SIST EN 61000-4-12:2007
Electromagnetic compatibility (EMC) -- Part 4-12: Testing and measurement techniques - Ring wave immunity test
Electromagnetic compatibility (EMC) -- Part 4-12: Testing and measurement techniques - Ring wave immunity test
This part of IEC 61000 relates to the immunity requirements and test methods for electrical and electronic equipment, under operational conditions, to non-repetitive damped oscillatory transients (ring waves) occurring in low-voltage power, control and signal lines supplied by public and non-public networks. The object of this basic standard is to establish the immunity requirements and a common reference for evaluating in a laboratory the performance of electrical and electronic equipment intended for residential, commercial and industrial applications, as well as of equipment intended for power stations and substations, as applicable.
Elektromagnetische Verträglichkeit (EMV) - Teil 4-12: Prüf- und Messverfahren - Störfestigkeit gegen gedämpfte Sinusschwingungen (Ringwave)
Compatibilité électromagnétique (CEM) -- Partie 4-12: Techniques d'essai et de mesure - Essai d'immunité à l'onde sinusoïdale amortie
La présente partie de la CEI 61000 traite des méthodes d'essai des matériels électriques et électroniques dans leurs conditions d'exploitation et des exigences en matière d'immunité contre les oscillations transitoires amorties non répétitives (ondes sinusoïdales amorties) se manifestant sur les alimentations basse tension ainsi que sur les lignes de commande et de signal raccordées aux réseaux publics ou privés. L'objet de cette norme fondamentale est d'établir les exigences d'immunité et de constituer une référence commune pour l'évaluation en laboratoire des performances d'équipements électriques et électroniques destinés aux applications résidentielles, commerciales et industrielles, ainsi que d'équipements destinés aux postes électriques et sous-stations, le cas échéant. La présente norme a pour objet de définir les paramètres suivants: - tension d'essai et formes d'ondes de courant; - gammes de niveaux d'essais; - matériel d'essai; - installation d'essai; - procédure d'essai.
Elektromagnetna združljivost (EMC) - 4-12. del: Preskusne in merilne tehnike - Preskus odpornosti proti zadušenemu nihajnemu valu (IEC 61000-4-12:2006)
General Information
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Standards Content (Sample)
SIST EN 61000-4-12:2007SLOVENSKImarec 2007
STANDARD(OHNWURPDJQHWQD]GUXåOMLYRVW(0&GHO3UHVNXVQHLQPHULOQHWHKQLNHPreskus odpornosti proti zadušenemu nihajnemu valu (IEC 61000-4-12:2006)Electromagnetic compatibility (EMC) - Part 4-12: Testing and measurement techniques - Ring wave immunity test (IEC 61000-4-12:2006) !#"$%&'& *+5HIHUHQþQDãWHYLONDSIST EN 61000-4-12:2007(en)ICS33.100.20
EUROPEAN STANDARD EN 61000-4-12 NORME EUROPÉENNE
EUROPÄISCHE NORM December 2006
CENELEC European Committee for Electrotechnical Standardization Comité Européen de Normalisation Electrotechnique Europäisches Komitee für Elektrotechnische Normung
Central Secretariat: rue de Stassart 35, B - 1050 Brussels
© 2006 CENELEC -
All rights of exploitation in any form and by any means reserved worldwide for CENELEC members.
Ref. No. EN 61000-4-12:2006 E
ICS 33.100.20 Supersedes EN 61000-4-12:1995 + A1:2001
English version
Electromagnetic compatibility (EMC) Part 4-12: Testing and measurement techniques -
Ring wave immunity test (IEC 61000-4-12:2006)
Compatibilité électromagnétique (CEM) Partie 4-12: Techniques d'essai
et de mesure -
Essai d'immunité à l'onde
sinusoïdale amortie (CEI 61000-4-12:2006)
Elektromagnetische Verträglichkeit (EMV) Teil 4-12: Prüf- und Messverfahren -
Störfestigkeit gegen gedämpfte Sinusschwingungen (Ringwave) (IEC 61000-4-12:2006)
This European Standard was approved by CENELEC on 2006-11-01. 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.
Up-to-date lists and bibliographical references concerning such national standards may be obtained on application to the Central Secretariat or to any CENELEC member.
This European Standard exists 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 Central Secretariat has the same status as the official versions.
CENELEC members are the national electrotechnical committees of Austria, Belgium, Cyprus, the Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, the Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland and the United Kingdom.
EN 61000-4-12:2006
- 2 -
Foreword The text of document 77B/509/FDIS, future edition 2 of IEC 61000-4-12, prepared by SC 77B, High frequency phenomena, of IEC TC 77, Electromagnetic compatibility, was submitted to the IEC-CENELEC parallel vote and was approved by CENELEC as EN 61000-4-12 on 2006-11-01. This European Standard supersedes EN 61000-4-12:1995 + A1:2001.
It constitutes a technical revision of the characteristics and performance of the test equipment. It only addresses the ring wave immunity test. The following dates were fixed: – latest date by which the EN has to be implemented
at national level by publication of an identical
national standard or by endorsement
(dop)
2007-08-01 – latest date by which the national standards conflicting
with the EN have to be withdrawn
(dow)
2009-11-01 Annex ZA has been added by CENELEC. __________ Endorsement notice The text of the International Standard IEC 61000-4-12:2006 was approved by CENELEC as a European Standard without any modification. In the official version, for Bibliography, the following notes have to be added for the standards indicated: IEC 60068-1 NOTE
Harmonized as EN 60068-1:1994 (not modified). IEC 61000-4-5 NOTE
Harmonized as EN 61000-4-5:2006 (not modified). IEC 61010-1 NOTE
Harmonized as EN 61010-1:2001 (not modified). __________
- 3 - EN 61000-4-12:2006 Annex ZA
(normative)
Normative references to international publications with their corresponding European publications
The following referenced documents are indispensable for the application 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.
NOTE
When an international publication has been modified by common modifications, indicated by (mod), the relevant EN/HD applies.
Publication Year Title EN/HD Year
IEC 60050-161 -1) International Electrotechnical Vocabulary (IEV)
Chapter 161: Electromagnetic compatibility - -
1) Undated reference.
NORME INTERNATIONALECEIIEC INTERNATIONAL STANDARD 61000-4-12Deuxième éditionSecond edition2006-09 Compatibilité électromagnétique (CEM) – Partie 4-12: Techniques d'essai et de mesure – Essai d'immunité à l'onde sinusoïdale amortie
Electromagnetic compatibility (EMC) – Part 4-12: Testing and measurement techniques – Ring wave immunity test
Pour prix, voir catalogue en vigueur For price, see current catalogue IEC 2006
Droits de reproduction réservés
Copyright - all rights reserved Aucune partie de cette publication ne peut être reproduite ni utilisée sous quelque forme que ce soit et par aucun procédé, électronique ou mécanique, y compris la photocopie et les microfilms, sans l'accord écrit de l'éditeur. No part of this publication may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying and microfilm, without permission in writing from the publisher. International Electrotechnical Commission,
3, rue de Varembé, PO Box 131, CH-1211 Geneva 20, SwitzerlandTelephone: +41 22 919 02 11 Telefax: +41 22 919 03 00 E-mail: inmail@iec.ch
Web: www.iec.ch CODE PRIX PRICE CODE V Commission Electrotechnique InternationaleInternational Electrotechnical CommissionPUBLICATION FONDAMENTALE EN CEM
BASIC EMC PUBLICATION
61000-4-12 IEC:2006 – 3 –
CONTENTS FOREWORD.7 INTRODUCTION.11
1 Scope.13 2 Normative references.13 3 Terms and definitions.13 4 General.17 4.1 Description of the phenomenon.17 4.2 Relevant parameters.19 5 Test levels.21 6 Test equipment.21 6.1 Test generator.21 6.2 Coupling/decoupling network specifications.27 7 Test set-up.29 7.1 Test of power supply ports.33 7.2 Test of input/output ports.33 7.3 Test of communication ports.33 7.4 Earthing connections.33 7.5 Equipment under test.35 7.6 Coupling/decoupling networks.37 8 Test procedure.37 8.1 Laboratory reference conditions.39 8.2 Execution of the test.39 9 Evaluation of test results.43 10 Test report.43
Annex A (informative)
Information on test levels for the ring wave.61
Bibliography.63
Figure 1 – Waveform of the ring wave (open circuit voltage and short circuit current).19 Figure 2 – Example of schematic circuit of the test generator for ring wave.23 Figure 3 – Example of test set-up for table-top equipment using the ground
reference plane.31 Figure 4 – Example of test set-up for floor-standing equipment using the ground reference plane.31 Figure 5 – AC/DC power supply port, single phase, line-to-line test.45 Figure 6 – AC/DC power supply port, single phase, line-to-ground test.45 Figure 7 – Example of test setup for capacitive coupling on a.c. lines (3 phases) –
line L3 to line L1 coupling.47 Figure 8 – Example of test setup for capacitive coupling on a.c. lines (3 phases) –
line L3 to ground coupling.49
61000-4-12 IEC:2006 – 5 –
Figure 9 – Example of test setup for unshielded unsymmetrical interconnection lines –line-to-line and line-to-ground coupling via capacitors.51 Figure 10 – Example of test setup for unshielded unsymmetrical interconnection lines –line-to-line and line-to-ground coupling via arrestors.53 Figure 11 – Example of test setup for unshielded unsymmetrical interconnection lines –line-to-line and line-to-ground coupling via a clamping circuit.55 Figure 12 – Example of test setup for unshielded symmetrical interconnection lines (communication lines) – lines-to-ground coupling via arrestors.57 Figure 13 – Test of a system with communication ports with fast operating signals (generator output earthed).59
Table 1 – Test levels for ring wave.21
61000-4-12 IEC:2006 – 7 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION ___________
ELECTROMAGNETIC COMPATIBILITY (EMC) –
Part 4-12: Testing and measurement techniques – Ring wave immunity test
FOREWORD 1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising all national electrotechnical committees (IEC National Committees). The object of IEC is to promote international co-operation on all questions concerning standardization in the electrical and electronic fields. To this end and in addition to other activities, IEC publishes International Standards, Technical Specifications, Technical Reports, Publicly Available Specifications (PAS) and Guides (hereafter referred to as “IEC Publication(s)”). Their preparation is entrusted to technical committees; any IEC National Committee interested in the subject dealt with may participate in this preparatory work. International, governmental and non-governmental organizations liaising with the IEC also participate in this preparation. IEC collaborates closely with the International Organization for Standardization (ISO) in accordance with conditions determined by agreement between the two organizations. 2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international consensus of opinion on the relevant subjects since each technical committee has representation from all interested IEC National Committees.
3) IEC Publications have the form of recommendations for international use and are accepted by IEC National Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any misinterpretation by any end user. 4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications transparently to the maximum extent possible in their national and regional publications. Any divergence between any IEC Publication and the corresponding national or regional publication shall be clearly indicated in the latter. 5) IEC provides no marking procedure to indicate its approval and cannot be rendered responsible for any equipment declared to be in conformity with an IEC Publication. 6) All users should ensure that they have the latest edition of this publication. 7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and members of its technical committees and IEC National Committees for any personal injury, property damage or other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC Publications.
8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is indispensable for the correct application of this publication. 9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of patent rights. IEC shall not be held responsible for identifying any or all such patent rights. International Standard IEC 61000-4-12 has been prepared by subcommittee 77B: High frequency phenomena, of IEC technical Committee 77: Electromagnetic compatibility. It has the status of a basic EMC publication in accordance with IEC Guide 107, Electro-magnetic compatibility – Guide to the drafting of electromagnetic compatibility publications. This second edition cancels and replaces the first edition published in 1995 and its amendment 1 (2000), and constitutes a technical revision of the characteristics and perform-ance of the test equipment. It only addresses the ring wave immunity test.
61000-4-12 IEC:2006 – 9 –
The text of this standard is based on the following documents: FDIS Report on voting 77B/509/FDIS 77B/519/RVD
Full information on the voting for the approval of this standard can be found in the report on voting indicated in the above table. This publication has been drafted in accordance with the ISO/IEC Directives, Part 2. The committee has decided that the contents of this publication will remain unchanged until the maintenance result date indicated on the IEC web site under "http://webstore.iec.ch" in the data related to the specific publication. At this date, the publication will be
• reconfirmed; • withdrawn; • replaced by a revised edition, or • amended.
61000-4-12 IEC:2006 – 11 –
INTRODUCTION
IEC 61000 is published in separate parts according to the following structure:
Part 1: General General considerations (introduction, fundamental principles)
Definitions, terminology Part 2: Environment Description of the environment Classification of the environment Compatibility levels
Part 3: Limits Emission limits Immunity limits (in so far as they do not fall under the responsibility of the product committees) Part 4: Testing and measurement techniques
Measurement techniques Testing techniques Part 5: Installation and mitigation guidelines
Installation guidelines Mitigation methods and devices
Part 6: Generic standards Part 9: Miscellaneous Each part is further subdivided into several parts, published either as international standards or as technical specifications or technical reports, some of which have already been published as sections. Others will be published with the part number followed by a dash and a second number identifying the subdivision (example: 61000-6-1). This part is an International Standard which gives immunity requirements and test procedures related to ring waves.
61000-4-12 IEC:2006 – 13 –
ELECTROMAGNETIC COMPATIBILITY (EMC) –
Part 4-12: Testing and measurement techniques –
Ring wave immunity test
1 Scope and object This part of IEC 61000 relates to the immunity requirements and test methods for electrical and electronic equipment, under operational conditions, to non-repetitive damped oscillatory transients (ring waves) occurring in low-voltage power, control and signal lines supplied by public and non-public networks. The object of this basic standard is to establish the immunity requirements and a common reference for evaluating in a laboratory the performance of electrical and electronic equipment intended for residential, commercial and industrial applications, as well as of equipment intended for power stations and substations, as applicable. NOTE As described in IEC Guide 107, this is a basic EMC publication for use by product committees of the IEC. As also stated in Guide 107, the IEC product committees are responsible for determining whether this immunity test standard should be applied or not, and if applied, they are responsible for determining the appropriate test levels and performance criteria. TC 77 and its sub-committees are prepared to co-operate with product committees in the evaluation of the value of particular immunity tests for their products. The purpose of this standard is to define: – test voltage and current waveforms; – ranges of test levels; – test equipment; – test set-up; – test procedure. 2 Normative references The following referenced documents are indispensable for the application 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 60050(161): International Electrotechnical Vocabulary (IEV) – Chapter 161: Electro-magnetic compatibility 3 Terms and definitions
For the purpose of this document, the following terms and definitions, together with those in IEC 60050-161 apply. NOTE Several of the most relevant terms and definitions from IEC 60050-161 are presented among the definitions below. 3.1
burst sequence of a limited number of distinct pulses or an oscillation of limited duration [IEV 161-02-07]
61000-4-12 IEC:2006 – 15 –
3.2
calibration set of operations which establishes, by reference to standards, the relationship which exists under specified conditions, between an indication and a result of a measurement NOTE 1 This term is based on the "uncertainty" approach. NOTE 2 The relationship between the indications and the results of measurement can be expressed, in principle, by a calibration diagram. [IEV 311-01-09] 3.3
coupling interaction between circuits, transferring energy from one circuit to another 3.4
coupling network electrical circuit for the purpose of transferring energy from one circuit to another 3.5
decoupling network electrical circuit for the purpose of preventing test voltages applied to the EUT (equipment under test) from affecting other devices, equipment, or systems which are not under test 3.6
immunity (to a disturbance) the ability of a device, equipment, or system to perform without degradation in the presence of an electromagnetic disturbance [IEV 161-01-20]
3.7
port particular interface of the EUT with the external electromagnetic environment 3.8
rise time the interval of time between the instants at which the instantaneous value of a pulse first reaches 10 % value and then the 90 % value [IEV 161-02-05, modified] 3.9
transient (adjective and noun) pertaining to or designating a phenomenon or a quantity which varies between two consecutive steady states during a time interval short compared with the time-scale of interest [IEV 161-02-01] 3.10
verification set of operations which is used to check the test equipment system (e.g. the test generator and the interconnecting cables) and to demonstrate that the test system is functioning within the specifications given in Clause 6 NOTE 1 The methods used for verification may be different from those used for calibration. NOTE 2 The procedure of 6.1.2 and 6.2.2 is meant as a guide to insure the correct operation of the test generator and other items making up the test set-up, so that the intended waveform is delivered to the EUT. NOTE 3 For the purpose of this basic EMC standard this definition is different from the definition given in IEV 311-01-13.
61000-4-12 IEC:2006 – 17 –
4 General 4.1 Description of the phenomenon The ring wave (described in Figure 1) is a typical oscillatory transient, induced in low-voltage cables due to the switching of electrical networks and reactive loads, faults and insulation breakdown of power supply circuits or lightning. It is, in fact, the most diffused phenomenon occurring in power supply (HV, MV, LV) networks, as well as in control and signal lines. The ring wave is representative of a wide range of electromagnetic environments of residential, as well as industrial installations. It is suitable for checking the immunity of equipment in respect of the above-mentioned phenomena, which give rise to pulses characterized by sharp front-waves that, in the absence of filtering actions, are in the order of 10 ns to a fraction of µs. The duration of these pulses may range from 10 µs to 100 µs. The rise time and duration of the parameters are subject to modification, depending on the propagation media and path. The propagation of the wave in the lines (power and signal) is always subject to reflections, due to the mismatching impedance (the lines are terminated on their own loads or connected to protection devices, input line filters, etc.). These reflections produce oscillations, whose frequency is related to the propagation speed. The presence of parasitic parameters (stray capacitance of components like motors, transformer windings, etc.) are other conditioning factors. The rise time is reduced to slowing down due to the low-pass characteristic of the line involved in the propagation. This modification is more relevant for the fast rise times (in the order of 10 ns), and less relevant for values in the range of a fraction of µs. The resultant phenomenon at the equipment ports is an oscillatory transient, or ring wave. This ring wave with a defined 0,5 µs rise time and 100 kHz oscillation frequency has been determined to be typical and is widely used by some industries for testing products. Another cause of the ring wave is lightning, which itself is characterized by a unidirectional waveform (standard 1,2/50 µs pulse). Circuits subjected to the indirect effects of lightning (inductive coupling among lines) are influenced by the derivative of the primary pulse and the coupling mechanisms involved, which gives rise to oscillations. The characteristics of the resulting ring wave depend on the reactive parameters of the ground circuits, metal structures involved in the lightning current flow, and the propagation in the low-voltage transmission lines.
61000-4-12 IEC:2006 – 19 –
U/I 100 % 90 % 10 % T1 T 110 % to 40 % t Pk3 Pk4 Pk2 Pk1 IEC
1453/06
Key T1 Rise time T
Oscillation period NOTE Only Pk1 is specified for the current waveform. Figure 1 – Waveform of the ring wave (open circuit voltage and short circuit current) Other IEC standards, such as IEC 61000-4-5, refer to the 1,2/50 µs standard lightning pulse, which may be considered to be complementary to ring wave described in this document. It is the responsibility of the product committees to define the most appropriate test, according to the phenomenon considered as relevant. 4.2 Relevant parameters 4.2.1 Repetition rate The repetition rate of the transient is directly related to the frequency of occurrence of the primary phenomenon. It is higher whenever the primary cause is the load switching in control lines, and less frequent in the case of faults and lightning. The occurrence may range from 1/s to 1/month or 1/year. The repetition rate may be increased in order to reduce the duration of the test. However, it should be selected according to the characteristics of the transient protectors involved. 4.2.2 Phase angle Equipment failures related to ring wave on power supply sources can depend on the phase angle of the ac mains at which the transient is applied. When a protection element operates during a ring-wave test, power-follow might occur depending on the phase angle at which the transient occurs. Power-follow is the current from the connected power source that flows through a protective element, or from any arc in the EUT both during and following the transient.
61000-4-12 IEC:2006 – 21 –
For semiconductors, the phenomenon may be related to the conduction state of the device at the time the ring wave occurs. Semiconductor parameters that might be involved include forward and reverse recovery characteristics and secondary breakdown performance. Devices most likely to fail in a phase-related way are semiconductors involved in the power input circuitry. Other devices in different areas of the EUT might also exhibit such failure modes. 5 Test levels The preferred test levels for the ring wave applicable to power, signal and control ports of the equipment, are given in Table 1. The test level is defined as the voltage of the first peak (maximum or minimum) in the test waveform (Pk1 in Figure 1). Different levels may apply to power, signal and control ports. The level(s) used for signal and control ports shall not differ by more than one level from that used for power supply ports. Table 1 – Test levels for ring wave Level Line-to-ground kV Line-to-line kV 1 0,5 0,25 2 1 0,5 3 2 1 4 4 2 x a x x a x
can be any level, above, below or in-between the other levels. This level can be given in the product standard.
The applicability of the ring wave test, shall refer to the product specification. The test levels from Table 1 should be selected in accordance with the most realistic installation and environmental conditions.
The immunity tests are correlated with these levels in order to establish a performance level for the environment in which the equipment is expected to operate, taking into account the primary phenomena and the installation practices which determine the classes of the electromagnetic environment. The selection of the test levels should be done on the basis of the applicability to a given location or installation.
6 Test equipment 6.1 Test generator The generator output shall have the capability to operate under short-circuit conditions. A block diagram of a representative ring wave generator is shown in Figure 2.
61000-4-12 IEC:2006 – 23 –
U R1 C1 R2 L1R3 C2R4 S1 S2 IEC
1454/06
Key U:
high-voltage source
R3: 30
Ω resistor C1: energy storage capacitor R4:
12
Ω resistor C2: filter capacitor L1:
oscillating circuit coil R1:
charging resistor
S1: high-voltage switch R2: filter resistor S2: output impedance selector
Figure 2 – Example of schematic circuit of the test generator for ring wave 6.1.1 Impedance values Two values of impedance have been selected to test power supply ports: 12 Ω and 30 Ω. These values are applicable, respectively, to short and long branch circuits, corresponding to the relative distance of the power supply source from the main distribution network. They represent a technical compromise, because they include the need to test EUT ports normally interfaced with low impedance circuits, according to the values given in IEC 60816. In addition, they cover the requirements to test the transients protection devices, such as the metal oxide varistors, Zener diodes, etc., installed in the EUT. 6.1.2 Characteristics and performance of the test generator The test generator is a single-shot ring wave generator with the following characteristics, measured as it is to be applied to the EUT port. If applied via a coupling/decoupling network, the characteristics are to be as specified at the output of that network. The generator output shall be floating. This condition is necessary to test EUT control and signal ports in line-to-line mode. It is not necessary for power ports and line-to-ground mode tests of control and signal ports. The generator shall have provisions to prevent the emission of heavy disturbances that may be injected in the power supply network, or may influence the test results. Specifications: – voltage rise time (T1 in Figure 1): 0,5 µs ± 30 % (open-circuit condition); – current rise time (T1 in Figure 1): ≤1 µs (short-circuit condition);
61000-4-12 IEC:2006 – 25 –
– voltage oscillation frequency, Note 1: 100 kHz ± 10 %; – decaying (Voltage only; see Figure 1) 0,4 < Ratio of Pk2 to Pk1 < 1,1
0,4 < Ratio of Pk3 to Pk2 < 0,8
0,4 < Ratio of Pk4 to Pk3 < 0,8
No requirements for other peaks. – transients' repetition: 1 to 60 transients per minute; – output impedance, Note 2: 12 Ω and 30 Ω ± 20 % (switchable)
...
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