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