EE3601 – Protection & Switchgear – Question Bank

UNIT I - PROT EC TIO N SCHE M ES

PART-A
1. State any four functions of protective relaying. (MAY -20 15 )

 To disconnect the abnormally operating part so as to avoid the damage

Within effective operation of the rest of the system.
 To prevent the subsequent faults arising due to the primary fault.
 To disconnect the faulty part as quickly as possible so as to minimize
the damage to the faulty part itself.
 To improve system performance, reliability and service continuity.
2. What is protective zone? (MAY -20 15 )
A protective zone is a separate zone which is established around each element of
power system remains unprotected. The area of a power system which remains unprotected
such that any fault occurring in that area would not be cleared at all is called dead spot or
blind spot of a power system.
3. List the basic requirements or essential qualities of pr otective relaying.
(DEC-2008)
(i)Reliability (ii) selectivity and discrimination (iii) speed and time (iv)sensitivity (v)
stability (vi) adequateness (vii) simplicity and economy.
4. what is backup protection? (DEC 2012)
The protection which comes in to the play when the primary protection fails is called
backup protection. When the primary protection is made inoperative for the maintenance
purpose then backup protection acts like main protection.
5. Define pickup value and plug setting multipl ier. (DEC -201 0)
Pickup value: it is the minimum value of an actuating quantity at which relay starts
operating. In most of the relays actuating quantity is current in the relay coil and pickup
value of current is indicated along with the relay.
Plug setting multiplier: the ratio of actual fault current in the relay coil to the pickup
current is called plug setting multiplier(P.S.M.).
6. Why the secondary of the C.T. should not be open? (MAY-2 01 5)
If the secondary of the C.T. is kept open then current through the secondary
becomes zero hence the ampere turns produced by secondary which generally oppose
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primary ampere turns becomes zero. As there is no counter m.m.f., unopposed primary
m.m.f. produce high flux in the core. This produces excessive core loss heating the core.
It also produces heavy e.m.f. on primary and secondary side which may damage the
insulation of the winding. This is dangerous from the operator point of view as well.
Hence the secondary of C.T. should not be open.
7. What is pickup current? (DEC -201 4)
The minimum value of the actuating current at which the relay starts operating
is called pickup current of the relay.
8. What are the different types of faults in a power system?(May 17)(MAY-2014)
Symmetrical faults: the fault which gives rise to equal fault currents in all the lines
with displacement of 120° between them. The example is line to line fault i.e. shorting
of all three lines.
Unsymmetrical faults: The fault which gives rise to unequal fault currents in all the lines
with unequal displacement between them. The example is line ground, line to line, line
to line to ground faults.
9. What are the causes of faults in a power system? (DEC -201 3)
The various causes are failure of insulation of conductor at one or more places,
conducting objects comes in contact with the live part of the system, mechanical failure,
excessive internal and external stress, over voltages due to switching surges, lightning
strokes, heavy winds and storms, falling of trees on the lines, accidents of vehicles with the
towers or poles, perching of birds on the lines, accidental short circuits due to snakes,
kites, strings etc.
10. What are the various methods of earthing in substatio ns? (MAY -20 15 )
 Solid or effective grounding
 Resistance grounding
 Reactance grounding
 Resonant grounding
11. Why earth wire is provided in overhead transmission lines? (DEC -201 5)
 To protect the line conductors from direct lightning strokes.
 To reduce the line outages
 To reduce the interference on neighbouring installations.

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  To transmit telecommunication signals.
12. What is the difference between a short circuit and an overload. (DEC-2015)
When there is a short circuit, the impedance at the fault point is almost zero and
the voltage at the fault point is zero. The short circuit current is very high. While an
overload means the load is higher than the rated load which is specified as the safe load.
Thus the current is also higher than the safe load. The overload does not causes damage
instantly but if persists for long time, can cause damage to the system.

PART - B
1. (i) Describe the Essential Qualities of Protective Relaying. (MAY-2014)
A protective relaying scheme should have certain important qualities. Such an
essential qualities of protective relaying are,
1. Reliability
2. Selectivity and Discrimination
3. Speed and Time
4. Sensitivity
5. Stability
6. Adequateness
7. Simplicity and Economy
1.1 Reliability
A protective relaying should be reliable is its basic quality. It indicates the ability
of the relay system to operate under the predetermined conditions. The reliability of a
protection system depends on the reliability of various components like circuit
breakers, relays, current transformers (C.T.s), potential transformers (P.T.s), cables,
trip circuits etc. The proper maintenance also plays an important role in improving the
reliable operation of the. This can be achieved by the factors like,
i) Simplicity ii) Robustness iii) High contact pressure iv) Dust free enclosure iv)
Good contact material vi) Good workmanship vii)Careful Maintenance
1.2 Selectivity and Discrimination
The selectivity is the ability of the protective system to identify the faulty part
correctly and disconnect that part without affecting the rest of the healthy part of

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system. The discrimination quality of the protective system is the abili ty to distinguish
between normal condition and abnormal condition and also between abnormal condition
within protective zone and elsewhere. The protective system should operate only at the time
of abnormal condition and not at the time of normal condition. Thus the protective system
should select the fault part and disconnect only the faulty part without disturbing the healthy
part of the system.
The protective system should not operate for the faults beyond its protective zone.
For example, consider the portion of a typical power system shown in the Fig.

It is clear from the Fig that if fault F2 occurs on transmission line then the circuit
breakers 2 and 3 should operate and disconnect the line from the remaining system. If the
protective system is not selective then it operates for the fault beyond its protective zones
and unnecessary the large part of the system gets isolated.
1.3 Speed and Time
a protective system must disconnect the faulty system as fast as possible. If the
faulty system is not disconnect for a long time then,
1. The devices carrying fault currents may get damaged.
2. The failure leads to the reduction in system voltage. Such low voltage may
affect the motors and generators running on the consumer sude.
3. If fault persists for long time, then subsequently other faults may get generated.
The total time required between the instant of fault and the instant of final arc
interruption in the circuit breaker is called fault clearing time. It is the sum of relay
time and circuit breaker time. The fault clearing time should be as small as possible to
have high speed operation of the protective system.

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1.4 Sensitivity
The protective system should be sufficiently sensitive so that it can operate reliably
when required. The sensitivity of the system is the ability of the relay system to operate
with low value of actuating quantity.It indicates the smallest value of the actuating quantity
at which the protection starts operating in relation with the minimum value of the fault
current in the protected zone.
The relay sensitivity is the function of the volt-amperes input to the relay coil necessary
to cause its operation. Smaller the value of volt-ampere input, more sensitive is the
relay. Thus 1 VA input relay is more sensitive than the 5VA input relay.
1.5 Stability
The stability is the quality of the protective system due to which the system remains
inoperative and stable under certain specified conditions such as transients, disturbance,
through faults etc. In most of the cases time delays, filter circuits, mechanical and electrical
bias are provided to achieve stable operation during the disturbances.
1.6 Adequateness
There are variety of faults and disturbance those may practically exists in a power
system. It is impossible to provide protection against each and every abnormal condition
which may exist in practice, due to economical reasons. But the protective system must
provide adequate protection for any element of the system. The adequateness of the system
can be assessed by considering following factors,
1. Ratings of various equipments
2. Cost of the equipments
3. Locations of the equipments
4. Probability of abnormal condition due to internal and external cause s.
5. Discontinuity of supply due to the failure of the equipment
1.7 Simplicity and Economy
In addition to all the important qualities, it is necessary that the cost of the system
should be well within limits. In practice sometimes it is not necessary to use ideal
protection scheme which is economically unjustified. In such cases compromise

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is done. As a rule, the protection cost should not be more than 5% of the total cost. But
if the equipments to be protected are very important, the economic constrains can be
relaxed.The protective system should be as simple as possible so that it can be easily
maintained. The simpler system are always more reliable.

(ii). Discuss the Natur e and Cause s of Faults in a powe r system. (DEC-2007)
Any fault in electrical apparatus is nothing but the defect in its electrical circuit which
makes current path directed from its intended path. Normally due to breaking of
conductors or failure of insulation, these faults occur. The other reasons for occurrence of
fault include mechanical failure, accidents, excessive internal and external stresses. The
impedance of the path in the fault is low and the fault currents are comparatively large.
The reduction of the insulation is not considered as a fault until its show some effects
such as excessive current flow or reduction of impedance between conductors or between
conductors and earth. When a fault occurs on a system, the voltages of the three phases
become unbalanced. As the fault currents are large, the apparatus may get damaged. The flow
of power is diverted towards the fault which affects the supply to the neighbouring zone.
A power system consists of generators, transformers, switchgear, transmission and
distribution circuits. There is always a possibility in such a large network that some fault
will occur in some part of the system. The maximum possibility of fault occurrence is on
transmission lines due to their greater lengths and exposure to atmospheric conditions. The
fault can not be totally eliminated from the system but their occurrence can be minimized
by improving system design, quality of the equipment and maintenance.
The faults can be classified according to causes their incidence. The breakdown may
occur at normal voltage due to deterioration of insulation. The breakdown may also occur
due to damage on account of unpredictable causes which include perching of birds,
accidental short circuiting by snakes, kite strings, three branches etc. The breakdown may
occur at abnormal voltages due to switching surges or surges caused by lighting.

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 The AC faults can also be classified as single line to ground fault, double line to ground
fault, three phase fault, that may occur in the system due to unbalance in current and
voltage, over voltages, reversal of power, power swings, under frequency, temperature rise
and instability.
It may be necessary to know the frequency of the fault occurrance on various parts
of the system which help in designing suitable protection circuit. Following table gives us
an idea as to how the faults are distributed in the various parts of the system.
Equipment % Total of Fault
1 Overhead line 50
2 Switchgear 15
3 Transformer 12
4 Cables 10
5 Miscellaneous 8
6 Control equipment 3
7 CTs and PTs 2
It can be seen from the above table that maximum number of faults are occurring on
overhead lines. In case of three phase system, the breakdown of insulation between one of
the phases and earth is known as line to ground fault. In line to line fault, there is insulation
breakdown between either of two phases. While the insulation breakdown between two
phases and earth forms double line to ground fault. The breakdown of insulation between
three phases is nothing but three phase fault. Following table gives occurance of these
faults.
It can be seen from the above table that most of the faults are line to ground faults
in case of overhead lines. A large number of these faults are transitory in nature. the word
transitory refers to the fault which remains for short duration of time. The fault current
varies with time. For example if a twig falls across a line and across arm and burns itself
out or just falls down then the fault is transient as it vanishes after few cycles. During first
one to three cycles, the fault current is very high but later on decreases very rapidly. This
zone in which the current is very high but decreases very rapidly is called 'sub transient'
state. After these first few cycles, the rate of current decreases is slower. This zone is called
'transient' state. This state remains for several

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cycles. After the transient state is over, steady state is reached. During the steady state , the
rms values of short circuit current remains constant. The circuit breaker operates during
transient state.

This fault current produced by line to ground fault has considerable magnitude. So
the protective system must be properly designed so as to have operation of relays under line
to ground fault.
The Line to Line to Line (L-L-L) fault is nothing but symmetrical three phase fault
which normally occurs due to carelessness of operating personnel. Usually the phase lines
are tried together with the help of a bare conductor so as to protect the lineman working on
the lines against inadvertent charging of the line. Sometimes after the work, if lineman
forgets to remove the tie up between phase lines and if the circuit breaker is closed then
three phase symmetrical fault occurs.
The most serious effect of uncleared fault is nothing but fire which destroys the
equipment, spreads up in the system and causes total failure. The most common type of
fault which may prove the dangerous is short circuit. Due to this fault, there is great
reduction in the line voltage over a major part of the power system. There is damage which
may result to the elements of the system by electric arc which accompanies short circuit.
The other apparatus in the system are damaged due to overheating and due to setting up
of abnormal mechanical forces. The stability of the power system is distributed which may
sometimes result in complete shut down of the power system. Due to reduction voltage,
currents drawn by motors are abnormally high. This may result into loss of industrial
protection. So such faults are avoided from occurring by designing suitable and reliable but
economical protective scheme.

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2. Explain the overlapping of protective zones with neat sketch. (DEC -201 5)
Protective Zones is a protective relaying scheme, the circuit breakers are placed
at the appropriate points such that any element of the entire power system can be
disconnected for repairing work, usual operation and maintenance requirements and also
under abnormal conditions like short circuits. Thus a protective covering is provided around
rich element of the system. A protective zone is the separate zone which Es established
around each system element. The significance of such a protective zone I B that any fault
occurring within cause the tripping of relays which causes opening of all the circuit breakers
within that zone. The various components which are provided with the protective zone are
generators, transformers, transmission lines, bus bars, cables, capacitors etc. No part of
the system is left unprotected The Fig. shows the various protective zones used in a system

The boundaries of protective zones are decided by the locations of the current

transformer. In practice, various protective zones are overlapped. The overlapping of

protective zones is done to ensure complete safety of each and every element of the system.
The zone which is unprotected is called dead spot. The zones are overlapped and hence
there is no chance of existence of a dead spot in a system. For the failures within the region
where two adjacent protective zones are overlapped, more circuit breakers get tripped than
minimum necessary to disconnect the faulty element If there are no overlaps, then dead
spot may exist, means the circuit breakers lying within the

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zone may not trip even though the fault occurs. This may cause damage to the healthy
system. The extent of overlapping of protective zones is relatively small. The probability
of the failures in the overlapped regions is very low; consequently the tripping of the too
many circuit breakers will be frequent. The figure shows the overlapping of protective
zones in primary relaying.
The circuit breakers are located in the connections to each power system element. This
provision makes it possible to disconnect only the faulty element from the system.
Occasionally for economy in the number of circuit breakers, a breaker between the two
adjacent sections may be omitted but in that Case both the power system are required to be
disconnected for the failure in either of the two. Each protective zone has certain protective
scheme and each scheme has number of protective systems.
Primary and Backup Protection:
The protection provided by the protective relaying equipment can be cate gorized with two
types as
1. Primary protection 2. Backup protection
The primary protection is the first line of defense and is responsible to protect all
the power system elements from all the types of faults. The backup protection comes
into play only when the primary protection fails.The backup protection is provided as the
main protection can fail due to many reasons like,
1. Failure in circuit breaker
2. Failure in protective relay
3. Failure in tripping circuit
4. Failure in d.c tripping voltage
Thus it the backup protection is absent and the main protection tails then there is a possibility
of severe damage to the system. When the primary protection is made inoperative for the
maintenance purpose, the backup protection acts like a main protection. The arrangement
of back up protective scheme should be such that the failure in main protection should
not the failure in bark up protection as well This is satisfied if back up relaying and primary
relaying do not have anything common. Hence generally backup protection is located at
different stations from the primary protection. Front the cast and economy point of sew.
The backup protection is

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employed only for the protection against short circuit and not for any other abnormal
conditions.

3. Classify the different faults in power system. Which of these are more
frequent?
Electrical fault is the deviation of voltages and currents from nominal values or
states. Under normal operating conditions, power system equipment or lines carry normal
voltages and currents which results in a safer operation of the system.But when fault
occurs, it causes excessively high currents to flow which causes the damage to
equipments and devices. Fault detection and analysis is necessary to select or design
suitable switchgear equipments, electromechanical relays, circuit breakers and other
protection devices.
Active Faults
The “Active” fault is when actual current flows from one phase conductor to another (phase-
to-phase) or alternatively from one phase conductor to earth (phase -to-earth). This type of
fault can also be further classified into two areas, namely the “solid” fault and the
“incipient” fault.
Passive Faults
Passive faults are not real faults in the true sense of the word but are rather conditions that
are stressing the system beyond its design capacity, so that ultimately active faults will
occur.
Typical examples are:

Overloading - leading to overheating of insulation (deteriorating quality,
reduced life and ultimate failure).

Overvoltage - stressing the insulation beyond its limits. Under frequency
- causing plant to behave incorrectly.

Power swings - generators going out-of-step or synchronism with each
other
Transient & Permanent Faults
Transient faults are faults which do not damage the insulation permanently and
allow the circuit to be safely re-energized after a short period of time. A typical

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example would be an insulator flashover following a lightning strike, which would be
successfully cleared on opening of the circuit breaker, which could then be automatically
reclosed. Transient faults occur mainly on outdoor equipment where air is the main
insulating medium. Permanent faults, as the name implies, are the result of permanent
damage to the insulation. In this case, the equipment has to be repaired and reclosing must
not be entertained. There are mainly two types of faults in the electrical power system. Those
are symmetrical and unsymmetrical faults.
1.Symmetrical faults
These are very severe faults and occur infrequently in the power systems. These
are also called as balanced faults and are of two types namely line to line to line to ground
(L-L-L-G) and line to line to line (L-L-L). Only 2-5 percent of system faults are
symmetrical faults. If these faults occur, system remains balanced but results in severe
damage to the electrical power system equipments. Above figure shows two types of three
phase symmetrical faults. Analysis of these fault is easy and usually carried by per phase
basis. Three phase fault analysis or information is required for selecting set-phase relays,
rupturing capacity of the circuit breakers and rating of the protective switchgear.

1. Unsymmetrical faults
These are very common and less severe than symmetrical faults. There are
mainly three types namely line to ground (L-G), line to line (L-L) and double line to
ground (LL-G) faults.

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Line to ground fault (L-G) is most common fault and 65-70 percent of faults are of this
type. It causes the conductor to make contact with earth or ground. 15 to 20 percent of
faults are double line to ground and causes the two conductors to make contact with
ground. Line to line faults occur when two conductors make contact with each other mainly
while swinging of lines due to winds and 5 - 10 percent of the faults are of this type.These
are also called unbalanced faults since their occurrence causes unbalance in the system.
Unbalance of the system means that that impedance values are different in each phase
causing unbalance current to flow in the phases. These are more difficult to analyze and are
carried by per phase basis similar to three phase balanced faults.

4. Explain the various methods of earthing the neutral point of the power system.
(DEC-2015) (May 2017)
“The process of connecting the metallic frame of electrical equipment or some electrical
part of the system (e.g. neutral point in a star-connected system) to earth (i.e. soil) is called
grounding or earthing.”
Methods of Neutral Grounding
i. Solid Grounding
ii. Resistance Grounding
iii. Reactance Grounding
iv. Resonant Groundings/Peterson coil Groundings
Solid Grounding
“When the neutral point of a 3-phase system (e.g. 3- phase generator,3-phase
transformer etc.) is directly connected to earth (i.e. soil) through a wire of negligible

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resistance and reactance, it is called solid grounding or effective grounding.” Fig. shows
the solid grounding of the neutral point. Since the neutral point is directly connected to
earth through a wire, the neutral point is held at earth potential under all conditions.

When there is an earth fault on any phase of the system, the phase to earth voltage of the
faulty phase becomes zero. However, the phase to earth voltages of the remaining two
healthy phases remain at normal phase voltage because the potential of the neutral is
fixed at earth potential. This permits to insulate the equipment for phase voltage. The
neutral is effectively held at earth potential. Therefore, there is a saving in the cost of
equipment.

When earth fault occurs on any phase, the resultant capacitive current IC is in phase
opposition to the fault current IF. The two currents completely cancel each other. Therefore,
no arcing ground or over-voltage conditions can occur.
DISADVANTAGES
The following are the disadvantages of solid grounding :
i. Since most of the faults on an overhead system are phase to earth faults, the system
has to bear a large number of severe shocks. This causes the system to become unstable.

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ii. The solid grounding results in heavy earth fault currents. Since the fault has to be
cleared by the circuit breakers, the heavy earth fault currents may cause the burning of
circuit breaker contacts.
iii. The increased earth fault current results in greater interference in the
neighbouring communication lines.
RESISTANCE GROUNDING
In order to limit the magnitude of earth fault current, it is a common practice to
connect the neutral point of a 3-phase system to earth through a resistor. This is called
resistance grounding. When the neutral point of a 3-phase system (e.g. 3-phase generator,
3-phase transformer etc.) is connected to earth (i.e. soil) through a resistor, it is called
resistance grounding.

If the earthing resistance R is very high, the system conditions become similar to
ungrounded System. If the value of earthing resistance R is very low, the earth fault current
will be large and the system becomes similar to the solid grounding system. The value
of R should neither be very low nor very high. Fig. shows the grounding of neutral point
through a resistor R.
In practice, that value of R is selected that limits the earth fault current to 2 times the
nor-mal full load current of the earthed generator or transformer. The value of R is so
chosen such that the earth fault current is limited to safe value but still sufficient to
permit the operation of earth fault protection system. The following are the advantages
of resistance earthing:

It improves the stability of the system.

The earth fault current is small due to the presence of earthing resistance.
Therefore, interference with communication circuits is reduced.

By adjusting the value of R, the arcing grounds can be minimized.

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The fault current IF lags behind the phase voltage of the faulted phase by a certain angle
depending upon the earthing resistance R. Suppose earth fault occurs in phase B as shown
in Fig. The capacitive currents IR and IY flow in the healthy phases R and Y respectively.
If the value of earthing resistance R is so adjusted that IF2= IC, the arcing ground is
completely eliminated and the operation of the system becomes that of solidly grounded
system. The lagging component IF2 is in phase opposition to the total capacitive current IC.
The fault current IF can be resolved into two components viz.
i. IF1 in phase with the faulty phase voltage.
ii. IF2 lagging behind the faulty phase voltage by 90°.
However, if R is so adjusted that IF2 < IC, the operation of the syst em becomes that of
ungrounded neutral system.
The following are the disadvantages of resistance grounding:

A large amount of energy is produced in the earthing resistance during
earth faults. Some-times it becomes difficult to dissipate this energy to atmosphere.

This system is costlier than the solidly grounded system.

Since the system neutral is displaced during earth faults, the equipment has
to be insulated for higher voltages.
REACTANCE GROUNDING
In this system, a reactance is inserted between the neutral and ground as shown in Fig. The
purpose of reactance is to limit the earth fault current. By changing the earthing reactance,
the earth fault current can to changed to obtain the conditions similar to that of solid
grounding.

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This method is not used these days because of the following disadvantages :

High transient voltages appear under fault conditions.

In this system, the fault current required to operate the protective device is
higher than that of resistance grounding for the same fault conditions.
RESONANT GROUNDING (OR) ARC SUPPRESSION COIL GROUNDING
(May 2017)
(OR) PETERSON COIL (MAY -20 15 )
We have seen that capacitive currents are responsible for producing arcing grounds. These
capacitive currents flow because capacitance exists between each line and earth. If
inductance L of appropriate value is connected in parallel with the capacitance of the
system, the fault current IF flowing through L will be in phase opposition to the capacitive
current IC of the system. If L is so adjusted that IL = IC, then resultant current in the fault
will be zero.
“When the value of L of arc suppression coil is such that the fault current IF exactly
balances the capacitive current IC, it is called resonant grounding.”

The reactor is provided with tappings to change the inductance of the coil. By adjusting
the tappings on the coil, resonant grounding can be achieved. An arc suppression coil (also
called Peterson coil) is an iron-cored coil connected between the neutral and earth as shown
in Fig.
The Peterson coil grounding has the following advantages:

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
The Peterson coil has the advantages of ungrounded neutral system

The Peterson coil is completely effective in preventing any damage by
an arcing ground.


The lines should be transposed.

Due to varying operational conditions, the capacitance of the network
changes from time to time. Therefore, inductance L of Peterson coil
requires readjustment.

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