3.2 Protection Equip- Equip-ment for Low-Voltage
3.2.1 Circuit-Breakers with Protective Functions
Protective functions of LV circuit-breakers
Circuit-breakers are used, first and foremost, for overload and short-cir-cuit protection. In order to increase their protective functions, they can also be equipped with additional releases, e.g. for clearance with undervoltage, or with supplementary modules for detecting fault/residual currents (also see Chapter 6).
The circuit-breakers are distinguished according to their protective function:
C Circuit-breakers for system protec-tion acc. to EN 2/ IEC 60947-2/DIN VDE 0660-101
C Circuit-breakers for motor protec tion acc. to EN 60947-2/
IEC 60947-2 / DIN VDE 0660-101
C Circuit-breakers used in motor star ters acc. to EN 60947-4-2/
IEC 60947-4-2 / DIN VDE 0660-102 C Miniature circuit-breakers for cable
and line protection acc. to EN 60898/ IEC 60898 / DIN VDE 0641-11
Zero-current interrupters / current limiters
Depending on their method of opera-tion, circuit-breakers are available as:
C Zero-current interrupters or C Current limiters (fuse-type
current limiting).
When configuring selective distribu-tion boards, zero-current interrupters are more suitable as upstream pro-tection devices and current limiters as downstream protection devices.
Overcurrent protection devices Standard Overload Short-circuit See Section
protection protection
Fuses gL EN 60 269/IEC 60 269/DIN VDE 0636 × × Section 6.2.2
Miniature circuit-breakers EN 60 898/IEC 60 898/DIN VDE 0641-11 × × Section 6.2.4 Circuit-breakers with overload EN 60 947-2/IEC 60 947-2/DIN VDE 0660-101 × × Section 6.2.1 and overcurrent releases
Switchgear EN 60 269/IEC 60 269/DIN VDE 0636 – × Section 6.2.2
fuses aM
Switchgear assemblies
with back-up fuse, utilization EN 60 269/IEC 60 269/DIN VDE 0636 – × category gL or aM, and
contactor with overload relay EN 60 947-4-1/IEC 60 947-4-1/DIN VDE 0660-102 × – or
starter circuit-breaker and EN 60 947-2/IEC 60 947-2/DIN VDE 0660-101 – × contactor with overload relay EN 60 947-4-1/IEC 60 947-4-1/DIN VDE 0660-102 × –
×Protection provided – No protection provided.
Table 3/1 Overview of line and cable overcurrent protection devices discussed in this manual together with their protection ranges
Protection devices MV Switch-disconnectors, Circuit-breakers, Switch-disconnectors,
HV HRC fuses transducer, time- HV HRC fuses
overcurrent protection
LV Circuit-breakers or Tie breaker Circuit-breakers Network circuit-breakers
LV HRC fuses and network master relays
Cost Low Justifiable High Low
Medium-voltage side
Transformers with thermal release or full thermal protection
Low-voltage side with various series-connected protection devices in radial systems, and parallel-connected LV HRC fuses in interconnected systems
Individual and parallel operating customary
Optional HV HRC
MV LV
≤ 630 A
LV HRC
≤ 50 A, ≤ 100 A
Individual and parallel operating customary
MV LV I >
I >>
Only parallel operation customary
HV HRC MV LV
(interconnected system)
S
HV or LV HRC fuses I >
I >>
Definite-time-overcurrent protection, two-level I> and I>>, via current transformer S Network master relay (directional power relay)
via current transformer and system voltage Power-factor correction controller
Switch-disconnector
Circuit-breaker
Drawout circuit-breaker (with safe clearance)
Contactor
Overload relay
Table 3/2 Overview of protection grading schemes discussed in this manual for outgoing transformer and LV feeders
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Overload and overcurrent protection Tables 3/3 and 3/4 provide an overview of releases and relays in LV circuit-breakers. Table 3/5 contains the operating ranges of the overcur-rent releases. According to the stan-dards specified in Table 3/1, the oper-ating value at which the releases trig-ger may deviate by ± 20% from the set value.
Overcurrent releases
The instantaneous electromagnetic overcurrent releases have either fixed or variable settings, whereas the electronic overcurrent releases used in Siemens circuit-breakers all have variable settings.
Modules
The overcurrent releases can be inte-grated in the circuit-breaker or sup-plied as separate modules for retro-fitting or replacement. Possible ex-ceptions are indicated in the manufacturer specifications.
Overload releases
Mechanical (thermal) inverse-time-de-lay overload releases (L-releases) are not always suitable for networks with a high harmonic content. Circuit-breakers with electronic overload re-leases must be used in such cases.
Short-circuit protection with S-releases
In the case of circuit-breakers with definite (short-)time-delay overcur-rent releases (S) used for time-grad-ing short-circuit protection, it should be noted that the circuit-breakers are designed for a specific maximum per-missible thermal and dynamic load.
If, in the event of a short circuit, the time delay results in this load to be exceeded, an I-release must also be used to ensure that the circuit-breaker is opened instantaneously with very high short-circuit currents.
The information supplied by the Table 3/4 Circuit-breaker releases and relays with protective functions
Function Release Relay
Overload Overload release Overload relay protection Inverse-time delay or Thermal delay or
electronic delay electronic delay
Thermistor protection release devices Short-circuit Overcurrent release Overcurrent relay
protection Instantaneous electromagnetic Instantaneous electromagnetic
or electronic release
Selective Overcurrent release – short-circuit Instantaneous electromagnetic protection or electronic
Protective Siemens Time-delay Graphical symbol acc. to function symbol characteristics EN 60 617/DIN 40 713
of release
Circuit diagram Block
or diagram
Overload L Inverse-time
protection delay
Selective S1) Definite-time
short-circuit delay by
protection timing element
or
inverse-time delay Fault current/ G1) Definite-time residual current/ delay
earth fault or
protection inverse-time
delay Short-circuit I Instantaneous protection
1)For SENTRON 3WL and SENTRON 3VL circuit-breakers, protection also includes
“zone-selective interlocking” (ZSI)
In the following, combinations of releases will be referred to by their code letters only (L-, S- and I-releases).
I >I >
I
I >
I >>
Table 3/3 Symbols for releases according to protective functions
manufacturer should be consulted when the release type is selected.
Reclosing lockout after short-circuit tripping
A number of circuit-breakers can be fitted with a mechanical and/or elec-trical reclosing lockout which pre-vents reclosing to the short-circuit after short-circuit tripping.
The circuit-breaker can only be closed again after the fault has been elimi-nated and the lockout has been reset manually.
Fault-current/residual-current protection
The global importance of fault-current protection devices has grown in the field of protection technology due to the high level of protection they pro-vide (protection of human life and property) and their extended scope of protection (alternating and pulsat-ing current sensitivity).
Apart from residual-current-operated breakers, miniature circuit-breaker assemblies, e. g. miniature circuit-breakers with fault-current trip-ping, are being used to an increasing extent for commercial and industrial applications.
MCBs with fault-current tripping These circuit-breaker assemblies are available as compact factory-built de-vices or may be assembled from a miniature circuit-breaker as the basic device and an add-on module.
Circuit-breakers with fault-current/
residual-current tripping
The assembly comprising a circuit-breaker and add-on module has es-tablished itself for circuit-breakers with rated currents Inof up to 400 A and fault-current/residual-current trip-ping.
Technical features
The add-on module for residual-cur-rent tripping used in system protec-tion applicaprotec-tions includes such tech-nical features as:
C Rated residual current I∆n, ad-justable in steps, e.g. 30 mA/
100 mA/300 mA/500 mA/1,000 mA/3,000 mA
C Tripping time ta, adjustable in steps,
e. g. instantaneous/60 ms/ 100 ms/250 ms/500 ms/1,000 ms C Operation depends on system
voltage
C Sensitivity: tripping with alternating and pulsating DC fault currents C Reset button ”R” for resetting after residual-current tripping
C Test button ”T” for testing the circuit-breaker assembly
C Status display for the current leak-age / residual current I∆in the downstream circuit, e. g. by means of colored LEDs:
– green: I∆≤0.,5 I∆n C Disconnection of the electronics
overvoltage protection prior to insu-lation measurement in the installa-tion
C ”Remote tripping”
C ”Auxiliary switch (AS)”
Interface to bus systems With appropriate interfaces, the circuit-breaker assemblies can be equipped to bus systems to enable the exchange of information and in-teraction with other components in the electrical installation.
AC/DC sensitive circuit-breaker assemblies
In industrial applications, circuit-breaker assemblies which are sensi-tive to AC/DC currents are required for electrical installations in which smooth DC fault currents or currents with a low residual ripple occur in the event of a fault.
Applications Time-delay Operating ranges of
(primarily for characteristic inverse-time-dealy overcurrent
short-circuit release as multiple of set
current clearance) value Ir
Circuit-breaker Instantaneous or Approx. 3 to 6 · Ir for generator short-time delay
protection
Circuit-breaker for Instantaneous Approx. 6 to 12 · Ir
line protection
Circuit-breaker for Instantaneous or Approx. 8 to 15 · Ir motor protection short-time delay1)
1) Poss. short-time delay for rush current shunting
Table 3/5 Operating ranges of the overcurrent releases (acc. to EN 60 947 / IEC 60 947/DIN VDE 0660)
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Standards
The standards EN 60947-2/
IEC 60947-2 / DIN VDE 0660-101 apply for circuit-breakers with add-on fault-current or residual-current modules.
Selection criteria for circuit-breakers
When selecting the appropriate cir-cuit-breaker for system protection, special attention must be paid to the following characteristics:
C Type of circuit-breaker and its re-leases according to the respective protective function and tasks C Rated voltages
C Short-circuit strength Icu/ Icsand rated short-circuit making (Icm)and breaking capacity (Icn)
C Rated and maximum load currents The system voltage and system frequency are crucial factors for selecting the circuit-breakers according to
C Rated insulation voltage Ui and
C Rated operating voltage Ue. Rated insulation voltage Ui The rated insulation voltage Uiis the standardized voltage value for which the insulation of the circuit-breakers and their associated components is rated in accordance with HD 625 / IEC 60664 / DIN VDE 0110, Insulation Group C.
Rated operating voltage Ue The rated operating voltage Ueof a circuit-breaker is the voltage value to which the rated short-circuit making and breaking capacities and the short-circuit performance category refer.
Short-circuit current
The maximum short-circuit current at the installation location is a crucial factor for selecting the circuit-breakers according to
C Short-circuit strength Icu/ Ics, as well as
C Rated short-circuit making Icmand breaking capacities Icn.
Dynamic short-circuit strength The permissible dynamic short-circuit strength is indicated as the peak short-circuit current. It is the highest permis-sible instantaneous value of the prospective short-circuit current along the conducting path with the highest load.
Thermal fault withstand capability (1-s current)
The permissible thermal short-circuit strength is referred to as the rated short-time current Icw. It is the maxi-mum current which the breaker is ca-pable of withstanding for X s without any damage occurring. Generally, the Icwcurrent refers to 1 s. Other time values can be converted assuming Icn= constant.
Rated switching capacity The rated switching capacity of the circuit-breakers is specified as the rated short-circuit making capacity and rated short-circuit breaking capacity.
Table 3/6 Correlation n between rated short-circuit making and breaking capacity and the respective power factor (for AC circuit-breakers)
Rated short-circuit Power factor Minimum value n
breaking capacityIcn cos ϕ n =short-circuit making capacity (r.m.s. value) kA short-circuit breaking capacity
4.5 < I≤ 6 0.7 1.5
6 < I≤ 10 0.5 1.7
10 < I≤ 20 0.3 2.0
20 < I≤ 50 0.25 2.1
50 < I 0.2 2.2
Table 3/7 Switching performance categories acc. to EN 60947 / IEC 60947 / DIN VDE 0660 and IEC 157-1
The rated short-circuit breaking capacity is indicated using two values:
Switching capacity Icu Ics
Rated ultimate Rated service
short-circuit short-circuit breaking capacity breaking capacity
Test sequence O-t-CO O-t-CO-t-CO
Test of • ultimate short-circuit • service short-circuit breaking capacity breaking capacity
Testing Testing
• the overload tripping • the overload tripping
• the insulation resistance • the insulation resistance
• the overheating • the overheating O Opening (O = Open)
CO Opening and closing (C = Close) t Interval (t = time)
Circuit-breaker type Rated current Application example Tripping characteristic
Air Protection of distribution systems, motors,
circuit-breaker (ACB) 630A to 6,300 A transformers and generators
SENTRON 3WL1 – High rated short-time current for time selectivity – Two series, SENTRON WL1 and SENTRON WL6 with high and medium rated switching capacity – Electronic, microprocessor-based overcurrent
releases independent of external voltages – Zone-selective interlocking (ZSI) with total delay
time of 50 ms
Current-limiting Designed and tested in compliance with circuit-breaker EN 60947 / IEC 60947 / DIN VDE 0660
(MCCB) Possible applications:
SENTRON 3VL
TM release: For system protection up to 1,600 A
16 A to 630 A Optional adjustable overload and overcurrent release:
ETU release: Precise adaptation to protection requirements 63 A to 1,600 A
ETU release: For motor protection up to 500 A
63 A to 500 A Electronic overload release with adjustable time-lag class:
Effective protection when motor is under full load
M release: For starter combinations up to 500 A 63 A to 500 A Unsusceptible to inrush currents:
Breaker not tripped by direct-on-line motor starting
M release: As isolating circuit-breaker (load interrupter) up to 2,000 A 100 A to 1,600 A with integrated overcurrent releases,
no back-up fuse required
Circuit-breaker 0.16 to 100 A 3 RV1 circuit-breaker for motor protection
3RV1 with overload and overcurrent protection
L Overload tripping S Short-time delay overcurrent tripping I Instantaneous overcurrent tripping G Ground fault tripping L
S G I
L
S I
L I
L I
I
I
L
I
I
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Rated short-circuit making capacity Icm
The rated short-circuit making capac-ity Icmis the short-circuit current which the circuit-breaker is capable of making at the rated operating volt-age +10%, rated frequency and a specified power factor. It is ex-pressed as the maximum peak value of the prospective short-circuit cur-rent, and is at least equal to the rated short-circuit breaking capacity Icn, multiplied by the factor n specified in Table 3/6.
Rated short-circuit breaking capacity Icn
The rated short-circuit breaking capac-ity Icnis the short-circuit current which the circuit-breaker is capable of break-ing at the rated operatbreak-ing voltage +10%, rated frequency and a speci-fied power factor cos ϕ. It is ex-pressed as the r.m.s. value of the al-ternating current component.
Switching capacity category Switching capacity categories, which specify how often a circuit-breaker can switch its rated making and breaking current as well as the condi-tion of the breaker after the specified switching cycle, are defined for circuit-breakers in EN 60947 / IEC 60947/ DIN VDE 0660 and in accordance with IEC 157-1 (Table 3/7). The rated short-circuit breaking capacity Icnis based on the test sequence O-t-CO-t-CO. The rated service short-circuit breaking capacity Icscan also be specified on the basis of the shortened switching sequence O-t-CO (see Table 3/7 for explanation of O, t, and C).
Rated circuit-breaker currents The rated duty, e.g. continuous operation, intermittent operation or short-time operation, plays a decisive role in selecting the switchgear according to its rated currents.
The following rated currents are dis-tinguished according to the thermal characteristics:
C Rated thermal current Ith C Rated continuous current Iu C Rated operating current Ie. Conventional rated thermal current Ith, rated continuous current Iu
The conventional rated thermal cur-rent Ithor Ithefor motor starters in enclosures is defined as an 8-h cur-rent in accordance with EN 60947-1, -4-1, -3 / IEC 60947-1, -4-1, -3 / DIN VDE 0660-100, -102, -107.
It is the maximum current which can be carried during this time without the temperature limit being ex-ceeded. The rated continuous current Iucan be carried for an unlimited time.
With adjustable inverse-time-delay releases and relays, the maximum current setting is the rated continu-ous current Iu.
Rated operating current Ie The rated operating current Ieis the current that is determined by the op-erating conditions of the switching device, the rated operating voltage and rated frequency, rated switching capacity, the rated duty, utilization category1), contact life and the de-gree of protection.
Application examples and tripping curves
Application examples for circuit-breakers with protection
The principal application examples and typical tripping curves of modern circuit-breakers currently available from Siemens are specified in Table 3/8.
1) The utilization category describes the switching devices’ application and stress, see device standards EN 60947 / IEC 60947 / DIN VDE 0660.