FprEN 50209:2026

• Abstract:

  This document applies to rotating electrical machines with rated voltages (UN) from 5 kV to 27 kV inclusive and with rated output from 5 MVA upwards for generators and from 5 MW upwards for motors. This document is also applicable to machines with rated outputs between 1 MVA (1 MW) and 5 MVA (5 MW) and with rated voltages of 5 kV and above, provided its use has been agreed beforehand. Requirements for machines with a rated voltage above 27 kV are the subject of individual agreement. In the case of machines whose windings are cured in the stator, tests on the separate winding elements are not possible; for these machines the requirements in Clause 6 apply. Converter fed machines are excluded from the scope of this standard. The described tests can be used for this type of machines however the criteria are mutually agreed upon between user and manufacturer.

• Status: Not Published
• Publication Date: 14-Jan-2027
• ICS: 29.080 - Insulation
• ICS: 29.160.10 - Components for rotating machines
• Technical Committee: CLC/TC 2 - Rotating machinery
• Current Stage: 4599 - Dispatch of FV draft to CMC - Finalization for Vote
• Start Date: 15-Jul-2026
• Due Date: 12-Nov-2025
• Completion Date: 15-Jul-2026
• Ref Project: oSIST prEN 50209:2026 - Test of insulation of bars and coils of high-voltage machines

• Revises: EN 50209:1998 - Test of insulation of bars and coils of high-voltage machines
• Effective Date: 21-Nov-2023

Overview

FprEN 50209:2026 is a draft European Standard developed by CLC/TC 2 for the testing of insulation in bars and coils of high-voltage rotating electrical machines. It applies to machines with rated voltages from 5 kV to 27 kV, including generators from 5 MVA upwards and motors from 5 MW upwards. The standard also covers machines within the 1 MVA (or MW) to 5 MVA (or MW) range at voltages of 5 kV and above, provided its application has been agreed upon. It supersedes EN 50209:1998 and introduces updated test procedures and expanded voltage ranges.

This standard aims to ensure the quality and reliability of electrical insulation in stator bars and coils, a critical aspect for high-voltage machine performance and operational safety. It is not applicable to converter-fed machines, although tests may be adapted through agreement between the user and manufacturer.

Key Topics

• Scope of Application

  • Rotating electrical machines rated 5 kV to 27 kV
  • Machines above 5 MVA output for generators and 5 MW for motors
  • Optionally, machines down to 1 MW/MVA by agreement
  • For machines above 27 kV, specific agreements are required

• Routine and Sample Testing

  • Detailed procedures for routine tests on a set percentage of machine bars/coils
  • Random sample (destructive) testing for additional quality assurance
  • Specialized procedures for globally impregnated windings where individual element tests are not possible

• Insulation Test Methods

  • Voltage withstand tests (AC and DC) between conductors
  • Interturn surge and AC withstand tests for multiturn coils
  • Surface resistance measurement of conductive slot coatings
  • Dissipation factor (tan δ) and capacitance measurements across voltage steps
  • Partial discharge measurement following international best practices
  • Optional black out (corona detection) testing on stress control systems

• Documentation and Reporting

  • Requirements for comprehensive test reports
  • Certification in line with EN 10204:2004
  • Recording of environmental conditions and characteristic values for traceability

Applications

FprEN 50209:2026 is widely relevant for:

• Electrical Machine Manufacturers
  Ensuring compliance with insulation quality and performance requirements during production, especially for rotating machines used in power generation and industrial drives.

• Power Plant Operators and Utilities
  Standardized insulation tests help assess long-term reliability and minimize unplanned outages due to insulation failures in high-voltage generators and large motors.

• Inspection and Testing Laboratories
  Provides detailed guidance on test procedures, sampling rates, and reporting required for third-party verification and certification of machine insulation systems.

• Industrial End-Users
  Enables informed procurement by referencing a recognized standard for insulation quality, reducing risk in critical motor and generator applications.

• Engineering Consultants
  Facilitates specification of insulation testing protocols in project documentation for new installations or major refurbishments.

Related Standards

• EN 60034-1:2010

  • Rotating electrical machines - Rating and performance

• EN IEC 60034-15:2025

  • Impulse voltage withstand levels of form-wound stator coils

• EN IEC 60034-27 series

  • Off-line partial discharge measurements (27-1), dissipation factor measurement (27-3), insulation resistance measurement (27-4)

• EN 10204:2004

  • Types of inspection documents

• IEEE 286

  • Power factor tip-up of electric machinery stator coil insulation

• IEEE 1799

  • Quality control testing of external discharges on stator coils, bars, and windings

By referencing FprEN 50209:2026, stakeholders can ensure best practices for insulation testing in high-voltage electrical machines, thereby enhancing machine reliability, safety, and lifecycle performance. For further optimization, its alignment with internationally recognized standards supports interoperability and global market acceptance.

SLOVENSKI STANDARD
01-februar-2026
Preskus izolacije palic in tuljav visokonapetostnih strojev
Test of insulation of bars and coils of high-voltage machines
Prüfung der Isolierung von Stäben und Spulen von Hochspannungsmaschinen
Essai de l’isolation des barres et des bobines des machines à haute tension
Ta slovenski standard je istoveten z: prEN 50209:2026
ICS:
29.080.01 Električna izolacija na Electrical insulation in
splošno general
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EUROPEAN STANDARD DRAFT
prEN 50209
NORME EUROPÉENNE
EUROPÄISCHE NORM
January 2026
ICS 29.080; 29.160.10 Will supersede EN 50209:1998
English Version
Test of insulation of bars and coils of high-voltage machines
Essai de l'isolation des barres et des bobines des machines Prüfung der Isolierung von Stäben und Spulen von
à haute tension Hochspannungsmaschinen
This draft European Standard is submitted to CENELEC members for enquiry.
Deadline for CENELEC: 2026-04-17.

It has been drawn up by CLC/TC 2.

If this draft becomes a European Standard, CENELEC members are bound to comply with the CEN/CENELEC Internal Regulations which
stipulate the conditions for giving this European Standard the status of a national standard without any alteration.

This draft European Standard was established by CENELEC in three official versions (English, French, German).
A version in any other language made by translation under the responsibility of a CENELEC member into its own language and notified to
the CEN-CENELEC Management Centre has the same status as the official versions.

CENELEC members are the national electrotechnical committees of Austria, Belgium, Bulgaria, Croatia, Cyprus, the Czech Republic,
Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, the
Netherlands, Norway, Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland,
Türkiye and the United Kingdom.

Recipients of this draft are invited to submit, with their comments, notification of any relevant patent rights of which they are aware and to
provide supporting documentation.

Warning : This document is not a European Standard. It is distributed for review and comments. It is subject to change without notice and
shall not be referred to as a European Standard.

European Committee for Electrotechnical Standardization
Comité Européen de Normalisation Electrotechnique
Europäisches Komitee für Elektrotechnische Normung
CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2026 CENELEC All rights of exploitation in any form and by any means reserved worldwide for CENELEC Members.
Project: 79007 Ref. No. prEN 50209:2026 E

prEN 50209:2026 (E)
Contents Page
1 European foreword . 3
2 Introduction . 4
3 1 Scope . 5
4 2 Normative references . 5
5 3 Terms and definitions . 5
6 4 Routine tests . 6
7 4.1 Number of elements to be tested . 6
8 4.2 Tests to be performed . 6
9 4.2.1 Voltage withstand test between parallel conductors (bars versus coils) . 6
10 4.2.2 Interturn test . 7
11 4.2.3 Measurement of the surface resistance of the conductive slot coating (OCP) . 7
12 4.2.4 Measurement of the dissipation factor (and capacitance) . 7
13 4.2.5 Voltage withstand test on the main wall insulation . 8
14 4.2.6 Partial discharge measurement . 8
15 4.2.7 Black out test . 9
16 4.3 Test reporting . 9
17 5 Random sample test . 9
18 5.1 Sampling . 9
19 5.2 Tests to be performed . 9
20 5.2.1 Insulation resistance measurement . 9
21 5.2.2 Thermal stability test . 9
22 5.2.3 PD measurement .10
23 5.2.4 AC Voltage test on end winding insulation .10
24 5.2.5 AC Voltage test on main wall insulation .10
25 5.2.6 Steep-front impulse test .10
26 5.3 Reporting .11
27 5.4 Result of the tests .11
28 6 Globally impregnated windings .11
29 6.1 General .11
30 6.2 Tests to be performed .11
31 Bibliography .14

prEN 50209:2026 (E)
32 European foreword
33 This document (prEN 50209:2026) has been prepared by CLC/TC 2 “Rotating Machinery”.
34 This document is currently submitted to the Enquiry.
35 The following dates are proposed:
• latest date by which the existence of this (doa) dav + 6 months
document has to be announced at national
level
• latest date by which this document has to be (dop) dav + 12 months
implemented at national level by publication of
an identical national standard or by
endorsement
• latest date by which the national standards (dow) dav + 36 months
conflicting with this document have to be (to be confirmed or
withdrawn modified when voting)
36 This document will supersede EN 50209:1998 and all of its amendments and corrigenda (if any).
37 prEN 50209:2026 includes the following significant technical changes with respect to EN 50209:1998:
38 — expanding the voltage range up to 27 kV;
39 — addition of some tests as routine tests (interturn test, surface resistance, voltage withstand test and partial
40 discharge measurement);
41 — modification of the criteria for the dissipation factor measurement;
42 — included the requirement to record the capacitance during the dissipation factor measurement.
prEN 50209:2026 (E)
43 Introduction
44 The purpose of this document is to assess the uniform quality of manufacturing and determine the dielectric
45 behaviour of the insulation of rotating electrical machines having rated voltages from 5 kV to 27 kV. It applies to
46 stator bars and stator coils.
prEN 50209:2026 (E)
47 1 Scope
48 This document applies to rotating electrical machines with rated voltages (U ) from 5 kV to 27 kV inclusive and
N
49 with rated output from 5 MVA upwards for generators and from 5 MW upwards for motors.
50 This document is also applicable to machines with rated outputs between 1 MVA (1 MW) and 5 MVA (5 MW)
51 and with rated voltages of 5 kV and above, provided its use has been agreed beforehand.
52 Requirements for machines with a rated voltage above 27 kV are the subject of individual agreement.
53 In the case of machines whose windings are cured in the stator, tests on the separate winding elements are not
54 possible; for these machines the requirements in Clause 6 apply.
55 Converter fed machines are excluded from the scope of this standard. The described tests can be used for this
56 type of machines however the criteria are mutually agreed upon between user and manufacturer.
57 2 Normative references
58 The following documents are referred to in the text in such a way that some or all of their content constitutes
59 requirements of this document. For dated references, only the edition cited applies. For undated references, the
60 latest edition of the referenced document (including any amendments) applies.
61 EN 10204:2004, Metallic products - Types of inspection documents
62 EN 60034-1:2010, Rotating electrical machines - Part 1: Rating and performance (IEC 60034-1:2010)
63 EN IEC 60034-15:2025, Rotating electrical machines - Part 15: Impulse voltage withstand levels of form-wound
64 stator coils for rotating a.c. machines (IEC 60034-15:2025)
65 EN IEC 60034-27-1:2018, Rotating electrical machines - Part 27-1: Off-line partial discharge measurements on
66 the winding insulation (IEC 60034-27-1:2017)
67 EN 60034-27-3:2016, Rotating electrical machines - Part 27-3: Dielectric dissipation factor measurement on
68 stator winding insulation of rotating electrical machines (IEC 60034-27-3:2015)
69 EN IEC 60034-27-4:2018, Rotating electrical machines - Part 27-4: Measurement of insulation resistance and
70 polarization index on winding insulation of rotating electrical machines (IEC 60034-27-4:2018)
71 3 Terms and definitions
72 For the purposes of this document, the following terms and definitions apply.
73 ISO and IEC maintain terminology databases for use in standardization at the following addresses:
74 — ISO Online browsing platform: available at https://www.iso.org/obp/
75 — IEC Electropedia: available at https://www.electropedia.org/
76 3.1
77 winding set
78 collection of winding elements needed to produce a complete stator including additional elements intended for
79 (destructive) quality assessment and spares
80 3.2
81 coil side
82 either of the two, normally straight, parts of a coil which lie essentially in the axial direction of the machine
83 [SOURCE: IEV 411-38-06]
prEN 50209:2026 (E)
84 3.3
85 multiturn coil
86 physical assembly of one or more electrical coil sections generally surrounded by common insulation comprising
87 a conductor or group of conductors formed into two or more conducting loops
88 [SOURCE: IEV 411-38-01, 411-38-03, modified]
89 3.4
90 conductive slot coating
91 OCP
92 conductive paint or tape layer in intimate contact with mainwall insulation in the slot portion of the coil/bar side,
93 often called semi-conductive coating
94 Note 1 to entry: The purpose is to prevent partial discharge from occurring between the coil/bar and the stator core.
95 4 Routine tests
96 4.1 Number of elements to be tested
97 The routine test shall be carried out on the number of bars or coil sides, including any spare bars or coils, as
98 shown in Table 1. Exception is the test on the parallel conductor insulation; this test shall be performed on all
99 elements.
100 Table 1 — Number of elements to be tested
Rated output
Number of poles Number of test elements
MVA (MW)
10 % of bars or coil sides
All pole numbers < 5
with a minimum of 20
All bars or coil sides
Two (2) and four (4) ≥ 50
(100 %)
At least 60 bars or coil
sides and, in addition,
Two (2) and four (4) < 50, ≥ 5
10 % of all bars or coil
sides
At least 60 bars or coil
sides and, in addition,
Six (6) and more ≥ 5
10 % of all bars or coil
sides
101 4.2 Tests to be performed
102 4.2.1 Voltage withstand test between parallel conductors (bars versus coils)
103 The parallel conductor insulation of Roebel bars shall be tested using an AC voltage of minimum 110 V, 50 Hz
104 before applying the main wall insulation. Alternatively, a DC voltage of 250 V may be used, particularly in case
105 the conductors are interconnected to the inner corona protection (when present). In that case a minimum
106 insulation resistance of 200 Ω is required.
107 In case of multiturn stator coils, the parallel conductor insulation shall be tested using an AC voltage sufficiently
108 high enough in relation with the nature of the single conductor insulation. The minimum level is 400 V, 50/60 Hz.
109 NOTE For half-coils without Roebel transpositions reference is made to the procedure for multiturn coils.
prEN 50209:2026 (E)
110 4.2.2 Interturn test
111 This test is only performed on multiturn coils. As a default the test is carried out as a steep-front impulse test
112 according to the provisions set in EN IEC 60034-15:2025, Clause 6 and Annex D. In case the coil is built up
113 from turns with parallel conductors stacked in the height and no dedicated turn insulation is present, an AC
114 voltage withstand test is preferably used as an alternative.
115 NOTE A multiturn coil can be built from parallel conductors positioned in the width of the stack and/or in the height of
116 the stack. Only in the latter case the AC test can be used to assess the interturn insulation capability.
117 The steep-front impulse test level is depending on whether the coil end winding insulation is in its final state or
118 not. When the coil insulation is in its final state the default test levels as given by EN IEC 60034-15:2025,
119 Table 1, shall be used for the steep-front test. For more details, see EN IEC 60034-15:2025, Annex D. In case
120 of an AC voltage withstand test the levels shall be agreed upon, recommended is a level of 0,3 × U for 1 min
N
121 between turns.
122 4.2.3 Measurement of the surface resistance of the conductive slot coating (OCP)
123 The surface resistance of the OCP shall be measured and recorded at a minimum of two positions along the
124 straight portion of the bar or coil and at least at every meter for long bars/coils over 2 m length. The manufacturer
125 shall state the criteria and measurement method beforehand.
126 4.2.4 Measurement of the dissipation factor (and capacitance)
127 The dissipation factor (tanδ) and capacitance shall be measured on the elements at room temperature in relation
128 to voltage (see Figure 1) over the range of 0,2 × U at least up to 1,0 × U at intervals of 0,2 × U with increasing
N N N
129 voltage.
131 Figure 1 — Curve of tanδ against the ratio U/U
N
132 In case of coils, both coil sides shall be measured separately.
133 The measurements shall be carried out by means of a dedicated instrument, using a guard ring arrangement to
134 exclude the influence of the end winding (for instance due to the stress grading) and thus only determining the
135 dielectric losses of the main wall insulation.
136 The outer surface of the straight part of the element shall be properly connected to the earth (or the sensing
137 terminal of the test equipment) over a length equal to that of the stator core. The guard ring arrangement shall
138 lie outside this zone. Refer to EN 60034-27-3 (grounding: 5.2.1, guard rings: 5.2.2) as well as IEEE 286 (driven
139 guard technique) for further details.
140 The initial value of tanδ , the tip-up (tanδ - tanδ ) and the increment Δtanδ per measuring step of 0,2 × U
0,2 0,6 0,2 N
141 shall not exceed the values in Table 2 for rated voltages up to 27 kV irrespective whether the voltage is
142 increased or decreased during the measurement.
143 If more than 5 % of the samples
...