IEC 60255-149:2013

Measuring relays and protection equipment - Part 149: Functional requirements for thermal electrical relays

IEC 60255-149:2013 specifies minimum requirements for thermal protection relays. This standard includes specification of the protection function, measurement characteristics and test methodologies. The object is to establish a common and reproducible reference for evaluating dependent time relays which protect equipment from thermal damage by measuring a.c. current flowing through the equipment. Complementary input energizing quantities such as ambient, coolant, top oil and winding temperature may be applicable for the thermal protection specification set forth in this standard. This standard covers protection relays based on a thermal model with memory function.

Relais de mesure et dispositifs de protection - Partie 149: Exigences fonctionnelles pour les relais électriques thermiques

La CEI 60255-149:2013 spécifie les exigences minimales relatives aux relais de protection thermique. Cette norme inclut la spécification de la fonction de protection, les caractéristiques de mesure et les méthodes d'essais. L'objet est d'établir une référence commune et reproductible pour des relais à temps dépendant qui protègent un dispositif contre les détériorations thermiques par la mesure du courant alternatif circulant dans le dispositif. Des grandeurs d'alimentation d'entrée complémentaire, telles que la mesure de la température ambiante, du liquide de refroidissement, de l'huile isolante, des enroulements, peuvent être applicables pour la spécification de protection thermique définie dans la présente norme. La présente norme couvre les relais de protection basés sur un modèle thermique à fonction de mémoire.

General Information

• Status: Published
• Publication Date: 29-Jul-2013
• ICS: 29.120.70 - Relays
• ICS: 29.140.20 - Incandescent lamps
• Technical Committee: TC 95 - Measuring relays and protection equipment
• Drafting Committee: MT 4 - TC 95/MT 4
• Current Stage: PPUB - Publication issued
• Start Date: 30-Jul-2013
• Completion Date: 31-Aug-2013

Overview

IEC 60255-149:2013 is an international standard developed by the International Electrotechnical Commission (IEC) focused on functional requirements for thermal electrical relays. These relays are critical devices used in electric power systems to protect equipment from thermal damage by accurately measuring alternating current (AC) and other thermal parameters. The standard establishes minimum performance criteria, definitions, and test methodologies to ensure consistent and reliable operation of thermal protection relays globally.

The purpose of IEC 60255-149 is to provide a common and reproducible reference for evaluating dependent time relays that protect electrical equipment from overheating, ensuring safety, equipment longevity, and reduced downtime in power systems and industrial installations.

---

Key Topics

• Thermal Protection Function Specification
  The standard defines the core protection function, detailing how relays measure AC current and calculate thermal loading on equipment based on a thermal model with memory function.

• Input Energizing Quantities
  Besides the primary AC current, other factors such as ambient temperature, coolant condition, top oil temperature, and winding temperature may influence relay operation.

• Functional Logic and Thermal Modeling
  IEC 60255-149 includes mathematical models for thermal level calculations, equivalent heating current, and time-current limit characteristics, enabling precise relay response to thermal stress.

• Binary Input and Output Signals
  The standard specifies interfaces and functional logic for binary signals such as operate (trip) outputs, alarm signals, and other control indicators.

• Performance and Testing
  Detailed performance specifications include accuracy of operating current measurement, operate times, response to frequency variations, and performance under harmonic distortions. It also describes test methodologies for steady-state errors and response under different thermal conditions.

• Environmental and Operational Considerations
  The standard addresses the effect of environmental factors like ambient temperature on relay behavior and requirements for thermal reset functionality and auxiliary power supply failure handling.

• Documentation Requirements
  IEC 60255-149 mandates comprehensive test reports and user documentation to support installation, operation, and maintenance of thermal relays.

---

Applications

Thermal electrical relays standardized as per IEC 60255-149:2013 are extensively used in:

• Power Generation and Distribution
  Protecting transformers, motors, generators, and other electrical apparatus from thermal overload.

• Industrial Automation
  Monitoring electrical equipment to prevent overheating and related failures in manufacturing processes.

• Energy Management Systems
  Integrating thermal protection relays for predictive maintenance and enhanced safety monitoring.

• Renewable Energy Installations
  Ensuring reliable thermal protection in wind turbines, solar inverters, and hybrid systems.

By adhering to this standard, engineers and utility operators achieve reliable, safe, and predictable relay protection, reducing the risk of costly equipment damage and improving system availability.

---

Related Standards

IEC 60255-149:2013 fits within a broader series of standards for measuring relays and protection equipment, including:

• IEC 60255-8 - Previous standard on thermal protection relays, now replaced.
• IEC 60085 - Insulation coordination and temperature limits for electrical equipment.
• IEC 60255 series in general - Covers diverse relay protection functions such as overcurrent, distance, and differential protection.
• ISO/IEC Directives Part 2 - Governs preparation and drafting of international standards related to electrotechnical equipment.

For electrical engineers and protection specialists, cross-referencing these standards ensures comprehensive understanding and compliance in relay protection system design.

---

By conforming to IEC 60255-149:2013, manufacturers and users of thermal electrical relays can guarantee operational safety, accuracy, and interoperability, maintaining global industry standards in electrical power protection systems.

Standards Content (Sample)

IEC 60255-149 ®
Edition 1.0 2013-07
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Measuring relays and protection equipment –
Part 149: Functional requirements for thermal electrical relays

Relais de mesure et dispositifs de protection –
Partie 149: Exigences fonctionnelles pour relais électriques thermiques

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IEC 60255-149 ®
Edition 1.0 2013-07
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Measuring relays and protection equipment –

Part 149: Functional requirements for thermal electrical relays

Relais de mesure et dispositifs de protection –

Partie 149: Exigences fonctionnelles pour relais électriques thermiques

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
CODE PRIX X
ICS 29.120.70 ISBN 978-2-8322-1005-5

– 2 – 60255-149 © IEC:2013
CONTENTS
FOREWORD . 4
1 Scope . 6
2 Normative references . 6
3 Terms and definitions . 7
4 Specification of the function . 8
4.1 General . 8
4.2 Input energizing quantities/energizing quantities . 9
4.3 Binary input signals . 9
4.4 Functional logic . 10
4.4.1 Equivalent heating current . 10
4.4.2 Basic (setting) and operating current values for thermal protection . 10
4.4.3 Thermal level calculation . 11
4.4.4 Time-current limit characteristic equations and curves . 12
4.4.5 Thermal level alarm threshold . 14
4.5 Binary output signals . 15
4.5.1 General . 15
4.5.2 Operate (trip) output signal . 15
4.5.3 Alarm signal . 15
4.5.4 Other binary output signals . 15
4.6 Additional influencing factors on thermal protection . 16
4.6.1 General . 16
4.6.2 Influence of ambient temperature on thermal protection . 16
4.6.3 Thermal reset facilities . 16
4.7 Behaviour of thermal protective device during auxiliary power supply failure . 17
5 Performance specification . 17
5.1 Accuracy related to the characteristic quantity . 17
5.2 Accuracy related to the operate time . 17
5.3 Performance during frequency variations . 18
6 Functional test methodology . 18
6.1 General . 18
6.2 Determination of steady-state errors related to the operating current value . 19
6.3 Determination of steady-state errors related to the characteristic quantity and
the operate time . 19
6.3.1 Accuracy determination of the cold curve . 19
6.3.2 Accuracy determination of the hot curves . 20
6.4 Performance with specific cooling thermal time constant . 21
6.5 Performance with harmonics . 22
6.6 Performance during frequency variations . 22
6.7 Performance during different ambient temperatures . 23
7 Documentation requirements . 24
7.1 Type test report . 24
7.2 Other user documentation . 24
Annex A (informative) Simple first-order thermal model of electrical equipment. 26
Annex B (informative) Thermal electrical relays which use temperature as setting
parameters . 41
Bibliography . 46

60255-149 © IEC:2013 – 3 –
Figure 1 – Simplified thermal protection function block diagram . 9
Figure 2 – Typical examples of characteristic curves for cold state of a first-order
thermal system with no previous load before overload occurs . 13
Figure 3 – Typical examples of characteristic curves for hot states of a first-order
thermal system for different values of previous load before overload occurs . 14
Figure A.1 – An electrical equipment to be thermally protected represented as a
simple first-order thermal system . 26
Figure A.2 – Equivalence between a first-order thermal system and an electric parallel
RC circuit . 30
Figure A.3 – Analogue thermal circuit representation of a simple first-order thermal
system . 31
Figure A.4 – Analogue thermal circuit representation of a simple first-order thermal
system – motor starting condition . 31
Figure A.5 – Analogue thermal circuit representation of a simple first-order thermal
system – motor stopped condition . 31
Figure A.6 – Dynamic step response of a simple first-order thermal system algorithm to
a current below pickup . 33
Figure A.7 – Dynamic step response of a first-order thermal system (cold initial state) . 34
Figure A.8 – Dynamic step response of a first-order thermal system (hot initial state) . 34
Figure A.9 – Dynamic step response of a first-order thermal system to a load current
followed by an overload current (initial state: cold) . 35
Figure A.10 – Dynamic step response of a first-order thermal system to a load current
followed by an overload current (initial state: hot) . 35

Table 1 – Limiting error as multiples of assigned error . 18
Table 2 – Test points of the cold curve . 20
Table 3 – Test points of the hot curve . 21
Table 4 – Test points of the cold curve with harmonics . 22
Table 5 – Test points of the cold curve during frequency variations . 22
Table A.1 – Thermal and electrical models . 30
Table A.2 – Thermal insulation classes and maximum temperatures, according to
IEC 60085. 40
Table A.3 – Example of correction factor values (F ) for class F equipment according
a
to the ambient temperature (T ) . 40
a
– 4 – 60255-149 © IEC:2013
INTERNATIONAL ELECTROTECHNICAL COMMISSION
______________
MEASURING RELAYS AND PROTECTION EQUIPMENT –

Part 149: Functional requirements for thermal electrical relays

FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
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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 60255-149 has been prepared by IEC technical committee 95:
Measuring relays and protection equipment.
This first edition cancels and replaces IEC 60255-8, published in 1990.
The text of this standard is based on the following documents:
FDIS Report on voting
95/313/FDIS 95/317/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.

60255-149 © IEC:2013 – 5 –
A list of all parts of IEC 60255 series, under the general title Measuring relays and protection
equipment, can be found on the IEC website.
Future standards in this series will carry the new general title as cited above. Titles of existing
standards in this series will be updated at the time of the next edition.
The committee has decided that the contents of this publication will remain unchanged until
the stability 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.
IMPORTANT – The 'colour inside' logo on the cover page of this publication 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.
– 6 – 60255-149 © IEC:2013
MEASURING RELAYS AND PROTECTION EQUIPMENT –

Part 149: Functional requirements for thermal electrical relays

1 Scope
This part of the IEC 60255 series specifies minimum requirements for thermal protection
relays. This standard includes specification of the protection function, measurement
characteristics and test methodologies.
The object of this standard is to establish a common and reproducible reference for evaluating
dependent time relays which protect equipment from thermal damage by measuring a.c.
current flowing through the equipment. Complementary input energizing quantities such as
ambient, coolant, top oil and winding temperature may be applicable for the thermal protection
specification set forth in this standard. This standard covers protection relays based on a
thermal model with memory function.
The test methodologies for verifying performance characteristics of the thermal protection
function and accuracy are also included in this Standard.
This standard does not intend to cover the thermal overload protection trip classes indicated
in IEC 60947-4-1 and IEC 60947-4-2, related to electromechanical and electronic protection
devices for low voltage motor-starters.
The thermal protection functions covered by this standard are as follows:

Protection function IEC 61850-7-4 IEEE C37.2
PTTR
Thermal overload protection 49
Rotor thermal overload protection PROL 49R
Stator thermal overload protection PSOL 49S

General requirements for measuring relays and protection equipment are specified in
IEC 60255-1.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and
are indispensable for its application. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any
amendments) applies.
IEC 60050 (all parts), International Electrotechnical Vocabulary (available at
http://www.electropedia.org)
IEC 60085, Electrical insulation – Thermal evaluation and designation
IEC 60255-1, Measuring relays and protection equipment – Part 1: Common requirements
IEC 61850-7-4, Communication networks and systems for power utility automation – Part 7-4:
Basic communication structure – Compatible logical node classes and data classes

60255-149 © IEC:2013 – 7 –
3 Terms and definitions
For the purpose of this document, the terms and definitions given in IEC 60050-447, as well
as the following apply.
3.1
hot curve
for a thermal electrical relay with a total memory function, characteristic curve representing
the relationship between specified operating time and current, taking into account thermal
effect of a specified steady-state load current before the overload occurs
Note 1 to entry: Hot curve is a plot of a particular time-current solution for a first-order thermal system differential
equation, assuming a specific constant overload current and a specific preload current.
3.2
cold curve
for a thermal electrical relay, characteristic curve representing the relationship between
specified operating time and current, with the relay at reference and steady-state conditions
with no-load current flowing before the overload occurs
Note 1 to entry: Cold curve is a plot of a particular time-current solution for a first-order thermal system
differential equation, assuming a specific constant overload current when there is no preload.
3.3
basic current
I
B
specified limiting (nominal) value of the current for which the relay is required not to operate
at steady-state conditions of the equipment to be thermally protected
Note 1 to entry: The basic current serves as a reference for the definition of the operational characteristics of
thermal electrical relays. The basic settings of a thermal electrical protection function are made in terms of this
basic current (I ) and the thermal time constant (τ) of the protected equipment.
B
3.4
equivalent heating current
I
eq
current which takes into account the additional heating sources such as imbalance currents
and/or harmonics
3.5
factor k
factor by which the basic current (I ) is multiplied to obtain the maximum permissible
B
continuous operating current value of the equipment to be thermally protected, which is used
in the thermal characteristic function
Note 1 to entry: The factor k indicates the maximum permissible constant between phase current (full load) and
the basic (nominal) current of the protected equipment.
3.6
previous load ratio
ratio of the load current preceding the overload to basic current under specified conditions
3.7
reference limiting error
limiting error determined under reference conditions
[SOURCE: IEC 60050:2010, 447-08-07]

– 8 – 60255-149 © IEC:2013
3.8
temperature rise
difference between the temperature of the part under consideration and a reference
temperature
Note 1 to entry: The reference temperature may be for example the ambient air temperature or the temperature of
a cooling fluid.
[SOURCE: IEC 60050:2001, 151-16-26]
3.9
thermal equilibrium
thermal state reached when the temperature rise of the several parts of the machine do not
vary by more than a gradient of 2 K per hour
[SOURCE: IEC 60050:1996, 411-51-08]
3.10
thermal time constant
T
th
time required for the temperature rise of the protected equipment relative to its initial
temperature, to reach 63,2 % of its final, asymptotic value following a step increase in current
Note 1 to entry: The initial temperature for example can be ambient temperature.
3.11
thermal level
H
ratio expressed in percentage between the estimated actual temperature of the equipment
and the temperature of the equipment when the equipment is operating at its maximum
current (k × I ) for a long period, enough to allow equipment to reach its thermal equilibrium
B
4 Specification of the function
4.1 General
An example of a thermal protection function with its input energizing quantities, binary input
signals, operate (trip), alarm and other binary outputs, and functional logic which includes
measuring element, thermal level calculation, settings, and thresholds are shown in Figure 1.
The manufacturer shall provide the functional block diagram of the specific thermal protection
implementation.
60255-149 © IEC:2013 – 9 –
Thermal protection functional logic
Thresholds
Operate (trip)
Settings
(trip, alarm) signal
Input
Energizing
Measuring
energizing
quantities
quantites element
(signal
(equivalent
processing)
heating
current)
Thermal
Alarm (pre-
operate) signal
level
calculation
Ambient / winding
To other
temperature
protection
measuring
functions
Binary
(option)
input
signals
Other binary
The exact and complete contents of this functional logic block diagram area
output signals
depends upon the implementation

IEC  1846/13
Figure 1 – Simplified thermal protection function block diagram
4.2 Input energizing quantities/energizing quantities
The input energizing quantities are the measuring signals, such as phase (or line) currents,
and ambient/environmental or winding temperatures (if required or applicable). Their ratings
and relevant requirements are specified in IEC 60255-1.
Input energizing quantities can be presented to the thermal protection functional logic either
hardwired from current transformers and any additional input quantities such as ambient or
winding temperature, or as a data packet over a communication ports using an appropriate
data communication protocol, such as IEC 61850-9-2.
The input energizing quantities used by the thermal protection function need not be the
current directly taken from the secondary side of the current transformers. Therefore the
protection relay documentation shall state the type of energizing quantities used by the
thermal protection function.
Examples of input energizing quantities are:
– single-phase current measurement;
– three-phase current measurement;
– positive and negative sequence current measurement;
– winding or ambient temperature sensor.
NOTE The ambient temperature, coolant temperature, top oil temperature or winding temperature of the
equipment to be thermally protected can be measured by temperature sensors, such as resistance temperature
detector (RTD), the values of which can be used for biasing the calculation of the thermal level replica specified in
this standard. Output signals or values of these temperature sensors can be taken into account for the first-order
thermal model algorithm, which can influence and compensate the calculated thermal level (based on the
equivalent heating current and heating thermal time constant values).
4.3 Binary input signals
If any binary input signals (externally or internally driven) are used, their influence on the
thermal protection function shall be clearly described on the functional logic diagram or in the
protective device manufacturer documentation. Additional textual description may also be
provided if this can further clarify the functionality of the input signals and their intended
application or implementation.

– 10 – 60255-149 © IEC:2013
Binary input signals to this function may emanate from a number of different sources.
Examples include:
• traditionally wired to physical inputs;
• via a communications port from external devices;
• via internal logical connections from other functional elements within the relay.
The method of receiving the signal is largely irrelevant except to conform to operational
requirements.
Definitions, ratings and standards for physical binary input signals are specified in
IEC 60255-1.
The following are examples of binary input signal application in thermal protection.
1) When the thermal protection function is implemented with two operating modes of the
protected equipment, such as power transformers with natural or forced ventilation, two-
speed motors or a star/delta starting motor, a binary input can be implemented to
discriminate the different operating modes and to select the required group of settings to
be used for proper thermal protection application.
2) Another example of a binary input is to implement a reset function of the thermal memory
during testing/commissioning procedures, using a binary input either directly hardwired or
through data communications.
4.4 Functional logic
4.4.1 Equivalent heating current
The equivalent heating current I takes into account the additional heating source such as
eq
imbalance currents and/or harmonics. The type of measurement of the equivalent heating
current shall be stated in the protection relay documentation.
For the rms measurement, the manufacturer shall specify the bandwidth of the rms current
measurement and define which harmonics are included in the equivalent heating current
calculation.
Annex A gives an explanation of the definition of the equivalent heating current and different
cases of implementation of thermal protection applications of electrical equipment.
4.4.2 Basic (setting) and operating current values for thermal protection
For the thermal electrical relay, the basic (setting) current value I is the specified limiting
B
value of the current for which the relay is required not to operate. For motor or transformer
applications, the basic current is usually set to the nominal current of the protected
equipment.
To take into account the maximum continuous load current of the protected equipment, a
factor k is applied to the basic (setting) current value, to determine the operating current for
the thermal protection.
Therefore the value k × I defines the operating current of the thermal protection relays,
B
where
k may be a constant value or a user setting, as declared by the thermal relay manufacturer;
I is the basic (setting) current value expressed as the permissible current of the equipment
B
to be thermally protected.
60255-149 © IEC:2013 – 11 –
With the factor k, no operation of the thermal relay is guaranteed for phase currents equal to
the setting value I . If the factor k is a user setting, it should include a range of at least 1,0 to
B
1,5. For motor or transformer applications, the factor k is usually set by the user, where k × I
B
is equal to or less than maximum operating (full load) current of the equipment to be thermally
protected. For relays which do not have a k factor setting (assumed to be fixed at 1,0) the
setting for I should be adjusted to account for the k factor.
B
In some cases a fixed value of k may be defined by the manufacturer, equal to the accuracy
of current measurement of the thermal electrical relay. This ensures that the thermal relay
shall not operate for an operating current of I . In this case the ratio between the overload and
B
the nominal current for the equipment being protected can be accommodated in the setting of
the base current I .
B
4.4.3 Thermal level calculation
The thermal level calculation of the protected equipment is based on the equivalent heating
phase current measurement and the recursive computation of a discrete-time equation of a
differential first-order thermal model.
The thermal level H(t) of the protected equipment is calculated by the following equation:

It()
∆τt
eq

Ht() .+ .Ht(− ∆t)  (1)

kI⋅ τ∆++t τ∆t
B

where
H(t) is the thermal level at time t;
H(t–∆t) is the thermal level at time t–∆t;
∆t is the sample period which is the time interval between two consecutives samples of
input currents;
I (t) is the equivalent heating phase current at time t (see 4.4.1 and Annex A);
eq
k·I is the value of the maximum continuous current, including k factor;
B
τ is the heating/cooling thermal time constant of the equipment to be thermally
protected, τ is assumed to be >>∆t.
Derivation of differential and time-current equations and dynamics for a simple first-order
thermal system are given in detail in Annex A.
For a particular steady-state case with a constant I , the thermal level H can be calculated
eq
by the following particular and simplified equation:
I
 
eq
(2)
H =  
 
kI⋅
 B 
The thermal electrical relay operates if the thermal level reaches 100 % of maximum thermal
level threshold.
According to the mechanical design of the electrical equipment to be thermally protected, the
heating thermal time constant and cooling thermal time constant can have different values.
For example, for electric motor protection application, the heating thermal time constant is
lower than the cooling thermal time constant due to the rotor rotation and self-ventilation
operation when the motor is running. In these cases, the thermal level is calculated according
to the phase current level, with two different thermal time constants, according to the following
equations.
=
– 12 – 60255-149 © IEC:2013
If I (t) ≥ 0 (or if I (t) is greater than a fixed input current threshold, stated by the thermal
eq eq
relay manufacturer), the thermal level can be computed by the following equation:
 
It()
∆t τ
eq
Ht() .+−.Ht( ∆t)
 
(3)
 
kI. τ ++∆t τ ∆t
B 11
 
If I (t) ≈ 0 (or if I (t) is lower than a fixed input current threshold, stated by the thermal relay
eq eq
manufacturer), the thermal level can be computed by the following equation:
τ
Ht() .Ht(− ∆t) (4)
τ∆+ t
where
τ is the heating thermal time constant of the equipment to be thermally protected;
τ is the cooling thermal time constant of the equipment to be thermally protected.
NOTE 1 Generally τ is used when the protected equipment is energized and τ is used when the protected
1 2
equipment is deenergized.
NOTE 2 The heating thermal time constant τ is also used when the equipment is energized and the phase
current is reduced to a lower level, which causes a lowering of the equipment thermal level, causing a decrease in
the equipment temperature.
NOTE 3 Manufacturers can implement multiple heating and multiple cooling time constants to cover the variety of
heating and cooling conditions. For example, during direct on-line motor starting the time constant used in the
thermal model can be changed (decreased) to allow for reduced cooling capability of the rotor at standstill/low
speed and then revert to a longer time constant when normal running speed is achieved.
For most thermal protection applications, such as self-ventilated motor and generator, two-
speed motors, star/delta starting motor, the thermal time constants τ and τ are different. For
1 2
some other applications, such as motors with separated, independent forced ventilation or
cooling systems, power transformers with or without forced ventilation cooling systems,
cables, and capacitors, the thermal time constants τ and τ may have the same value. Some
1 2
specific applications, such as two-speed motors or where star/delta starting is used,
additional heating time constants may be used.
4.4.4 Time-current limit characteristic equations and curves
4.4.4.1 General
The time-current characteristics shall be published by the relay manufacturer either in the
form of equations or by graphical methods. The time-current equations for a simple thermal
model are given here for cold state and hot state.
4.4.4.2 Cold curve
The cold curve for thermal protection relays is a particular solution of the first-order
differential Equation (1) for the following conditions.
– Starting from a thermal level with no load current before the overload occurs. Therefore,
the equipment temperature is considered as the ambient temperature and its thermal level
is considered equal to zero.
– A constant phase current during the overload.
The cold time-current limit characteristic is given by the following time-current equation:
=
=
60255-149 © IEC:2013 – 13 –

I
eq
t(I ) τ⋅ln (5)

eq
2 2

I −⋅()kI
eq B

where
t(I ) is the theoretical operate time with a constant phase current I , with no load current
eq eq
before (prior) the overload occurs;
I is the equivalent heating current;
eq
τ is the heating thermal time constant of the protected equipment;
k is a constant (fixed) value or a setting, declared by the thermal relay manufacturer;
I is the basic current value expressed as permissible current of the equipment to be
B
thermally protected.
A typical example of time-current characteristic curve for cold state of a first-order thermal
system with no previous load before overload occurs is shown in Figure 2.
t
I k ⋅ I
B B
I
eq
IEC  1847/13
Figure 2 – Typical examples of characteristic curves for cold state of a first-order
thermal system with no previous load before overload occurs
A detailed differential equation derivation, algorithm, dynamics, and cold time-current
characteristic solution for the first-order thermal system are developed and given in Annex A.
4.4.4.3 Hot curve
The hot curve for thermal protection relays is a particular solution of the first-order differential
Equation (1) and it is given by the following time-current equation:

II−
eq p
t(I ) τ⋅ln (6)

eq
2 2

I −⋅()kI
eq B

=
=
– 14 – 60255-149 © IEC:2013
where
t(I ) is the theoretical operate time with a constant phase current I with a constant current
eq eq
of I prior to the overload;
p
I is the equivalent heating current;
eq
I is the steady-state load current prior to the overload for a duration which would result in
p
constant thermal level (duration is greater than several heating thermal time constants
τ); I = 0 results in the cold curve;
p
τ is the heating thermal time constant of the equipment to be thermally protected;
k is a constant value (fixed) value or a setting, declared by the thermal relay
manufacturer;
I is the basic current value expressed as permissible current of the equipment to be
B
thermally protected.
The relay manufacturer can publish thermal tripping curves as in the example given below
with the previous load ratio p as a parameter, described by the following equation:
I
P
p = (7)
I
B
Typical examples of current-time characteristic curves for hot states of a first-order thermal
system for different values of previous load before overload occurs are shown in Figure 3.
t
p = 0
p = 0,6
p = 0,8
p = 0,9
k ⋅ I
B
I
eq
IEC  1848/13
Figure 3 – Typical examples of characteristic curves for hot states of a first-order
thermal system for different values of previous load before overload occurs
A detailed differential equation derivation, algorithm, dynamics, and hot time-current
characteristic solution for the first-order thermal system are developed and given in Annex A.
4.4.5 Thermal level alarm threshold
If the thermal protection relay contains an alarm threshold level it can produce an alarm
output signal when the thermal level exceeds a predetermined setting alarm threshold. This
threshold can be defined as a percentage of the nominal (rated) thermal limit of the equipment
to be thermally protected.
60255-149 © IEC:2013 – 15 –
Nominal (rated) thermal limit (H = 100 %) is considered as the maximum thermal level
nominal
to which the equipment to be thermally protected can continuously withstand to avoid over
temperature. An over temperature above the permitted limit could damage the
chemical/physical properties of the materials component of the insulation system, reducing its
expected life time.
This predictive overload alarm threshold level, if provided, shall include at least a range of
50 % to 100 % of the nominal (rated) thermal limit.
NOTE 1 The thermal level H can be compensated for the ambient temperature level of the equipment this is
detailed in Equations (8) and (9).
NOTE 2 For motor thermal protection applications, the actual thermal level, measured by the thermal protection
device using the equations shown in this standard, can be used as a restart blocking signal, as an input reference
for the restarting blocking protection function (function 66), for a motor in a stopped condition (at rest), at a hot
state, after operation. For this application, the remaining time for the next allowed motor start attempt can be
indicated in the thermal protection device display, taking into account the cooling thermal time constant for the
stopped motor, the actual thermal level of the motor at rest and the estimated or calculated thermal level required
for motor starting (calculated based on the motor heating thermal time constant, starting current and starting time).
4.5 Binary output signals
4.5.1 General
Binary output signals from this function may be available in a number of different
forms. Examples include:
• traditionally wired from physical relay output contacts,
• via a communications port to external devices,
• via internal logical connections to other functional elements within the relay.
The method of providing the signal is largely irrelevant except to conform to functional
requirements.
Definitions, ratings and standards for physical binary output signals are specified in
IEC 60255-1.
4.5.2 Operate (trip) output signal
The operate (trip) signal is the output of measuring and threshold elements, when the
calculated thermal level H(t), defined in Equation (1), exceeds 100 % (1,0 pu) of the nominal
(rated) thermal level of the equipment to be thermally protected.
NOTE The trip signal could operate when the calculated thermal level of any of the three phases exceeds the
nominal thermal level.
4.5.3 Alarm signal
The alarm signal is the output of measuring and threshold elements, when the calculated
thermal level H(t), defined in Equation (1), exceeds a predetermined overload alarm threshold
setting.
4.5.4 Other binary output signals
If any other binary output signals are available for use, their method of operation shall be
clearly shown on the functional logic diagram or in the protective device manufacturer
documentation. Additional textual description may also be provided if this can further clarify
the functionality of the output signal and its intended usage.

– 16 – 60255-149 © IEC:2013
4.6 Additional influencing factors on thermal protection
4.6.1 General
The manufacturer shall declare if any specific algorithms are implemented in the relay. These
algorithms shall be described by the manufacturer in the thermal protective device
documentation.
For example, if the thermal protection relay is equipped with temperature m
...