SIST EN IEC 61757-4-3:2020

SLOVENSKI STANDARD
01-november-2020
Optični senzorji - 4-3. del: Merjenje električnega toka - Polarimetrijska metoda (IEC
61757-4-3:2020)
Fibre optic sensors - Part 4-3: Electric current measurement - Polarimetric method (IEC
61757-4-3:2020)
Lichtwellenleitersensoren - Teil 4-3: Strommessung - Polarimetrisches Verfahren (IEC
61757-4-3:2020)
Capteurs fibroniques - Partie 4-3: Mesure du courant électrique - Méthode polarimétrique
(IEC 61757-4-3:2020)
Ta slovenski standard je istoveten z: EN IEC 61757-4-3:2020
ICS:
33.180.99 Druga oprema za optična Other fibre optic equipment
vlakna
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EUROPEAN STANDARD EN IEC 61757-4-3

NORME EUROPÉENNE
EUROPÄISCHE NORM
September 2020
ICS 33.180.90
English Version
Fibre optic sensors - Part 4-3: Electric current measurement -
Polarimetric method
(IEC 61757-4-3:2020)
Capteurs fibroniques - Partie 4-3: Mesure du courant Lichtwellenleitersensoren - Teil 4-3: Strommessung -
électrique - Méthode polarimétrique Polarimetrisches Verfahren
(IEC 61757-4-3:2020) (IEC 61757-4-3:2020)
This European Standard was approved by CENELEC on 2020-09-03. 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 CEN-CENELEC
Management Centre 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 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,
Turkey and the United Kingdom.

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
© 2020 CENELEC All rights of exploitation in any form and by any means reserved worldwide for CENELEC Members.
Ref. No. EN IEC 61757-4-3:2020 E

European foreword
The text of document 86C/1578/CDV, future edition 1 of IEC 61757-4-3, prepared by SC 86C "Fibre
optic systems and active devices" of IEC/TC 86 "Fibre optics" was submitted to the IEC-CENELEC
parallel vote and approved by CENELEC as EN IEC 61757-4-3:2020.
The following dates are fixed:
• latest date by which the document has to be implemented at national (dop) 2021-06-03
level by publication of an identical national standard or by endorsement
• latest date by which the national standards conflicting with the (dow) 2023-09-03
document have to be withdrawn
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CENELEC shall not be held responsible for identifying any or all such patent rights.
Endorsement notice
The text of the International Standard IEC 61757-4-3:2020 was approved by CENELEC as a
European Standard without any modification.
Annex ZA
(normative)
Normative references to international publications
with their corresponding European publications
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.
NOTE 1 Where an International Publication has been modified by common modifications, indicated by (mod), the
relevant EN/HD applies.
NOTE 2 Up-to-date information on the latest versions of the European Standards listed in this annex is available
here: www.cenelec.eu.
Publication Year Title EN/HD Year
IEC 61757 - Fibre optic sensors - Generic specification EN IEC 61757 -

IEC 61757-4-3 ®
Edition 1.0 2020-07
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Fibre optic sensors –
Part 4-3: Electric current measurement – Polarimetric method

Capteurs fibroniques –
Partie 4-3: Mesure du courant électrique – Méthode polarimétrique

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 33.180.99 ISBN 978-2-8322-8729-3

– 2 – IEC 61757-4-3:2020 © IEC 2020
CONTENTS
FOREWORD . 5
INTRODUCTION . 7
1 Scope . 8
2 Normative references . 8
3 Terms and definitions . 8
4 Components of optical current sensor using polarimetric method . 10
4.1 General descriptions . 10
4.2 Classification of Faraday elements . 11
5 Characteristic test . 12
5.1 General information . 12
5.2 Output power of light source . 12
5.3 Input power of light detector . 13
5.4 I/O characteristics . 13
5.4.1 General . 13
5.4.2 Test method . 14
5.4.3 Test procedure . 16
5.4.4 Evaluation . 18
5.5 Warm-up time . 19
5.5.1 General . 19
5.5.2 Test method . 19
5.5.3 Evaluation . 19
5.6 Current conditions for obtaining each parameter . 19
5.7 Input parameter dependency . 20
5.7.1 Frequency characteristic . 20
5.7.2 Transient characteristic . 20
5.8 External environment dependency . 22
5.8.1 Steady state temperature characteristic test . 22
5.8.2 Transient temperature characteristic test . 25
5.8.3 External magnetic field test . 27
5.8.4 Conductor position test . 28
5.8.5 Vibration test . 29
Annex A (informative) Principle of optical current sensor . 30
A.1 Outline . 30
A.2 Faraday effect . 30
A.3 Types of Faraday element . 31
A.4 Conversion of the Faraday effect into an electric signal . 31
A.4.1 Detection of the Faraday effect of nonmagnetic material . 31
A.4.2 Detection of the Faraday effect of ferromagnetic material . 31
A.5 Current detection method . 32
A.5.1 General . 32
A.5.2 Examples of current detection method . 32
Annex B (informative)  Features of optical current sensor technology . 35
Annex C (informative)  Design considerations . 36
C.1 General information . 36
C.2 Performance restricting factors . 36
C.3 Procedure for determining the specifications of the equipment . 37

IEC 61757-4-3:2020 © IEC 2020 – 3 –
Annex D (informative)  Optical current sensor output in the application of other phase
magnetic fields. 39
D.1 Ampere's circulation integral law . 39
D.2 Influence of other phase magnetic fields . 39
Annex E (informative) Measurement parameter performance table . 41
E.1 General . 41
E.2 Output power of light source . 41
E.3 Input power of light detector . 41
E.4 I/O characteristics . 41
E.5 Frequency characteristics . 42
E.6 Transient characteristics . 43
E.7 Steady state temperature characteristics . 43
E.8 Transient temperature characteristics . 44
E.9 External magnetic field. 45
E.10 Conductor positions . 46
E.11 Vibration . 47
Bibliography . 48

Figure 1 – Measurement system using optical current sensor . 10
Figure 2 – Construction of optical current sensor . 11
Figure 3 – Classification of Faraday elements . 12
Figure 4 – Example of an optical power monitor . 13
Figure 5 – Example of the amplifying circuit of a light detector . 13
Figure 6 – I/O characteristics of an optical current sensor . 14
Figure 7 – Measurement system of waveform comparison method . 15
Figure 8 – Measurement system of AC bridge method . 16
Figure 9 – Transient characteristics of AC dedicated system . 21
Figure 10 – Transient characteristics of DC/AC system . 22
Figure 11 – Configuration example of steady state temperature characteristic test and
transient temperature characteristic test of sensor part . 24
Figure 12 – Example of temperature profile. 24
Figure 13 – Birefringence change during temperature change . 25
Figure 14 – Example of temperature programme . 27
Figure 15 – Position of the outer conductor in the external magnetic field test when the
Faraday element is an optical fibre . 28
Figure 16 – Position of the conductor in the conductor position test when the Faraday
element is an optical fibre . 29
Figure A.1 – Faraday effect. 30
Figure A.2 – Configuration of current detection method using Faraday effect . 32
Figure A.3 – Basic configuration of intensity modulation type optical current sensor . 33
Figure A.4 – Configuration example of intensity modulation type reflective optical
current sensor . 33
Figure A.5 – Configuration example of interference type optical current sensor. 34
Figure D.1 – The law of Ampere's circulation integral . 39
Figure D.2 – Image diagram of incomplete closed loop . 40
Figure E.1 – Example of the transient characteristic . 43

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Figure E.2 – Example of the temperature characteristics at current 0 . 43
Figure E.3 – Example of the temperature characteristics at rated current . 44
Figure E.4 – Example of the transient temperature characteristics at input current 0 . 44
Figure E.5 – Example of the transient temperature characteristics at rated current . 45
Figure E.6 – Positions of the outer conductor . 46
Figure E.7 – Positions of the conductor in the conductor positions test . 46
Figure E.8 – Example of the vibration test at current 0 . 47
Figure E.9 – Example of the vibration test at rated current . 47

Table 1 – List of parameters to be obtained . 12
Table 2 – Test method . 15
Table 3 – Current conditions for obtaining each parameter . 19
Table E.1 – Output power of light source . 41
Table E.2 – Input power of light detector . 41
Table E.3 – I/O characteristics . 41
Table E.4 – Frequency characteristics . 42
Table E.5 – External magnetic field . 45
Table E.6 – Conductor position . 46

IEC 61757-4-3:2020 © IEC 2020 – 5 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
FIBRE OPTIC SENSORS –
Part 4-3: Electric current measurement –
Polarimetric method
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 itself does not provide any attestation of conformity. Independent certification bodies provide conformity
assessment services and, in some areas, access to IEC marks of conformity. IEC is not responsible for any
services carried out by independent certification bodies.
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 61757-4-3 has been prepared by subcommittee SC 86C: Fibre optic
systems and active devices, of IEC technical committee TC 86: Fibre optics.
The text of this International Standard is based on the following documents:
CDV Report on voting
86C/1578/CDV 86C/1611/RVC
Full information on the voting for the approval of this International Standard can be found in the
report on voting indicated in the above table.
This document has been drafted in accordance with the ISO/IEC Directives, Part 2.
A list of all parts in the IEC 61757 series, published under the general title Fibre optic sensors,
can be found on the IEC website.

– 6 – IEC 61757-4-3:2020 © IEC 2020
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under "http://webstore.iec.ch" in the data related to
the specific document. At this date, the document will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
IEC 61757-4-3:2020 © IEC 2020 – 7 –
INTRODUCTION
Current measuring techniques are essential for controlling and diagnosing apparatus that
support industry and society. As current measuring devices, optical current sensors based on
magneto-optic effect have been developed. As these sensors enable advanced current
measurement free from the issues related to conventional current sensors based on
electromagnetic induction, they have been applied in various fields including power systems.
Given the expectations for the potential of this sensing technology, various kinds of optical
current sensors for various applications have been proposed by manufacturers. With this
background, there are many kinds (target current for measurement, configuration of sensor,
signal processing method, installation method) of optical current sensors for various
applications. When developing a new optical current sensor, the evaluation and design of
performance and characteristics are carried out in each case.
For promoting the dissemination of optical current sensors, it is important to define the terms
representing performance and functionality of the optical current sensor, which is manufactured
on the basis of sensing technology. It is also important to make clear how to evaluate such
terms. This makes it possible to design the sensor efficiently and properly and to transfer the
sensor smoothly from a supplier to a user by settling these issues. Under these circumstances,
a set of methods is summarized in this document for evaluating the performance and
characteristics of optical current sensors. As the required performance for a sensor depends
on its application, the performance is not defined quantitatively in this document. However, with
the help of this document, the quantitative measures of sensor performance will be defined in
designing the sensor itself in anticipation of its practical application.
This document is based on standard OITDA FS 01 published by the Optoelectronics Industry
and Technology Development Association (OITDA).

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FIBRE OPTIC SENSORS –
Part 4-3: Electric current measurement –
Polarimetric method
1 Scope
This part of IEC 61757 defines terminology, structure, and a characteristic test method of an
optical current sensor using the polarimetric method. It addresses the current sensing element
only and not the additional devices that are unique to each application. Generic specifications
for fibre optic sensors are defined in IEC 61757.
As the specifications of optical polarimetric fibre current sensors required by each user vary
depending on the application, this document does not define the required performance values.
The required performance values are defined when designing a sensor according to the specific
application.
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 61757, Fibre optic sensors – Generic specification
3 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC 61757 and the
following apply.
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
current conducting coil
air-core coil made of lead wires that applies electric current, which is used to apply the equal
magnetic field generated by the current to be measured to an optical fibre or a bulk-form
Faraday element when conducting a characteristic test of optical current sensor
3.2
external magnetic field
magnetic field generated from anywhere other than the conductor where the current to be
measured is passing in an optical current sensor
3.3
Faraday effect
circular birefringence that is generated when an external magnetic field is imposed on a
substance
IEC 61757-4-3:2020 © IEC 2020 – 9 –
Note 1 to entry: The Faraday effect is a kind of magneto-optical effect. "Magneto-optical effect" is a general term
that means the effect of a magnetic field on the optical characteristics of particles or crystal. In addition to the Faraday
effect, there are other magneto-optical effects such as the Zeeman effect, magnetic birefringence, magnetic circular
dichroism, the magnetic Kerr effect, and magneto-oscillatory absorption.
3.4
Faraday element
optical element for detecting Faraday effect
3.5
Faraday mirror
reflecting mirror that rotates the polarization angle by the Faraday effect
3.6
intensity modulation method
method of converting the rotation of a polarization plane to light intensity and generating an
optical signal that corresponds to the current to be measured by passing light, first through a
sensing element and then through a polarization separation element, in an optical current
sensor
3.7
interferometric method
method of generating an optical signal that corresponds to the current to be measured by an
optical current sensor by converting the left-handed and right-handed circularly polarized light
that passed through the Faraday element to the same polarization, then interfering with each
other to convert the polarized light to the light intensity
3.8
maximum measurable current
maximum measurable value of the current to be measured by an optical current sensor
3.9
maximum measurable frequency
maximum measurable frequency of the current to be measured by an optical current sensor
3.10
minimum measurable frequency
minimum measurable frequency of the current to be measured by an optical current sensor
3.11
operating temperature range
range of temperatures within which an optical current sensor satisfies the defined performances
3.12
optical current sensor
part, module, sub-assembly, assembly, or equipment that measures the electric current using
fibre optic technology
Note 1 to entry: Optical current sensors are commonly used with power supplies, user interface, and
electromagnetic shields, as shown in Figure 1. The output signal is arranged in a signal form required by the output
interface, and a signal is sent to an application system such as an oscilloscope or a control system. An optical current
sensor using the polarimetric method consists of the sensor part, optical transmission part, and signal processing
part (see Clause 4).
– 10 – IEC 61757-4-3:2020 © IEC 2020

Key
I/F interface
Figure 1 – Measurement system using optical current sensor
3.13
optical part
part consisting of lens, prism, mirror, and optical element, such as a phase modulator, in an
optical current sensor
Note 1 to entry: While the term "sensor part" focuses on the component position (see Clause 4), the term "optical
part" focuses on the component materials.
3.14
outer conductor
conductor other than the conductor in which the current to be measured is passing in an optical
current sensor
3.15
rated current
value of the current to be measured by an optical current sensor that is used as a basis for
showing the performance in a given test under defined conditions
3.16
required specifications
list of specifications an optical current sensor satisfies
3.17
spun optical fibre
optical fibre that is manufactured by rotating a preform at high speed in the drawing process
3.18
transient characteristic
phenomenon of changing the current value that is output from an optical current sensor when
the current to be measured fluctuates from the defined current value in a short period of time
4 Components of optical current sensor using polarimetric method
4.1 General descriptions
Figure 2 shows the construction of an optical current sensor. For exposing each part to a
different environment, this document focuses on the position of each part and divides the sensor
into three parts.
IEC 61757-4-3:2020 © IEC 2020 – 11 –
In the optical current sensor, a portion of a Faraday element that is adjacent to the conductor
and is affected by the Faraday effect is connected with a portion that houses a processing
circuit outputting a current calculation value via optical fibre, and each portion is exposed to a
different environment in general. In this case, while a portion in which the Faraday element is
arranged and exposed to the same environment is called a sensor part, a portion containing a
processing circuit for outputting the current calculation value is called the signal processing
part, and an optical fibre that connects the sensor part and the signal processing part is called
the optical transmission part. For the specific functions of each part, see Annex A.
NOTE The sensor part can have elements of controlling polarization and phase at the same time in addition to
Faraday elements. The signal processing part can have elements of controlling polarization and phase at the same
time in addition to a light source, a power supply, and a light detector.
For the features expected of an optical current sensor, see Annex B. For design considerations
of an optical current sensor, see Annex C.

Figure 2 – Construction of optical current sensor
4.2 Classification of Faraday elements
The classification of Faraday elements is shown in Figure 3. As a method of propagating light
in a Faraday element, a waveguide type element is used. This is an element having a waveguide
structure for guiding light by providing difference in refractive index in a sensor. While
candidates for the waveguide type element include optical fibres and planar waveguides, only
the optical fibre type is currently put to practical use.
There is also a bulk type element that guides light into a sensor using a lens and mirror without
utilizing a difference in refractive index. The element made of a nonmagnetic material and that
made of a ferromagnetic material are distinguished from each other based on the element
material used.
Bulk type sensors use optical fibres for signal transmission, and the optical fibres themselves
are not Faraday elements.
This document is applicable to both waveguide element and bulk element sensors using optical
fibre technology.
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Figure 3 – Classification of Faraday elements
5 Characteristic test
5.1 General information
Clause 5 specifies a characteristic test method of the optical current sensor. Output power of
light source, described in 5.2, and input power of light detector, described in 5.3, shall be
measured at the beginning of a characteristics test to confirm normal operation of the light
source and optical part. The input-output (I/O) characteristics described in 5.4 are the basis of
the test. Subclause 5.5 describes the warm-up time, which often is not considered in
conventional current sensors. In 5.6 through 5.8, definition of the dependency of parameters is
made that is recommended for testing and the dependence on external environment in the test
method for each factor. Subclause 5.6 describes the current condition for obtaining each
parameter. The parameter list to be acquired is shown in Table 1. For each parameter,
requirement (R) or option (O) is shown. Measurement results are summarized in the inspection
report (see Annex E) and shown to the user.
Table 1 – List of parameters to be obtained
No. Parameters Requirement (R) or option (O)
1 Output power of light source R
2 Input power of light detector
3 I/O characteristics
4 Warm-up time
5 Parameter dependency Input parameter Frequency characteristic R for type test
dependency
Transient characteristic R for type test
External environment Steady state temperature R for type test
dependency characteristic
O in routine test for outdoor
use sensor
Transient temperature R for type test
characteristic
O for routine test for outdoor
use sensor
External magnetic field O
Conductor position O
Vibration O
5.2 Output power of light source
Output power of the light source shall be measured in a routine test using one of the following
methods:
a) measure the output power of the light source by optical power meter;
b) measure the output signal of the optical power monitor.

IEC 61757-4-3:2020 © IEC 2020 – 13 –
Figure 4 is an example of the optical power monitor. The light source is often provided with a
photodiode for the power monitor, which can be used. By amplifying the signal of the photodiode,
an output of optical power monitor is obtained.

Figure 4 – Example of an optical power monitor
5.3 Input power of light detector
Input power of the light detector shall be measured in a routine test using one of the following
methods:
a) measure the input power of the light detector by optical power meter;
b) measure the output signal of the light detector.
For example, as shown in Figure 5, the output of the light detector can be amplified.

Figure 5 – Example of the amplifying circuit of a light detector
5.4 I/O characteristics
5.4.1 General
I/O characteristics are the most basic characteristics of optical current sensors. Figure 6 shows
the I/O characteristics of the optical current sensor. Ideally, the current to be measured and the
output instruction value are the same, but an error actually occurs. The error is classified into
the following three factors.
a) Noise
Unnecessary output. Particularly, the DC components are called offsets and should be
distinguished from noise. Some noises are correlated with the current to be measured, and
some noises are not. Therefore, they shall be acquired separately.

– 14 – IEC 61757-4-3:2020 © IEC 2020
b) Sensitivity change
A phenomenon of changing sensitivity that is the proportionality coefficient of the output
against the input.
c) Non-linearity
The phenomenon that the sensitivity changes, and the relationship between the output and
the input deviates from a straight line.
NOTE 1 When a ferromagnetic material is used as a Faraday element, nonlinearity due to magnetic saturation
appears.
NOTE 2 When a non-magnetic material is used as a Faraday element, no saturation phenomenon appears.
However, nonlinearity due to the signal processing method sometimes appears.

Figure 6 – I/O characteristics of an optical current sensor
These errors give the sensor output fluctuation as indicated by the dotted line in Figure 6.
Since the above error behaves differently against error factors defined in 5.8.1 and 5.8.5, etc.,
these errors shall be obtained for noise, sensitivity change, and nonlinearity, respectively.
Generally, in the optical current sensor, when the current to be measured is larger than a certain
value, the output saturates, and the phenomenon of output decrease is seen, as shown in Figure
6. The manufacturer of the optical current sensor specifies the maximum measurable current
at a value that is lower than the current at which the output signal no longer increases with
increasing current due to such saturation and defines said value in the specifications. The
manufacturer also determines the rated current as the upper limit of current measurement with
high accuracy without being affected by saturation and defines it in the specifications.
Due to noise, sensitivity change and non-linearity, the actual sensor output varies between the
solid line and the broken line in Figure 6.
5.4.2 Test method
Table 2 shows test methods for grasping error components consisting of noise, sensitivity
change, and nonlinearity of the optical current sensor. The test shall be performed by a
waveform comparison method using a waveform recording device such as an oscilloscope.
However, when testing a nonlinearity of 1 % or less with an analogue output, and if sufficient
accuracy is not achieved in a waveform recording device such as an oscilloscope, the bridge
method can apply in addition to the waveform comparison one.

IEC 61757-4-3:2020 © IEC 2020 – 15 –
Table 2 – Test method
Items In the case of In the case of
digital output analogue output
General measurement General measurement High accuracy
measurement
a
Noise Waveform comparison Waveform comparison
NA
method method
Sensitivity change Waveform comparison Waveform comparison Bridge method
method method
Nonlinearity Waveform comparison Waveform comparison Bridge method
method method
a
Not applicable.
Following are examples of test configurations using the waveform comparison method and the
AC bridge method.
a) Waveform comparison method
Figure 7 shows an example of the test configuration of the characteristic test using the
waveform comparison method. A current transformer or a shunt resistor can be used as a
standard for comparison. Figure 7 is an example using a current transformer. The current
measured by the current transformer and the output of the optical current sensor are
measured and recorded in a waveform recording device such as an oscilloscope or a data
logger. The current transformer or shunt resistor and the waveform recording apparatus
shall have sufficient accuracy in the test frequency band. Data shall be recorded as digital
data in order to calculate errors and phase difference in 5.4.4. To avoid aliasing errors
occurring at the time of digitization, an anti-alias filter that sufficiently attenuates signals
other than the measuring frequency band of the waveform recording apparatus shall be
installed before the waveform recording apparatus.

Figure 7 – Measurement system of waveform comparison method
b) AC bridge method
In the case of a waveform recording apparatus such as an oscilloscope, if the accuracy is
insufficient, a null method should be used concurrently. Figure 8 shows an example of a test
configuration with an optical current sensor with analogue voltage output. A current
transformer or a shunt resistor can be used as a standard for comparison. An impedance

– 16 – IEC 61757-4-3:2020 © IEC 2020
used for balancing with the optical current sensor is a combination of a shunt resistor, a
variable resistor, and a variable capacitor. A sensitivity change of the optical current sensor
is obtained from the equilibrium point. As only sensitivity change and nonlinearity prescribed
in 5.4.1 can be obtained in this method, noise shall be acquired by the waveform comparison
method as described in a).
Figure 8 – Measurement system of AC bridge method
5.4.3 Test procedure
5.4.3.1 In the case of AC or AC/DC dual use
To understand the noise, sensitivity change, and nonlinearity described in 5.4.1, the waveform
of the standard for comparison shall be compared with the output waveform of the optical
current sensor while changing the current to be measured. This comparison is conducted at the
test current values listed below, where the current to be measured ranges from zero to the
maximum measurable current.
a) At zero current
1) Noise
The noise intensity should be obtained at each frequency using a frequency analysis
(fast Fourier transform) function of an oscilloscope as much as possible. In particular, it
is highly likely that the frequency component of the current to be measured and its
harmonics adversely affect the system.
b) At the noise equivalent current and at approximately twice that value
1) Separate the noise into a component that is correlated with the current and into one that
is not correlated with the current.
2) In the case where the noise I is included, the amplitude of the AC component of the
noise
optical current sensor output I is expressed by Equation (1).
O
I = I + I (1)
O signal noise
where
I is the amplitude of the AC component of the optical fibre current output;
O
I is the value of the current to be measured;
signal
I is the noise.
noise
IEC 61757-4-3:2020 © IEC 2020 – 17 –
2) If I is not correlated with the current, which is the case when the noise I
noise noise
disappears in Equation (1) after a sufficiently long measurement time over which the
waveform is averaged, then Equation (1) becomes Equation (2).
I = I (2)
O signal
c) At several points between the current that are approximately twice
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