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...)