IEC TR 61912-2:2009

Low-voltage switchgear and controlgear - Over-current protective devices - Part 2: Selectivity under over-current conditions

IEC 61912-2:2009(E), which serves as an application guide for the determination of selectivity between over-current protective devices of low-voltage switchgear and controlgear, summarises the definitions of the terminology and provides examples of application.

General Information

• Status: Published
• Publication Date: 12-May-2009
• ICS: 29.130.20 - Low voltage switchgear and controlgear
• Technical Committee: SC 121A - Low-voltage switchgear and controlgear
• Drafting Committee: MT 9 - TC 121/SC 121A/MT 9
• Current Stage: PPUB - Publication issued
• Start Date: 31-Jul-2009
• Completion Date: 13-May-2009

Standards Content (Sample)

IEC/TR 61912-2 ®
Edition 1.0 2009-05
TECHNICAL
REPORT
Low-voltage switchgear and controlgear – Over-current protective devices –
Part 2: Selectivity under over-current conditions

IEC/TR 61912-2:2009(E)
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IEC/TR 61912-2 ®
Edition 1.0 2009-05
TECHNICAL
REPORT
Low-voltage switchgear and controlgear – Over-current protective devices –
Part 2: Selectivity under over-current conditions

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
PRICE CODE
V
ICS 29.130 ISBN 978-2-88910-585-4
– 2 – TR 61912-2 © IEC:2009(E)
CONTENTS
FOREWORD.4
INTRODUCTION.6
1 Scope.7
2 Normative references .7
3 Terms, definitions and abbreviated terms .8
3.1 Alphabetical index of terms .8
3.2 Terms and definitions .9
3.3 Abbreviated terms .10
4 Scope of selectivity requirements .11
4.1 General .11
4.2 Motor protection circuit-breaker / Manual motor starter.12
5 Selectivity determination .12
5.1 Circuit-breaker as UD.12
5.1.1 Selectivity between circuit-breakers.12
5.1.2 Selectivity between a circuit-breaker (UD) and a fuse to IEC 60269-1
(DD) .15
5.1.3 CB/CPS – Selectivity between a circuit-breaker to IEC 60947-2 (UD)
and a CPS to IEC 60947-6-2 (DD) .16
5.1.4 Circuit-breaker/MOR – Selectivity between a circuit-breaker (UD) and
a motor protection overload relay to IEC 60947-4-1 or IEC 60947-4-2
(DD) .17
5.2 Fuse(s) to IEC 60269-1 as UD.18
5.2.1 Fuse/circuit-breaker – Selectivity between a fuse to IEC 60269-1
(UD) and a circuit-breaker (DD) .18
5.2.2 FU/FU – Selectivity between fuses to IEC 60269-1 (UD and DD) .20
5.2.3 FU/CPS – Selectivity between fuse(s) to IEC 60269-1 (UD) and a
CPS to IEC 60947-6-2 (DD).21
5.2.4 FU/MOR – Selectivity between fuse(s) to IEC 60269-1 (UD) and a
motor overload protection relay to IEC 60947-4-1 or IEC 60947-4-2
(DD) .21
6 Residual current devices (RCDs).21
6.1 General .21
6.2 Selectivity – RCD/RCD.22
6.2.1 Selectivity between RCDs in the case of earth-leakage current.22
6.2.2 Selectivity between RCDs in the case of earth-fault (ground-fault)
current.23
7 Zone Selective Interlocking (ZSI).24
7.1 General .24
7.2 Operating principle .24
7.3 Example .25
8 Over-current protection relay (OCR) – Single input energizing quantity measuring
relays with dependent or independent time.26
Annex A (informative) Examples of selectivity between over-current protection devices
Examples of the grades of selectivity applicable to circuit-breakers .27
Annex B (informative) Standing loads – Effect of standing loads on selectivity in the
overload zone .30
Bibliography.33

TR 61912-2 © IEC:2009(E) – 3 –
Figure 1 – Comparison of the operating characteristics of circuit-breakers in the
overload zone .13
Figure 2 – Example of selectivity in the fault current zone with time-delay short-circuit
release .14
Figure 3 – Selectivity in the overload zone between a circuit-breaker (UD) and a fuse
(DD).16
Figure 4 – Circuit-breaker/MOR – Circuit-breaker selectivity with motor overload relay .17
Figure 5 – ICB/MOR – ICB selectivity with motor overload relay .18
Figure 6 – Fuse/circuit -breaker - Verification of selectivity between fuse and circuit-
breaker for operating time in the overload zone (t ≥ 0,1 s for the fuse).19
Figure 7 – FU/CB – Verification of selectivity between fuse and circuit-breaker for
operating time t < 0,1 s .19
Figure 8 – FU/FU – Verification of selectivity between fuses for operating time t ≥ 0,1 s .20
Figure 9 – RCD characteristics showing selectivity on earth-leakage – time-delay
Type S versus non-time delay.23
Figure 10 – Schematic diagram of an installation designed for multiple supplies with
zone selective interlocking .25
Figure 11 – Schematic diagram of mains distribution system with OCR protection .26
Figure A.1 – Circuit-breaker coordination example – 50 kA/9 kA fault levels .27
Figure A.2 – Time-current curves (examples 1 and 2) .28
Figure A.3 – Operation in the fault current zone (examples 1 and 2) .28
Figure A.4 – Circuit-breaker coordination example – 50 kA/20 kA fault levels .29
Figure B.1 – Overload and short-circuit zones.30
Figure B.2 – OCPDs in series .31

Table 1 – Type of selectivity and corresponding subclause number .

– 4 – TR 61912-2 © IEC:2009(E)
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
LOW-VOLTAGE SWITCHGEAR AND CONTROLGEAR –
OVER-CURRENT PROTECTIVE DEVICES –

Part 2: Selectivity under over-current conditions

FOREWORD
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The main task of IEC technical committees is to prepare International Standards. However, a
technical committee may propose the publication of a technical report when it has collected
data of a different kind from that which is normally published as an International Standard, for
example "state of the art".
IEC 61912-2, which is a technical report, has been prepared by subcommittee 17B: Low-
voltage switchgear and controlgear, of IEC technical committee 17: Switchgear and
controlgear.
The text of this technical report is based on the following documents:
Enquiry draft Report on voting
17B/1606/DTR 17B/1666/RVC
Full information on the voting for the approval of this technical report can be found in the
report on voting indicated in the above table.

TR 61912-2 © IEC:2009(E) – 5 –
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
A list of all parts of IEC 61912 series, published under the general title Low-voltage
switchgear and controlgear – Over-current protective devices, can be found on the IEC
website.
The committee has decided that the contents of this publication will remain unchanged until
the maintenance result date indicated on the IEC web site under "http://webstore.iec.ch" in
the data related to the specific publication. At this date, the publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
A bilingual version of this publication may be issued at a later date.

– 6 – TR 61912-2 © IEC:2009(E)
INTRODUCTION
Low-voltage equipment standards IEC 60947, IEC 60269, IEC 60898-1 and IEC 61009-1
currently include operating characteristics for over-current protective devices, defined in terms
of the ability of the equipment to operate at levels of over-current up to their maximum short-
circuit current ratings. In practice, the installation of such devices in series requires
consideration of the relationship between the device characteristics to achieve the optimum in
supply availability in the event of an over-current causing operation of any device. The ability
of an over-current device to perform selectively in combination with other such devices needs
to be fully understood by the circuit designer to avoid leaving a circuit vulnerable to
unnecessary loss of supply, particularly where critical supplies are concerned. It is also useful
to take full advantage of the capability of devices and systems to avoid over-engineering, with
the consequent unnecessary additional cost. Selectivity over the whole range of fault current
up to the prospective fault current at the point of installation is not always possible or
necessary. A more economic solution may be found in many cases by accepting a limited
selectivity, particularly taking into account the low probability of a high short-circuit fault
current.
Where a short-circuit protective device is used to provide back-up protection to a downstream
device, guidance on the application is provided in IEC/TR 61912-1.

TR 61912-2 © IEC:2009(E) – 7 –
LOW-VOLTAGE SWITCHGEAR AND CONTROLGEAR –
OVER-CURRENT PROTECTIVE DEVICES –

Part 2: Selectivity under over-current conditions

1 Scope
This technical report, which serves as an application guide for the determination of selectivity
between over-current protective devices of low-voltage switchgear and controlgear,
summarises the definitions of the terminology and provides examples of application.
The following standards for devices are considered in this technical report:
− IEC 60255-3; IEC 60255-6; IEC 60255-8, IEC 60255-12
− IEC 60269-1, IEC 60269-2, IEC 60269-3;IEC 60269-4;
− IEC 60898-1;
− IEC 60947 series;
− IEC 61008-1;
− IEC 61009-1.
This report does not deal with other forms of protection, such as power-reversal protection,
directional protection and arc-protection systems.
2 Normative references
The following referenced documents are indispensable for the application of this document.
For dated references, only the edition cited applies. For undated references, the latest edition
of the referenced document (including any amendments) applies.
IEC 60255 (all parts), Electrical relays
IEC 60269-1, Low-voltage fuses – Part 1: General requirements
IEC 60269-2, Low-voltage fuses – Part 2: Supplementary requirements for fuses for use by
authorized persons (fuses mainly for industrial application) – Examples of standardized
systems of fuses A to I
IEC 60269-3, Low-voltage fuses – Part 3: Supplementary requirements for fuses for use by unskilled
persons (fuses mainly for household and similar applications)
IEC 60269-4, Low-voltage fuses – Part 4: Supplementary requirements for fuse-links for the
protection of semiconductor devices
IEC 60898-1, Electrical accessories – Circuit-breakers for over-current protection for
household and similar installations – Part 1: Circuit-breakers for a.c. operation
IEC 60947-2, Low-voltage switchgear and controlgear − Part 2: Circuit-breakers
IEC 60947-4-1, Low-voltage switchgear and controlgear − Part 4-1: Contactors and motor-
starters − Electromechanical contactors and motor-starters

– 8 – TR 61912-2 © IEC:2009(E)
IEC 60947-4-2, Low-voltage switchgear and controlgear − Part 4-2: Contactors and motor-
starters − AC semiconductor motor controllers and starters
IEC 60947-6-2, Low-voltage switchgear and controlgear − Part 6-2: Multiple function
equipment – Control and protective switching devices (or equipment) (CPS)
IEC 61008-1, Residual current operated circuit-breakers without integral over-current
protection for household and similar uses (RCCBs) – Part 1: General rules
IEC 61009-1, Residual current operated circuit-breakers with integral over-current protection
for household and similar uses (RCBOs) – Part 1: General rules
IEC/TR 61459, Coordination between fuses and contactors/motor-starters – Application guide
IEC/TR 61818, Application guide for low-voltage fuses
IEC/TR 61912-1, Low-voltage switchgear and controlgear – Overcurrent protective devices –
Part 1: Application of short-circuit ratings
3 Terms, definitions and abbreviated terms
For the purposes of this document, the following terms, definitions and abbreviated terms
apply.
3.1 Alphabetical index of terms
Reference
B
back-up protection. 3.2.6
C
coordination of over-current protective devices . 3.2.1
D
downstream device (DD). 3.2.8
F
fault current zone (of over-current). 3.2.10
O
over-current discrimination. 3.2.2
over-current protective device (OCPD). 3.2.5
overload zone (of over-current). 3.2.9
S
selectivity of protection . 3.2.3
selectivity limit current . 3.2.4
U
upstream device (UD) . 3.2.7

TR 61912-2 © IEC:2009(E) – 9 –
3.2 Terms and definitions
3.2.1
coordination of over-current protective devices
coordination of two or more over-current protective devices in series to ensure over-current
discrimination (selectivity) and/or back-up protection
NOTE This report deals with selectivity. Guidance on back-up protection is given in IEC/TR 61912-1.
3.2.2
over-current discrimination
coordination of the operating characteristics of two or more over-current protective devices
such that, on the incidence of over-currents within stated limits, the device intended to
operate within these limits does so, while the other(s) does (do) not
NOTE Distinction is made between series discrimination, involving different over-current protective devices
passing substantially the same over-current, and network discrimination involving identical protective devices
passing different proportions of the over-current.
[IEV 441-17-15]
3.2.3
selectivity of protection
ability of a protection to identify the faulty section and/or phase(s) of a power system
[IEV 448-11-06]
NOTE Whereas the terms “selectivity” and “discrimination” have a similar meaning according to the IEV
definitions, this report prefers and uses the term “selectivity” to express the ability of one over-current device to
operate in preference to another over-current device in series, over a given range of over-current. The effect of
standing load current on selectivity in the overload zone is also considered.
3.2.4
selectivity limit current
current coordinate (Is) of the intersection between the maximum break time-current
characteristic of the downstream over-current protective device and the pre-arcing (for fuses)
or tripping (for circuit-breakers) time-current characteristic of the upstream over-current
protective device
[IEV 442-05-60 modified]
NOTE 1 In the case of a combination of circuit-breakers without intentional time-delay, in the short-circuit zone
the selectivity limit-current is not a simple function of time and must be established from test data.
NOTE 2 In the case of a combination of fuses, in the short-circuit zone the selectivity limit-current is a function of
energy let-through I t.
3.2.5
over-current protective device
OCPD
device provided to interrupt an electric circuit in the case of the current in the circuit
exceeding a predetermined value for a specified duration
[IEV 826-14-14 modified]
NOTE The term OCPD includes the use of an over-current protective relay in combination with a separate
switching device.
3.2.6
back-up protection
over-current coordination of two over-current protective devices in series where the protective
device, generally but not necessarily on the supply side, effects the over-current protection
with or without the assistance of the other protective device and prevents any excessive
stress on the latter
– 10 – TR 61912-2 © IEC:2009(E)
[IEC 60947-1, definition 2.5.24]
NOTE When referred to particular devices in combination, back-up protection is sometimes known as
"series rating".
3.2.7
upstream device
UD
in considering selectivity between two OCPDs, the OCPD connected in the circuit nearest to
the source of supply
3.2.8
downstream device
DD
in considering selectivity between two OCPDs, the OCPD connected in the circuit immediately
following the upstream device, on the load side
3.2.9
overload zone (of over-current)
range of current, exceeding the rated current of the OCPD, produced by the circuit loading in
the absence of a fault in the circuit
NOTE 1 The overload zone operation of the OCPD is in the range from a few seconds, up to four hours, following
an inverse time/current characteristic.
NOTE 2 In the case of a distribution circuit, the overload zone is not strictly defined since it depends on the
capability of the load to draw excessive current. It may be defined by the characteristics of the OCPD as follows:
− in the case of a circuit-breaker, this is the point when the tripping characteristic changes from inverse time
dependent to virtually instantaneous, at which level the operation will be in less than 0,2 s. Typically, this
occurs in the region of 10 times the nominal full-load current, dependent on the setting.
MCBs to IEC 60898-1 have defined limits of instantaneous tripping given, in Table 7 of the standard, as three
types B, C and D;
− in the case of a fuse, the overload zone may be considered as values of over-current that result in operation in
more than 0,1 s, typically below 10-20 times rated current.
NOTE 3 In the case of a circuit supplying an individual motor, the overload zone is limited to the stalled current of
the motor, typically 6-15 times motor full-load current (I ), exceptionally higher values are found.
e
NOTE 4 Within the overload zone, transitory conditions may occur, for example transformer inrush currents, of
only a few milliseconds duration.
3.2.10
fault current zone (of over-current)
range of current exceeding the overload current, produced by a fault in the circuit
NOTE 1 In the fault current zone, operation of a circuit-breaker as OCPD is typically in the range from a few
milliseconds (instantaneous), up to three seconds with a definite short-time delay function.
Below 50 ms the time/current characteristic is no longer useful. Reference should be made to current limitation
and/or energy let-through characteristics.
NOTE 2 In the case of a fuse, the fault current zone may be considered as values of over-current that result in
operation in less than 0,1 s.
The time current characteristic of a fuse uses the pre-arcing time, i.e. the time after which the fuse will operate.
Above 0,1 s pre-arcing time, on an a.c. supply, the arcing time of the fuse is not considered significant. However
below 0,1 s pre-arcing time, the arcing time is a significant portion of the total time and hence the time/current characteristic is
no longer useful and the I t characteristic is used.
NOTE 3 The fault current zone is also referred to as the short-circuit zone.
3.3 Abbreviated terms
ACB Air circuit-breaker
TR 61912-2 © IEC:2009(E) – 11 –
CB Circuit-breaker to IEC 60947-2 (includes ACB, MCCB and ICB)
CBR Residual current circuit-breaker to IEC 60947-2, Annex B
CPS Control and protective switching device to IEC 60947-6-2
DD Downstream device
FU Fuse to IEC 60269-1, IEC 60269-2, IEC 60269-3 or IEC 60269-4
ICB Instantaneous trip (only) circuit-breaker to Annex O of IEC 60947-2
I Ultimate short-circuit breaking capacity of an MCB
cn
I Ultimate short-circuit breaking capacity of a CB
cu
I Prospective short-circuit current
p
I Selectivity limit current
s
LV Low voltage
MCB Circuit-breaker to IEC 60898-1 for over-current protection in household and
similar installations
MCCB Moulded case circuit-breaker to IEC 60947-2
MOR Motor overload relay
MRCD Modular residual current device to IEC 60947-2, Annex M
MV Medium voltage
OCPD Over-current protective device
OCR Over-current relay to IEC 60255 series
RCBO Residual current circuit-breaker to IEC 61009-1
RCD Residual current device to IEC 61008-1, IEC 61009-1, IEC 60947-2
SCPD Short-circuit protective device
t Delay time
d
Tx Transformer
UD Upstream device
ZSI Zone selective interlocking
4 Scope of selectivity requirements
4.1 General
Table 1 shows the range of OCPDs considered and gives a designation for each type of
selectivity and the corresponding clause number of this report.
Where an OCPD is equipped with an undervoltage coil dependent on line voltage, selectivity
may be affected by operation of an upstream device due to voltage drop on short-circuit. To
improve selectivity any undervoltage release may need to be time-delayed.

– 12 – TR 61912-2 © IEC:2009(E)
Table 1 – Type of selectivity and corresponding subclause number
Downstream device (DD)
Upstream device
(UD)
CB/MCB FU CPS MOR RCD
CB/MCB 5.1.1 5.1.2 5.1.3 5.1.4
FU 5.2.1 5.2.2 5.2.3 5.2.4
RCD
6.2
4.2 Motor protection circuit-breaker / Manual motor starter
These devices are not covered by a single IEC standard but combine the characteristics of a
circuit-breaker to IEC 60947-2 and a motor overload relay to IEC 60947-4-1. For selectivity
purposes with upstream devices they are treated in the same manner as circuit-breakers.
5 Selectivity determination
This clause deals with the method of determination of selectivity between two OCPDs in
series at any point in a system. The complete coordination study requires the application of
this methodology to all the OCPDs, from the supply to the load.
In determining selectivity limit currents the applicable tolerances on the operating
characteristics need to be taken into account. In the following figures, the characteristics are
shown without tolerance bands for simplicity. In using published time-current characteristics,
the maximum operating time curve must be taken for the downstream device (DD) and the
minimum operating time curve must be taken for the upstream device (UD).
NOTE Strictly for greater accuracy, the operating temperature of thermal overload devices should be taken into
account by considering both the hot and cold characteristics. In practice, it is sufficient to compare either the two
cold curves or the two hot curves to arrive at a satisfactory solution in most cases.
5.1 Circuit-breaker as UD
Characteristics of the over-current protection provided by an MCB, MCCB or ACB are as
follows:
Integral: − Thermal/magnetic
− Electronic MCCB, ACB only
External/remote: − Over-current protection relay in combination with an MCCB or ACB.
5.1.1 Selectivity between circuit-breakers
Methods for the determination of selectivity between circuit-breakers are given in 5.1.1.1, for
the overload zone of operation, and in 5.1.1.2, for the fault current zone of operation.

TR 61912-2 © IEC:2009(E) – 13 –

Circuit-breaker Circuit-breaker

SUPPLY CB/UD CB/DD LOAD
UD DD
5.1.1.1 Circuit-breakers – Selectivity in the overload zone
Selectivity in the overload zone is verified by comparison of the time/current characteristics,
see Figure 1a and Figure 1b (Figure 1b is applicable to MCCB and ACB only). Separation of
the characteristics in both the time and current axes ensures selective operation of DD with
respect to UD, in this zone. There will be a tolerance applicable to the characteristic, which
should be taken into account. The manufacturer’s data should show a tolerance band or
otherwise indicate the tolerance applicable, as required by the product standard.

DD
DD
UD
UD
Adjustable
Adjustable
Current
Current
IEC  768/09
NOTE 1 The characteristics are subject to tolerances, which must be taken into account, see
introduction of Clause 5.
NOTE 2 It is necessary that the current scales are in amperes (or kA) for comparison of these
curves. Manufacturer’s published characteristics may be given in either amperes or multiples of rated
current.
NOTE 3 A combination of thermal/magnetic and electronic characteristics is also commonly used.
Figure 1a – Comparison of thermal Figure 1b – Comparison of electronic
time/current characteristics in the time/current characteristics in the
overload zone overload zone
Figure 1 – Comparison of the operating characteristics
of circuit-breakers in the overload zone
5.1.1.2 Circuit-breakers – Selectivity in the fault current zone
Selectivity between circuit-breakers is elaborated in the product standards IEC 60947-2,
Annex A and IEC 60898-1, Annex D, specifying the tests, where applicable, which show
Time
Time
– 14 – TR 61912-2 © IEC:2009(E)
determination of selectivity in the fault-current zone. The methods applicable are described in
5.1.1.2.1 and 5.1.1.2.2.
NOTE Due to their basic construction MCBs to IEC 60898-1 are generally highly selective against MCCBs to
IEC 60947-2.
5.1.1.2.1 Circuit-breakers – Selectivity determination in the fault current zone by
comparison of characteristics
Determination from characteristics, of selectivity between two circuit-breakers in the fault
current zone, is limited to the case where UD has a short-circuit release time-delay function
provided by an electronic release (see Figure 2). Selectivity at fault currents producing
instantaneous tripping of UD, for both electronic and electromagnetic releases, needs to be
determined from test data provided by the manufacturer (see 5.1.1.2.2).
In the absence of specific test data in the case where the instantaneous tripping of UD
depends on an electromagnetic effect, the minimum level of selectivity between two circuit-
breakers in the fault current zone may be determined as follows:
– selectivity is assured up to the fault current level at which the peak current let-through of
DD is less than the peak value corresponding to the instantaneous tripping level of UD.
EXAMPLE UD = 800 A MCCB; I = 8 – 12 kA r.m.s. (10 kA setting ±20 %)
inst
DD =125 A MCCB.
Minimum tripping level of UD is 8 x 1,414 = 11,3 kA peak.
Let-through current of DD at 15 kA r.m.s. prospective, due to the current limitation of DD, is 11 kA peak, from
manufacturer’s data.
Therefore the system is selective to at least 15 kA r.m.s. prospective.
Note that the selectivity limit obtained by this method will err on the low side and the actual limit determined by test
(see 5.1.1.2.2) will be significantly higher in most cases.

DD
UD
t = 80 ms
d
Instantaneous
Current
Fault-current zone
IEC  769/09
NOTE The characteristics are subject to tolerances, which must be taken into account, see introduction of
Clause 5.
Figure 2 – Example of selectivity in the fault current zone with
time-delay short-circuit release
Time
TR 61912-2 © IEC:2009(E) – 15 –
5.1.1.2.2 Circuit-breakers – Selectivity determination by test in the fault current
zone at fault currents producing instantaneous tripping
The selectivity limit currents, for each combination of circuit-breakers, is determined from test
and the manufacturer will supply data, usually in the form of a chart. In each case selectivity
may be total or partial as follows:
a) total selectivity: this means selectivity for all over-currents up to the value of the breaking
capacity, i.e. DD alone will operate (to the TRIPPED position) on over-current up to this
level. In the case where UD is a current-limiting circuit-breaker, dynamic contact action
can allow total selectivity to a level higher than the breaking capacity of DD (dynamic
contact action means momentary opening of the contacts of UD (typically < 10 ms)). See
also Note 3;
b) partial selectivity: this means selectivity up to a value of over-current less than the
breaking capacity of DD. The selectivity limit current is obtained by comparison of
time/current characteristics or, in the case of instantaneous tripping of UD, from data
obtained from tests and provided by the manufacturer. In the case where UD is a current-
limiting circuit-breaker, dynamic contact action may occur.
Examples of the grades of selectivity applicable to circuit-breaker applications are given in
Annex A.
In certain applications, the momentary contact opening may not be acceptable, in which case
the selection and/or settings of the circuit-breakers may be made such that this does not
occur, for example application of short-time delay to the upstream device. However, it should
be appreciated that a severe voltage dip will occur during a short-circuit fault, regardless of
the type of SCPD (fuse or circuit-breaker), dependent on the level of fault current and the
impedance of the circuit to the location of the fault.
NOTE 1 The data for selectivity limit current in the fault current zone producing instantaneous tripping of the
circuit-breaker is obtained from test data and is specific to each device type. There is no recognised method of
substitution for devices of different manufacture.
NOTE 2 Proprietary software systems for determination of selectivity are available, working from time/current data
supplied by individual manufacturers.
NOTE 3 Application of DD at fault current levels above the breaking capacity of DD, relying on dynamic contact
action for back-up protection, cannot be used where there is significant short-circuit current contribution from
inductive load(s) at the input side of DD, for example motors.
5.1.2 Selectivity between a circuit-breaker (UD) and a fuse to IEC 60269-1 (DD)
Circuit-breaker
CB/UD FU/DD
SUPPLY LOAD
UD
5.1.2.1 Circuit-breaker/fuse – Selectivity in the overload zone
Selectivity in the overload zone is determined by the comparison of time/current
characteristics (see Figure 3).

– 16 – TR 61912-2 © IEC:2009(E)

CB
Fuse
Current
IEC  770/09
NOTE The characteristics are subject to tolerances, which must be taken into account, see introduction of
Clause 5.
Figure 3 – Selectivity in the overload zone between
a circuit-breaker (UD) and a fuse (DD)
5.1.2.2 Circuit- breaker/fuse – Selectivity in the fault current zone
Selectivity in the fault current zone may be determined by comparison of time/current
characteristics in the case of circuit-breaker with short-circuit time-delay release. In the zone
of instantaneous tripping and for a circuit-breaker without short-circuit time-delay release, the
selectivity limit current must be determined from test data provided by the manufacturer.
In the absence of specific test data in the case where UD is a thermal/magnetic circuit-
breaker, the minimum level of selectivity between the fuse and the circuit-breaker in the fault
current zone may be determined as follows. Selectivity is assured up to the fault current level
at which the peak current let-through of DD is less than the peak value corresponding to the
instantaneous tripping level of UD.
Note that the selectivity limit obtained by this method will err on the low side and the actual
level determined by test will be significantly higher in most cases.
5.1.3 CB/CPS – Selectivity between a circuit-breaker to IEC 60947-2 (UD) and a CPS
to IEC 60947-6-2 (DD)
CB/UD CPS/DD
SUPPLY LOAD
In general, the CPS to IEC 60947-6-2 is a final circuit device, for example a motor controller.
Since it has integral over-current releases and a short-circuit breaking capacity, it is treated
for selectivity purposes in the same way as a circuit-breaker (see 5.1.1).
Time
TR 61912-2 © IEC:2009(E) – 17 –
5.1.4 Circuit-breaker/MOR – Selectivity between a circuit-breaker (UD) and a motor
protection overload relay to IEC 60947-4-1 or IEC 60947-4-2 (DD)
The methods for determination of selectivity, taking into account the characteristics given in
the product standards, are given in this subclause.
Circuit-breaker
SUPPLY CB/UD MOR/DD MOTOR
UD
The motor protection overload relay in a motor starter or starter assembly provides overload
protection to the motor and the circuit conductors. A circuit-breaker as UD is required to
provide short-circuit protection to the circuit conductors and the starter itself. Coordination
between the overload relay and the circuit-breaker is determined by test in accordance with
IEC 60947-4-1, Clause B.4, or IEC 60947-4-2, Annex C.
Determination of the coordination either Type 1 or Type 2, ensures selectivity up to the stalled
current of the motor.
A circuit-breaker, with overload and fault current functions, may be used for this purpose (see
Figure 4). However, only the instantaneous tripping function is required and an ICB to
IEC 60947-2, Annex O is specifically intended for this purpose (see Figure 5).
Note that circuit-breakers to IEC 60898-1 have defined overload characteristics, graded by
instantaneous tripping level with specified tolerance bands. Thus selectivity in the overload
zone may be established by reference to the type of characteristic B, C or D. In general, type
D will be used in this application due to the inrush current of the motor.

Type B – Instantaneous tripping band = 3 to 5 I
n
Type C – Instantaneous tripping band = 5 to 10 I
n
Type D – Instantaneous tripping band = 10 to 20 I
n
CB
MOR
Motor
Current
Overload zone
Fault-current zone
IEC  771/09
NOTE The characteristics are subject to tolerances, which must be taken into account, see introduction of
Clause 5.
Figure 4 – Circuit-breaker/MOR – Circuit-breaker selectivity with motor overload relay
Time
– 18 – TR 61912-2 © IEC:2009(E)

MOR (DD)
ICB (UD)
Motor
Current
Overload zone Fault-current zone
IEC  772/09
NOTE The characteristics are subject to tolerances, which must be taken into account, see introduction of
Clause 5.
Figure 5 – ICB/MOR – ICB selectivity with motor overload relay
5.2 Fuse(s) to IEC 60269-1 as UD
IEC/TR 61818 provides a detailed application guide for low-voltage fuses.
IEC/TR 61459 provides a detailed application guide for coordination between fuses and
contactors/motor starters.
5.2.1 Fuse/circuit-breaker – Selectivity between a fuse to IEC 60269-1 (UD) and a
circuit-breaker (DD)
Methods for the determination of selectivity between a fuse and a circuit-breaker are given
in 5.2.1.1, for the overload zone of operation, and in 5.2.1.2, for the fault current zone of
operation.
FU/UD CB/DD
SUPPLY LOAD
Installations are generally either circuit-breaker based or fuse based, except at the appliance
level, however this particular case does arise where the fuse(s) is(are) used as back-up
protection for a circuit-breaker at exceptionally high prospective fault levels.
5.2.1.1 Fuse/circuit-breaker – Selectivity in the overload zone
Selectivity in the overload zone (see 3.11) is determined by the comparison of time/current
characteristics (see Figure 6).
Time
TR 61912-2 © IEC:2009(E) – 19 –

Fuse (UD)
CB (DD)
Current
IEC  773/09
NOTE The characteristics are subject to tolerances, which must be taken into account, see introduction of
Clause 5.
Figure 6 –
...