SIST IEC 60255-8:1995

SLOVENSKI STANDARD
01-avgust-1995
Electrical relays - Part 8: Thermal electrical relays
Electrical relays - Part 8: Thermal electrical relays
Relais électriques - Huitième partie: Relais électriques thermiques
Ta slovenski standard je istoveten z: IEC 60255-8
ICS:
29.120.70 Releji Relays
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

NORME
CEI
INTERNATIONALE
IEC
255-8
INTERNATIONAL
Deuxième édition
Second edition
STANDARD
1990-09
Relais électriques
Huitième partie:
Relais électriques thermiques
Electrical relays
Part 8:
Thermal electrical relays
© CEI 1990 Droits de reproduction réservés
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255-8 © CET
CONTENTS
Page
FOREWORD 5
PREFACE 5
Clause
SECTION ONE GENERAL
1 Scope and object 7
2 Definitions 7
SECTION TWO — REQUIREMENTS
3 Standard values
4 Accuracy 17
SECTION THREE — TEST METHODS
5 Tests related to operating characteristics and accuracy 19
6 Tests for thermal requirements 21
Figures 22
Appendix A – Characteristic curves, cold curves
Appendix B - Characteristic curves, hot curves
Appendix C – Example for determining accuracy.

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255-8 © I E C
INTERNATIONAL ELECTROTECHNICAL COMMISSION
ELECTRICAL RELAYS
Part 8: Thermal electrical relays
FOREWORD
The formal decisions or agreements of the IEC on technical matters, prepared by Technical Committees on which all the National
1)
an international consensus of opinion on the
Committees having a special interest therein are represented, express, as nearly as possible,
subjects dealt with.
ational use and they are accepted by the National Committees in that sense.
2) They have the form of recommendations for inte rn
resses the wish that all National Committees should adopt the text of the I E C
3) In order to promote inte rnational unification, the I E C exp
d
an
recommendation for their national rules in so far as national conditions will permit. Any divergence between the I E C recommendation
possible, be clearly indicated in the latter.
the corresponding national rules should, as far as
PREFACE
This standard has been prepared by Sub-Committee 41B: Measuring relays and protection equipment, of IEC
Technical Committee 41: Electrical relays.
and replaces the first edition (1978). It
This standard constitutes the second edition of I E C Publication 255-8
the second impression (1987) of I E C Publication 255-17.
also replaces the first edition (1982) and
The text of this standard is based on the following documents:
Six Months' Rule Report on Voting
41 B(CO)47 41 B(CO)51
indicated in
Full information on the voting for the approval of this standard can be found in the Voting Repo rt
the above table.
This standard is a third-level specification.
The following IEC publications are quoted in this standard:
Publications Nos. 50: International electrotechnical vocabulary (IEV).
255: Electrical Relays.
protection equipment.
255-6 (1988): Part 6: Measuring relays and

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ELECTRICAL RELAYS
Part 8: Thermal electrical relays
SECTION ONE – GENERAL
1 Scope and object
This stan
dard is applicable to dependent specified time electrical measuring relays which protect equipment from
electrical thermal damage by the measurement of current flowing in the protected equipment.
1.1 This standard covers the following two types of relays:
a)
thermal electrical relays having a total memory function of the load-current conditions before the conditions
which caused the switching of the relay;
b) thermal electrical relays having a pa rt
ial memory function, i.e. of the overload current conditions only.
1.2 This stan
dard also covers the particular requirements for thermal electrical relays used for motor protection.
The object of this stan
dard is to state the particular requirements for thermal electrical relays. It is to be read in
conjunction with higher level documents in the IEC Publication 255 series.
2 Definitions
For definitions of general terms not defined in this standard, reference should be made to the Inte rnational
Electrotechnical Vocabulary (I E V) (IEC Publication 50)
and higher level documents.
For the purpose of this standard the following definitions shall apply:
2.1 hot curve:
For a thermal electrical relay with a total memory function, the characteristic curve representing
the relationship between specified operating time
and current, taking account of the thermal effect of a specified
steady-state load current before the overload occurs.
2.2 cold curve:
For a thermal electrical relay, the 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.
2.3
correcting quantity (compensating quantity): A quantity modifying the specified characteristics of the
relay in a specified manner. Such quantities can be oil temperature, etc.
2.4 basic current:
The specified limiting value of the current for which the relay is required not to operate.
NOTE — The basic current se
rves as a reference for the definition of the characteristics of thermal electrical relays. Settings of a thermal
electrical relay are made in terms of this current.
2.5 constant k:
The constant by which the basic current is multiplied to obtain the current value to which the
accuracy of the minimum operating current referred.
2.6 previous load ratio:
The ratio of the load current preceding the overload to basic current under specified
conditions.
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SECTION TWO – REQUIREMENTS
3 Standard values
3.1 Characteristic curves
The characteristics of time with respect to current can be stated either by equations or by graphical methods. The
equations for a simple thermal model are given below under Sub-clauses 3.1.1 and 3.1.2. Other characteristic
curves are permitted and should be declared by the manufacturer. As an example, see Appendix A.
NOTES
1— For practical purpose, e.g. testing, it is convenient to give the characteristic curve as a combination of current and time values.
2 — The time constant used in an equation should be as specified in National St andards or as declared by the m anufacturer.
3.1.1 Cold curve
A general curve for thermal electrical relays, based on the heating effect and on the time constant, is given by the
following formula:
I2
t= z.ln
I2 (k . IB) 2
–
where
t = operating time;
T = time constant;
IB = basic current;
k = constant;
I = relay current.
IB k . IB I
Cold curve
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- 1 1 —
3.1.2 Hot curve
With regard to preheating on a relay with a total memory function, the hot curve is relev ant. For example, the
equation obtained by modification of the general cold curve is given by the following formula which is derived in
Appendix B:
jp2
IZ–
t = z . ln
IZ – (k . lB) 2
where Ip is the specified load current before the overload occurs.
3.2 Nominal ranges of auxiliary energizing quantities
For other nominal ranges than the preferred 80% to 110% the limits of the nominal r ange shall be as specified in
National Standards or as declared by the m anufacturer.
3.3 Standard reference values of influencing quantities and factors
The st
andard reference values and test tolerances of influencing quantities and factors, previous current values and
correcting quantities are given in Tables I, II and III, respectively.
Table I – Reference conditions and test tolerances of influencing quantities and factors
Reference condition
Influencing quantity and factor
Test tolerances
(see note)
Position As specified in National St
andards or as 2° in any direction or, for static relays,
declared by the manufacturer as specified in National Standards
or as declared by the manufacturer
Setting of basic current Rated current or as specified in As specified in National Stand
ards or
â
National Standards or as declared
as declared by the manufacturer
c
by the manufacturer
g
i c
Magnitude (reference for determination As specified in National Standards or
As specified in National Staridards or
o
a'
of variations)
as declared by the manufacturer. as declared by the manufacturer.
C3 T.,
.
For motor 'protection, two times and
For motor protection: ±1%
six times b
asic current
c
Lcdi
tt
tt
U
D.C. transient component in a.c. Zero 2% of peak value
Setting As specified in National Standards or
As specified in National Standards or
as declared by the m
anufacturer as declared by the manufacturer
F
Setting parameter(s) of the curve As specified in National Standards or
As specified in National Standards or
as declared by the man
ufacturer as declared by the m anufacturer
en
.N
Voltage Rated value(s)
As specified in National St andards or
;, as declared by the manufacturer
a.
component in a.c. Zero 2% of peak value
7e
d
NOTE — National Standar
ds: special conditions of application or the character of the relay may necessitate the use of non-standard values.
In such cases, the m an
ufacturer shall declare the reference values and tolerances. For example, special applications may necessitate the
use of 40 °C as
the reference value of ambient temperature instead of 20 °C.

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13 —
Table II – Previous current values when measuring the effect of influencing quantities
Quantity
Reference condition Test tolerances
For cold curve: zero Not applicable
Specified load current before overload
occurs
For hot curve: as specified in National As specified in National Standards or
Standards or as declared by the as declared by the manufacturer
manufacturer
Previous ratio load for motor For hot curves: 1.0 or 0.9 as chosen by ±1%
protection the manufacturer
Table III – Standard reference conditions and test tolerances of correcting quantities when measuring
the effect of influencing quantities
Correcting quantity
Reference condition Test tolerances
Unbalance current in a sinusoidal poly- Balanced
See Note 8 of Table II of IEC 255-6
phase system
Speed of protected rotating machine
As specified in National Standards or As specified in National Standards or
as declared by the manufacturer. as declared by the manufacturer
For motor protection: rated speed of
motor
Temperatures of different parts of the
protected equipment (see Note 1)
±
Temperature of cooling medium of the 20 °C or as declared by manufacturer 2 °C
protected equipment (see Note 2)
NOTES
1 — These values of correcting quantities represent steady-state thermal conditions of the protected equipment before overload
occurs. Their application depends on the principle of the relay used.
2 — To be declared by the m an
ufacturer mainly when a cooling medium other than air is used.
3.4
Standard values of the limits of the nominal range of influencing quantities and factors
The standard values of the limits of the nominal range of influencing quantities
and factors, previous current
values and correcting quantities are given in tables IV, V
and VI respectively.
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Table
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