IEC 61225:2025

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
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