It covers both DC and AC static UPS solutions (systems without rotating parts) that draw, store and deliver electrical energy to maintain uninterrupted power to instrumentation, control and electrical power systems. Component‑level design (detailed battery or inverter internals) is excluded and handled by referenced IEC standards.

This edition expands and clarifies requirements to ease application to small modular reactors (SMRs) and passive designs.

Key topics and technical requirements

• System requirements and boundaries - definitions of SUPS scope, system divisions and interfaces with plant power systems.
• Functional requirements - requirements for SUPS supporting systems important to safety; specifications for batteries, battery chargers, inverters, bypass switches, converters (DC/DC, AC/DC) and associated performance.
• Distribution systems - load allocation, electrical characteristics, earthing/grounding, and architecture examples for redundant divisions.
• Supply quality and load effects - immunity to voltage/frequency variations, transients, open‑phase and asymmetrical conditions, electromagnetic interference (EMI) and load current impacts.
• Monitoring, protection and control - continuous monitoring, alarms, electrical protection requirements, and human factors considerations.
• Qualification, ageing, testing and maintenance - qualification criteria, aging mitigation, test procedures and maintenance practices to ensure long‑term reliability.
• Informative annexes - examples of voltage variations (Annex A), sample specifications (Annex B) and specific guidance for SMRs/passive plants (Annex C).

Practical applications and who uses this standard

IEC 61225:2025 is used to:

• Specify SUPS performance for safety‑related and non‑safety I&C equipment in NPPs.
• Inform procurement and vendor requirements for static UPS, batteries and converters.
• Guide system architects and electrical engineers on distribution design, redundancy and earthing for uninterrupted DC/AC supplies.
• Support safety assessors, licensing authorities and plant operators in demonstrating robustness against grid disturbances and ageing.
• Provide test, qualification and maintenance criteria for engineering, commissioning and long‑term plant operation.

Typical users: NPP electrical/I&C engineers, plant designers, equipment manufacturers, safety analysts, regulatory bodies and maintenance teams.

Related standards

IEC 61225:2025 is to be used with IEC 61513, IEC 60709, IEC 60880, IEC 62138, IEC 62855, IEC 63046 and other normative references covering component‑level design, software, and qualification of nuclear I&C and electrical systems.

Keywords: IEC 61225:2025, static uninterruptible power supply, SUPS, UPS, nuclear power plants, NPP, DC power, AC power, SMR, passive design, batteries, inverters, distribution, EMC, testing.

Standards Content (Sample)

IEC 61225 ®
Edition 4.0 2025-07
INTERNATIONAL
STANDARD
REDLINE VERSION
Nuclear power plants - Instrumentation, control and electrical power systems -
Requirements for static uninterruptible DC and AC power supply systems
ICS 27.120.20 ISBN 978-2-8327-0575-9
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CONTENTS
FOREWORD . 3
INTRODUCTION . 1
1 Scope . 8
2 Normative references . 8
3 Terms and definitions . 9
4 Abbreviated terms . 11
5 System requirements . 12
5.1 General . 12
5.2 Function and description . 12
5.2.1 Preamble . 12
5.2.2 Designations. 15
5.2.3 Direct current systems . 15
5.2.4 Alternating current systems . 15
5.3 System divisions . 16
5.4 System boundaries . 16
6 Functional requirements for static uninterruptible power supplies . 17
6.1 Static uninterruptible power supplies for systems important to safety . 17
6.2 Batteries and battery chargers . 17
6.3 Inverters and bypass switches . 19
6.4 UPS . 20
6.5 Converters used for voltage stabilization . 20
6.6 I&C power supply using DC/DC converters and AC/DC converters . 20
7 Requirements for distribution systems . 21
7.1 System aspects . 21
7.2 Load allocation . 21
7.3 Electrical aspects . 23
7.4 Earthing . 24
8 Effects of loads on supply quality . 24
8.1 General . 24
8.2 Electromagnetic interference . 24
8.3 Transients . 25
8.4 Load current . 25
8.5 Power supplies to loads of lower safety classification . 25
9 Monitoring and protection . 26
9.1 General . 26
9.2 Monitoring . 26
9.3 Electrical protection . 27
10 Qualification of equipment . 27
11 Design to cope with ageing . 27
12 Testing . 28
13 Maintenance . 28
Annex A (informative) Examples of voltage input variations . 30
Annex B (informative) Examples of specifications . 32
B.1 Example 1: Specification for a DC power supply for equipment requiring a
non-interruptible supply . 32
B.2 Example 2: Specification for AC power supply for equipment requiring a non-
interruptible supply . 33
B.3 Example 3: Specification for DC power supply with DC/DC converter for
equipment . 34
B.4 Human factor engineering programme . 35
Annex C (normative) Uninterruptible power supplies and distribution systems for
power plants with passive design and SMRs . 36
C.1 General . 36
C.2 Recommendations and requirements . 37
C.3 Safety assessment . 37
Bibliography . 38

Figure 1 – Boundary of a SUPS . 14
Figure 2 – Example of one division of a SUPS system . 22
Figure 3 – Example of I&C uninterruptible AC power supply system . 23
Figure A.1 – Example of voltage variations on the on-site AC power system during
clearing of a transmission system fault . 30
Figure A.2 – Example of on-site voltage profile after loss of load (transfer to house
load operation) . 30
Figure A.3 – Example of simulated safety bus voltages, double open phase condition in
the 400 kV line to the unit transformer . 31

Table B.1 – Example 1: Specification for a DC power supply for equipment requiring a
non-interruptible supply . 32
Table B.2 – Example 2: Specification for AC power supply for equipment requiring a
non-interruptible supply . 33
Table B.3 – Example 3: Specification for DC power supply with DC/DC converter for
equipment . 34
Table C.1 – Features of electrical power systems that support the different levels of
defence in depth as stated in IAEA Safety Standard Series SSR-2/1 (Rev. 1) . 36

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
Nuclear power plants - instrumentation, control and electrical power
systems - requirements for static uninterruptible dc and ac
power supply systems
FOREWORD
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This redline version of the official IEC Standard allows the user to identify the changes made
to the previous edition IEC 61225:2019. A vertical bar appears in the margin wherever a change
has been made. Additions are in green text, deletions are in strikethrough red text.

IEC 61225 has been prepared by subcommittee 45A: Instrumentation, control and electrical
power systems of nuclear facilities, of IEC technical committee 45: Nuclear instrumentation. It
is an International Standard.
This fourth edition cancels and replaces the third edition published in 2019. This edition
constitutes a technical revision.
This edition includes the following significant technical changes with respect to the previous
edition:
a) expansion and clarification of the requirements for static uninterruptible DC and AC power
supply systems to ease the application in SMRs and passive designs.
This International Standard is to be used in conjunction with IEC 61513:2011, IEC 60709:2018,
IEC 60880:2006, IEC 62138:2018, IEC 62855:2016 and IEC 63046:2020.
The text of this International Standard is based on the following documents:
Draft Report on voting
45A/1591/FDIS 45A/1610/RVD
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 International Standard 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.
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.
INTRODUCTION
a) Technical background, main issues and organization of the standard
The 1993 issue of IEC 61225 was developed for specifying the requirements relevant to the
design of electrical supplies for I&C systems in nuclear power plants. Considering the
experience gathered worldwide on this subject, in 2003 working group A2 recommended a
revision to this document so that to allow a new revision, IEC 61225 Ed. 2 (2005), could to be
consistently integrated into the SC 45A standard series. In 2015, working group A11
recommended a revision to this document following the publication of the revision of IAEA SSG-
34 and that the scope of the standard should cover static uninterruptible power supplies for all
types of connected equipment. In 2022, working group A11 recommended a revision to this
document to ease the application to SMRs and passive designs.
International operating experience with electrical supply systems in nuclear power plants has
highlighted a number of supply voltage variations and malfunctions, such as:
• voltage perturbations due to disturbances on the internal AC distribution system (with origin
off-site or on-site).
• voltage overshoot on loss of grid.
• open phase conditions (one or two phases).
• asymmetrical faults.
These types of perturbations can degrade the performance of static uninterruptible power
supplies and ultimately result in failure of connected equipment.
One of the objectives of the uninterruptible power supplies is to protect connected equipment
from voltage variations on the on-site AC interruptible distribution system (the immunity
concept). The power supplies shall also guarantee an output voltage with specified magnitude
and waveform (in case of AC) to connected loads. The power supplies shall have the capacity
to supply the relevant loads during a specified time regardless of any voltage variations on the
on-site AC interruptible distribution system.
Examples of voltage and frequency variations in the incoming feeder to the supplies can be
found in informative Annex A. Examples of specifications for static uninterruptible power
supplies can be found in informative Annex B. Requirements for SMRs and passive designs are
given in Annex C.
This document is applicable to the design of static uninterruptible electrical power supplies in
new nuclear power plants (including SMRs and passive designs) when design work is initiated
after the publication of this document and in general for nuclear facilities. It also serves as a
reference for upgrading and modernizing existing nuclear power plants and facilities.
b) Situation of the current standard in the structure of the SC 45A standard series
IEC 61225 is a second level document specifically addressing the particular topic of
requirements for electrical supplies.
For more details on the structure of the SC 45A standard series, see item d) of this introduction.
c) Recommendations and limitations regarding the application of this document
It is important to note that this document establishes no additional functional requirements for
safety systems.
To ensure that the standard will continue to be relevant in future years, the emphasis has been
placed on issues of principle, rather than specific technologies.
This standard is to be applied in conjunction with IEC 61513, IEC 60709, IEC 60880,
IEC 62138, IEC 62855 and IEC 63046 (to be published).
d) Description of the structure of the IEC SC 45A standard series and relationships with
other IEC documents and other bodies documents (IAEA, ISO)
The top-level documents of the IEC SC 45A standard series are IEC 61513 and IEC 63046.
IEC 61513 provides general requirements for I&C systems and equipment that are used to
perform functions important to safety in NPPs. IEC 63046 provides general requirements for
electrical power systems of NPPs; it covers power supply systems including the supply systems
of the I&C systems. IEC 61513 and IEC 63046 are to be considered in conjunction and at the
same level. IEC 61513 and IEC 63046 structure the IEC SC 45A standard series and shape a
complete framework establishing general requirements for instrumentation, control and
electrical systems for nuclear power plants.
IEC 61513 and IEC 63046 refer directly to other IEC SC 45A standards for general topics
related to the categorization of functions and the classification of systems, qualification,
separation, defence against common cause failure, control room design, electromagnetic
compatibility, cybersecurity, software and hardware aspects for programmable digital systems,
coordination of safety and security requirements and management of ageing. The standards
referenced directly at this second level should be considered together with IEC 61513 and
IEC 63046 as a consistent document set.
At a third level, IEC SC 45A standards not directly referenced by IEC 61513 or by IEC 63046
are standards related to specific equipment, technical methods, or specific activities. Usually
these documents, which make reference to second-level documents for general topics, can be
used on their own.
A fourth level extending the IEC SC 45 standard series, corresponds to the Technical Reports
which are not normative.
The IEC SC 45A standards series consistently implements and details the safety and security
principles and basic aspects provided in the relevant IAEA safety standards and in the relevant
documents of the IAEA nuclear security series (NSS). In particular this includes the IAEA
requirements SSR-2/1, establishing safety requirements related to the design of nuclear power
plants (NPPs), the IAEA safety guide SSG-30 dealing with the safety classification of structures,
systems and components in NPPs, the IAEA safety guide SSG-39 dealing with the design of
instrumentation and control systems for NPPs, the IAEA safety guide SSG-34 dealing with the
design of electrical power systems for NPPs and the implementing guide NSS17 for computer
security at nuclear facilities. The safety and security terminology and definitions used by
SC 45A standards are consistent with those used by the IAEA.
IEC 61513 and IEC 63046 have adopted a presentation format similar to the basic safety
publication IEC 61508 with an overall life-cycle framework and a system life-cycle framework.
Regarding nuclear safety, IEC 61513 and IEC 63046 provide the interpretation of the general
requirements of IEC 61508-1, IEC 61508-2 and IEC 61508-4, for the nuclear application sector.
In this framework IEC 60880, IEC 62138 and IEC 62566 correspond to IEC 61508-3 for the
nuclear application sector. IEC 61513 and IEC 63046 refer to ISO as well as to IAEA GS-R
part 2 and IAEA GS-G-3.1 and IAEA GS-G-3.5 for topics related to quality assurance (QA). At
level 2, regarding nuclear security, IEC 62645 is the entry document for the IEC/SC 45A
security standards. It builds upon the valid high-level principles and main concepts of the
generic security standards, in particular ISO/IEC 27001 and ISO/IEC 27002; it adapts them and
completes them to fit the nuclear context and coordinates with the IEC 62443 series. At level 2,
IEC 60964 is the entry document for the IEC/SC 45A control rooms standards and IEC 62342
is the entry document for the ageing management standards.
NOTE 1 It is assumed that for the design of I&C systems in NPPs that implement conventional safety functions (e.g.
to address worker safety, asset protection, chemical hazards, and process energy hazards) international or national
standards would be are applied.

NOTE 2 IEC/SC 45A domain was extended in 2013 to cover electrical systems. In 2014 and 2015 discussions were
held in IEC/SC 45A to decide how and where general requirements for the design of electrical systems were to be
considered. IEC/SC 45A experts recommended that an independent standard be developed at the same level as
IEC 61513 to establish general requirements for electrical systems. Project IEC 63046 is now launched to cover this
objective. When IEC 63046 is published, this NOTE 2 of the introduction of IEC/SC 45A standards will be suppressed.

1 Scope
This document specifies the performance and the functional characteristics of the low voltage
static uninterruptible power supply (SUPS) systems in a nuclear power plant (NPP) and, when
applicable, in nuclear facilities in general. An uninterruptible power supply (UPS) is an electrical
equipment which draws electrical energy from a source, stores it, and maintains the supply in
a specified form by means inside the equipment to output terminals. A SUPS has no rotating
parts to perform its functions.
The specific design requirements for the components of the power supply system are covered
by IEC standards and other standards listed in the normative references and. Otherwise,
specific component-level design requirements are otherwise outside the scope of this
document.
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, IEC standard voltages
IEC 60146-1-1, Semiconductor converters – General requirements and line commutated
converters – Part 1-1: Specification of basic requirements
IEC 60146-2, Semiconductor converters – Part 2: Self-commutated semiconductor converters
including direct d.c. converters
IEC 60364-4-41, Low voltage electrical installations – Part 4-41: Protection for safety –
Protection against electric shock
IEC 60709, Nuclear power plants – Instrumentation, control and electrical power systems
important to safety – Separation
IEC/IEEE 60780-323, Nuclear power plants – Electrical equipment important to safety –
Qualification
IEC 60880, Nuclear power plants – Instrumentation and control systems important to safety –
Software aspects for computer-based systems performing category A functions
IEC 60980, Recommended practices for seismic qualification of electrical equipment of the
safety system for nuclear generating stations
IEC 61000 (all parts), Electromagnetic compatibility (EMC)
IEC 61508 (all parts), Functional safety of electrical/electronic/programmable electronic safety-
related systems
IEC 61513, Nuclear power plants – Instrumentation and control important to safety – General
requirements for systems
IEC 62003, Nuclear power plants – Instrumentation, control important to safety and electrical
power systems – Requirements for electromagnetic compatibility testing
IEC 62040 (all parts), Uninterruptible power systems (UPS)
IEC 62138, Nuclear power plants – Instrumentation and control systems important to safety –
Software aspects for computer-based systems performing category B or C functions
IEC 62566, Nuclear power plants – Instrumentation and control important to safety –
Development of HDL-programmed integrated circuits for systems performing category A
functions
IEC 62566-2, Nuclear power plants – Instrumentation and control important to safety –
Development of HDL-programmed integrated circuits – Part 2: HDL-programmed integrated
circuits for systems performing category B or C functions (to be published)
IEC/IEEE 60780-323, Nuclear facilities – Electrical equipment important to safety –
Qualification
IEC/IEEE 60980-344, Nuclear facilities – Equipment important to safety – Seismic qualification
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
NOTE Other terms not defined below are defined in IAEA Safety Guides SSG-34 and SSG-39.
ISO and IEC maintain terminological databases for use in standardization at the following
addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
3.1
anticipated operational occurrence
AOO
deviation of an operational process from normal operation that is expected to occur at least
once during the operating lifetime of a facility but which, in view of appropriate design
provisions, does not cause any significant damage to items important to safety or lead to
accident conditions
[SOURCE: IAEA Nuclear Safety and Security Glossary, 20162022 (Interim) Edition]
3.2
battery charger
electrical item used to convert AC into DC to charge batteries and to supply power to DC loads
during normal operation
Note 1 to entry: The battery charger provides transformer isolation of the DC output from the AC input and is
equipped with regulation and monitoring.
[SOURCE: IEEE 946, 2004]
3.3
common cause failure
CCF
failure of two or more structures, systems and or components due to a single specific event or
cause
[SOURCE: IAEA Nuclear Safety and Security Glossary, 20162022 (Interim) Edition]
3.4
common mode failure
CMF
failure of two or more structures, systems or components in the same manner or mode due to
a single specific event or cause
[SOURCE: IAEA Nuclear Safety and Security Glossary, 2022 (Interim) Edition]
3.5
diversity
presence of two or more independent (redundant) systems or components to perform an
identified function, where the different systems or components have different attributes so as
to reduce the possibility of common cause failure, including common mode failure
[SOURCE: IAEA Nuclear Safety and Security Glossary, 20162022 (Interim) Edition]
3.6
division
collection of items, including their interconnections, that form one redundancy of a redundant
system or safety group
Note 1 to entry: Divisions may can include multiple channels.
Note 2 to entry: Designation that enables the establishment and maintenance of physical, electrical, and functional
independence from other redundant sets of items.
[SOURCE: IAEA SSG-39, 2016]
3.7
item important to safety
item that is part of a safety group and/or whose malfunction or failure could lead to radiation
exposure of the site personnel or members of the public
Note 1 to entry: Items important to safety include those structures, systems and components whose malfunction
or failure could lead to undue radiation exposure of site personnel or members of the public;
[SOURCE: IAEA Nuclear Safety and Security Glossary, 2022 (Interim) Edition]
3.8
plant states
Operational states Accident conditions
Normal Anticipated operational Design basis Design extension conditions
operation occurrences accidents
Without significant fuel degradation With core melting

[SOURCE: IAEA Nuclear Safety and Security Glossary, 20162022 (Interim) Edition]
3.9
redundancy
provision of alternative (identical or diverse) structures, systems and components, so that any
one single structure, system or component can perform the required function regardless of the
state of operation or failure of any other
[SOURCE: IAEA Nuclear Safety and Security Glossary, 20162022 (Interim) Edition]
3.8
safety related system
system important to safety that is not part of a safety system
[SOURCE: IAEA Safety Glossary, 2016 edition]
3.9
safety system
system important to safety, provided to ensure the safe shutdown of the reactor or residual heat
removal from the core, or to limit the consequences of anticipated operational occurrences and
design basis accidents
[SOURCE: IAEA Safety Glossary, 2016 Edition]
3.10
single failure
failure which results in the loss of capability of a single system or component to perform its
intended safety function(s), and any consequential failure(s) which result from it
[SOURCE: IAEA Nuclear Safety and Security Glossary, 20162022 (Interim) Edition]
3.11
single failure criterion
criterion (or requirement) applied to a system such that it must be capable of performing its task
in the presence of any single failure
[SOURCE: IAEA Nuclear Safety and Security Glossary, 20162022 (Interim) Edition]
3.12
station blackout
SBO
plant condition with complete loss of all AC power from off-site sources, from the main generator
and from standby AC power sources
Note 1 to entry: Uninterruptible DC and AC power supplies may be available as long as batteries can supply the
loads.é
Note 2 to entry: Alternate AC supplies according to IAEA SSG-34 are available.
[SOURCE: IAEA SSG-34, 2016]
4 Abbreviated terms
AC alternating current
AOO Anticipated operational occurrence
CCF common cause failure
CMF common mode failure
DC direct current
EMC electromagnetic compatibility
EMP electromagnetic pulse
I&C instrumentation and control
IGBT insulated-gate bipolar transistor
NPP nuclear power plant
SBO Station blackout
SSC Structures, systems and components
SMR small modular reactor
SUPS static uninterruptible power supply
UPS uninterruptible power supply
5 System requirements
5.1 General
This document defines requirements for reliable and robust SUPS systems in nuclear power
plants (NPPs). Even though more stringent criteria are applied to safety power supplies and
more verification is necessary, the entire onsite and off-site power systems contribute to the
reliability and robustness of the NPP power systems. It is recommended that The same
requirements are applied to SUPS systems regardless of safety classification with a graded
approach to verification and validation.
This document defines requirements for reliable and robust SUPS systems in nuclear facilities
including NPPs. In general, more stringent criteria are applied to items important to safety, such
as power supplies, requiring additional verifications. However, the entire on-site and off-site
power systems contribute to the reliability and robustness of the nuclear facilities power system.
The same requirements should be applied to SUPS systems regardless of their importance to
safety with a graded approach to verification and validation.
For SMRs and power plants with passive designs, the term "(highly) reliable power supply" is
often used instead of "power supply important to safety". Provisions for power supplies
important to safety apply in general for a reliable power system. These supplies are required
for monitoring and actuation (if necessary) of systems important to safety.
Robust power supply systems shall have sufficient margins and built-in conservatism to ensure
that:
• equipment ratings, capabilities and capacities required to meet intended goals are not
exceeded during all postulated conditions considered in the safety case;
• equipment protection set points are selected to accommodate anticipated perturbations in
the NPP nuclear facility's electrical distribution system during all modes of operation and
plant states;
• the equipment has the capacity and capability to support emergency operations.
The design of the power supplies shall include analysis of:
• transient, dynamic, and quasi-stationary variations in voltage and frequency (in case of AC
power source);
• power interruptions on the incoming feeder (see 5.2.2) (or voltage /or frequency dips
exceeding the allowed dynamic variation range) lasting from milliseconds up to SBO
conditions extended loss of all AC interruptible power supplies;
• asymmetrical conditions;
• independence of redundant parts of the power supply system and resilience against
common cause failures (CCFs).
5.2 Function and description
5.2.1 Preamble
The SUPS system shall provide a continuous uninterruptible supply to each connected load
within the specified tolerances of voltage, and (for AC systems) waveform and frequency for all
input conditions. The system required to perform safety functions shall have sufficient
independence, redundancy and testability assuming a single failure in the system.
A SUPS generally consists of one or both of the following systems (see Figure 1):
• a DC power system with battery chargers and batteries, supplying DC loads.
• an AC power system with battery chargers, back-up batteries, static switches, and inverters,
supplying AC loads.
In certain designs the battery chargers and batteries could can be common to both the DC and
AC power systems.
The functions and the classification of the power supplies should be designated according to
the safety classification scheme of the NPP. In general, the classification should be based on
the highest classification of the load supplied by the power supply as discussed in 7.2.
The constraints on connecting loads with different classifications are discussed in 8.5.

NOTE DC/DC converters, AC/DC converters, and converters for voltage stabilization and maintenance bypass of the AC power supply system are not shown. Uninterruptible
AC power supply can also be provided by an UPS. An UPS is normally an integrated assembly of battery charger, battery and inverter as shown on the right-hand side of the
figure.
a
The box represents:
– a direct connection;
– a transformer; or
– a regulated transformer.
Figure 1 – Boundary of a SUPS
5.2.2 Designations
Incoming feeder: the SUPS has an incoming feeder, generally from a low voltage AC power bus
which is normally part of the electrical safety buses .
Outgoing feeder: the SUPS utilizes a set of battery charger/ and battery or battery charger/,
battery/ and inverter system with an outgoing feeder that is connected to the distribution system.
The SUPS supplies the loads from a distribution system (see Figure 1).
5.2.3 Direct current systems
Each division of an uninterruptible DC power supply system shall consist of at least:
• a battery;
• a battery charger;
• a distribution system.
In order to increase the availability and the robustness of the division, but also for increased
maintenance flexibility, the provision of internal redundancy in each division should be
considered. The provision of redundancy is discussed in 5.3.
The DC power supply system shall supply the DC loads during normal operation (including
outages), anticipated operational occurrences, accident conditions and all plant states
considered in the safety case.
The DC power supply system shall supply the DC loads for a specified minimum duration
defined by the safety case when power is lost on the incoming feeder with no perturbation of
the outgoing feeder voltage outside specified tolerances.
5.2.4 Alternating current systems
Each division of an uninterruptible AC power supply system shall consist of:
• a power supply from a DC power system to an inverter;
• alternatively, a power supply from an interruptible AC bus, a dedicated battery charger, a
battery and an inverter;
• a distribution system.
A back-up power supply to the distribution system from an interruptible AC bus shall be installed
together with a device for automatically switching between the power supply from the inverter
and the back-up supply. Additionally, a maintenance bypass should be implemented.
To increase the availability and the robustness of the division and for more flexibility for
maintenance, the provision of internal redundancy should be considered as discussed in 5.3.
The uninterruptible AC power supply system shall supply the AC loads during normal operation
(including outages), anticipated operational occurrences, accident conditions and all plant
states considered in the safety case.
The uninterruptible AC power supply system shall supply the AC loads for a specified minimum
duration, defined by the safety case, when power is lost on the incoming feeder with no
perturbation of the outgoing feeder voltage outside specified tolerances.
___________
See figures in IAEA SSG-34.
Due to the higher reliability of DC systems, supplying a safety I&C system from the DC system
should be considered. If possible, a combination of DC and AC supply should be used to
minimize the risks of common mode failure. If only DC loads are supplied, the use of DC supply
should be the preferred solution. Such an approach eliminates a source of failure.
Due to the higher reliability of DC systems, supplying an I&C system of importance to safety
from the DC system should be considered. If determined by the safety case, a double infeed
combining DC and AC supply for each train should be used to minimize the risks of common
mode failure.
The design should consider the use of DC loads supplied by DC supply. Avoiding use of
uninterruptible AC power supply system eliminates a source of failure in the critical power
supply system.
5.3 System divisions
The DC power supply system and the AC power supply system shall be divided into redundant
and independent divisions. The safety loads Loads important to safety shall be supplied from
at least one dedicated supply source for each division, to ensure physical and functional
separation. The required number of redundant sources depends on the design of the reactor
protection system and the redundancy in the NPP design.
The power supplies for loads important to safety loads covered by this document shall be in
accordance with the single failure criterion as described in IEC 61513 and the requirements on
separation and failure tolerance described in IEC 60709.
AC and DC systems that are important to safety should not be shared between units.
Any deviations from the divisional structure and proposals to share facilities for highly reliable
AC and DC systems in SMR modules or modules of passive designs should be supported by
safety assessment. This safety assessment shall demonstrate that single failure criteria is met,
that faults should not propagate between units or modules, maintenance activities can be
performed safely and that there is no risk of common cause failure. When AC or DC systems
are shared, operating constraints should be considered for impact on overall operation of units
or modules when maintenance or surveillance actions are necessary.
The functional diversity of AC and DC systems shall be considered for I&C power supplies to
improve the availability of the overall power supply. The principle of independence and diversity
shall be applied to protect agains
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