prEN IEC 60255-21:2026

• Status: Not Published
• Publication Date: 16-Feb-2028
• Technical Committee: CLC/TC 95X - Measuring relays and protection equipment
• Current Stage: 4020 - Enquiry circulated - Enquiry
• Start Date: 11-Sep-2026
• Due Date: 17-May-2024
• Completion Date: 11-Sep-2026
• Ref Project: oSIST prEN IEC 60255-21:2026 - Measuring relays and protection equipment - Part 21: Vibration, shock, bump and seismic tests requirements

• Revises: EN 60255-21-3:1995 - Electrical relays - Part 21: Vibration, shock, bump and seismic tests on measuring relays and protection equipment - Section 3: Seismic tests
• Effective Date: 16-Jun-2026
• Revises: EN 60255-21-1:1995 - Electrical relays - Part 21: Vibration, shock, bump and seismic tests on measuring relays and protection equipment - Section 1: Vibration tests (sinusoidal)
• Effective Date: 16-Jun-2026
• Revises: EN 60255-21-2:1995 - Electrical relays - Part 21: Vibration, shock, bump and seismic tests on measuring relays and protection equipment - Section 2: Shock and bump tests
• Effective Date: 16-Jun-2026

prEN IEC 60255-21:2026 is a CLC/IEC document for measuring relays and protection equipment. It specifies vibration, shock, bump and seismic test requirements for equipment used in power system environments, so manufacturers, test labs and buyers can check whether a relay or protection device can withstand transport, handling and service stresses.

What does prEN IEC 60255-21:2026 specify?

prEN IEC 60255-21:2026 specifies the withstand levels and associated tests for measuring relays and protection equipment exposed to vibration, shocks, bumps and seismic events. It also sets the acceptance criteria and the information that belongs in the test report.

The document applies to equipment used for protection within the power system environment, typically in power stations, substations and industrial buildings. It also notes that special applications such as marine, railway, aerospace, explosive atmospheres and computer centres may need extra requirements.

The document is organized into clauses on scope, normative references, terms and definitions, environmental levels, test requirements, severities, test procedures, acceptance criteria and test report. The front matter states that the tests are type tests unless otherwise declared, and that compliance means meeting all tests specified in the document.

What are the key requirements of prEN IEC 60255-21:2026?

Applicability and compliance

prEN IEC 60255-21:2026 requires all specified tests to be performed to claim compliance. The order of tests may be changed to reduce test time, but the pre- and post-checks from IEC 60255-1 are still required.

In practice, this means the qualification plan has to cover the full test set, not just one vibration or shock case. It also means the equipment has to be assessed before and after the test sequence in a consistent way.

Environmental class selection

prEN IEC 60255-21:2026 defines environmental classes to help select the test level from the real installation and transport conditions. The document gives two classes: a typical industrial setting and a severe setting.

This matters because the chosen class affects the severity of the tests. The standard also says that if equipment is shipped in packaging, the packaging has to be designed so the severity class parameters are not exceeded during transportation.

Mounting and test condition control

prEN IEC 60255-21:2026 requires the equipment to be mounted on a fixture that is equivalent to the manufacturer’s recommended arrangement, but rigid enough to avoid amplification and unwanted motion. The attachment method, bolt size and torque have to follow the technical documentation, and cable routing must not add unrealistic restraint or mass.

The tests are run under the reference conditions from IEC 60255-1, and the protection settings are taken from IEC 60255-1 Annex A. This is important because the measured response has to reflect the normal operating condition of the relay or protection equipment, not a loosely mounted lab setup.

Test families and energised or non-energised condition

prEN IEC 60255-21:2026 separates the tests into vibration response, broadband random vibration, shock response, shock withstand, bump and seismic tests. It also distinguishes between tests on energised equipment and tests on non-energised equipment.

For vibration response, shock response and seismic testing, the equipment is energised under the specified reference conditions. For broadband random vibration, shock withstand and bump testing, the equipment is non-energised. In practice, this means the user has to plan both functional testing and purely mechanical robustness testing.

Severity classes and seismic methods

prEN IEC 60255-21:2026 uses severity classes for the different test families, with the detailed parameters given in tables. The document also provides two acceptable seismic methods: single axis sine sweep seismic test, called method A, and biaxial multi-frequency random seismic test, called method B.

Method A and method B are both acceptable and neither is identified as the referee method. Method B uses a standard response spectrum and a damping ratio, while method A uses sweep motion in three orthogonal axes. For the reader, this means the qualification route has to match the selected method and the equipment’s intended seismic environment.

Monitoring, acceptance and reporting

prEN IEC 60255-21:2026 requires the state of the binary output ports to be monitored during vibration response, shock response and seismic tests with a device capable of capturing high-speed signals. The acceptance criteria focus on maloperation, mechanical damage and post-test electrical integrity.

During the functional response tests, the equipment must not maloperate, and the normal state of its binary output ports must not change permanently or for more than 2 ms. Indicators such as mechanical flags, LEDs or LCD indications must not change permanently. After testing, the equipment must still comply with its technical documentation and pass insulation resistance and dielectric withstand tests in IEC 60255-1.

What terms does prEN IEC 60255-21:2026 define?

• Biaxial multi-frequency random seismic test - a seismic test where equipment is stressed simultaneously in horizontal and vertical axes with random motion shaped to match a standard response spectrum.
• Bump test - a test on non-energised equipment using a limited number of bumps in three axes to check resistance to transport-type impacts.
• Check point - a rigid point on the fixture, table or specimen close to the fixing point, used to help verify that the test requirements are met.
• Response spectrum - a plot of maximum response versus natural frequency for a family of single-degree-of-freedom bodies at a specified damping ratio.
• Shock response test - a shock test on energised equipment to see how it responds to shocks likely to occur occasionally in service.
• Shock withstand test - a high-level shock test on non-energised equipment to check resistance to shocks likely during transport and handling.
• Standard response spectrum - the response spectrum shape defined by Figure 1, with damping ratio and zero-period acceleration as the main parameters.
• Zero-period acceleration (ZPA) - the largest peak acceleration represented by the response spectrum, used as a key seismic parameter.

Who uses prEN IEC 60255-21:2026?

prEN IEC 60255-21:2026 is used by manufacturers of measuring relays and protection equipment, test laboratories, and engineering teams qualifying products for power system use. It is also relevant to quality managers and buyers who need evidence that the equipment can survive transport, handling, storage and service vibration or seismic exposure.

The standard is aimed at equipment installed near the electrical systems it protects, including power stations, substations and industrial buildings. It is especially useful when the equipment is mounted in racks, panels or cubicles and has to be checked as a complete assembly.

What changed in prEN IEC 60255-21:2026 from the previous edition?

prEN IEC 60255-21:2026 replaces IEC 60255-21-1:1988, IEC 60255-21-2:1988 and IEC 60255-21-3:1993, and the foreword describes it as a technical revision.

The foreword lists these main changes:

• the three IEC 60255-21 standards have been merged into one mechanical standard
• unused terms were removed from the terminology
• duplicated parameters were removed and replaced with references to the base standards
• settings and pre- and post-testing references were added from IEC 60255-1
• vibration endurance using a sinusoidal sweep was removed
• random vibration tests were added
• transportation tests were clarified for equipment mounted in panels or cubicles

Which standards are used with prEN IEC 60255-21:2026?

Standard	What it contributes
IEC 60068-2-6	Vibration response testing and the sine-sweep basis used for method A
IEC 60068-2-27	Shock and bump testing, including the shock test basis and pulse shape
IEC 60068-2-47	Mounting of specimens for vibration, impact and similar dynamic tests
IEC 60068-2-57	Seismic testing method B and terms such as response spectrum and damping ratio
IEC 60068-2-64	Broadband random vibration testing
IEC 60255-1	Common requirements, reference test conditions, settings, and pre- and post-test checks

The bibliography also cites ISO 2041:2018 for general mechanical vibration, shock and condition-monitoring vocabulary.

What does the prEN IEC 60255-21:2026 document contain?

The document contains tables for environmental classes, vibration, shock, bump and seismic severities, plus figures for the standard response spectrum, break points and seismic sweep behaviour. It also includes the procedures for preconditioning, test conditions, mounting, the three test families, recovery, final measurements and acceptance criteria.

For test execution, the document sets out how to run the equipment in three mutually perpendicular axes, how to monitor binary output ports, and how to handle the two seismic methods. For reporting, it lists the mounting arrangement, excitation axis, sensor location, filters, measurement uncertainties, control strategy, test severities, results, observations and the test manager signature.

SLOVENSKI STANDARD
01-november-2026
Merilni releji in zaščitna oprema - 21. del: Zahteve za preskuse vibracij, udarcev,
sunkov in potresov
Measuring relays and protection equipment - Part 21: Vibration, shock, bump and
seismic tests requirements
Relais de mesure et équipements de protection - Partie 21: Exigences pour les essais de
vibrations, de chocs, de secousses et sismiques
Ta slovenski standard je istoveten z: prEN IEC 60255-21:2026
ICS:
29.120.70 Releji Relays
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

95/647/CDV
COMMITTEE DRAFT FOR VOTE (CDV)
PROJECT NUMBER:
IEC 60255-21 ED1
DATE OF CIRCULATION: CLOSING DATE FOR VOTING:
2026-09-11 2026-12-04
SUPERSEDES DOCUMENTS:
95/586A/CD, 95/605A/CC
IEC TC 95 : MEASURING RELAYS AND PROTECTION EQUIPMENT
SECRETARIAT: SECRETARY:
France Mr Thierry BARDOU
OF INTEREST TO THE FOLLOWING COMMITTEES: HORIZONTAL FUNCTION(S):
TC 17,TC 38
ASPECTS CONCERNED:
Electricity transmission and distribution
SUBMITTED FOR CENELEC PARALLEL VOTING NOT SUBMITTED FOR CENELEC PARALLEL VOTING
Attention IEC-CENELEC parallel voting
The attention of IEC National Committees, members of
CENELEC, is drawn to the fact that this Committee Draft
for Vote (CDV) is submitted for parallel voting.
The CENELEC members are invited to vote through the
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This document is still under study and subject to change. It should not be used for reference purposes.
Recipients of this document are invited to submit, with their comments, notification of any relevant patent rights of
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Recipients of this document are invited to submit, with their comments, notification of any relevant “In Some
Countries” clauses to be included should this proposal proceed. Recipients are reminded that the CDV stage is
the final stage for submitting ISC clauses. (SEE AC/22/2007 OR NEW GUIDANCE DOC).

TITLE:
Measuring relays and protection equipment - Part 21: Vibration, shock, bump and seismic tests
requirements
PROPOSED STABILITY DATE: 2032
NOTE FROM TC/SC OFFICERS:
download this electronic file, to make a copy and to print out the content for the sole purpose of preparing National
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IEC CDV 60255-21 © IEC 2026
1 CONTENTS
3 FOREWORD . 3
4 INTRODUCTION . 5
5 1 Scope . 6
6 1.1 Vibration Tests . 6
7 1.2 Shock and Bump Tests . 6
8 1.3 Seismic Tests . 6
9 2 Normative references . 7
10 3 Terms, definitions and abbreviated terms . 7
11 3.1 Terms and definitions . 7
12 3.2 Abbreviated terms . 11
13 4 Definition of environmental levels . 11
14 5 Requirements for testing . 12
15 5.1 General . 12
16 5.2 Required characteristics . 12
17 5.2.1 Vibration tests . 12
18 5.2.2 Shock and bump tests . 12
19 5.2.3 Seismic tests . 12
20 5.2.4 Mounting . 12
21 6 Severities . 12
22 6.1 General . 12
23 6.2 Vibration tests . 12
24 6.2.1 General . 12
25 6.2.2 Severity classes for vibration response test . 12
26 6.2.3 Severity classes for broadband random vibration . 13
27 6.3 Shock and bump tests . 13
28 6.3.1 General . 13
29 6.3.2 Severity classes for shock response test . 14
30 6.3.3 Severity classes for shock withstand test . 14
31 6.3.4 Severity classes for bump test . 14
32 6.4 Seismic tests . 14
33 6.4.1 Severities of single axis sine sweep seismic test (method A) . 14
34 6.4.2 Severity classes for biaxial multi-frequency random seismic test
35 (method B) . 16
36 7 Test procedures . 17
37 7.1 Preconditioning . 17
38 7.2 Test condition . 17
39 7.2.1 Test reference conditions . 17
40 7.2.2 Settings . 17
41 7.2.3 Auxiliary energising quantities, input energising quantities and loading . 17
42 7.3 Vibration testing . 17
43 7.4 Shock and bump testing . 17
44 7.5 Seismic testing . 18
45 7.5.1 Single axis sine sweep test (Method A). 18
46 7.5.2 Biaxial multi-frequency random seismic test (method B) . 18
47 7.6 Recovery . 18
IEC CDV 60255-21 © IEC 2026
48 7.7 Final measurements . 18
49 8 Acceptance criteria . 18
50 8.1 Acceptance criteria for vibration response, shock response random
51 broadband vibration and seismic test . 18
52 8.2 Acceptance criteria for shock withstand and bump test . 18
53 9 Test report . 18
54 Bibliography . 20
56 Table 1 – Environmental classes . 11
57 Table 2 – Vibration response test parameters for different severity classes . 13
58 Table 3 – Break points for spectrum – for different severity classes . 13
59 Table 4 – Shock response test parameters for different severity classes . 14
60 Table 5 – Shock withstand test parameters for different severity classes . 14
61 Table 6 – Bump test parameters for different severity classes . 14
62 Table 7 – Single axis sine sweep seismic test parameters for different severity classes . 15
63 Table 8 – Biaxial multi-frequency random seismic test parameters for different severity
64 classes . 17
66 Figure 1 – Multi-frequency broad-band standard response spectrum shape . 10
67 Figure 2 – Frequency / amplitude break points . 13
68 Figure 3 – Acceleration versus frequency for the single axis sine sweep seismic test . 16
IEC CDV 60255-21 © IEC 2026
71 INTERNATIONAL ELECTROTECHNICAL COMMISSION
72 ____________
74 MEASURING RELAYS AND PROTECTION EQUIPMENT –
76 Part 21: Vibration, shock, bump and seismic test requirements
78 FOREWORD
79 1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
80 all national electrotechnical committees (IEC National Committees). The object of IEC is to promote international
81 co-operation on all questions concerning standardization in the electrical and electronic fields. To this end and
82 in addition to other activities, IEC publishes International Standards, Technical Specifications, Technical Reports,
83 Publicly Available Specifications (PAS) and Guides (hereafter referred to as “IEC Publication(s)”). Their
84 preparation is entrusted to technical committees; any IEC National Committee interested in the subject dealt with
85 may participate in this preparatory work. International, governmental and non-governmental organizations liaising
86 with the IEC also participate in this preparation. IEC collaborates closely with the International Organization for
87 Standardization (ISO) in accordance with conditions determined by agreement between the two organizations.
88 2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
89 consensus of opinion on the relevant subjects since each technical committee has representation from all
90 interested IEC National Committees.
91 3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
92 Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC
93 Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any
94 misinterpretation by any end user.
95 4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
96 transparently to the maximum extent possible in their national and regional publications. Any divergence between
97 any IEC Publication and the corresponding national or regional publication shall be clearly indicated in the latter.
98 5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity
99 assessment services and, in some areas, access to IEC marks of conformity. IEC is not responsible for any
100 services carried out by independent certification bodies.
101 6) All users should ensure that they have the latest edition of this publication.
102 7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and
103 members of its technical committees and IEC National Committees for any personal injury, property damage or
104 other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and
105 expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC Publications.
106 8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
107 indispensable for the correct application of this publication.
108 9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of patent
109 rights. IEC shall not be held responsible for identifying any or all such patent rights.
110 International Document IEC 60255-21 has been prepared by IEC technical committee 95:
111 Measuring relays and protection equipment.
112 This document cancels and replace the documents IEC 60255-21-1:1988, IEC 60255-21-2:
113 1988 and IEC 60255-21-3:1993. This document constitutes a technical revision.
114 This document includes the following significant technical changes with respect to the
115 documents IEC 60255-21-1: 1998, IEC 60255-21-2: 1988 and IEC 60255-21-3: 1993:
116 a) The three IEC 60255-21 standards have been merged into a single mechanical standard;
117 b) terminology updated to remove unused terms;
118 c) duplication of parameters from the base standards have been removed replaced with references to
119 the base standards;
120 d) references have been included for settings and for pre testing and post testing in accordance with
121 IEC 60255-1;
122 e) vibration endurance using a sinusoidal sweep has been removed
123 f) random vibration tests have been added;
124 g) transportation tests have been clarified that they are for equipment mounted in panels or cubicles.
125 The text of this International Document is based on the following documents:
IEC CDV 60255-21 © IEC 2026
FDIS Report on voting
95/XX/FDIS 95/XX/RVD
127 Full information on the voting for the approval of this International Document can be found in
128 the report on voting indicated in the above table.
129 This document has been drafted in accordance with the ISO/IEC Directives, Part 2.
130 The committee has decided that the contents of this document will remain unchanged until the
131 stability date indicated on the IEC website under "http://webstore.iec.ch" in the data related to
132 the specific document. At this date, the document will be
133 • reconfirmed,
134 • withdrawn,
135 • replaced by a revised edition, or
136 • amended.
IEC CDV 60255-21 © IEC 2026
137 INTRODUCTION
138 This part of IEC 60255 specifies tests applicable to measuring relays or protection equipment
139 that are expected to experience various vibration, shock, bump, and seismic environments
140 during transportation, handling, storage, or in service.
141 The main part of this document provides, in detail, the procedure for each test. For vibration
142 and seismic tests, the requirements for the vibration motion, choice of severities including
143 frequency ranges, amplitudes, endurance times, and response spectrum are also specified. In
144 the case of shock and bump tests, the requirements for the test, choice of severities including
145 acceleration, duration of the nominal pulse, and number of pulses are defined herein. In each
146 case, each selected severity level is represented by a series of parameters.
147 VIBRATION TESTS
148 This portion of the document defines tests intended to emulate conditions involving vibration of
149 a harmonic pattern. Such vibration environments are generated primarily by rotating, pulsating,
150 or oscillating forces. These are essentially caused by machinery.
151 SHOCK AND BUMP TESTS
152 This portion of the document defines tests intended to emulate conditions involving relatively
153 infrequent, non-repetitive or repetitive shocks e.g. circuit breaker operation.
154 SEISMIC TESTS
155 This portion of the document defines tests intended to emulate random or oscillating dynamic
156 forces for short duration that are induced in equipment as a result of earthquakes . The
157 characteristics of these forces and the damping of the equipment could be such that the
158 vibration response of the equipment will not reach a steady-state condition.
IEC CDV 60255-21 © IEC 2026
159 MEASURING RELAYS AND PROTECTION EQUIPMENT –
161 Part 21: Vibration, shock, bump and seismic tests
165 1 Scope
166 This part of IEC 60255 specifies the withstand levels and associated tests for vibration response,
167 broadband random vibration, shock response, shock withstand, bump, and seismic tests
168 applicable to measuring relays and protection equipment. The purpose of these tests is to prove
169 the capability of a measuring relay or protection equipment to withstand conditions of vibration,
170 shocks, and bumps encountered in service, during transportation unpackaged in final
171 arrangement e.g. relays mounted in cubicle. They also demonstrate the capability of a
172 measuring relay or protection equipment to perform its required function during and after the
173 time it is subjected to the stresses and displacements resulting from an earthquake . To claim
174 compliance a product shall meet all tests specified in this document.
175 All measuring relays and protection equipment used for protection within the power system
176 environment are covered by this document. Other documents in this series can define their own
177 requirements which in such cases take precedence. The measuring relays and protection
178 equipment are generally located close to the electrical installations to be protected : power
179 stations, substation, and industrial buildings.
180 Measuring relays and protection equipment installed in special applications (marine, railways,
181 aerospace, explosive atmospheres, computer centres, etc.) could have additional requirements
182 required by that application.
183 The requirements specified in this document are applicable to measuring relays and protection
184 equipment in a condition representative of how new equipment is provided by the manufacturer.
185 All tests in this document are type tests, unless otherwise declared. As each category of test
186 has a unique procedure and requirements, the below details are also provided.
187 1.1 Vibration Tests
188 This document includes two types of vibration test: the vibration response test on energised
189 equipment under test, carried out in accordance with test Fc of IEC 60068-2-6. and the
190 broadband random vibration test on non-energised equipment under test, carried out in
191 accordance with IEC 60068-2-64.
192 1.2 Shock and Bump Tests
193 This document includes three types of tests: the shock response test on energised equipment
194 under test, the shock withstand test on non-energised equipment under test, and the bump test
195 on non-energised equipment under test, carried out in accordance with test Ea of IEC 60068-
196 2-27. However, some test parameters included herein are not exactly the same as those found
197 in IEC 60068-2-27. This is because the values of test parameters in this document have been
198 selected to always allow a direct comparison between severity levels (Class 1 and 2).
199 1.3 Seismic Tests
200 This document includes two alternative types of seismic test:
201 • the single axis sine sweep seismic test (method A); and
202 • the biaxial multi-frequency random seismic test (method B).
203 Both methods are acceptable, and neither has been identified as reference (or referee) method.
204 The tests are carried out in accordance with test Fc of IEC 60068-2-6 (method A) and the test
205 Ff of IEC 60068-2-57 (method B).
IEC CDV 60255-21 © IEC 2026
206 2 Normative references
207 The following documents are referred to in the text in such a way that some or all of their content
208 constitutes requirements of this document. For dated references, only the edition cited applies.
209 For undated references, the latest edition of the referenced document (including any
210 amendments) applies.
211 IEC 60068-2-6:2007 Environmental testing - Part 2-6: Tests - Test Fc: Vibration (sinusoidal)
212 IEC 60068-2-27:2008 Environmental testing - Part 2-27: Tests - Test Ea and guidance: Shock
213 IEC 60068-2-47:2005 Environmental testing - Part 2-47: Test - Mounting of specimens for
214 vibration, impact and similar dynamic tests
215 IEC 60068-2-57:2013 Environmental testing - Part 2-57: Tests - Test Ff: Vibration - Time-history
216 and sine-beat method
217 IEC 60068-2-64:2008+AMD1:2019 CSV Environmental testing - Part 2-64: Tests – Test Fh:
218 Vibration, broadband random and guidance
219 IEC 60255-1:2022 Measuring relays and protection equipment - Part 1: Common requirements
220 3 Terms, definitions and abbreviated terms
221 3.1 Terms and definitions
222 For the purposes of this document, the following terms and definitions apply.
223 ISO and IEC maintain terminological databases for use in standardization at the following
224 addresses:
225 • IEC Electropedia: available at http://www.electropedia.org/
226 • ISO Online browsing platform: available at http://www.iso.org/obp
228 3.1.1
229 biaxial multi-frequency random seismic test
230 a test during which an equipment is submitted to a random sequence of stresses with a test
231 response spectrum which reproduces the standard response spectrum by a biaxial multi -
232 frequency input motion
233 3.1.2
234 biaxial testing
235 a test during which an equipment is submitted to stresses in the horizontal and vertical axes
236 simultaneously
237 3.1.3
238 bump test
239 a test during which a non-energised measuring relay or protection equipment is subjected to a
240 limited number of bumps, in three different axes of the equipment in turn, to determine its
241 capability to withstand the effects of bumps likely to be encountered during transportation
242 3.1.4
243 check point
244 point located on the fixture, on the table surface of the shock-testing machine or on the
245 specimen as close as possible to the fixing point, and in any case rigidly connected to it
246 Note 1 to entry: A number of check points are used as a means of ensuring that the test requirements are satisfied.
247 Note 2 to entry: If more than four fixing points exist, the relevant specification should state the number of fixing
248 points to be used as checkpoints.
249 Note 3 to entry: In special cases, for example, for large or complex specimens, the check points will be prescribed
250 by the relevant specification if not close to the fixing points.
251 Note 4 to entry: Where a large number of small specimens are mounted on one fixture, or in the case of a small
252 specimen where there are a number of fixing points, a single checkpoint (that is the reference point) may be selected
...