260664511-IEC-Standards-Power-Factor-Correction

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© ABB Group 
March 10, 2015 | Slide 1
Lionel Ng, LPBS - Low Voltage Products
Welcome To ABB 
Technical Sharing Session 
Circuit Breakers
Standards Guidelines IEC 60947-2
© ABB Group 
March 10, 2015 | Slide 3
 IEC 60947-2
Circuit Breaker Standard, for industrial application
– Definitions for MCCBs and ACBs
– Choice criteria based on rated and limit values
Agenda
© ABB Group 
March 10, 2015 | Slide 4
– International Standard IEC 60947
– European Standard EN 60947
 IEC 60947-1 Part 1: General rules
 IEC 60947-2 Part 2: Circuit breakers
 IEC 60947-3 Part 3: Switch disconnectors 
 IEC 60947-4-1 Part 4: Contactors
 IEC 60947-5-1 Part 5: Control circuit devices 
 IEC 60947-6-1 Part 6: Multifunction devices
 IEC 60947-7-1 Part 7: Auxiliary materials
Standard for LV apparatus
 IEC 60947 Standard for industrial application
© ABB Group 
March 10, 2015 | Slide 5
A mechanical switching device capable of breaking, carrying and 
making currents under normal circuit conditions and also making, 
carrying, for a specified time, and breaking currents under specified 
abnormal circuit conditions such as those of short-circuit.
 BREAKING Breaking Capacity 
 WITHSTAND Short time withstand 
 MAKING Making Capacity
IEC Standard definitions
 Circuit Breaker - IEC 60947-2 
© ABB Group 
March 10, 2015 | Slide 6
A mechanical switching device capable of breaking, making and 
carrying currents under normal circuit conditions but only making and 
carrying, for a specified time, currents under specified abnormal circuit 
conditions such as those of short-circuit.
 BREAKING Breaking Capacity
 WITHSTAND Short time withstand 
 MAKING Making Capacity
IEC Standard definitions
 Switch Disconnector - IEC 60947-3
© ABB Group 
March 10, 2015 | Slide 7
Moulded case circuit breaker (MCCB): a circuit breaker having a supporting 
housing of moulding insulating material, forming an integral part of the circuit 
breaker (Tmax-XT).
IEC Standard definitions 
IEC Standard definitions
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March 10, 2015 | Slide 8
Air circuit breaker (ACB): a circuit breaker having a 
supporting housing of moulding insulating material and a 
metallic frame, forming an integral part of the circuit 
breaker (Emax & Emax 2).
© ABB Group 
March 10, 2015 | Slide 9
A circuit breaker with a break-time short enough to prevent the short-circuit 
current from reaching its peak value.
Current limiting circuit breaker 
 Current limiting circuit breaker (IEC 60947-2 def. 2.3)
A current-limiting circuit 
breaker is able to reduce the 
stress, both thermal and 
dynamic, because it has been 
designed to start the opening 
operation before the short-
circuit current has reached its 
first peak, and to quickly 
extinguish the arc between the 
contacts.
Current limiting circuit breaker
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March 10, 2015 | Slide 10
A = Direction of arc due to the magnetic field
R= Repulsion of moving contacts due to the short circuit current
A
I
A
R
R
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March 10, 2015 | Slide 11
Time
Current
Current limiting circuit breaker
 Energy limitation
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March 10, 2015 | Slide 12
Value of the limited peak 
of the short circuit current 
according to the value of 
the symmetrical short 
circuit current Irms.
Current limiting circuit breaker
 Peak limitation curves
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March 10, 2015 | Slide 13
Value of the let-through 
energy according to the 
value of the symmetrical 
short circuit current Irms.
Current limiting circuit breaker
 I2t curves
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March 10, 2015 | Slide 14
Protection against short-circuit (IEC 60364)
To protect a cable against short-circuit, the specific let-through energy of 
the protective device must be lower or equal to the withstanding energy of 
the cable:
where
– I2 t is the specific let-through energy of 
the protective device which can be read on 
the curves supplied by the manufacturer;
– S is the cable cross section [mm2]; in the 
case of conductors in parallel it is the 
cross section of the single conductor;
– k is a factor that depends on the cable 
insulating and conducting material.
0.1kA 1kA 10kA 100kA
1E-2MA²s
0.1MA²s
1MA²s
10MA²s
100MA²s
1E3MA²s
Specific let through energy curve LLL
Current limiting circuit breaker
 Energy limitation
© ABB Group 
March 10, 2015 | Slide 15
 Rated values (Iu, Ue)
 Limit values (Icu, Ics, Icw, Icm)
 Insulation values (Ui, Uimp)
Choice criteria
 Rated values (Iu, Ue)
© ABB Group 
March 10, 2015 | Slide 16
the rated uninterrupted current of an equipment is a value of 
current, stated by the manufacturer, that the equipment can carry 
in uninterrupted duty (at 40 °C)
IEC 60947-1 def. 4.3.2.4
Rated value Iu
 Rated uninterrupted current Iu
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March 10, 2015 | Slide 17
Rated value Iu
 The rated uninterrupted current Iu is different from the rated 
current In, which is the rated current of the thermomagnetic or 
electronic trip unit and is lower or equal to Iu.
A new concept 
for setting the 
current In: the 
rating plug
© ABB Group 
March 10, 2015 | Slide 18
XT1 160
XT4 250 
 Rated uninterrupted current Iu
Some factors may reduce the Iu of a circuit breaker
like temperature, altitude or frequency.
Rated value Iu
© ABB Group 
March 10, 2015 | Slide 19
the rated operational voltage of an equipment is a value of voltage 
which, combined with a rated operational current, determines the 
application of the equipment and to which the relevant tests and 
the utilization categories are referred.
IEC 60947-1 def. 4.3.1.1
Rated value Ue
 Rated operational voltage Ue
© ABB Group 
March 10, 2015 | Slide 20
Breaking capacity is always referred to the operational voltage; the 
breaking capacity decreases when the voltage increases.
Rated value Ue
 Rated operational voltage Ue
© ABB Group 
March 10, 2015 | Slide 21
 Some factors may reduce the Ue of a circuit breaker
Rated value Ue
© ABB Group 
March 10, 2015 | Slide 22
 Rated values (Iu, Ue)
 Limit values (Icu, Ics, Icw, Icm)
 Insulation values (Ui, Uimp)
Choice criteria
 Limit values (Icu, Ics, Icw, Icm)
© ABB Group 
March 10, 2015 | Slide 23
Breaking capacity according to a specified test sequence.
Do not include after the short circuit test, the capability of the 
circuit breaker to carry its rated current continuously.
- test sequence: O - 3 min - CO 
- dielectric withstand at 2 x Ue
- verification of overload release at 2.5 x I1
Limit value Icu
Icu = RATED ULTIMATE SHORT 
CIRCUIT BREAKING CAPACITY
IEC 60947-2 
def. 4.3.5.2.1
© ABB Group 
March 10, 2015 | Slide 24
Breaking capacity according to a specified test sequence. 
Include after the short circuit test, the capability of the circuit 
breaker to carry its rated current continuously
- test sequence: O - 3 min - CO - 3 min – CO 
- dielectric withstand at 2 x Ue
- verification of temperature rise at Iu
- verification of overload release at 1.45 x I1
- verification of the electrical life
Ics = RATED SERVICE SHORT 
CIRCUIT BREAKING CAPACITY
IEC 60947-2 
def. 4.3.5.2.2
Limit value Ics
© ABB Group 
March 10, 2015 | Slide 25
Limit values Icu and Ics
The service breaking capacity Ics can be expressed as 
a value of breaking current, in kA;
Standard ratios between Ics and Icu
 Relation between Ics and Icu
This relation is always true!!!
Ics ≤ Icu
a percentage of Icu, rounded up 
to the lowest whole number, 
in accordance with the table (for 
example Ics = 25% Icu).
When is Icu required?
 Where continuity of service is not a fundamental requirement.
 For protection of single terminal load.
 For motor protection.
 Where maintenance work is easily carried out without much 
disruption. 
 Generally for circuit breaker installed on terminals part of 
plant.
When is Ics required?
 Where continuity of service is a fundamental requirement.
 For installation in power center. Where is more difficult to make maintenance.
 When is difficult to manage spare breakers.
 Generally for installation in main distribution board 
immediately downstream transformer or generator. 
© ABB Group 
March 10, 2015 | Slide 28
Main circuit breakers or circuit breakers for which 
a long out-of-service period can not be accepted 
(for example naval installation)
CB selection 
based on
Ics
Icu
circuit breakers tor termlnal circuits or 
circuit breakers for economic application
Limit values Icu and Ics
 Icu and Ics: selection criteria
Icu or Ics ?
 Application of Icu / Ics circuit breakers
When Isc = 100 % of Icu is not necessary ?
 When the real short circuit current in the point of 
installation is lower than the maximum Ics breaking 
capacity.
U LOAD
B
A
Breaker A:
Icu =100 kA
with Ics = 100 % of Icu
Breaker B:
Icu = 100 kA
with Ics = 75 % of Icu
70 kA
50 kA !!!
Please also consider
that short circuit current 
at the end of the line is
still lower
When Isc = 100 % of Icu is not necessary ?
 Motor Protection according to IEC 60947- 4-1
Duty cycle:
O - 3mins - CO at “Iq” current (maximum short circuit current)
O - 3mins - CO at “r” current (critical short circuit current depending from the contactor size)
Where:
O: Tripping of the circuit breaker under short circuit condition.
CO: Closing by the contactor under short circuit condition and tripping of the 
circuit breaker.
Icu or Ics ? Conclusion
 Consider that not always Ics = 100% of Icu for all the employ 
voltage range, i.e. (from 220 V a.c. to 690 V a.c.duty, and 250 
V d.c.).
 Selection of circuit breaker with breaking capacity Icu or Ics 
must be done according to the real technical installation 
requirement.
 Independently from the duty cycle selected the safety of the 
plant is strictly dependent from the maximum circuit breaking 
capacity (in most of cases Icu). 
© ABB Group 
March 10, 2015 | Slide 33
Limit value Icw
Icw = RATED SHORT-TIME 
WITHSTAND CURRENT
IEC 60947-2 
def. 4.3.5.4
Example of use of category B circuit breakers 
in electrical plant
Trafo 630kVA
Ucc%=4%
400V
ACB E1B12
MCCB XT4
22.7kA
MCCB XT3
The upstream circuit 
breaker can withstand 
the fault current up to 1 
sec, thus guaranteeing 
an excellent selectivity 
with downstream 
apparatus
© ABB Group 
March 10, 2015 | Slide 34
Circuit breakers specifically intended for selectivity in short 
circuit conditions in relation to other protection devices in 
load-side series, that is with an intentional delay (adjustable) 
applicable in short circuit conditions. 
These circuit breakers have a specified rated short-time 
withstand current Icw.
IEC 60947-2 
Table 4
CATEGORY B
CIRCUIT BREAKER
Limit value Icw
© ABB Group 
March 10, 2015 | Slide 35
Circuit-breakers “not specifically” intended for selectivity under 
short circuit conditions with respect to other protection devices 
in series on the load side, that is without intentional short-time 
delay provided for selectivity under short-circuit conditions. 
These circuit-breakers have not a specified rated short-time 
withstand current value Icw.
Limit value Icw
IEC 60947-2 
Table 4
CATEGORY A
CIRCUIT BREAKER
© ABB Group 
March 10, 2015 | Slide 36
It is the value of short-time withstand current assigned to the 
circuit-breaker by the manufacturer under specified test 
conditions. This value is referred to a specified time (usually 1s or 3s).
It must be stated when the circuit-breaker is classified in 
category B and its value must be greater than:
 The highest value between 12 Iu and 5 kA for CBs with Iu  2500A 
 30 kA for CBs with Iu > 2500A 
Circuit breakers without Icw value are classified in category A
Limit value Icw
IEC 60947-2 
Table 3
Icw = RATED SHORT-TIME 
WITHSTAND CURRENT
Selectivity Categories 
© ABB Group 
March 10, 2015 | Slide 38
IEC 60947-2 
def. 4.3.5.1
Icm = RATED SHORT-CIRCUIT 
MAKING CAPACITY
Making capacity for which the prescribed conditions according 
to a specified test sequence include the capability of the circuit 
breaker to make the peak current corresponding to that rated 
capacity at the appropriate applied voltage.
Limit value Icm
It is always necessary to verify that:
Icm  Ipeak
© ABB Group 
March 10, 2015 | Slide 39
Limit value Icm
Icm ≥ n x Icu
For a.c. the rated short-circuit making 
capacity of a circuit-breaker shall be not 
less than its rated ultimate short-circuit 
breaking capacity, multiplied by the factor 
n of the table.
IEC 60947-2 
Table 2
© ABB Group 
March 10, 2015 | Slide 40
16,8kA
50kA
54kA
Peak
Irms
105kA
10kA 100kA
10kA
100kA
T6L800 In800
XT2L 160 In160
Example
Current limiting circuit breaker
© ABB Group 
March 10, 2015 | Slide 41
If the cos of the plant is higher than the standard prescribed 
value, it is not necessary to take into account the rated short-
circuit making capacity of the circuit-breakers (Icm).
If the cos of the plant is lower than the standard 
prescribed value, usually near to the transformer and/or 
generator, it is necessary to verify Icm  Ipeak.
Limit value Icm
© ABB Group 
March 10, 2015 | Slide 42
If the cosk of the plant is equal to 0.16 (lower than the standard 
prescribed value) the evaluated Ip = 175 kA.
Short circuit current of the plant is Icc = 75kA ; 
The used circuit breaker has an Icu = 75 kA; 
According to the table 2, cosk=0.2 and n=2,2 so Icm = n x Icu = 165 kA.
Since Ip > Icm the CB selected is not correct. I will use a CB with a greater
value of Icu in order to have an Icm value suitable to the peak current of the 
plant. 
Sometimes it can happen
Limit value Icm
© ABB Group 
March 10, 2015 | Slide 43
Limit value Icm
© ABB Group 
March 10, 2015 | Slide 44
 Rated values (Iu, Ue)
 Limit values (Icu, Ics, Icw, Icm)
 Insulation values (Ui, Uimp)
Choice criteria
 Insulation values (Ui, Uimp)
© ABB Group 
March 10, 2015 | Slide 45
IEC 60947-1 
def. 4.3.1.2
Ui = RATED INSULATION 
VOLTAGE
The rated insulation voltage of an equipment is the value 
of voltage to which dielectric tests and creepage 
distances are referred.
It shall be always verified that: 
Ue < Ui
Limit value Ui
© ABB Group 
March 10, 2015 | Slide 46
IEC 60947-1 
def. 4.3.1.3
Uimp = RATED IMPULSE 
WITHSTAND VOLTAGE
The peak value of an impulse voltage of prescribed form and 
polarity (1,2/50ms) which the equipment is capable of 
withstanding without failure under specified conditions of test 
and to which the values of the clearances are referred.
It shall be always verified that: 
Uimp > transient overvoltage in 
the plant
Limit value Uimp
Temperature-rise for terminals and accessible parts
© ABB Group 
March 10, 2015 | Slide 47
IEC 60947- 2 
Table 7
Overload protection
© ABB Group 
March 10, 2015 | Slide 48 i
t
IEC 60947- 2 
Table 6
Short circuit protection
© ABB Group 
March 10, 2015 | Slide 49
i
S
I
t
IEC 60947- 2 
8.3.3.1.2
Type Tests
The tests to verify the characteristics of 
circuit breakers are:
• type tests carried out on samples:
IEC 60947- 2 
8.3
Type Tests
© ABB Group 
March 10, 2015 | Slide 51
Routine Tests
© ABB Group 
March 10, 2015 | Slide 52
• routine tests carried out on 
all circuit breakers and 
including the following tests:
IEC 60947- 2 
8.4
Tests of EMC for circuit breakers with electronic overcurrent protection
Immunity
Emission
•Electrostatic discharges
•Radiated radio-frequency electromagnetic fields
•Electrical fast transients/bursts
•Surges
•Conducted disturbances induced by radio-frequency fields
•Harmonics
•Voltage fluctuations
•Conducted disturbances
•Radiated disturbances
Climatic tests
•Dry heat test Damp heat test 
•Temperature variation cycles at a specified rate of change
Annex F - J
CE Marking
© ABB Group 
March 10, 2015 | Slide 54
According to european directives:
Low Voltage Directive 73/23 EECElectromagnetic Compatibility 89/336 EEC
Annex H
Test sequence for circuit-breakers for IT systems
This test is intended to cover the case of a second fault to earth in presence of a first 
fault on the opposite side of a circuit breaker when installed in IT systems.
In this test at each pole the applied voltage shall be the phase-to-phase voltage 
corresponding to the maximum rated operational voltage of the circuit breaker at which it 
is suitable for applications on IT systems. 
Circuit Breakers
Standards Guidelines IEC 60898
IEC Standard definitions
International Standard References 
IEC 60898
Applicable to circuit-breakers for protection of wiring installation 
in buildings and similar applications, and designed for use by 
uninstructed persons, and for not being maintained. 
Part 1: Circuit-breakers for a.c. operation
Part 2: Circuit-breakers for a.c. and d.c. operation (additional requirements)
Miniature Circuit Breakers MCB
 Rated values (In, Ue)
 Limit values (Icn, Ics)
 Rated values (In, Ue)
Choice criteria
Rated uninterrupted current (In):
 the rated uninterrupted current of an equipment is a value
of current, stated by the manufacturer, which the equipment
can carry in uninterrupted duty, at a specified reference
ambient air temperature (30 °C).
 The rated current doesn’t exceed the 125A.
IEC 60898-1 def. 5.2.2
Rated value In
Rated operational voltage (Ue):
• The rated operational voltage of a circuit-breaker is the 
value of voltage, assigned by the manufacturer, to which 
its performances (particularly the short-circuit 
performance) are referred. 
• The rated operational voltage doesn’t exceed the 440Vac 
220Vdc.
IEC 60898-1 def. 5.2.1.1
Rated value Ue
 Rated values (In, Ue)
 Limit values (Icn, Ics) Limit values (Icn, Ics)
Choice criteria
The rated short-circuit capacity is the value of the ultimate 
short-circuit breaking capacity for which the prescribed 
conditions, according to a specified test sequence, do not 
include the capability of the circuit-breaker to carry 0.85 times 
its non-tripping current for the conventional time. 
The rated short circuit capacity doesn’t exceed the 
25kA in ac and 10kA in dc
test sequence: O - 3 min - CO 
- leakage current at 1.1 Ue (< 2 mA)
- dielectric strength test at 900 V 
- verification of overload release at 2.8 x In
IEC 60898-1
def. 5.2.4
Icn = RATED SHORT CIRCUIT 
CAPACITY
Limit value Icn
The service short-circuit capacity of a circuit-breaker is the 
value of the breaking capacity for which the prescribed 
conditions according to a specified test sequence include the 
capability of the circuit-breaker to carry 0.85 times its non-
tripping current for the conventional time. 
IEC 60898-1
def. 3.5.5.2
Ics = RATED SERVICE SHORT 
CIRCUIT CAPACITY
Limit value Ics
Service Short Circuit capacity (Ics):
- test seq. : O - 3 min - O - 3 min – CO (for one or two poles cb)
O - 3 min - CO - 3 min – CO (for three or four poles cb)
- leakage current at 1.1 Ue (< 2 mA)
- dielectric strength test
- verification of no tripping at 0,85 x In
A circuit-breaker with a rated short-circuit capacity (Icn) has a corresponding service short-
circuit capacity (Ics) as from this table:
The circuit breaker with 
Icn < 6000A Ics is equal to 1xIcn 
6000A < Icn < 10000A Ics is equal to 0,75xIcn Minimum value of Ics is 6000A. 
Icn > 10000A Ics is equal to 0,5xIcn Minimum value of Ics is 7500A. 
Limit value Ics
Ics Test 
The main difference between the overload protection curve of the CBs responding to 
IEC 60947 or IEC 60898 are referred to the conventional non tripping current.
The prescibed conditions are given in this table:
Overload characteristics
Tripping Curves 
The CBs according to IEC 60947 usually have the instantaneous threshold at 5 or 10 times 
the rated current with a tolerance of + 20%.
The CBs according to IEC 60898-1 (ac applications) have different instantaneous 
threshold referred to the type B , C , D as indicated in the table below:
Magnetic characteristics
Tripping Curves 
Tripping Curves 
In some cases, the conditions IB < In < IZ
and I2 < 1.45 IZ do not guarantee complete 
protection, e.g. when overcurrents are 
present for long periods which are smaller 
than I2. They also do not necessarily lead 
to an economical solution. It is therefore 
assumed that the circuit is designed so 
that minor overloads of a long duration will 
not occur regularly.
IEC 60364-4-43
Tripping Curves 
Tripping Curves 
IEC 60947-2 IEC 60898-1
People Instructed Uninstructed
Maintenance Possible Not possible
Rated Voltage (Ue)
< 1000 Vac
< 1500 Vdc
< 440 Vac
< 220 Vdc
Ambient 
Temperature
40° C 30° C
Rated Current
No limits 
(Iu < 6300 A)
In = 125 A
Short circuit 
breaking current
No limits for Icu
Icn = 25 kA (ac)
Icn = 10 kA (dc)
Comparison IEC 60947-2 vs IEC 60898 
Generalities about the main electrical parameters
 Don’t forget
 Ue  Un
 Icu or Ics  Ik
 Icm  Ip
Ue, Icu, Ics, Icm?
Selection of protective Devices
Protection of feeders
 against overload
Ib ≤ In or I1 ≤ Iz
 against short-circuit
I2t ≤ k2S2
In
Iz S
Ib
Selection of protective Devices
The correct circuit breaker must be selected to satisfy the following 
conditions:
•It must own short circuit breaking power (lcu or eventually lcs) greater or 
equal to the short circuit current lcc
•It must use a protection release so that its overload setting current ln (l1) 
satisfies the relation lB < ln < lZ
•The let through energy (l2t) that flows through the circuit breaker must be 
lesser or equal to the maximal one allowed by the cable (K²S²)
Selection of protective Devices
Selection of protective Devices
As far as the verification required by IEC 60364, according to which the
overload protection must have an intervention current lf that assures the
operation for a value lesser than 1,45 lz (lf < 1,45 lz), we must state that it
is always verified for ABB Circuit breakers, since according to IEC 60947-2
the required value is less than 1,3 ln.
Selection of protective Devices
Selection of protective Devices
Protection of generators
 Ingen ≤ I1
 I3 or I2 ≤ 2.5-4 x Ingen
G
Selection of protective Devices
Protection of transformers
 InT ≤ I1
 Upstream CB
 I3 or I2  Iinrush
Selection of protective Devices
Steps
 determining the short-circuit 
currents
 choosing the CB
 setting of the MV overcurrent 
protection …
 setting of the LV overcurrent 
protection …
20kV
400V
Selection of protective Devices
20kV
400V
Selection of protective Devices
20kV
400V
Selection of protective Devices
As to be able to protect LV/MV transformers LV side, we must mainly 
take into account:
• Rated current of the protected transformer, LV side, from which 
the rated current of the circuit breaker and the setting depend on 
(In);
• The maximum estimated short circuit current in the installation 
point which defines the minimal breaking power of the protection 
circuit breaker (Isc).
Protection of Transformers
Sn
In
Isc
U20
Protection of Transformers
Switchboards with one transformer 
The rated current of the transformers LV side is defined by the 
following expression
where
Sn = rated power of the transformer [kVA]
U20 = rated secondary voltage (no load) of the transformer [V]
ln = rated current of the transformer, LV side [A]
In =
Sn x 103
3 x U20
The full voltage three-phase short circuit current immediately after the LV 
side of the transformer can be expressed by the following relation once we 
suppose infinite power at the primary:
where
Ucc %= short circuit voltage of the transformer [%]
ln = rated current, LV side, [A]
lsc = three-phase rated short circuit current, LV side, [A]
Isc =
In x 100
Ucc %
Protection of Transformers
The short circuit current is normally lesser than the preceding deduced 
value if the circuit breaker is installedat a certain distance by means of 
a cable or bar connection, according to the connection impedance.
Protection of Transformers
The following table shows some possible choices within the SACE Emax 
ACB range according to the characteristics of the CB to protect.
Attention
Those indications are valid at the conditions that we declare in the table; 
different conditions will lead us to repeat calculations and modify the 
choices.
Protection of Transformers
(1) For values of the percent short circuit voltage U’cc% different from the Ucc% values as per table, the rated three-phase short 
circuit current I’cn becomes:
(2) The calculated values refer to a U20 voltage of 400 V. for different U’20 values, do multiply In and Isc the following k times:
I’sc =
Ucc %
U’cc %
Isc
U’20 [V] 220 380 400 415 440 480 500 660 690
k 1.82 1.05 1 0.96 0.91 0.83 0.8 0.606 0.580
Protection of Transformers
Sn [kVA] 500 630 800 1000 1250 1600 2000 2500 3150
Ucc (1) % 4 4 5 5 5 6,25 6,25 6,25 6,25
In (2) [A] 722 909 1154 1443 1804 2309 2887 3608 4547
Isc (2) [kA] 18 22.7 23.1 28.9 36.1 37 46.2 57.7 72.7
SACE Emax E1B08 E1B12 E1B12 E2B16 E2B20 E3B25 E3B32 E4S40 E6H50
Protection of Transformers
Switchboards with more than 1 transformer in Parallel
C
irc
u
it b
re
a
k
e
r B
I1 I2 I3
1 2 3
Isc2 + Isc3
Isc1 + Isc2 + Isc3
I4 I5
C
irc
u
it b
re
a
k
e
r A
Isc1
As far as the calculation of the rated current of the transformer is
concerned, the rules beforehand indicated are completely valid.
The minimum breaking capacity of each circuit breaker LV side must be
greater than the highest of the following values: (the example refers to
machine 1 of the figure and it is valid for the three machines in parallel):
•lsc 1 (short circuit current of transformer 1) in case of fault
immediately downstream circuit breaker 1;
•lsc2 + lsc3 (short circuit currents of transformer 2 and 3) in case of
fault immediately upstream circuit breaker 1;
Protection of Transformers
Circuit breakers l4 and l5 on the load side must have a short circuit
capacity greater than lsc1 + lsc2 + lsc3; naturally every transformer
contribution in the short circuit current calculation is to be lessened by the
connection line transformer - circuit breaker (to be defined case by case).
Protection of Transformers
© ABB Group 
March 10, 2015 | Slide 92
Low voltage selectivity
with ABB circuit breakers
Selectivity definitions and Standards
 Definitions and Standards 
 Selectivity techniques
 Definitions and Standards 
 Back-up protection
Agenda
Low voltage selectivity with ABB circuit breakers
Selectivity (or discrimination)
is a type of coordination of two or 
more protective devices in series.
Selectivity is done between 
one circuit breaker on the supply side 
and one circuit breaker, or more than 
one, on the load side.
A is the supply side circuit 
breaker (or upstream)
B and C are the load side circuit 
breakers (or downstream)
Introduction
What is selectivity?
 Better selectivity
FAULT CONTINUITY OF SERVICEDAMAGE REDUCTION
 Fast fault elimination
 Reduce the stress and prevent damage
 Minimize the area and the duration of 
power loss
Introduction
Protection system philosophy
Selective coordination among devices
is fundamental for economical and technical reasons
It is studied in order to:
 rapidly identify the area involved in the problem; 
 bound the effects of a fault by excluding just the affected zone of the 
network;
 preserve the continuity of service and good power quality to the sound 
parts of the network;
 provide a quick and precise identification of the fault to the personnel in 
charge of maintenance or to management system, in order to restore 
the service as rapidly as possible;
 achieve a valid compromise between reliability, simplicity and cost 
effectiveness.
Main purposes of coordination
Selectivity purpose
The definition of selectivity
“Trip selectivity (for overcurrent) is a coordination between the 
operating characteristics of two or more overcurrent protection 
devices, so that, when an overcurrent within established limits 
occurs, the device destined to operate within those limits trips 
whereas the others do not trip”
IEC 60947-1 Standard: “Low voltage equipment
Part 1: General rules for low voltage equipment”
Standards definition
Selectivity
IEC 60947-1 
def. 2.5.23
In occurrence of a fault 
(an overload or a short circuit) 
if selectivity is provided
only the downstream circuit 
breaker opens.
Overcurrent selectivity
Example
All the system is out of service!
In occurrence of a fault 
(an overload or a short circuit)
if selectivity is not provided
both the upstream and the 
downstream circuit breakers 
could open
Overcurrent selectivity
Example
A and B connected in series:
partial selectivity and total selectivity.
Standards definition
Partial and total selectivity
IEC 60947-2 
def. 2.17.2 - 2.17.3
“Partial selectivity is an overcurrent selectivity where, in the
presence of two protection devices against overcurrent in series,
the load side protection device carries out the protection up to a
given level of overcurrent, without making the other device trip.”
B opens only according to fault current 
lower than a certain current value; 
values equal or greater than Is
will give the trip of both A and B.
Is is the ultimate 
selectivity 
value!
Is = ImA
Standards definition 
Partial selectivity
Only B trips for every current value 
lower or equal to the maximum 
short-circuit current.
“Total selectivity is an overcurrent selectivity where, in the
presence of two protection devices against overcurrent in series,
the load side protection device carries out the protection without
making the other device trip.”
B A
Is = Ik
Standards definition
Total selectivity
Upstream circuit breaker A
T4N 250 PR221DS In = 250 (Icu = 36kA)
Downstream circuit breaker B
S 294 C100 (Icu = 15kA)
Standards definition
Partial and total selectivity
 Overload zone
Thermal protection
L protection
 Short-circuit zone
Magnetic protection
S, D, I and EF protections
Time-current selectivity
Current, time, energy, zone, 
directional, zone directional selectivity
Selectivity analysis
Time-current curves
Real currents circulating through the circuit breakers
I>A
B I> I> I>
A
B
I>
I>
I>
I> I>
A
B
I>
I>
IA = IB
IA IB
tA
tB
tA
tB
IAIBIA=IB
tA
tB
IA = IB + Iloads IA = (IB + Iloads) / 2
Selectivity analysis
Real currents
© ABB Group, BU Breakers and Switches 
March 10, 2015 | Slide 106
 Definitions and Standards 
 Selectivity techniques Selectivity techniques
 Back-up protection
Agenda
Low voltage selectivity with ABB circuit breakers
© ABB Group, BU Breakers and Switches 
March 10, 2015 | Slide 107
 Current selectivity 
 Time selectivity
 Energy selectivity
 Zone (logical) selectivity
Introduction
Selectivity techniques
 The ultimate selectivity value 
is equal to the instantaneous trip threshold 
of the upstream protection device
 Other methods are needed to have a total 
selectivity 
AB
ImB ImA
 Current selectivity: closer to the power supply 
the fault point is, higher the fault current is
 In order to guarantee selectivity,
the protections must be set to different 
values of current thresholds
Ultimate 
selectivity 
value
1kA
3kA
tB
tA
tA
Current selectivity
Base concept
A
B
Here the selectivity is a total selectivity, 
because it is guaranteed up to the maximum 
value of the short-circuit current, 1kA.
Circuit breaker A will be set to a value which does not
trip for faults which occur on the load side of B.
(I3Amin >1kA)
Circuit breaker B will be set to tripfor faults which
occur on its load side (I3Bmax < 1kA)
0.1kA 1kA 10kA
10-2s
10-1s
1s
10s
102s
103s
104s
3kA
Is Is = I3Amin
Current selectivity
Example
Plus
Easy to be realized 
Economic
Instantaneous
Minus
Selectivity is often only partial
Current thresholds rise very quickly
CURRENT SELECTIVITY
Current selectivity 
Plus and minus
 Time selectivity is based on a trip delay of the upstream 
circuit breaker, so to let to the downstream protection the 
time suitable to trip
B A
 Setting strategy:
progressively increase the 
trip delays getting closer to 
the power supply source 
 On the supply side 
the S function is required
Time selectivity
Base concept
0.1kA 10kA 100kA
10-2s
10-1s
1s
10s
102s
103s
104s
1kA
The ultimate selectivity value is: 
Is = IcwA (if function I = OFF)
Is = I3minA (if function I = ON)
I
k
A will be set with the current threshold I2
adjusted so as not to create trip overlapping
and with a trip time t2 adjusted so that
B always clears the fault before A
B will be set with an instantaneous trip
against short-circuit
B
I2
t2
Is
Time selectivity
Example
0.1kA 10kA 100kA
10-2s
10-1s
1s
10s
102s
103s
104s
1kA
The network must withstand high values of 
let-through energy!
If there are many hierarchical levels, the 
progressive delays could be significant!
I
k
Which is the problem of time selectivity?
In the case of fault occurring at the busbars, 
circuit breaker A takes a delayed trip time t2
B
t2
Time selectivity 
Example
Plus
Economic solution
Easy to be realized
Minus
TIME SELECTIVITY
Time selectivity
Plus and minus
Quick rise of setting levels
High values of let-through energy
 Energy selectivity is based on the current-
limiting characteristics of some circuit breakers
A
B
0.1kA 1kA 10kA
10-2s
10-1s
1s
10s
102s
103s
104s
Current-limiting circuit breaker
has an extremely fast trip time,
short enough to prevent the
current from reaching its peak
 The ultimate current 
selectivity values
is given by the 
manufacturer 
(Coordination tables)
Energy selectivity
Base concept
1kA 10kA0.1kA
10-2s
10-1s
1s
10s
102s
103s
104s
Circuit breaker A conditions:
I3=OFF
S as for time selectivity
A
B
Is = 20kA
Energy selectivity
Example
PLUS
MINUS
ENERGY SELECTIVITY
Energy selectivity
Plus and minus
High selectivity values
Reduced tripping times
Low stress and network disturbance
Increasing of circuit breakers size
 Zone selectivity is an evolution of the time 
selectivity, obtained by means of a electrical 
interlock between devices
 The circuit breaker which detects a fault 
communicates this to the one on the supply side,
sending a locking signal
Fault 
locking 
signal
 Only the downstream circuit breaker opens, 
with no need to increase the intentional time 
delay
Zone selectivity
Base concept
A Does Not Open 
B Does Not Open 
C Opens
A
B
C
Z
o
n
e
 1
Z
o
n
e
 2
Z
o
n
e
 3
Zone selectivity
Example
 Is up to 100kA for Tmax
 Is up to Icw for Emax
 It is possible to obtain zone selectivity between Tmax and Emax
Z
o
n
e
 1
Z
o
n
e
 2
Z
o
n
e
 3
Zone selectivity needs:
 a shielded twisted pair cable
 an external source of 24V
 dedicated trip units
 PR223EF for Tmax T4, T5 and T6
 PR332/P for Tmax T7 and T8
 PR122/P and PR123/P for Emax
 PR332/P and PR333/P for X1
Zone selectivity
Specifications
PLUS
MINUS
ZONE SELECTIVITY
Zone selectivity
Plus and minus
Trip times reduced
Low thermal and dynamic stress
High number of hierarchical levels 
Can be made between same size circuit breakers
Cost and complexity of the installation
Additional wiring and components
© ABB Group, BU Breakers and Switches
March 10, 2015 | Slide 122
 Definitions and Standards 
 Selectivity techniques
 Back-up protection Back-up protection
Agenda
Low voltage selectivity with ABB circuit breakers
Back-up protection (or cascading)
is a type of coordination of two protective 
devices in series which is done in electrical 
installations where continuous operation is 
not an essential requirement.
Back-up protection
What is back-up protection?
Back-up protection 
excludes the use
of selectivity!!!
The definition of back-up is given by the 
“Back-up is a coordination of two overcurrent protective 
devices in series, where the protective device on the supply 
side, with or without the assistance of the other protective 
device, trips first in order to prevents any excessive stress on 
downstream devices”.
IEC 60947-1 Standard: “Low voltage equipment
Part 1: General rules for low voltage equipment”
Back-up protection
Standards definition
IEC 60947-1 
def. 2.5.24
 Back-up is used by those who need 
to contain the plant costs
 The use of a current-limiting circuit 
breaker on the supply side 
permits the installation of lower performance 
circuit breakers on the load side
 Both the continuity of service and the selectivity are sacrificed
Back-up protection
Base concept
T4L 250
T1N 160 T1N 160 T1N 160
Ik = 100 kA
T4L 250 T4L 250 T4L 250 Icu = 120kA
Icu = 36kA
Icu (T4L+T1N) = 100kA
Back-up protection 
Application example
Back-up protection tables
T4L 250
T1N 160 T1N 160 T1N 160
Ik = 100kA
Icu (T4L+T1N) = 100kA
Ik = 100kA
A
B C D
Back-up protection 
Application example
General power supply 
is always lost
Plus
Economic solution 
Quick tripping times
Minus
No selectivity
Low power quality
BACK-UP PROTECTION
Back-up protection
Plus and minus
Incoming = T5H 630A (70kA 
rating) Outgoing = T3N 160A 
(36kA rating)
Results: The co-ordination 
resulted in a conditional short-
circuit of 65kA for the T3 mccb!
The discrimination is up to 20kA.
Example of Selectivity 
Iz
T5H 630A 70kA
T3N 160A 36kA
65kA
~
Example of Selectivity 
Discrimination
Example of Selectivity 
Back-Up
T5H 70kA
T3N 36kA
Example of Selectivity 
Meaning of Selectivity Value
T3N 36kA
T5H 70kA
Y is 20kA
Fault level at Y is 20kA
T3N 36kA
T5H 70kA
 T5H 
T3N 20kA 
 
 
 
Example of Selectivity 
Meaning of Selectivity Value
5kA
 T5H T3N 
5kA fault ON Trip 
 
 
 
T3N 36kA
T5H 70kA
Example of Selectivity 
Meaning of Selectivity Value
 T5H T3N 
5kA fault ON Trip 
10kA fault ON Trip 
 
 
 
10kA
T3N 36kA
T5H 70kA
Example of Selectivity 
Meaning of Selectivity Value
T3N 36kA
20kA
T5H 70kA T5H T3N 
5kA fault ON Trip 
10kA fault ON Trip 
20kA fault Trip Trip 
 
 
 
Example of Selectivity 
Meaning of Selectivity Value
T3N 36kA
36kA
T5H 70kA T5H T3N 
5kA fault ON Trip 
10kA fault ON Trip 
20kA fault Trip Trip 
36kA fault Trip Trip 
 
 
 
Example of Selectivity 
Meaning of Selectivity Value
T3N
65kA
T5H 70kA T5H T3N 
5kA fault ON Trip 
10kA fault ON Trip 
20kA fault Trip Trip 
36kA fault Trip Trip 
65kA fault Trip Trip 
 
 
 
36kA
Example of Selectivity 
Meaning of Selectivity Value
Motor co-ordination
 ABB offers co-ordination tables
MV/LV Transformer Substations
Selection of Protective & Control Devices
Co-ordination between CBs and switch-disconnectors
T2S160
T1D160
400V
MV/LV Transformer Substations
Selection of Protective & Control Devices
© ABB Group 
March 10, 2015 | Slide 142
Power Factor Correction 
© ABB Group
March 10, 2015 | Slide 143
Power Factor Correction
Generalities on Power Factor Correction
In alternating current circuits, current is absorbed by a load 
which can be represented by two components:
 The Active component 
 In phase with the supply voltage 
 Directly related to the output
 The Reactive component 
 Quadrature to the voltage 
 Used to generate the flow necessary for the 
conversion of powers through the electric or 
magnetic field 
 In most installations the presence of inductive type 
loads, the current lags the active component (IR).
Generalities
© ABB Group
March 10, 2015 | Slide 144
In order to generateand transmit active power (P) a certain 
reactive power (Q) is essential for the conversion of the 
electrical energy but is not available to the load.
The power generated and transmitted make up the apparent 
power (S).
Power factor (cos ) is defined as the ratio between the 
active component (IR) and the total value of current (I).
 is the phase angle between the voltage and the current.
Generalities on Power Factor Correction
Power Factor Correction
Generalities
© ABB Group
March 10, 2015 | Slide 145
Generalities on Power Factor Correction
Power Factor Correction
Generalities
© ABB Group
March 10, 2015 | Slide 146
Typical Power Factors of some electrical equipment
Power Factor Correction
Generalities
© ABB Group
March 10, 2015 | Slide 147
Advantages of Power Factor Correction 
Power Factor Correction
Generalities
© ABB Group
March 10, 2015 | Slide 148
Advantages of Power Factor Correction 
 Better utilization of electrical machines
 Generators & transformers are sized according to the 
apparent power (S). With the same active power (P), 
the smaller the reactive power (Q) delivered, the 
apparent power will be smaller.
 Better utilization of cables 
 The reduction in current allows the use of smaller 
cables in the installation.
Power Factor Correction
Generalities
© ABB Group
March 10, 2015 | Slide 149
 Reduction in losses
 By improving the power factor, power losses is reduced 
in all parts of the installation.
 Reduction in voltage drop 
 The higher the power factor the Voltage drop will be 
lower at the same level of Active power.
Power Factor Correction
Generalities
© ABB Group
March 10, 2015 | Slide 150
 Economical savings 
 Power supply utilities apply penalties for energy used 
with poor factor. An improved power factor will reduce 
such penalties from the utilities. 
Power Factor Correction
Generalities
© ABB Group
March 10, 2015 | Slide 151
Advantages of Power Factor Correction 
 Improve capacity of transformers and cables
 By improving the power factor, you reduce the kVA load on the 
transformer and the current carried by the cables
 Thus additional transformer capacity is available if upgrade or 
expansion is required in the future
 Or new cables might not be needed if new loads are connected to 
an existing switchboard
Apparent Power (VA)
e.g 2MVA Transformer
At 100% capacity
Real Power (W)
eg. 500kW Load
Reactive Power (VAR)
e.g Motors (inductive)
100kW at 0.7pf = 102kVAR
Reactive Power (VAR)
eg. 50kVAR Capacitors
Power Factor Correction
Generalities
© ABB Group
March 10, 2015 | Slide 152
 Distributed power factor correction
 It is achieved by connecting a capacitor bank properly 
sized according to the load and is connected directly to 
the terminals of the load.
Power Factor Correction
Different Methods 
© ABB Group
March 10, 2015 | Slide 153
 Group power factor correction
 It is achieved by connecting a capacitor bank properly 
sized according to a group of loads and is connected to 
the upstream of the loads to be corrected.
Power Factor Correction
Different Methods
© ABB Group
March 10, 2015 | Slide 154
Types of Power Factor correction 
 Centralized power factor correction
 It is achieved by installing an automatic power factor 
correction bank capacitor bank directly to the main 
distribution boards.
Power Factor Correction
Different Methods
© ABB Group
March 10, 2015 | Slide 155
Types of Power Factor correction 
 Combined power factor correction
 This solution is derived from a compromise between a 
distributed & centralized power factor correction.
 Distributed power factor correction is used mainly 
for higher loads and a smaller centralized power 
factor correction is used for the small loads.
Power Factor Correction
Different Methods
© ABB Group
March 10, 2015 | Slide 156
Switching and Protection
 Electrical switching phenomena
 The switching of a capacitor bank causes an electric 
transient due to the phenomena of electric charging of 
the bank.
 The overcurrents at the moment of switching depends 
greatly on both the inductance of the upstream network 
as well as from the number of connected capacitor 
banks.
Power Factor Correction
Capacitor Switching 
© ABB Group
March 10, 2015 | Slide 157
Switching and Protection
 Choice of protective device 
Power Factor Correction
Capacitor Switching 
In
Resistance
In
Motor
In
Capacitor
AC-1 AC-3 AC-6b
Power Factor Correction
Capacitor Switching
Single step capacitor
In
30 times In
Power Factor Correction
Capacitor Switching
Multi steps capacitor bank
In
> 100 times In
Power Factor Correction
Capacitor Switching
Ith = 1.3 x 1.15 x Inc = 1.5 Inc
Thermal current
Up to 30% for harmonics and voltage fluctuations on main
Up to 15% for tolerances on capacitor power
Contactor have to support Ith
Contactor sizing: Thermal current + peak current
Power Factor Correction
Contactor Sizing
© ABB Group
March 10, 2015 | Slide 162
Example 
Power Factor Correction
Example 
kVARh is billed if it is higher than the contracted level.
Apparent power (kVA) is significantly higher than the Active power (kW)
The excess current causes losses (kWh) which is billed.
The design of the installation has to be over-dimensioned.
The installation requires 850kW at power factor of 0.75.
The transformer will have to be overloaded to 850k / 0.75 = 1.133MVA.
Current taken by the system is 
Losses in the cables 
P = I2R 
The Transformer, Circuit breaker & Cable has to be increased. 
P
I = 
3 * U * Cos 
= 1636A
I = 1636A
Cos  = 0.75
kVA
kW kVar
Cos  = 0.75
850kW Load
1MVA
400V
© ABB Group
March 10, 2015 | Slide 163
Example 
Power Factor Correction
Example 
kVARh is reduced to lower than the contracted level or eliminated.
Apparent power (kVA) is significantly higher than the Active power (kW)
The charges based on the contracted kVA demand is close to the active 
power.
The installation requires 850kW at a power factor of 0.9.
The transformer will not be overloaded to 850k / 0.90 = 945 kVA.
Current taken by the system is 
Losses in the cables 
P = I2R 
There is not need to increase the Transformer, Circuit breaker & Cable. 
P
I = 
3 * U * Cos 
= 1364A
I = 1364A
Cos  = 0.90
kVA
kW kVar
Cos  = 0.90
850kW Load
1MVA
400V
© ABB Group
March 10, 2015 | Slide 164
Technical Application Paper 
Power Factor Correction