ABB Is-Limiter – (Original)
ABB Is-Limiter – (Original)
The world’s fastest switching device
– Reduces substation cost
– Solves short-circuit problems in new substations and
– Optimum solution for interconnection of switchboards
– In most cases the only technical solution
– Reliability and function proofed in thousands of
installations since 1960
– In service worldwide
– The peak short-circuit current will never be reached
– The short-circuit current is limited at the very first
S-limiter | 3
The IS-limiter, a switching device with extremely short
operating time, solves the problem.
A short-circuit downstream from an outgoing feeder breaker
is assumed. The oscillogram shown below indicates the
course of the short-circuit currents in the first half wave.
A short-circuit current of 31.5 kA can flow to the fault location through each transformer. This would result in a total
short-circuit current of 63 kA, which is twice as much as the
The course of the current through the IS-limiter in such an
event is shown below as current i2.
It can be seen that the IS-limiter is operating so rapidly, that
there is no contribution via the transformer T2, to the total
peak short-circuit current (i1 + i2). Therefore, a switchgear
with a rating of 31.5 kA is suitable for this application.
Single line diagram
of a bus tie for a
I‘‘k = 31.5 kA and
with an IS-limiter
Short-circuit currents too high?
Current i = i1 + i2
at the fault location
Current i = i1 + i2 at the fault location
4 | I
I k I k I k I k
1 2 3 4
Questions and answers regarding the IS-limiter
1. What is the peak short-circuit current?
The peak short-circuit current ipeak is the maximum instantaneous value of the current after the short-circuit occurs.
2. Why the peak short-circuit current have to be limited?
Because otherwise insufficiently dimensioned switchboards,
switches, current transformers, cables, etc. would be
destroyed due to the magnetic forces caused by the current.
3. How can switchboards which are only dimensioned for
2 x IK be operated with four transformers infeeds and a
total short-circuit current of 4 x IK without any risk of
overload and without losses?
By installing an IS-limiter between the busbar sections 1 – 2
and 3 – 4. (This is only one of the many possibilities for
the use of an IS-limiter (see page 15 for further examples).
4. How does the IS-limiter work?
The IS-limiter consists of two parallel conductors.
The main conductor carries the high rated normal
current (up to 5.000 A).
After tripping, the parallel fuse limits the short-circuit
current during the first current rise (in less than 1 ms).
Due to the peak short-circuit current the electrical
system is subjected to the maximum mechanical
stress created by magnetic forces.
Due to the AC short-circuit current duration
the system is subjected to thermal stress.
1 2 3 4
1 x I 2 x I 3 x I 4 x I k k k k
Transformers: 1 1 + 2 1 + 2 + 3 1 + 2 + 3 + 4 t
S-limiter | 5
6. What overvoltages occurs as a result of the sudden
interruption of the current?
The main conductor is suddenly opened, but not the
entire current path. With the opening of the main current
path the current commutates to the parallel fuse, which
interrupts the current. The overvoltage occuring at the
interruption by the fuse is considerably below the
permissible levels stated in the IEC / VDE standards,
e.g. IEC 60282-1 / VDE 0670 part 4.
8. Does the IS-limiter trip on every short-circuit?
No! The IS-limiter only trips when the system is at risk.
Small short-circuit currents are interrupted by the
1. Current transformer (detects the short-circuit current)
2. Measuring and tripping device (measures the current
and provides the triggering energy)
3. Pulse transformer (converts the tripping pulse to busbar potential)
4. Insert holder with insert (carries the rated normal current and limits
the short-circuit current)
Short-ciruit current limited
by the fuse element
7. Can IS-limiter inserts be refurbished after interruption of
Yes! They can be refurbished at the manufacturer’s works.
The costs are low. The opened main conductor, the parallel
fuse and the charge will be replaced. All other parts can be
5. How is the main conductor opened in less than a
thousandth of a second?
Switching devices with mechanical mechanisms and this
high rating are not able to open the main current path in
such a short time. For this reason we use an electronically
triggered charge as switching mechanism.
6 | I
9. How does the IS-limiter distinguish between minor and
The measuring and tripping device of the IS-limiter detects
the instantaneous current level and the rate of current
rise. The IS-limiter only trips when both set response
values are reached.
11. How often does an IS-limiter trip?
Experience shows that an IS-limiter trips once every four
years on average (based on a statistic with approximately
3000 IS-limiters in service).
10. What experience is available with the operation of
Since the invention of the IS-limiter by ABB Calor Emag
in 1955, several thousand devices have been successfully used in DC, AC and particularly in three phase
We have over 50 years of good operating experience
worldwide. More and more customers are selecting the
S-limiter when they need high short-circuit currents to
be safely limited and electrical systems and distribution
networks to be economically built or expanded.
12. Which short-circuit current the IS-limiter can interrupt?
The rate of current rise (—)
– is high with high short-circuit currents
– is low with low short-circuit currents
Tests at KEMA to date have
12 kV up to 210 kARMS
17.5 kV up to 210 kARMS
24 kV up to 140 kARMS
36/40.5 kV up to 140 kARMS
The function and applications of the IS-limiter are explained
in the following pages with various examples. Discuss your
short-circuit problems with us. We always find a commercially
interesting and technically elegant solution with the IS-limiter.
S-limiter | 7
The rising demand for energy world-wide requires more
powerful or additional transformers and generators, and an
increasing interconnection of the individual supply networks.
This can lead to the permissible short-circuit currents for the
equipment being exceeded and thus parts of the equipment
being dynamically or thermally destroyed.
The replacement of existing switchgear and cable connections
by new equipment with higher short-circuit strength is often
technically impossible or uneconomical for the user. The use of
S-limiters reduces the short-circuit current in new systems and
expansions to existing systems, thus saving cost.
Circuit-breakers cannot provide any protection against unduly
high peak short-circuit currents, as they are too slow. Only the
S-limiter is capable of detecting and limiting a short-circuit
The function of the IS-limiter
current at the first rise, i.e. in less than 1 ms. The maximum
instantaneous current occurring remains well below the level
of the peak short-circuit current.
In comparison with complex conventional solutions, the
S-limiter has both technical and economic advantages
when used in transformer or generator feeders, in switchgear sectionalizing and connected in parallel with reactors.
The IS-limiter is in every regard the ideal switching device
to solve the short-circuit problems for switchgear in power
stations, in heavy industry and at utilities.
S-limiter connected in parallel with a reactor – fixed mounted –
8 | I
S-limiters for three-phase systems basically consist of:
– three IS-limiter insert holders,
– three IS-limiter inserts,
– three tripping current transformers,
– a measuring and tripping device.
S-limiter insert holders
The IS-limiter insert holder comprises:
– base plate 1,
– insulator 2,
– insulator with pulse transformer 6 and
telescopic contact 5,
– pole heads with clamping device 3 for
the reception of the IS-limiter insert.
The operation of the clamping device will be done with two
levers. Only for insert holders Ir
≥ 2500 A and 12 kV/17.5 kV
the inserts are fixed with two bolts.
The location of the pulse transformer depends on the rated
– for ≤ 17.5 kV, in the lower insulator 6 only
– for 24 / 36 kV, in the upper and lower insulators.
The pulse transformer transmits the tripping pulse from the
tripping device (Figure 3) to the charge 10 in the IS-limiter
insert, and at the same time ensures electrical isolation of
the tripping device from the charge which is at system
The IS-limiter insert is the switching element. In a sturdy
insulating tube 8, the insert contains the main conductor,
designed as a bursting bridge 9, which encloses a charge 10.
On tripping, this charge is triggered and the main conductor
opens at the rupture point.
Figure 1: I
S-limiter insert holder with insert for 12 kV, 2000 A
1 Base plate
3 Pole head with clamping device
5 Telescopic contact
6 Insulator with pulse transformer
Figure 2: I
7 Fuse indicator
8 Insulating tube
9 Bursting bridge
11 Main conductor indicator
12 Fuse element
S-limiter | 9
Width (W) : 600 mm
Heigth (H) : 1450 mm
Depth (D) : 300 mm
Weight : 100 kg
Figure 3: Measuring and tripping device
The current commutates to the parallel high rupture capacity
(HRC) fuse 4. The fuse element 12 in the HRC fuse melts,
thus limiting the further current rise. The current is interrupted
at the next voltage zero passage.
Tripping current transformer
The tripping current transformers are used to measure the
current flowing through the IS-limiter. They are located directly
in series with the IS-limiter.
The IS-limiter current transformer is externally identical to a
conventional current transformer and is designed as a post or
bushing type current transformer. It´s remarkable features are:
– an extremely high overcurrent factor,
– an iron core with air gap to keep the remanent induction
– a low impedance shield between the primary and secondary winding.
Measuring and tripping device
The measuring and tripping device is accommodated in a
sheet steel control cabinet (Figure 3) or in the low voltage
compartment of the IS-limiter panel.
The functional groups within the control cabinet or low voltage
compartment are combined such as to form replaceable units
and are partly mounted on hinged frames.
The measuring and tripping device includes:
– a power unit to provide the necessary auxiliary DC voltages, a main switch which allows the tripping system to
be switched on and off at any time, and additionally a
– one tripping unit for each phase, which monitors the current flowing in the relevant phase and on tripping provides
the energy for triggering of the charge in the corresponding
– an indication unit with five flag indicator relays:
– one relay per phase for trip signalling,
– one relay for monitoring of readiness for operation,
– one relay for monitoring of the supply voltages,
– an anti-interference unit to protect the measuring and tripping assemblies from interference pulses from the outside,
which could possibly cause malfunction. The connecting
wires from the measuring and tripping device to the current
transformers, to the IS-limiter insert holders and to the AC
voltage supplies are routed via the anti-interference unit.
10 | I
The IS-limiter consists in principle of an extremely fast switch,
able to carry a high rated current but having a low switching
capacity, and a high rupturing capacity (HRC) fuse arranged
in parallel. In order to achieve the desired short opening time,
a small charge is used as the energy store for opening of the
switch (main conductor). When the main conductor is opened,
the current continues to flow through the parallel fuse, where
it is limited within 0.5 ms and then finally interrupted at the
next voltage zero passage.
The current flowing through the IS-limiter is monitored by an
electronic measuring and tripping device. At the very first
rise of a short-circuit current, this device decides whether
tripping of the IS-limiter is necessary. In order to reach this
decision, the instantaneous current and rate of current rise at
the IS-limiter are constantly measured and evaluated. When
the setpoints are simultaneously reached or exceeded, the
S-limiter trips. The three phases are operated independently
of one another.
The loss-free conduction of a high operating current on the
one hand and the limitation of the short-circuit current at
the first current rise on the other hand are made possible by
distributing these two functions of the IS-limiter between two
conductors. In comparison with reactors, the IS-limiter avoids
voltage drops and does not contribute to the peak shortcircuit current.
A DC voltage of 150 V generated in the power unit is used as
the charging voltage for the tripping capacitors and at the
same time as the supply voltage for the electronics. As far as
necessary, the supply voltage is divided and stabilized within
the individual assemblies. A watchdog module in the power
unit constantly monitors the most important functions of the
three tripping units.
Figure 4: Schematic diagram of the IS-limiter equipment
Function of the IS-limiter
G 1 A 2 L1 A 2 L2 A 2 L3
H 106 H 116 H 1L1 H 1L2 H 1L3
Anti-inteference unit A 3
L 1 L 2 L 3
Q 6 L 1 T1 L1 Q 6 L 2 T1 L2 Q 6 L 3 T1 L3
G1 Power unit
A2 Tripping units
A3 Anti-interference unit
A4 Indication unit
F116 Miniature circuit-breaker for supply voltage
S-limiter (insert holder and insert)
S-limiter tripping transformer
S-limiter | 11
The current supplied by the tripping transformers for the
corresponding phases is monitored in the tripping units.
The three tripping units work independently of each other.
Both the rate of current rise and the instantaneous current
value are used as criteria for tripping.
Both variables are converted into proportional voltages and
supplied via logical gates to an electronic measuring element.
The latter provides an output signal when the rate of current
rise and the instantaneous current value have both simultaneously reached the response value of the measuring element.
The output signal from the measuring element then activates a thyristor, which discharges a capacitor via the pulse
transformer in the IS-limiter insert holder to the charge. At
the same time, this discharge excites the corresponding flag
indicator relay “IS-limiter tripped” in the indication unit.
Figure 5: Schematic diagram of a measuring and tripping unit
S-limiter tripping transformer
T2 Intermediate transformer of the tripping unit
T3 Pulse transformer
L1 Measuring inductance
R1… R6 Setting resistors
C1 Tripping capacitor
RS Discharge resistor
12 | I
As with every other protective device, IS-limiters should
also be checked at regular intervals. There are special testing
sets available for those tests which can be performed by the
operator or by ABB AG. These test sets consist of a test
equipment and a test insert or a test plug and a test insert.
The test plug is used to check the voltages and the functions
of the tripping system. The user friendly test equipment
facilitates further tests such as determination of the response
voltages of the measuring elements, and testing and setting
of the modules of the measuring circuits.
During testing, the IS-limiter insert is replaced by the test
insert. The test insert contains a neon lamp as an indicator,
which lights up when a tripping pulse is received.
Figure 6: 1 IS-limiter insert holder with test insert | 2 Test plug | 3 Test equipment
Testing the IS-limiter
1 2 3
Width (W) : 400 mm
Heigth (H) : 215 mm
Depth (D) : 320 mm
Weight : 11 kg
S-limiter | 13
S-limiter as loose equipment supply
In this case the insert holders, the inserts and the
tripping current transformers are installed in an already
Width (W) : 600 mm
Heigth (H) : 1450 mm
Depth (D) : 300 mm
Weight : 100 kg
Technical data 1 2 3 4 5 6 7
Rated voltage V 750 12000 12000 17500 17500 24000 36000 / 40500
Rated current A 1250
withstand voltage kV 3 28 28 38 38 50 75
Rated lightning impulse
withstand voltage kV – 75 75 95 95 125 200
current kARMS up to 140 up to 210 up to 210 up to 210 up to 210 up to 140 up to 140
S-limiter insert holder kg 10.5 23 / 27.5 65 23 / 27.5 65 27 / 31.5 / 33 60
S-limiter insert kg 17.0 12 / 12.5 15.5 14 / 14.5 17.5 19 / 19.5 / 24 42
S-limiter Width W mm
insert holder Height H mm
with insert Depth D mm
637 / 651
503 / 510
637 / 651
503 / 510
740 / 754 / 837
553 / 560 / 560
1) With cooling fan
Frequency: 50/60 Hz. For higher rated currents, insert holders with inserts are connected in parallel.
1 2 3 4
5 6 7
The equipment supply generally comprises:
– three insert holders,
– three inserts,
– three tripping current transformers,
– one measuring and tripping device (Figure 3).
14 | I
28 75 2500 1000 1300
38 95 2200 1000 1300
50 125 2325 1000 1500 approx. 1300
1) With cooling fan
For higher currents, insert holders with inserts are connected in parallel.
B. Truck mounted IS-limiter in a switchgear panel
The IS-limiters can also be installed in a metal-clad
switchgear panel. The withdrawable truck with the three
S-limiter insert holders and inserts has the function of a
disconnector. The three tripping current transformers are
fixed mounted in the panel and the measuring and tripping
device is mounted in the low voltage compartment.
C. Fixed mounted IS-limiter in a switchgear panel
The IS-limiters for low voltage, 12 kV, 17.5 kV and 24 kV
are also available as fixed mounted equipment in a metal
enclosed switchgear panel. The three IS-limiter insert holders
with the IS-limiter inserts and the three tripping current transformers are fixed mounted in the panel.
The measuring and tripping device is mounted in the low
voltage compartment. The IS-limiter (fixed mounted) for
36 kV/40.5 kV is available in a metal-enclosed switchgear
panel. Same as for loose equipment supply, the measuring
and tripping device is installed in a separate sheet steel
cabinet (Figure 3).
For all fixed mounted IS-limiters the electrical data are the
same as for loose equipment supply. Dimensions and
weights on request.
Truck mounted IS-limiter in a switchgear panel
S-limiter | 15
S-limiters in system interconnections
S-limiters are frequently used in interconnections between
systems or in bus sections which would not be adequately
short-circuit proof when connected by a circuit-breaker.
Each partial system should have at least one incoming feeder,
so that power supply to each partial system can be maintained
on tripping of the IS-limiter (Figure 9). There is a large number
of advantages for the operation under normal conditions of
bus sections connected by IS-limiters:
– Reduction of the series network impedance. The voltage
drops caused by load surges (e.g. of starting of motors)
can be significantly reduced.
– Improvement of the current distribution at the feeder
– The load dependent losses of the feeder transformers
– Increased reliability of the power supply. On failure of one
feeder transformer, the load is taken over by the other
feeder transformers without current interruption. The cost
for an otherwise required new switchboard with higher
short-circuit capacity will be saved.
If a short-circuit occurs within a system or in an outgoing
feeder, the IS-limiter trips at the first rise of the short-circuit
current and divides the busbar system into two sections before
the instantaneous current reaches an inadmissible high level.
After tripping of the IS-limiter, the short-circuit is only fed by the
transformer in the part of the system affected by the shortcircuit. The short-circuit current is now selectively interrupted
by the circuit-breaker.
A remarkable advantage of the use of an IS-limiter is that the
voltage in the part of the system not affected by the shortcircuit only drops for a fraction of a millisecond so that even
sensitive loads (e.g. computers) remain protected from drops
in the system voltage.
For this reason the IS-limiter can also excellently be used as
a reliable switchgear suitable between an “unprotected” and
a “protected” switchboard or section of a switchboard.
Figure 9: IS-limiter in a bus section
16 | I
S-limiters used as a link between public networks
and consumer owned power supply systems.
The decentralization of power supply leads to systems with
their own power generating facilities being interconnected
with public supply networks. The additional short-circuit
current from generators leads to the permissible short-circuit
current in the utility network being exceeded. The most
appropriate technical solution – and mostly the only one –
is the installation of an IS-limiter in the interconnection with
the public utility network (Figure 10).
If necessary, the IS-limiter can be provided with a directional
tripping criterion. This requires three additional current transformers in the neutral connections of the generators. The
S-limiter then only trips on short-circuits in the public supply
network if a generator is in operation.
S-limiter in parallel with a reactor
The IS-limiter can also be connected in parallel with a reactor
(Figure 11). If a short-circuit occurs behind the reactor, the
S-limiter trips and the current commutates at the first current
rise to the parallel reactor, which then limits the short-circuit
current to the permissible level.
For normal operation, the IS-limiter bridges the reactor coil.
– Current dependent copper losses and the associated
operating costs of the reactor.
– Current dependent voltage drop at the reactor, which
frequently causes major difficulties on start-up of big
– Control problems with the generator.
Figure 10: I
S-limiter in connecting point with a public supply network Figure 11: I
S-limiter in parallel with a reactor in a generator feeder
3 ~ 50 Hz I” = 25 kA
I” = 15 kA
I” = 16 kA k perm
I” = 3 kA kG
S-limiter | 17
Use of more than one IS-limiter with selectivity
In order to achieve selectivity in a switchboard or switchboards with more than one IS-limiter installed, additional
tripping criteria as current summation or differences or
comparison of current directions are required.
If in case of two IS-limiters installed in a switchboard selective tripping is required, a measurement of the total current
becomes necessary. The IS-limiter trips as follows:
– Short-circuit in section A:
Only IS-limiter no. 1 trips.
– Short-circuit in section B:
S-limiter no. 1 and no. 2 trip.
– Short-circuit in section C:
Only IS-limiter no. 2 trips.
For measurement of the total current, transformer feeders
must be additionally equipped with one CT set each.
The total current Isum1 is equal to the current (IT1) of transformer
T1 plus the current (IIs-1) flowing through the IS-limiter 1.
The total current Isum2 is equal to the current of transformer T2
plus the currents flowing through IS-limiter 1 and 2.
The total current Isum3 is equal to the current of transformer T3
plus the current flowing through IS-limiter 2.
The tripping criteria of the IS-limiters correspond to a logic
”and“ function. The IS-limiter 1 trips in case of short-circuits
in section A, if the current of IS-limiter 1 and the total current
sum1 reach or exceed their response values simulaneously.
The same is applicable for section C. In case of a short-circuit
in section B IS-limiters 1 and 2 trip.
The summation of the currents corresponds to the principle
of the adding up of currents in a busbar protection system.
The only difference is the non-requirement of current transformers in the outgoing feeders, i.e. the requirement of
material is negligable. With this principle up to 5 transformers
have so far been connected in parallel, using 4 IS-limiters only.
The principle ensures that always only the IS-limiter or these
S-limiters trip, which are closest to the point of short-circuit.
Figure 12: Schematic diagram-IS-limiter with summation of currents
18 | I
Questionnaire on the use of IS-limiters in medium and low
voltage three-phase systems.
We require the following data for a quotation and design of
1. Operating Voltage:
2. Rated Current:
4. In order to calculate the tripping
and setting values we need:
– Single line diagram of the installation
with the following data:
– Initial symmerical short-circuit current Ik
generators, transformers, the grid, motor
contribution and the permissible short-circuit
current of the switchboard.
– Rated power of motors over 2 MW connected to
the same voltage level the IS-limiter is installed:
– Rated capacity of capacitor banks and the
inductance in series connected to the same
voltage level the IS-limiter is installed.
– Rated power of the biggest transformer, energised from the same voltage level where the
S-limiter is located.
– Single Line Diagram.