Siemens SENTRON Instruction sheet

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SENTRON
Protective devices
Selectivity for 3VA molded case
circuit breakers
Configuration Manual
08/2016
A5E03603181010-01
Introduction
1
Selectivity - terms and
definitions
2
Methods of implementing
selectivity using circuit
breakers
3
Selectivity implemented by
combinations of protective
devices
4
Selectivity with parallel
incoming feeders
5
Selectivity and undervoltage
protection
6
3VA selectivity tables
7
List of abbreviations
A

Siemens AG
Division Energy Management
Postfach 32 20
91050 ERLANGEN
GERMANY
Document order number: 3ZW1012-0VA20-0AC1
Ⓟ 10/2016 Subject to change
Copyright © Siemens AG 2016.
All rights reserved
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Selectivity for 3VA molded case circuit breakers
Configuration Manual, 08/2016, A5E03603181010-01 5
Table of contents
1 Introduction ................................................................................................................................................ 7
2 Selectivity - terms and definitions .............................................................................................................. 9
3 Methods of implementing selectivity using circuit breakers ...................................................................... 13
3.1 Starting point ........................................................................................................................... 15
3.2 Selectivity in the overload zone .............................................................................................. 15
3.2.1 Example .................................................................................................................................. 16
3.2.2 Circuit breaker parameters ..................................................................................................... 17
3.2.3 Tripping characteristics of circuit breakers ............................................................................. 18
3.3 Current selectivity ................................................................................................................... 19
3.3.1 Circuit breaker parameters ..................................................................................................... 20
3.3.2 Tripping characteristics of circuit breakers ............................................................................. 21
3.3.3 Different short-circuit currents at different fault locations ....................................................... 22
3.4 Time selectivity ....................................................................................................................... 23
3.4.1 Circuit breaker parameters ..................................................................................................... 26
3.4.2 Tripping characteristics of circuit breakers ............................................................................. 27
3.4.3 Implementation of total selectivity ........................................................................................... 28
3.5 Zone selective interlocking ..................................................................................................... 30
3.5.1 Zone selective interlocking and time selectivity ...................................................................... 33
3.5.2 Setting of circuit breakers ....................................................................................................... 36
3.6 Dynamic selectivity ................................................................................................................. 40
3.6.1 Implementation of dynamic selectivity .................................................................................... 41
3.6.2 Let-through energy diagram ................................................................................................... 43
3.6.3 Example .................................................................................................................................. 44
4 Selectivity implemented by combinations of protective devices ............................................................... 47
4.1 Molded case circuit breakers and downstream fuse .............................................................. 47
4.2 Fuse and downstream molded case circuit breaker ............................................................... 50
4.3 Fuse and downstream miniature circuit breaker ..................................................................... 53
4.4 Fuse and downstream fuse .................................................................................................... 53
5 Selectivity with parallel incoming feeders ................................................................................................. 57
5.1 Selectivity with two identical incoming feeders ....................................................................... 58
5.1.1 Symmetrical distribution (ideal scenario) ................................................................................ 59
5.1.2 Asymmetrical distribution ........................................................................................................ 59
5.2 Selectivity with three identical incoming feeders .................................................................... 60
5.2.1 Fault within the protection zone of the downstream protective device ................................... 60
5.2.2 Fault between transformer terminals and incoming feeder circuit breaker ............................. 61
5.3 Selectivity with bus-couplers ................................................................................................... 63
5.4 Sources of information and other documentation ................................................................... 68

Table of contents
Selectivity for 3VA molded case circuit breakers
6 Configuration Manual, 08/2016, A5E03603181010-01
6 Selectivity and undervoltage protection ................................................................................................... 69
7 3VA selectivity tables ............................................................................................................................... 71
A List of abbreviations ................................................................................................................................. 73
A.1 List of abbreviations ............................................................................................................... 73
Glossary .................................................................................................................................................. 75

Selectivity for 3VA molded case circuit breakers
Configuration Manual, 08/2016, A5E03603181010-01 7
Introduction 1
There are growing demands in many sectors for more reliable, secure electricity supplies for
installations, whether these are data centers, airports, industrial production facilities, power
plants, etc. (Over-current) selectivity can make an important contribution to fulfilling these
demands. It is capable of minimizing the effects of a fault in terms of its duration and the
area it affects, thereby ensuring that plant sections that are not affected by the fault will
remain in operation.
This "Selectivity" Configuration Manual introduces you to the subject of selectivity and
describes the relevant implementation methods as well as the relationships between
selectivity and infeed and/or selectivity and undervoltage.
The manual is aimed at engineers, planners and project managers.
Please note that the principles of selectivity described in this manual apply only to low-
voltage switchboard and distribution systems.
Further information about implementing selectivity with 3VA molded case circuit breakers
can be found in the following documents:
● 3VA Manual (order number 3ZW1012-0VA10-0AB1)
● 3VA Communications Manual (order number 3ZW1012-0VA20-0BB0)
Structure of the manual
The manual is divided into the following chapters:
● Selectivity - terms, definitions and methods (Page 9)
This chapter explains important terms relating to selectivity and presents possible
methods for implementing selectivity.
● Application examples (Page 13)
The methods by which selectivity can be implemented using circuit breakers are
explained in greater depth and demonstrated by specific examples.
● Selectivity implemented by combinations of protective devices (Page 47)
Conditions are demonstrated under which different protective devices can be combined in
order to afford selectivity.
● Selectivity with parallel incoming feeders (Page 57)
The effects of parallel incoming feeders on selectivity are explained.
● Selectivity and undervoltage protection (Page 69)
The boundary conditions for implementing selectivity using circuit breakers equipped with
an undervoltage release are illustrated.

Introduction
Selectivity for 3VA molded case circuit breakers
8 Configuration Manual, 08/2016, A5E03603181010-01
Navigation around the manual
The following guides to accessing information are provided:
● Table of contents
● Section "Structure of the manual"
● Index

Selectivity for 3VA molded case circuit breakers
Configuration Manual, 08/2016, A5E03603181010-01 9
Selectivity - terms and definitions 2
This chapter introduces you to the subject of "selectivity" on the basis of the key terms
associated with the concept.
For additional information, especially in relation to selectivity methods, please refer to
chapter "Methods of implementing selectivity using circuit breakers (Page 13)".
Selectivity
Selectivity exists in a low-voltage network if only the protective device connected directly
upstream of the fault location (as viewed in the direction of current flow) trips in response to
over-currents.
Selectivity is achieved by combining two or more protective devices with appropriately
coordinated operating characteristics.
Example
Selectivity implemented by series-connected LV HRC fuses of the same operating class
① Time-current characteristic with tolerance range
In the event of a 1000 A over-current, the 100 A fuse directly upstream of the fault location
disconnects the fault location from the mains supply, while the 160 A fuse does not respond
due to its longer pre-arcing time. Sections of the plant unaffected by the fault continue to be
supplied with power.
This graph showing the time-current characteristic for determining selectivity applies only to
tripping times in excess of 100 ms. For tripping times of less than 100 ms, the I2t values
defined according to DIN VDE 0636-2 are applied.

Selectivity - terms and definitions
Selectivity for 3VA molded case circuit breakers
10 Configuration Manual, 08/2016, A5E03603181010-01
Over-current selectivity
Excerpt from EN 60947-1, 2.5.23:
Over-current selectivity (discrimination) is the "co-ordination 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".
See also DIN VDE 100-530: 2011-06 or IEC 60364-7-710 or DIN VDE 0100-710.
Partial selectivity
Partial selectivity exists in cases where the protective devices behave selectively only up to a
specific current, i.e. in addition to the protective device directly at the fault location, other
upstream protective devices also operate even though the maximum over-current at the fault
location is not yet reached.
Total selectivity is not achieved, i.e. sections of plant that are not immediately affected by the
over-current fault are also disconnected from the supply system.
If the actual short-circuit current is lower than the selectivity limit current, then total selectivity
is also afforded (see EN 60947-2, 2.17.4).
Excerpt from EN 60947-2, 2.17.3:
Partial selectivity is "over-current selectivity where, in the presence of two over-current
protective devices in series, the protective device on the load side effects the protection up
to a given level of over-current without causing the other protective device to operate".
Total selectivity
Total selectivity exists if only the protective device situated immediately upstream of the fault
location operates across the entire over-current zone (in the direction of current flow) that
can occur at the fault location.
Excerpt from EN 60947-2, 2.17.2:
Total selectivity is "over-current selectivity where, in the presence of two over-current
protective devices in series, the protective device on the load side effects the protection
without causing the other protective device to operate".
Total selectivity
Total selectivity is afforded if only the protective device situated immediately upstream of the
fault location operates across the entire over-current zone up to
● the rated ultimate short-circuit breaking capacity Icu of the downstream circuit breaker
according to IEC 60947-2, or
● the rated short-circuit breaking capacity Icn of the downstream miniature circuit breaker
according to IEC 60898, or
● up to the rated breaking capacity of the fuse according to IEC 60269-3.

Selectivity - terms and definitions
Selectivity for 3VA molded case circuit breakers
Configuration Manual, 08/2016, A5E03603181010-01 11
Selectivity limit current IS
The selectivity limit current corresponds exactly to the over-current value at which an
upstream protective device operates in addition to the protective device situated immediately
upstream of the fault source.
In other words, the operating characteristics of the protective devices touch or intersect at a
specific current magnitude. The point of contact or intersection determines the selectivity
limit current.
Only partial selectivity is afforded if the maximum potential over-current is higher than the
selectivity limit current. Total selectivity is afforded if the maximum potential over-current is
lower than the selectivity limit current.
Over-current
According to EN 60947-1, 2.1.4, over-current is a "[...] current that exceeds rated current
[...]", in other words, it is a short-circuit or overload current.
Excerpt from EN 60947-2, 2.17.4:
The "selectivity limit current is the current co ordinate of the intersection between the total
time-current characteristic of the protective device on the load side and the pre-arcing (for
fuses), or tripping (for circuit breakers) time-current characteristic of the other protective
device".

Selectivity for 3VA molded case circuit breakers
Configuration Manual, 08/2016, A5E03603181010-01 13
Methods of implementing selectivity using circuit
breakers 3
Using the example of a fault scenario in a power distribution system, this chapter explores
the potential methods for implementing selectivity. The description in this chapter focuses on
the implementation of selectivity between circuit breakers. The subject of implementing a
selectivity concept using combinations of different protective devices is discussed in chapter
"Selectivity implemented by combinations of protective devices (Page 47)".
Definition of selectivity
The following different methods can be used to implement selectivity with low-voltage circuit
breakers:
● Selectivity in the overload zone
● Selectivity in the short-circuit zone
– Current selectivity
– Time selectivity
– Dynamic selectivity (energy selectivity)
– Zone selective interlocking (ZSI, zone selectivity, logical selectivity)

Methods of implementing selectivity using circuit breakers
Selectivity for 3VA molded case circuit breakers
14 Configuration Manual, 08/2016, A5E03603181010-01
Overload zone and short-circuit zone for a circuit breaker
1) The boundary between the overload and short-circuit zones (definition Ikmin and Ikmax) is
described in chapter "Setting of circuit breakers (Page 36)".
2) For further information, please see chapter "Zone selective interlocking (Page 30)".

Methods of implementing selectivity using circuit breakers
3.1 Starting point
Selectivity for 3VA molded case circuit breakers
Configuration Manual, 08/2016, A5E03603181010-01 15
3.1 Starting point
The starting point is a fault scenario in a low-voltage power distribution system that is
protected by circuit breakers.
S1 Incoming feeder circuit breaker
S2.1 - S2.3 Main distribution board circuit breaker
S3.1 - S3.3 Subdistribution board circuit breaker
Figure 3-1 Fault scenario in a low-voltage power distribution system
The fault occurs downstream of circuit breaker S3.3. The three circuit breakers in the energy
line marked in red are all affected by the over-current.
If all of the circuit breakers are operating selectively, only circuit breaker S3.3 situated
directly at the fault source is permitted to trip. Circuit breakers S1 and S2.2 that are further
upstream must not trip even though they might detect the over-current.
3.2 Selectivity in the overload zone
Selectivity in the overload zone is implemented by using a suitable combination of protective
devices. It is determined by plotting the tripping characteristics in a graph.
Circuit breakers suitable for this application
Devices suitable for implementing selectivity in the overload zone include, for example,
molded case circuit breakers with an electronic and a thermal-magnetic trip unit and air
circuit breakers.
A fictitious example will be assessed below; this comprises a combination of a
downstream 100 A molded case circuit breaker and an upstream 250 A molded case circuit

Methods of implementing selectivity using circuit breakers
3.2 Selectivity in the overload zone
Selectivity for 3VA molded case circuit breakers
16 Configuration Manual, 08/2016, A5E03603181010-01
breaker, both of which are equipped with an electronic trip unit with adjustable LI/LSI
protective function. To make the examples shown below in chapters 3.2 - 3.4 easier to
understand, the tolerances of the tripping characteristics are not shown.
3.2.1 Example
The rated operational currents are as follows in the starting situation described in chapter
3.1:
•S1
•S2.2
•S3.3
Rated operational current Irated = 630 A
Rated operational current Irated = 250 A
Rated operational current Irated = 100 A
Figure 3-2 Fault scenario with overload zone selectivity
The analysis below will focus solely on the selectivity between circuit breaker S2.2 and
circuit breaker S3.3.

Methods of implementing selectivity using circuit breakers
3.2 Selectivity in the overload zone
Selectivity for 3VA molded case circuit breakers
Configuration Manual, 08/2016, A5E03603181010-01 17
3.2.2 Circuit breaker parameters
In order to ensure selectivity, the protection parameters of the LI protective function must be
set appropriately for the given situation.
The following parameters are important for the overload zone:
● Settings of the inverse-time delay L release (Ir)
● Delay time of the inverse-time delay L release (tr)
The inverse-time delay L release (Ir) trips at the following overload currents when the
settings shown in the table are applied:
● Setting at L release (Ir) of 100 A circuit breaker = 100 A
● Setting at L release (Ir) of 250 A circuit breaker = 250 A
The delay time of the L release (tr) ensures that there is a sufficient distance (i.e. no contact)
between the characteristics of the two circuit breakers in the overload zone.
The setting of the instantaneous I release (Ii) trips in the short-circuit zone. This value has
been set to 1000 A for both circuit breakers. Since this parameter has no relevance in the
overload zone, it will not be discussed in any detail until later on.
Note
Note the tolerance ranges
The tolerances of the setting values and the delay times must be taken into account when
the parameters are set:
•Tolerances of setting values (Ir, Isd, Ii): ± 10 %
•Tolerances of delay times (tr, tsd): ± 10 %

Methods of implementing selectivity using circuit breakers
3.3 Current selectivity
Selectivity for 3VA molded case circuit breakers
18 Configuration Manual, 08/2016, A5E03603181010-01
3.2.3 Tripping characteristics of circuit breakers
Figure 3-3 Current-time diagram: tripping characteristics of 100 A and 250 A circuit breakers with
selectivity in the overload zone
In order to achieve successful selectivity in the overload zone, the overload tripping
characteristics of the two circuit breakers must meet the following criteria:
● No contact or overlapping in the overload zone, including tolerance ranges.
For the purpose of the selectivity evaluation, the characteristics of the upstream circuit
breaker must be in the top, right-hand area (blue characteristic) and those of the
downstream circuit breaker in the bottom, left-hand area (green characteristic).
● There is a sufficient distance between all tolerance ranges in branched power distribution
boards.
The distances are dependent on the overload protection setting values of the circuit
breakers in the other branches.
The tripping characteristics fulfill these criteria in the overload zone. Selectivity is thus
assured in this zone.
The selectivity limit current (Is) equals 1000 A. When the current reaches this limit, both
circuit breakers trip simultaneously. As a result, the selectivity in the overload zone is only
partial.

Methods of implementing selectivity using circuit breakers
3.3 Current selectivity
Selectivity for 3VA molded case circuit breakers
Configuration Manual, 08/2016, A5E03603181010-01 19
3.3 Current selectivity
Current selectivity is a form of selectivity in which the (instantaneous) protective devices trip
at different over-current values.
In the event of a fault, the following currents flow:
● A high current flows through the circuit breakers close to the infeed.
● A low current flows through the circuit breakers at a far distance from the infeed.
The tripping characteristics are plotted in a graph in order to determine current selectivity.
Advantages and limitations
Advantages of current selectivity:
● Simple to implement
● Inexpensive
Limitations of current selectivity:
● Equipment exposed to extreme current loads as a result of potentially prolonged flow of
short-circuit currents:
If the fault location is situated between the two protective devices, IEC 60364-4-41 states
that it might take up to 5 seconds for the protective devices in TN systems to trip at a
rated current of > 32 A.
● In order to reduce the thermal and mechanical loading of equipment by high short-circuit
currents, the selectivity limit current is normally set to a relatively low value. As a general
rule, only partial selectivity can be achieved with current selectivity.
Area of application
Circuit breakers can be combined to provide current selectivity in applications in which the
maximum short-circuit currents between the upstream and downstream circuit breakers are
very different.
Circuit breakers suitable for this application
Current selectivity can be implemented using air circuit breakers or molded case circuit
breakers with an electronic and a thermal-magnetic trip unit.
The circuit breakers require neither short-time delay over-current releases nor dynamic
selectivity coordination.

Methods of implementing selectivity using circuit breakers
3.3 Current selectivity
Selectivity for 3VA molded case circuit breakers
20 Configuration Manual, 08/2016, A5E03603181010-01
3.3.1 Circuit breaker parameters
The following parameters are important for current selectivity:
● Setting of the instantaneous I release (Ii)
The parameters of the L release (Ir and tr) remain unchanged from the example given in
chapter "Selectivity in the overload zone (Page 15)".
The settings of the instantaneous I release (Ii) trip at different short-circuit currents in this
case:
● Setting of I release (Ii) of 100 A circuit breaker = 500 A
● Setting of I release (Ii) of 250 A circuit breaker = 2500 A
The effects are clearly illustrated in the current-time curve diagram, see below.
Note
Note the tolerance ranges
The tolerances of the operating currents and the delay times must be taken into account
when the parameters are set:
•Tolerances of setting values (Ir, Isd, Ii): ± 10 %
•Tolerances of delay times (tr, tsd): ± 10 %.