HIGH VOLTAGE AC POWER CIRCUIT BREAKER SHORT CIRCUIT RATING

The short-circuit rating of a circuit breaker is established by the symmetrical component of short-circuit current in rms amperes, designated rated short-circuit current, to which all required short-circuit capabilities are related. All values apply to both grounded and ungrounded short circuits on predominantly inductive or resistive 3-phase circuits with a normal frequency phase-to-phase recovery voltage equal to the operating voltage.

Figure 1— Relation of Symmetrical Interrupting Capability, Closing Capability, Latching Capability,

and Carrying Capability to Rated Short-Circuit Current

Rated Short-Circuit Current.
The rated short-circuit current of a circuit breaker is the highest value of the symmetrical component of the polyphase or phase-to-phase short-circuit current in rms amperes measured from the envelope of the current wave at the instant of primary arcing contact separation which the circuit breaker shall be required to interrupt at rated maximum voltage and on the standard operating duty.

It also establishes, by fixed ratios, the highest currents which the breaker shall be required to close and latch against, to carry, and to interrupt. For numerical values of rated shortcircuit current, refer to the tables of preferred ratings in ANSI C37.06-1979.

The relationship of rated short-circuit current to the other required capabilities is illustrated graphically in Figs 1 and 2.

Figure 2— Ratio of Circuit Breaker Asymmetrical to Symmetrical Interrupting Capabilities

Related Required Capabilities.
The circuit breaker shall have the following related required capabilities which are based on a relay time of one-half cycle, but may be used with any permissible tripping delay.


Required Symmetrical Interrupting Capability for Polyphase and Phase-to-Phase Faults.
For polyphase and phase-to-phase faults, the required symmetrical interrupting capability of a circuit breaker is the highest value of the symmetrical component of the short-circuit current in rms amperes at the instant of primary arcing contact separation which the circuit breaker shall be required to interrupt at a specified operating voltage on the standard operating duty and irrespective of the direct current component of the total short-circuit current.

The numerical value at an operating voltage between 1/K times rated maximum voltage and rated maximum voltage shall be determined by the following formula: In no case shall the required symmetrical interrupting capability exceed K times short-circuit current.


The numerical value shall be equal to the product of a ratio S, specified below and illustrated in Fig 2, times the required symmetrical interrupting capability of the breaker determined for the operating voltage. The values of S shall be 1.4, 1.3, 1.2, 1.1, or 1.0 for breakers having primary arcing contact parting times of 1, 1.5, 2, 3, 4, or more cycles, respectively.

The values of S for primary arcing contact parting times between those given above shall be determined by linear interpolation. The primary arcing contact parting time shall be considered equal to the sum of one-half cycle (present practical minimum tripping delay) plus the lesser of:

1) The actual opening time of the particular breaker, or
2) 1.0, 1.5, 2.5, or 3.5 cycles for breakers having a rated interrupting time of 2, 3, 5, or 8 cycles, respectively.

NOTE — Any combination of symmetrical and direct current components is permissible, provided that the following conditions are met at the instant of primary arcing contact separation:

1) The symmetrical component does not exceed the required symmetrical interrupting capability
2) The degree of asymmetry does not exceed 100 percent
3) The total short-circuit current does not exceed the required asymmetrical interrupting capability

Required Interrupting Capability for Single Line-to-Ground Faults.
For single line-to-ground faults, the required symmetrical interrupting capability and the required asymmetrical interrupting capability of a circuit breaker shall be 1.15 times the corresponding values for polyphase and phase-to-phase faults. In no case are the capabilities for single line-to-ground faults required to exceed K times the symmetrical interrupting capability (that is, K times rated short-circuit current) and K times asymmetrical interrupting capability, respectively, determined at rated maximum voltage.

Required Closing-Latching-Carrying-Interrupting Capabilities.
The circuit breaker shall be capable of performing the following duties in immediate succession:

1) Closing and, immediately thereafter, latching any normal-frequency making current which does not exceed 1.6K times the rated short-circuit or whose maximum crest (peak making current) does not exceed 2.7K times the rated short-circuit current.

2) Carrying a short-circuit current I for any time up to the permissible tripping delay determined in accordance with 5.8.

3) Interrupting any short-circuit current which, at the instant of primary arcing contact separation, has a symmetrical value not exceeding the required asymmetrical interrupting capability.

PROTECTIVE RELAYING – DEFINITION, INFORMATION AND PURPOSE

Our power system is made of of different electrical components, big and small, and super expensice at that. From power transformers, to insulators and conductors.

One of the most important facets in the power system is the protection side of it. And one of the most important factor involved is the protective relaying.

Objectives Of Protective Relaying

The objectives of electrical system protection and coordination are to

a. Limit the extent and duration of service interruption whenever equipment failure, human error, or adverse natural events occur on any portion of the system

b. Minimize damage to the system components involved in the failure

The circumstances causing system malfunction are usually unpredictable; however, sound design and preventive maintenance can reduce the likelihood of system problems. The electrical system, therefore, should be designed and maintained to protect itself automatically.

Protective Relaying's Role In Ensuring Safety

Prevention of human injury is the most important objective when designing electrical systems. Interrupting devices should have adequate interrupting capability. Energized parts should be sufficiently enclosed or isolated to avoid exposing personnel to explosion, fire, arcing, or shock. Safety should always take priority over service continuity, equipment damage, or economics.

These fundamental principles of safety have always been adhered to by responsible engineers engaged in the design and operation of electrical systems. The National Electrical Code ® (NEC®) (NFPA 70 1999), National Electrical Safety Code® (NESC®) (Accredited Standards Committee C2-2002), NFPA 70E-2000, and state and local codes all have prescribed practices intended to enhance the safety of electrical systems.

In recent years, an increased concern about safety has led to many studies resulting in detailed recommendations [from the National Institute of Occupational Safety and Health (NIOSH)] and regulations relating to electrical systems. Prominent among these are the regulations of the Occupational Safety and Health Administration (OSHA) of the U.S. Department of Labor.

Engineers and other personnel engaged in the design and operation of electrical system protection should be familiar with the most recent OSHA regulations, NIOSH Safely Alerts, and all other applicable codes and regulations relating to human safety.

The essential electrical infrastructure in industrial and commercial establishments employs protective devices, many of which are addressed in this recommended practice, that function to de-energize the electrical system in the event of a malfunction. However, electric shock can result in serious injury far more quickly than the available technology of interruption can perform its task.

Therefore, the limitation of shock hazard, such as the step and touch potentials in electrical substations, depends upon the speed of interrupting the fault. Rapid fault isolation is important (see IEEE Std 80-2000). Therefore, the most efficient form of electrical shock protection is to avoid shock altogether, and such avoidance is best accomplished through proper system design and operation, and through effective maintenance.

Equipment damage versus service continuity

Whether minimizing the risk of equipment damage or preserving service continuity is the more important objective depends upon the operating philosophy of the particular industrial plant or commercial business. Some operations can afford limited service interruptions to minimize the possibility of equipment repair or replacement costs, while others would regard such an expense as small compared with even a brief interruption of service.

In most cases, electrical protection should be designed for the best compromise between equipment damage and service continuity. One of the prime objectives of system protection is to obtain selectivity to minimize the extent of equipment shutdown in case of a fault.

Therefore, many protection engineers would prefer that faulted equipment be de-energized as soon as the fault is detected. However, for certain continuous process industry plants, high-resistance grounding systems that allow the first ground fault to be alarmed instead of automatically cleared are employed.

Economic and reliability considerations

The cost of system protection determines the degree of protection that can be feasibly designed into a system. Many features may be added that improve system performance, reliability, and flexibility, but incur an increased initial cost.

However, failure to design into a system at least the minimum safety and reliability requirements inevitably results in unsatisfactory performance, with a higher probability of expensive downtime. Modifying a system that proves inadequate is more expensive and, in most cases, less satisfactory than initially designing these features into a system.

The system should always be designed to isolate faults with a minimum disturbance to the system and should have features to give the maximum dependability consistent with the plant requirements and justifiable costs. Evaluation of costs should also include equipment maintenance requirements.

In many instances, plant requirements make planned system outages for routine maintenance difficult to schedule. Such factors should weigh into the economic versus reliability decision process. When costs of downtime and equipment maintenance are factored into the protection system cost evaluation, decisions can then be based upon total cost over the useful life of the equipment rather than simply the first cost of the system. In-depth coverage of reliability versus economic decisions can be found in IEEE Std 493-1997.