A PROLOGUE TO FAULT CALCULATIONS (Part IV)

A PROLOGUE TO FAULT CALCULATIONS
(Last Part of a Series)

SELECTION OF EQUIPMENT

To recap, short circuits must therefore be quickly removed from the power system and this is the reason why protective devices such as circuit breakers and fuses are being placed there. To accomplish this, the protective devices must have the ability to interrupt the maximum short circuit current without injury; otherwise the injured breaker becomes the fault itself. The maximum value of short circuit current is frequently referred to as the “available short circuit current” or in some other terminology, the “available SC MVA”. The protective devices at any point of the system must be sized such that in any event of fault wherever they might be in the system, the associated upstream device closest to the fault must first interrupt the fault without disturbing others. The value of short circuit current is directly related to the size and capacity of the reference power source and is independent of the load current of the circuit being protected. The larger the capacity of the reference power source (e.g. larger transformer sizes), the greater the short circuit will be, while cable impedances along its path tend to lower them down.

Once the short-circuit levels are determined, the engineer can specify proper interrupting rating requirements, selectively coordinate the system and provide component protection. With personnel safety while minimizing equipment damage in mind, it is essential to use equipment with short circuit ratings greater than the available short circuit current that can occur at the equipment location. The 2005 National Electrical Code states that: “Art. 110.9 Interrupting Rating. Equipment intended to interrupt current at fault levels shall have an interrupting rating sufficient for the nominal circuit voltage and the current that is available at the line terminals of the equipment. Equipment intended to interrupt current at other than fault levels shall have an interrupting rating at nominal circuit voltage sufficient for the current that must be interrupted”.

When these requirements are applied to a circuit breaker, busway or to switchgear bracing, the maximum 3-phase fault current the breaker will be required to interrupt must be calculated. This current is applied as the short-circuit current available at the terminals of the protective device.

Distribution equipment, such as circuit breakers, fuses, busways, switchgears and MCCs have interrupting or withstand ratings defined by the maximum rms values of symmetrical current. In MV systems, as GE Publication GET-3550F say: “a circuit breaker can't interrupt a circuit at the instant of inception of a short circuit. Instead, due to the relay time delay and breaker contact parting time, it will interrupt the current after a period of five to eight cycles, by which time the DC component would have decayed to nearly zero and the fault will be virtually symmetrical.”

“Closing a breaker against an existing fault makes it possible to intercept the peak asymmetrical short-circuit current, which is greater than the rms value of the symmetrical current. Fault analysis must thus calculate and compare symmetrical and asymmetrical current values in order to select a protective device to adequately protect a piece of electrical distribution equipment. For this reason, equipment is also tested at a particular test X/R ratio value typical to a particular electrical apparatus, such as switchgear, switchboards, or circuit breakers, and is designed and rated to withstand and/or close and latch the peak asymmetrical current described above.” It is also for this reason that modern circuit breakers have built-in rms momentary rating equal to 1.6 times the symmetrical current rating for medium voltage circuit breakers and typically 1.25 times for low voltage circuit breakers.

Calculations seeking for maximum fault duties for purposes of establishing KAIC or MVA ratings of circuit breakers or switchgears must not be misinterpreted as applied to breaker trippings. As explained earlier, the KAIC or MVA rating of a protective device is intended to be over & above the actual fault currents flowing into it such that the same shall not disintegrate or injure itself in the course of interrupting such faults. That’s why in breaker KAIC sizing, it is important to know the maximum possible fault current at any condition because the interrupting ratings of the devices placed in the system must be capable of enduring the tremendous energy associated with faults.

‘Breaker Tripping’ in MV systems on the other hand is applied to ‘Protective Relaying’. Tripping points are settable and can be programmed, in fact, contrary to KAIC capabilities; minimum fault current values are sought in protective relaying. Arming up relays are programmed not at maximum fault currents ‘per se’, but on lesser fault values suited for its operation depending on the Time Current Performance Curve selected and the way the engineer wants them to perform. These settings also include varying degrees of overloads.

Therefore, a high interrupting capacity rated circuit breaker does not mean that it can not trip during low magnitude faults. Nor that it would only trip during high fault current condition. The relay trips the breaker according to how the relay is set. This is true to all power circuit breakers whether medium or low voltage.

For low voltage Molded Case Circuit Breaker applications, the principle of KAIC sizing remains in effect. The only difference is the absence of protective relays in MCCB’s. In MCCB’s, its built-in thermal element takes care of overload conditions as can be selected through its Continuous Current Rating while its magnetic trip element for faults.

DIMENSIONING THE CIRCUIT BREAKER

We have to establish and completely specify the KAIC or MVA ratings of circuit breakers. What then is KAIC all about?

To start with, each circuit breaker has three most important ratings – a continuous current rating, a voltage rating and an interrupting capacity (IC) rating. The IC rating is the maximum amount of current that the device will open safely to relieve a fault condition - without injuring itself. The ‘injury’ means the condition where after interrupting a fault, the breaker ceased to be operable - or worse, the breaker disintegrated because the fault current is too much for the breaker to handle. It must be remembered that breakers are placed in the system to protect the system itself. What if during a fault, it is the protective device that disintegrated? Isn’t it the design engineer’s lapses? Well and good, “the installation is operating for five years and no untoward incident like this happened, if it is my fault, why only now?”, says the engineer. Fine.., because in five years time there was no fault! It’s worthwhile to mention that breakers are not placed in the system for normal conditions; breakers are expected to perform its intended work during abnormal conditions. Abnormal conditions don’t just happen during the first energization event. They happen when the installations are going older where faults are lurking and loads are becoming more demanding.

New generation circuit breakers do not include anymore the “momentary rating” in its nameplates. The “momentary rating” (in answer to the asymmetrical current during the inception of the fault) as expressed in the old generation breakers refers to the greatest current that the device must withstand during the first half-cycle of the fault - where the transient contributions of the synchronous & induction motors in the system along with the DC component (which make the fault current asymmetrical) do occur.

It should not be understood that the momentary rating is already scrapped out. What was done was that the momentary rating is already built-in the KAIC symmetrical rating of the modern day protective device. In the interest of simplicity in the calculation and specification, the recent general procedure in breaker sizing is only to compute for the symmetrical fault current values and specify the KAIC ratings based on those values. New generation breakers do express in their nameplates the symmetrical KAIC or MVA but in reality, the momentary rating is already built-in (normally with the factor of 1.6 times the nameplate KAIC for high & medium voltage and 1.25 for low voltage circuit breakers).
.
Therefore, it is not enough to specify the Continuous Current Rating and Voltage Rating of the breaker but most importantly, the engineer must specify the KAIC ratings of these protective devices. This does not only cover the large breakers but must transcend to all breakers including the smallest branch circuits at the end points of the system.

THE VALUE OF FAULT CALCULATIONS

The output of a fault calculation exercise will bear in the engineer’s frame of mind, the following realities. During fault conditions, i.e., three-phase faults, phase-to-phase faults, single line-to-ground faults whether bolted or arcing faults, the system behaves tremendously different from normal conditions:

1) During fault conditions, the system current behavior is to skyrocket to magnitudes in the KA levels at fraction of a second time condition where sub-transient reactances (not steady state) in the system prevail,

2) That during these conditions the protective devices must work as intended to,

3) That the protective device must not disintegrate or injure itself during these abnormal conditions and it is the responsibility of the engineer in providing such technical expertise in the design of the system in the first place,

4) That too much over-sizing or under-sizing of breaker KAIC ratings because of oversight is not supposed to be acceptable. Too much over-sizing results to tremendously large costs than what is needed while KAIC undersizing might result to disintegration of the breaker,

5) That lengths & sizes cables, sizes & impedances of transformers, fault duties of utility companies influence determination of the KAIC specifications of the breakers,

6) That the design engineer must perform a “Power System Study/Analysis” on the industrial plant he is supposed to be designing, especially in breaker KAIC sizing, protective relay settings and coordination/ discrimination,


MEDIUM VOLTAGE POWER CIRCUIT BREAKERS

Power Circuit Breakers for medium voltage application in the Philippines industries are in the 2.4, 4.16, 6.9, 13.8, 34.5 & 69 KV levels. These circuit breakers are in the forms of: Air, Oil, SF6 Gas or Vacuum types. Air & Oil Circuit Breakers are now obsolete while Vacuum Circuit Breakers are in the trend today – soon to replace the SF6 types which are lately found as non-environmentally friendly.

Vacuum Circuit Breakers at the time of this writing have reached the confidence level at the 25 KV Class while a few manufacturers claimed to have produced vacuum breakers at 36 KV levels. Leading electrical manufacturing countries in the world have been developing vacuum circuit breakers for applications at higher voltage classes with the intention that new generation vacuum breakers will replace the existing SF6 breakers in the immediate future. In the meantime, breakers at 36 KV, 69 KV and above have yet to contend with SF6 types.

LOW VOLTAGE CIRCUIT BREAKERS

The larger low voltage circuit breakers with trip units are known in the USA as "LV Power Circuit Breaker” while in Europe this type is called “Air Circuit Breaker”. LV Power Circuit Breakers are usually used in Power Transformers with low voltage secondaries. Consequently, Molded Case Circuit Breakers downstream the distribution feeders are recommended. Note the KAIC ratings of these circuit breakers on the manufacturers’ standard tables.

The Industrial Plant Systems Designer (IPSD) must know the fault duties of circuit breakers at any point of the system including the low voltage side. These fault duties will be pegged as the minimum KAIC ratings of the circuit breakers which will then be matched with standard ratings of circuit breakers available in the market. The engineer then has to specify the KAIC ratings in terms of standard sizes. The process may look easy in words but because of so many frame sizes & types with corresponding KAIC ratings in a continuous current rating of the breaker, the exercise is confusing than it may suggest. Before performing any fault calculation exercise, it is imperative to look at the ratings of modern-day molded circuit breakers. Note however that other manufacturers especially European–based have different KAIC ratings. It is therefore recommended to consult product journals/catalogs so that specifications will be done accordingly.

(End of Article)

A PROLOGUE TO FAULT CALCULATIONS (Part III)

A PROLOGUE TO FAULT CALCULATIONS
(Third of a Series)

TRUE-TO-LIFE STORY

In an effort to highlight system fault protection, it is deemed worthwhile to share a true-to-life story - the lessons learned from which, are worthy to recall:

In one afternoon of July 1999, four explosions in succession were heard over the whole plant area, the last of which was the loudest. One of the four XLPE 500 MCM phase “X1” (Phase “A”) main secondary conductors of the 5 MVA, 69KV/4.16KV Substation “B” developed an single-line-to-ground fault with the steel structure supporting the cables & the AVR by-pass switch. All the four events of faults did not trip the 69 KV SF6 primary circuit breaker that could have isolated the fault. The fourth fault eventually melted & cut-off the faulted cable while cutting a portion of the 8” steel channel member of the structure. In other words, the fault cleared itself when the faulted cable was cut-off - likened to the operation of a 500 MCM ‘fuse link’. Power from the Utility remained on line until plant personnel shut off the primary breaker. Utility System did not trip off because to the eyes of the utility it was only a throughfault. The nearest Utility breaker is some 25 kilometers away.

Each substation system was provided with GEC Alsthom top-of-the-line KCGG 140 phase overcurrent with residual ground relay at the primary circuit. Likewise, the transformer was provided with GEC Alsthom MBCH differential relay coupled with a lockout relay. The lock-out relay also accommodates signals on transformer over-temperature and over-pressure parameters that will trigger trippings of both the primary and secondary breakers. There was also a back-up ground fault protective relay 51G at the transformer neutral circuit. The system therefore was provided with millions of peso worth of protection equipment & devices the books could offer.

After the smoke cleared, subsequent investigation revealed the following:

 "The GEC Alsthom KCGG 140 Relay (50-51/50N-51N) for the primary circuit was found not ‘armed’ or ‘set up’. Parameters found were factory default values and therefore not “enabled”. Likewise, the tripping contact circuit was erroneously wired such that even if the relay worked, no breaker tripping could be realized.

 "The GEC Alsthom MBCH differential relay (87T) for transformer protection was tested and found OK, but the lockout relay (86T) did not trigger tripping because the lock-out relay dc power supply was incompletely wired. When the lock-out relay was finally made activated and simulated, no trippings of the 69 KV primary breaker and the main 5 KV secondary breaker happened. Again, the wirings were found erroneous.

 "The back-up ground protection (51G) was not set up as it was not correctly wired. If correctly wired and set up, this relay could have tripped the primary breaker and could have isolated the transformer should all other relays failed.

The subject plant is located somewhere in the southern part of the country. Built in nice quality aesthetics with recognition as ISO 9002 & ISO 14000 certified, the plant was made operational in 1995. However, it took four (4) years to discover the uselessness of the installed protective devices because of a simple reason, i. e., failing to arm the protective system into putting it to work when the need comes. A check with the other 5 MVA Substation "A" revealed the same condition as the faulted Substation "B". The rest is now history…

By sheer luck, there was a good thing that did not happen in the above story: the transformer was not burned, at least for the moment. Otherwise the transformer could have joined the archives of electrical disasters.

According to IEEE publications, damage to transformers brought by faults is cumulative. It means that in this case, there were already four faults that the transformer had been subjected to! Thanks, it did not give way in the fourth fault! But luck will surely run out. The engineer must not rely on luck, because the possibility of a disaster is always there - like a Sword of Damocles hanging over his neck.

In such a ground fault, how much destructive current was involved? Among others, this is what this paper seeks to answer.

POWER SYSTEM STUDY FOR EXISTING PLANTS

For existing installations where there are no visible fault calculation records, a Power System Study is necessary to establish the integrity of the power system. The value of Power System Study confirming correct ratings of circuit breakers leading to effective protective relaying can not be appreciated until a major fault occurred, after all - the protection system only comes to work during these abnormal times. Beyond a nice looking system, any industrial plant must review and see to it that the protection scheme is properly commissioned, set to predetermined performance levels, tested, simulated and live up to predictable expectations. The new Distribution Code and the new Grid Code of the Philippines require these commissioning chores.

Moreover, it seems that budgets for power system studies & commissioning most of the time, are forgotten or even not considered as necessary when it is in fact, the most important phase of a power system project. The truth of the matter is that commissioning the power system of a plant is never easy. It requires expertise & confidence acquired over long years of experience & practice. It also requires a battery of tests and test equipments that are not cheap, and the activity itself is NOT CHEAP.

In fact it needs a Power System Study to correctly set or arm the system protection devices. What happens next is that systems are energized with protective equipment & devices not commissioned to respond to abnormalities. Again, as long as when the system is energized, that’s it. Why mess with success? With lots of luck, the system operates for a few years, until an explosion is heard. Then, the electrical engineer will be brought to the center stage – grilled! Electrical Engineering is perceived by many as an easy job! No! Electrical Engineering is never easy! And nobody can substitute electrical engineers in his playing field.

(to be continued...)

A PROLOGUE TO FAULT CALCULATION (Part II)

A PROLOGUE TO FAULT CALCULATIONS
(Second of a Series)

THE NEED FOR FAULT CALCULATIONS IN DESIGN

Fault Calculations must precede any effort to procure system protection devices. This activity is supposed to be one major part of the design process, but is oftentimes skipped or omitted. Several provisions of the National Electrical Code, the Philippine Electrical Code & ANSI/IEEE Publications relate to proper system protection. Safe and reliable operation of the industrial plant based on these provisions mandate that electrical systems must be protected adequately & effectively.

While over-current protective devices (OCPD’s) are provided for overload protection for system components such as switchgears, busses, wires & cables, motor controllers, etc; it is also necessary as discussed earlier to place protection for more damaging events such as faults. To obtain a reliable operation and to assure that system components are protected from damage during abnormal events, it is necessary to first calculate the fault duties at various points in the electrical system while still on the drawing boards and adequate protective devices must subsequently be in place to anticipate these faults.

For all possible conditions, it is the responsibility of the system designer to design electric power systems with adequate control of short circuits as one major consideration. It is also the plant engineer’s responsibility to see to it that the protective relaying devices are armed to pre-determined settings either by himself or by consultants. As can be recalled, uncontrolled short circuits can cause service outages with accompanying production downtime and associated inconvenience, interruption of essential facilities, extensive equipment damage, personnel injury or fatality and a possible full-blown fire.

It is therefore important to de-mystify the stigma of faults and its counter-measures. Again, the system designer is responsible for the selection of the right equipment; and would generally have the task of calculating system short circuits. But alas… - procedures & techniques for these calculations are not generally available in one dissertation but are scattered among many publications & technical papers.

Fault Calculations also result to at least three very significant outputs which will become the bases of the following:

1) First: Proper selection of protective equipment ratings as circuit breakers or fuses that suit to system requirements;

2) Second: Realistic arming up of protective relays to trigger operation of circuit breakers once faults do occur;

3) Thirdly, Proper coordination of operation of these protective devices to effect selective interruptions of the only required breakers to trip faulted circuits without the hassle of rendering the other portions of the system powerless.

TYPES OF POWER SYSTEM SHORT CIRCUITS

Contributing sources of Short Circuit Currents into an industrial plant in focus include: Utility Generation thru the Industry Substation, Local Generation, In-Plant Synchronous Motors and Induction Motors. Capacitor discharge currents are normally neglected due to their short time duration.

In several forms or ways, short circuits can occur on a three-phase system. Again, for any type of faults, the protective devices must interrupt the faulted circuits while at the same time be able to withstand these faults.

a) THREE-PHASE BOLTED SHORT CIRCUITS

Three-phase bolted short-circuit describes the condition where the three conductors are physically held as if they were bolted together. In this condition, the impedance between these conductors or terminals is zero. This establishes a “worst case” condition, which results in maximum thermal and mechanical stress in the system. While ‘bolted short circuit condition’ seldom occurs (only in cases of errors in connecting buses or cables), it generally results in maximum short-circuit values and for this reason that the “basic short circuit calculation” in power systems is employed.

Subsequently, it is from these maximum values that the selection of fault duty ratings of circuit breakers, other protective devices and switchgear withstand ratings shall be based, busways included.

b) LINE-TO-LINE BOLTED SHORT CIRCUIT

From the three-phase fault calculation, other types of fault conditions can be obtained. The levels of line-to-line bolted short circuit currents in most three-phase power systems are approximately 87% of three-phase bolted short circuit currents, but this calculation is seldom required because it is not the maximum value, especially for establishing circuit breaker ratings. But then these values are needed as basis in relay settings or other purposes.

c) LINE-TO-GROUND BOLTED SHORT CIRCUITS

In solidly grounded systems, single line-to-ground bolted short circuit current in general, can be almost equal to the three-phase bolted short circuit current. Most of the time, SLG fault currents are lower than the 3Φ short circuit current due to the impedance of the ground return circuit and due to the non-zero-sequence current contribution from the motors which are usually ungrounded. Line-to-ground fault calculations are seldom necessary in solidly grounded low voltage industrial or commercial systems, but are needed in medium voltage systems for relay setting purposes.

In resistance-grounded medium voltage systems (common in 4.16 to 23kV) the neutral grounding resistor is generally selected to limit ground fault current to a value ranging between a few hundreds & a few thousand amps. Magnitudes of Line-to-Ground fault currents on these systems are controlled primarily by the resistor itself and a line-to-ground fault calculation using symmetrical components is generally required to size up the resistor.

d) ARCING SHORT CIRCUITS

In the real world, faults in many power systems tend to be arcing in nature. Statistics say that Single-Line-to-Ground Arcing Faults are the most frequent faults experienced in any power system.

Arcing faults are much lower level short circuit currents than the bolted ones at the same fault point. These lower levels of currents are due to the impedance of the arc ‘inserted’ into the circuit. Normally, arcing fault currents fall in the range from 40% to 50% of the bolted values.

(to be continued...)

A PROLOGUE TO FAULT CALCULATIONS

A PROLOGUE TO FAULT CALCULATIONS
(First of a Series)

GENERAL DISCUSSION

Experts say that, “continuity of operation in an industrial plant is as good as its electric power system. Its value speaks for itself the instant power is interrupted during production days or much more, during the visit of company officers. Even with the best design and top-of-the line materials available, the likelihood of a fault in power systems increases with age". According to experience, faults usually surface out after about five years operation of a brand-new plant. The industrial plant therefore must be designed to anticipate these faults, thus the need for fault calculations - the end objective of which is to design a plant sturdy enough to survive the disastrous effects of faults.

Electrical practitioners should be aware of the responsibility that an electrical system should not only be designed and constructed as a ‘safe system’ under normal service conditions but also during abnormal conditions. Its protection schemes designed & selectively coordinated as well, to insure continuity of service even during abnormal times. Abnormal conditions are always associated with electrical faults and a well-planned fault control system from the medium voltage level down to the last low voltage circuit is one where only the faulted circuit is isolated without disturbing any ‘unfaulted’ parts of the system. Although electrical systems are supposedly designed by responsible system designers to be free from short circuits as possible, but even with these precautions, the plant can not escape from faults because short circuits will surely do occur – perhaps not today but in the future.

It is worthwhile to mention that Fault Calculations may be easy to utility system engineers who may have been assigned in the ‘System Protection Department’ for years; but may be mystifying to operation or maintenance engineers in the industrial plant. Fault Calculations as a subject can not be understood nor mastered without constant practice & exposure to it over a long period of time. The task may seem overwhelming at least … at first, but constantly practicing a methodical step-by-step procedure can keep a novice engineer from getting tripped up…

IEEE says that power systems should be designed so that “protective relays operate to sense and to cause the quick isolation of faults in the end view of limiting the extent & duration of service interruptions. Because of their function, protective relays are the so-called "watchdogs or silent sentinels in a power system.” In medium voltage power systems, relays are considered as the “brains” while the breakers, the “muscles”. While smaller circuit breakers in the LV Systems are ‘electrical devices’, it should be noted that MV Power Circuit Breakers are not real electrical equipment. Being the muscles, they are in fact, ‘more of a mechanical device’ as the only electrical in them are the charging motor, tripping & closing circuits. Protective Relays guard the power system from the ever-present threat of damage caused by over-currents that can result to equipment loss, system failure and a much greater production loss.

But before attempting to set-up any system protection scheme, faults as complex phenomena must first be understood & their magnitudes calculated.

OVER-CURRENTS

An “over-current” as a widely-held phrase is either an overload or a short-circuit current. ‘Overload’ is defined as an excessive current in reference to normal operating current, but one which is confined to the normal conductive paths provided by the conductors & other components of the distribution system. On the other hand, as the name implies, a ‘short-circuit’ current is one which flows outside the normal conducting paths.

OVERLOADS

How ‘over’ are overloads? Overloads are usually between one and six times the normal current level. Generally, these are undisruptive fleeting surges that occur in events of motor or transformer starts-up. Such ‘transient overloads’ are normal events. Since they are of very brief duration; any temperature rise is trivial & has no harmful effect on the circuit components. In such cases, protective devices should not react to them.

Another form of overload is the ‘sustained overload’ that may have been caused by defective motors (such as worn motor bearings), overloaded equipment, or too many loads on a circuit. Such sustained overloads are destructive and must be disconnected by protective devices before they damage the distribution system or system loads. However, since they are relatively lower in extent compared to short-circuit currents, removal of the overload current within a few seconds is an acceptable practice. A sustained overload current if not cut-off results in overheating of conductors & circuit components and will cause deterioration of insulation, which may subsequently result in more severe damage such as short-circuits if not interrupted.

SHORT-CIRCUITS

While overloads do occur at somewhat modest levels, the ‘short-circuit’ or ‘fault current’ can be hundred times larger than the normal operating current. A high level fault may be 50,000 amperes and larger. If not interrupted within a matter of a few thousandths of a second, damage & destruction can become rampant. There can be severe insulation damage, melting of conductors, vaporization of metal, ionization of gases, arcing & fires. Moreover, high level short-circuit currents can develop huge magnetic-field stresses between switchgear buses that can reach destructive forces that even heavy bracing may not be able to keep them from being distorted beyond repair.

INTERRUPTING RATING

A protective device must be rated to withstand the destructive energy of short-circuits. If a fault current exceeds a level beyond the capability of the protective device, the device may rupture and disintegrate. The rating which defines the capacity of a protective device to maintain its integrity when reacting to fault currents is termed as “interrupting rating”. The interrupting rating of most branch-circuit molded case circuit breakers typically used in residential service entrance panels is 10,000 amperes. Larger circuit breakers used in industrial plants may have interrupting ratings of 18 kA or 60 kA or higher. In contrast, modern current-limiting fuses have an interrupting rating of 200 kA or 300kA and are commonly used as current-limiting devices.

SELECTIVE COORDINATION - PREVENTION OF BLACKOUTS

Note that proper coordination or discrimination of the operation of protective devices will prevent total system power outages or blackouts. When only the protective device nearest a faulted circuit opens and the larger upstream OCPD’s remain closed, the protective devices are “selectively coordinated” or in other words, “discriminated”. The word “selective” is to denote total coordination – meaning, isolation of a faulted circuit by the opening of only the affected circuit protective device. In other terminologies, this is also known as “fault control”.

(to be continued...)