IEC 60947-8:2021
(Main)Low-voltage switchgear and controlgear - Part 8: Control units for built-in thermal protection (PTC) for rotating electrical machines
Low-voltage switchgear and controlgear - Part 8: Control units for built-in thermal protection (PTC) for rotating electrical machines
IEC 60947-8:2021 specifies requirements for control units, which control a switching device in response to the PTC thermistors incorporated in rotating electrical machines and the industrial application.
This second edition cancels and replaces the first edition published in 2003, Amendment 1:2006 and Amendment 2:2011. This edition constitutes a technical revision.
This edition includes the following significant technical changes with respect to the previous edition:
safety aspects related to:
general aspects;
limited energy circuits;
electronic circuits;
alignment to IEC 60947-1:2020;
wire break detection function;
the term detector is replaced by thermistor;
reference to IEC 60738-1-4.
Appareillage à basse tension - Partie 8: Unités de commande pour la protection thermique incorporée (CTP) aux machines électriques tournantes
L'IEC 60947-8:2021 spécifie les exigences pour les unités de commande qui commandent un appareil de connexion réagissant aux thermistances CTP incorporées dans les machines électriques tournantes et l’application industrielle.
Cette deuxième édition annule et remplace la première édition parue en 2003, son Amendement 1:2006 et son Amendement 2:2011. Cette édition constitue une révision technique.
Cette édition inclut les modifications techniques majeures suivantes par rapport à l’édition précédente:
des aspects liés à la sécurité concernant:
les aspects généraux;
les circuits à énergie limitée;
les circuits électroniques;
l’alignement avec l’IEC 60947-1:2020;
la fonction de détection de rupture de conducteur;
le terme détecteur est remplacé par thermistance;
la référence à l’IEC 60738-1-4.
General Information
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Standards Content (Sample)
IEC 60947-8 ®
Edition 2.0 2021-07
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Low-voltage switchgear and controlgear –
Part 8: Control units for built-in thermal protection (PTC) for rotating electrical
machines
Appareillage à basse tension –
Partie 8: Unités de commande pour la protection thermique incorporée (CTP)
aux machines électriques tournantes
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IEC 60947-8 ®
Edition 2.0 2021-07
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Low-voltage switchgear and controlgear –
Part 8: Control units for built-in thermal protection (PTC) for rotating electrical
machines
Appareillage à basse tension –
Partie 8: Unités de commande pour la protection thermique incorporée (CTP)
aux machines électriques tournantes
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 29.130.20 ISBN 978-2-8322-1009-4
– 2 – IEC 60947-8:2021 © IEC 2021
CONTENTS
FOREWORD . 5
INTRODUCTION . 7
1 Scope . 8
2 Normative references . 8
3 Terms and definitions, symbols and abbreviated terms . 9
3.1 Terms and definitions . 9
3.2 Symbols and abbreviated terms . 11
4 Void . 11
5 Characteristics of control units for built-in thermal protection (PTC) . 11
5.1 Summary of characteristics . 11
5.2 Type of equipment . 11
5.2.1 Operating temperatures of protection systems . 11
5.2.2 Rated PTC thermistor operating temperature . 11
5.2.3 Rated system operating temperature . 12
5.2.4 Maximum permissible rated operating temperature for the system . 12
5.2.5 Control unit with reset temperature . 12
5.2.6 Control unit with sensor short-circuit detection . 12
5.2.7 Control unit with sensor wire break detection . 12
5.3 Rated electrical values of the switching device of the control unit . 13
5.3.1 Rated electrical values of switching devices . 13
5.3.2 Rated voltages of a control unit . 13
5.3.3 Rated currents of a control unit . 13
5.3.4 Rated making and breaking capacities of a control unit . 13
5.4 Rated electrical values of characteristic variation related to PTC thermistors . 13
5.4.1 General . 13
5.4.2 Electrical data/ratings and characteristics related to the PTC thermistor . 13
5.4.3 Rated voltage of the PTC thermistor circuit of the control unit . 14
5.5 Control supply circuit . 14
5.6 Auxiliary circuits . 14
6 Product information . 14
6.1 Nature of information . 14
6.2 Marking . 15
6.3 Instructions for installation, operation and maintenance, decommissioning
and dismantling. 15
6.4 Environmental information . 15
7 Normal service, mounting and transport conditions . 15
8 Constructional and performance requirements . 16
8.1 Constructional requirements . 16
8.1.1 General . 16
8.1.2 Materials . 17
8.1.3 Current-carrying parts and their connections . 17
8.1.4 Clearances and creepage distances . 17
8.1.5 Vacant . 17
8.1.6 Vacant . 17
8.1.7 Vacant . 17
8.1.8 Terminals . 17
8.1.9 Vacant . 18
8.1.10 Provisions for protective earthing . 18
8.1.11 Dedicated enclosures for equipment . 18
8.1.12 Degrees of protection of enclosed equipment . 18
8.1.13 Conduit pull-out, torque and bending with metallic conduits . 18
8.1.14 Limited energy source . 18
8.1.15 Stored charge energy circuit . 20
8.1.16 Fault and abnormal conditions . 20
8.1.17 Short-circuit and overload protection of ports . 21
8.2 Performance requirements . 21
8.2.1 Operating conditions . 21
8.2.2 Abnormal conditions of service . 22
8.2.3 Dielectric properties . 22
8.2.4 Temperature rise . 22
8.2.5 Conditional short-circuit current . 22
8.2.6 Making and breaking capacities for control and auxiliary circuits . 22
8.2.7 Additional requirements and tests for equipment with protective
separation . 22
8.2.8 Operating temperature variation . 22
8.2.9 Damp heat . 22
8.2.10 Shock and vibration . 22
8.2.11 Requirements for short-circuit detection within the PTC thermistor
circuit . 23
8.2.12 Requirements for wire break detection within the PTC thermistor circuit . 23
8.3 Electromagnetic compatibility (EMC) . 23
8.3.1 General . 23
8.3.2 Immunity . 23
8.3.3 Emission . 24
9 Tests . 24
9.1 Kinds of tests . 24
9.1.1 General . 24
9.1.2 Type tests . 24
9.1.3 Routine tests . 24
9.1.4 Sampling tests . 25
9.1.5 Special tests . 25
9.2 Compliance with constructional requirements . 25
9.2.1 General . 25
9.2.2 Electrical performance of screwless-type clamping units . 25
9.2.3 Ageing test for screwless-type clamping units . 25
9.2.4 Limited energy source test . 25
9.2.5 Breakdown of components . 26
9.3 Compliance with performance requirements . 27
9.3.1 Test sequences . 27
9.3.2 General test conditions . 28
9.3.3 Performance . 28
9.3.4 Performance under conditional short-circuit current . 32
9.4 EMC tests . 32
9.4.1 General . 32
9.4.2 Immunity . 32
– 4 – IEC 60947-8:2021 © IEC 2021
9.4.3 Emission . 33
9.5 Routine and sampling tests . 34
9.5.1 General . 34
9.5.2 Operating tests on control units . 34
9.5.3 Dielectric tests . 34
9.5.4 Routine verification of switch on and switch off of Mark A control units . 34
Annex A (normative) PTC thermistors used in thermal protection systems . 35
A.1 Characteristics of association of Mark A thermistors . 35
A.2 Verification of interchangeability characteristics . 35
A.2.1 Type tests on Mark A thermistors. 35
A.2.2 Routine tests on Mark A thermistors . 36
Annex B (normative) Additional requirements and tests for equipment with protective
separation . 37
B.1 General . 37
B.2 Definitions. 37
B.3 Requirements . 37
B.3.1 Test method for implementing protective impedance . 37
B.3.2 Touch current measurement . 38
Bibliography . 40
Figure 1 – Measurement of wire break detection . 31
Figure A.1 – Characteristic curve of a typical Mark A thermistor. 36
Figure B.1 – Protection by means of protective impedance . 38
Figure B.2 – Measuring instrument . 39
Table 1 – Limits for limited energy sources without an over-current protective device . 19
Table 2 – Limits for limited energy sources with an over-current protective device . 19
Table 3 – Limits for limited energy source with current limiting impedance . 20
Table 4 – Terminal disturbance voltage limits for conducted radio-frequency emission
(for control supply input) . 33
Table 5 – Radiated emission test limits . 33
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
LOW-VOLTAGE SWITCHGEAR AND CONTROLGEAR –
Part 8: Control units for built-in thermal protection (PTC)
for rotating electrical machines
FOREWORD
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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.
IEC 60947-8 has been prepared by subcommittee 121A: Low voltage switchgear and
controlgear, of IEC technical committee 121: Switchgear and controlgear and their assemblies
for low voltage. It is an International Standard.
This second edition cancels and replaces the first edition published in 2003, Amendment 1:2006
and Amendment 2:2011. This edition constitutes a technical revision.
This edition includes the following significant technical changes with respect to the previous
edition:
a) safety aspects related to:
• general aspects;
• limited energy circuits;
• electronic circuits;
b) alignment to IEC 60947-1:2020;
– 6 – IEC 60947-8:2021 © IEC 2021
c) wire break detection function;
d) the term detector is replaced by thermistor;
e) reference to IEC 60738-1-4.
The provisions of the general rules dealt with IEC 60947-1 are applicable to this part of
IEC 60947 series where specifically called for. Clauses and subclauses, tables, figures and
annexes of the general rules thus applicable are identified by reference to IEC 60947‑1:2020.
The text of this International Standard is based on the following documents:
FDIS Report on voting
121A/424/FDIS 121A/436/RVD
Full information on the voting for its approval can be found in the report on voting indicated in
the above table.
The language used for the development of this International Standard is English.
This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in
accordance with ISO/IEC Directives, Part 1 and ISO/IEC Directives, IEC Supplement, available
at www.iec.ch/members_experts/refdocs. The main document types developed by IEC are
described in greater detail at www.iec.ch/standardsdev/publications.
A list of all the parts in the IEC 60947 series, under the general title Low-voltage switchgear
and controlgear, can be found on the IEC website.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under 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.
IMPORTANT – The "colour inside" logo on the cover page of this document indicates that it
contains colours which are considered to be useful for the correct understanding of its
contents. Users should therefore print this document using a colour printer.
INTRODUCTION
Thermal protection systems which are based on the principle of monitoring the temperature of
the protected parts constitute a simple and effective means of protecting rotating electrical
machines, called also electric motors, against excessive temperature rises, including those
caused by faults in the cooling system, or excessively high ambient temperature, whereas
systems of protection based only on monitoring the current absorbed do not ensure this type of
protection in every circumstances.
Since the operating temperature and response times of thermal protection systems are fixed in
advance, they are not often adjusted in relation to the conditions of use of the machine and,
hence, they are not completely effective for all fault conditions, or improper use of the machine.
A thermal protection system in accordance with this document can consist of a characteristic
change thermal detector which has an associated control unit to convert a point on the
characteristic of the detector to a switching function. A very large number of thermal protection
systems are in use and, in all cases, the machine manufacturer will fit the detectors in the
machine. The machine manufacturer will either supply the control unit with the machine or
specify particulars of the control unit to be used.
It is also customary for the control units to be considered as part of the control system and not
necessarily supplied with the machine. For this reason, it is considered useful to have an
interchangeable system, where the characteristics of association between the detector and the
control unit are specified. This particular system is not considered superior in any way to other
systems complying with the requirements of this document, but in some fields the practice is
likely to be that this interchangeable system will be used, as indicated by the designation
“Mark A”.
– 8 – IEC 60947-8:2021 © IEC 2021
LOW-VOLTAGE SWITCHGEAR AND CONTROLGEAR –
Part 8: Control units for built-in thermal protection (PTC)
for rotating electrical machines
1 Scope
This part of IEC 60947 series specifies requirements for control units, which control a switching
device in response to the PTC thermistors incorporated in rotating electrical machines and the
industrial application.
It specifies requirements for that type of system comprising a positive temperature coefficient
(PTC) thermistor having particular characteristics, and its associated control unit.
This document includes:
– the characteristics, construction, performance and tests of the control unit; and
– its association with a PTC thermistor designated “Mark A”.
This document does not cover:
– the incorporation of thermal protections into rotating machines and their maximum winding
temperature. See IEC 60034-11;
– use of the product within explosive atmospheres (see IEC 60079 (all parts));
– software and firmware requirements;
NOTE 1 Guidance on embedded software is given in IEC TR 63201.
– cyber security aspects (see IEC TS 63208).
NOTE 2 It is not possible to specify all the requirements for the operating characteristics of a control unit, as they
are dependent on some aspects of the PTC thermistors. Some aspects of the requirements of the thermal protector
system can only be specified when account is taken of the characteristics of the rotating machine to be protected
and the method of installation of the PTC thermistor within the machine.
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.
CISPR 11:2015, Industrial, scientific and medical equipment – Radio-frequency disturbance
characteristics – Limits and methods of measurement
CISPR 11:2015/AMD1:2016
IEC 60068-2-6:2007, Environmental testing – Part 2-6: Tests – Test Fc: Vibration (sinusoidal)
IEC 60068-2-27:2008, Environmental testing – Part 2-27: Tests – Test Ea and guidance: Shock
IEC 60417, Graphical symbols for use on equipment (available at http://www.graphical-
symbols.info/equipment)
IEC 60445, Basic and safety principles for man-machine interface, marking and identification –
Identification of equipment terminals, conductor terminations and conductors
IEC 60730-1, Automatic electrical controls – Part 1: General requirements
IEC 60738-1:2006, Thermistors – Directly heated positive temperature coefficient – Part 1:
Generic specification
IEC 60738-1:2006/AMD1:2009
IEC 60738-1-4:2008, Thermistors – Directly heated positive step-function temperature
coefficient – Part 1-4: Blank detail specification – Sensing application – Assessment level EZ
IEC 60947-1:2020, Low-voltage switchgear and controlgear – Part 1: General rules
IEC 60947-5-1:2016, Low-voltage switchgear and controlgear – Part 5-1: Control circuit devices
and switching elements – Electromechanical control circuit devices
IEC 61140:2016, Protection against electric shock – Common aspects for installation and
equipment
ISO 2859-1:1999, Sampling procedures for inspection by attributes – Part 1: Sampling schemes
indexed by acceptance quality limit (AQL) for lot-by-lot inspection
ISO 2859-1:1999/AMD1:2011
3 Terms and definitions, symbols and abbreviated terms
3.1 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC 60947-1:2020, as
well as the following terms and definitions, 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
3.1.1
built-in thermal protection
protection of certain parts (called protected parts) of a rotating electrical machine against
excessive temperatures resulting from certain conditions of thermal overload, achieved by
means of a thermal protection system, the whole or part of which is a thermally sensitive device
incorporated within the machine
3.1.2
thermal protection system
system intended to ensure the thermal protection of a rotating electrical machine by means of
a built-in PTC thermistor together with a control unit
3.1.3
control system
system to translate a particular point on the characteristic of a PTC thermistor to a switching
function on the supply to the rotating electrical machine
Note 1 to entry: The system is capable of being reset (either manually or automatically) when the temperature falls
to the reset value.
3.1.4
protected part
part of a rotating electrical machine, the temperature of which is limited to a predetermined
value by the action of the thermal protection system
– 10 – IEC 60947-8:2021 © IEC 2021
3.1.5
abrupt characteristic change PTC thermistor
PTC thermistor for which the abrupt change in resistance for one fixed temperature is able to
initiate a switching operation in the control system
3.1.6
control unit
device which converts into a switching function the variation of the characteristic of a PTC
thermistor
Note 1 to entry: The control unit can be part of other devices or systems.
3.1.7
control circuit
circuit controlling the switching device which makes and breaks the power supply
3.1.8
PTC thermistor operating temperature
thermistor temperature at which, during an increase variation of the characteristic
related to the temperature is such as to cause operation of the associated control unit
3.1.9
system operating temperature
thermistor temperature at which, during an increase of temperature, the thermistor and
control unit together cause the operation of the control unit
3.1.10
reset temperature
thermistor temperature at which, during a decrease variation of the characteristic related to the
temperature is such that it permits the resetting of the control unit
3.1.11
positive temperature coefficient thermistor
PTC thermistor
thermistor, the resistance of which increases with its increasing temperature throughout the
useful part of its characteristic
[SOURCE: IEC 60738-1:2006, 3.4, modified – Addition of alternative term.]
3.1.12
mark A thermistor
PTC thermistor having the particular characteristics described in Annex A
3.1.13
mark A control unit
control unit having the particular characteristics specified in this document and intended for
operation in conjunction with a mark A thermistor
3.1.14
short-circuit detection
control unit capable of detecting short-circuited thermistor circuits
3.1.15
wire break detection
control unit capable of detecting wire breaks within the PTC thermistor circuits
3.2 Symbols and abbreviated terms
EMC Electromagnetic compatibility
I Rated operational current (5.3.3)
e
I Conventional free air thermal current (5.3.3)
th
PTC Positive temperature coefficient
Q Amplification factor (9.3.3.14.3)
TFS System operating temperature (3.1.9)
TNF PTC thermistor operating temperature (3.1.8)
U Rated operational voltage (5.3.2)
e
U Rated insulation voltage (5.3.2)
i
U Rated impulse withstand voltage (6.1)
imp
U Rated voltage of the thermistor circuit (6.1)
r
Rated control circuit supply voltage (6.1)
U
s
4 Void
5 Characteristics of control units for built-in thermal protection (PTC)
5.1 Summary of characteristics
The characteristics of a control unit shall be stated in the following terms, where such terms are
applicable:
– type of equipment (see 5.2);
– rated electrical values of the control unit (see 5.3);
– rated electrical values of the characteristic of the PTC thermistors and its circuit (see 5.4);
– rated electrical values of the control supply circuit (see 5.5).
5.2 Type of equipment
5.2.1 Operating temperatures of protection systems
Each PTC thermistor, or PTC thermistor with control unit, shall have either a declared rated
operating temperature in accordance with 5.2.2 (TNF), or a declared rated system operating
temperature in accordance with 5.2.3 (TFS), or both.
EXAMPLE
a) Abrupt characteristic change PTC thermistor: TNF to be declared; TFS is not applicable.
b) Abrupt characteristic change PTC thermistor with its control unit: TFS can be declared. For such systems the
value of TFS can coincide with the value of TNF for the PTC thermistor itself.
5.2.2 Rated PTC thermistor operating temperature
In the case of an abrupt characteristic change PTC thermistor, the value of the TNF shall be
declared under item o) of 6.1.
It is recommended that the normal value of TNF, expressed in degrees Celsius, be selected
from the series of numbers which are multiples of five.
– 12 – IEC 60947-8:2021 © IEC 2021
5.2.3 Rated system operating temperature
When the PTC thermistor and the control unit are supplied as a combined unit, the TFS shall
be declared under item o) of 6.1.
The tolerance on the declared value of the TFS shall be ±6 K unless otherwise specified.
NOTE The tolerance is the sum of the tolerances of the PTC thermistor and the control unit.
Routine tests shall be carried out to verify correct operation under normal operating conditions
in accordance with 8.2.1.1.
5.2.4 Maximum permissible rated operating temperature for the system
The maximum permissible value of the TFS depends on both the particular PTC thermistor and
the control unit. It shall be declared under item o) of 6.1.
NOTE For any particular device, the maximum value of the TFS will be dependent on the characteristics and the
materials used in the manufacture of the PTC thermistor, or by the limits on the characteristics of the PTC thermistor
which can be modified by the range of settings available with the control unit design.
5.2.5 Control unit with reset temperature
When the reset function is provided, the reset temperature value and tolerances shall be
provided under item o) of 6.1.
The reset temperature is verified in accordance with 9.3.3.8.
To restart the machine (electric motor) after the tripping of the control unit (switched off), it is
important for the machine winding and the PTC thermistor to cool sufficiently to permit normal
machine acceleration without nuisance tripping, especially with a high inertia load. The
temperature value for restarting depends on installation and service conditions. The control unit
can be designed in order to permit a selection of different temperature values.
For a manual restarting system, the maximum temperature should be considered. For automatic
restarting systems, the machine manufacturer should consider the minimum and maximum
differential temperatures which result from choices of TNF or TFS and reset temperature with
the declared tolerance values. Differential values which are too narrow might not permit
sufficient cool-down for restarting without nuisance tripping. Differential temperatures that are
too wide can result in an excessively long machine cooling down time or resetting can be
prevented in high ambient temperatures.
5.2.6 Control unit with sensor short-circuit detection
Control unit can be declared with sensor short-circuit detection. PTC thermistors have a low
resistance and therefore a special measure is necessary to recognize a reduction of the
resistance to nearly zero by a short-circuit. For safety applications, or to increase the lifetime
of a rotating electrical machine, it is recommended to establish a short-circuit detection system
within the PTC thermistor circuit. The safety of the thermal protection, in particular, is increased
by such a short-circuit detection.
Such a short-circuit detection only identifies a short-circuit but it does not automatically cover
a defined action. All following actions depend on the configuration of the control unit and the
application.
5.2.7 Control unit with sensor wire break detection
Control unit can be declared with sensor wire break detection. The wire break detection shall
monitor the PTC thermistor circuit, detect any interruption and give a corresponding signal.
The wire break detection is based on the evaluation of the steepness of the resistance-rise in
the PTC thermistor circuit, caused by wire break in the PTC thermistor cable or caused by
improper temperature-rise of the motor. In both cases the resistance in the PTC thermistor
circuits can be higher than 100 kΩ.
5.3 Rated electrical values of the switching device of the control unit
5.3.1 Rated electrical values of switching devices
The rated electrical values of the switching devices of control units shall be declared in
accordance with 5.3.2 to 5.3.4, as appropriate.
5.3.2 Rated voltages of a control unit
The rated voltages of a control unit are the rated insulation voltage (U ) and the rated operational
i
voltage (U ) as defined in 5.3.1.2 and 5.3.1.1 of IEC 60947-1:2020.
e
5.3.3 Rated currents of a control unit
The rated currents of a control unit are the conventional free air thermal current (I ) and the
th
rated operational current (I ) as defined in 5.3.2.1 and 5.3.2.3 of IEC 60947-1:2020.
e
NOTE A control unit can be assigned a number of combinations of rated operational voltage and rated operational
current.
5.3.4 Rated making and breaking capacities of a control unit
For a control unit to which a utilization category is assigned, the utilization category shall be
declared according to 4.4 of IEC 60947-5-1:2016 and it is unnecessary to specify rated making
and breaking capacities, since these values depend directly on the utilization category and on
the rated operational voltages and currents.
5.4 Rated electrical values of characteristic variation related to PTC thermistors
5.4.1 General
The rated electrical values of the characteristic variation related to PTC thermistors shall be
declared.
5.4.2 Electrical data/ratings and characteristics related to the PTC thermistor
The detail specification of IEC 60738-1-4:2008 shall be used as sensor input reference for
control units, tolerances or limiting values for the following parameters. If necessary, electrical
data can be listed in tabular form, with reference to styles and codes as follows:
– upper/lower category temperatures (UCT/LCT);
– operating temperature range at maximum voltage;
– maximum voltage (U );
max
– zero-power resistance (RT);
– lowest resistance value (R );
min
– resistance RNF at TNF (if applicable, see Figure 2 of IEC 60738-1:2006);
– isolation test voltage (insulated PTC thermistors only);
– insulation resistance (insulated PTC thermistors only);
– response time.
– 14 – IEC 60947-8:2021 © IEC 2021
5.4.3 Rated voltage of the PTC thermistor circuit of the control unit
The rated voltage of the PTC thermistor circuit (U ) intended to be used with characteristic
r
variation related to PTC thermistors having a defined rated operational voltage shall be
declared.
The voltage U is the maximum value of voltage which appears between each pair of terminals
r
intended for the connection of a PTC thermistor circuit when a resistance, determined as below,
is connected between these terminals and when the control unit is supplied at its rated voltage.
The resistance to be used corresponds to the value of the characteristic curve when the control
unit is switched off and takes into account the number of PTC thermistors in the circuit. This
may be a maximum or minimum value depending upon the shape of the characteristic curve.
NOTE If the circuit is an AC circuit, the rated voltage is t
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