Our Services
Short Circuit Study
The primary reason for performing a Short Circuit Study on a power system is to calculate the available short circuit current at each bus location to facilitate the selection of protective device interrupting ratings. A protective device must be able to interrupt the available short circuit current, at its location in the power system, therefore the protective device must have an interrupting rating equal to or greater than that available short circuit current. The three-phase symmetrical RMS fault current is typically considered to be the maximum available fault current and hence is usually used as the basis for the selection of the protective device interrupting ratings. Switchboards and Panelboards are rated based on the lowest AIC rating of the installed device, including the bracing ratings for the gear. The short circuit study aids in the selection in regard to breaker coordination, for the coordination study.
Coordination Study
The primary reason for performing a Selective Coordination Study on a power system is to determine if the protective devices in that system provide a system of selective coordination. Coordination is defined as “properly localizing a fault condition to restrict outages to the equipment affected, accomplished by the choice of selective fault protective devices”. Maximum protection requires that the overcurrent devices be rated, selected, and adjusted to allow the normal load currents to flow while instantaneously opening the circuit when abnormal currents flow. This maximum protection to the electrical distribution system is a result of good engineering practices. Selective coordination, while always desirable, is not required by the NEC, except in certain situations as follows: • In health care facilities, per NEC 517.17[C]. • Elevator circuits when more than one elevator motor is fed by a single feeder, NEC 620.62 • Emergency and legally required stanby power systems per NEC 700.27 and NEC 701.18 • Exceptions to selective coordination as per NEC 700.28, two devices are in series feeding a common load. Interruption of one or both interrupts power to the exact same portion of the power system
Arc Flash Study
An Arc Flash Analysis is a study of the facility’s power system to determine the incident energy available at specific electrical devices that employees would be exposed to while “interacting with” the electrical equipment at the facility. The output of the Analysis is a determination and labeling of the PPE required of the electrical devices, suitability of the devices for service as installed and if there are any miss-coordination issues involved. NFPA 70E-2018, Standard for Electrical Safety in the Workplace, indicates appropriate safety related work practices shall be determined before any person is exposed to electrical hazards involved by using both shock risk assessment and arc flash risk assessment. The electrical arc incident energy levels and arc flash boundary distances were calculated by SKM Power Tools Arc Flash Evaluation module. This module uses the formulas contained in IEEE 1584-2018, NFPA 70E-2018-Annex D.4.Incident energy is a function of arcing current magnitude and time. Thus, for a small amount of arcing current a long clearing time results in a high incident energy value. These high incident energy values can be somewhat misleading because a “jump back” response would presumable allow one to remove himself from harm’s way before serious injury occurred. IEEE 1584 Annex B1.2 states • “If the time is longer than two seconds, consider how long a person is likely to remain in the location of the arc flash. It is likely that a person exposed to an arc flash will move away quickly if it is physically possible and two seconds is a reasonable maximum time for calculations.” The calculations used to evaluate the arc flash incident energy take this into account by setting the maximum arcing duration time to two seconds. If the trip time read from the TCC plus the breaker time is greater than the Maximum Arcing Duration, the trip time will be set at maximum arc duration of two seconds, indicated by [*N9] on the
Short Circuit Results
All devices are evaluated on the basis of ANSI/IEEE standards C37.13-1981, C37.010-1979, and C37.5-1979. The calculations comply with the National Electrical Code 110-9 and 110-10.
Coordination Methodology
Maximum protection to electrical equipment is a result of good engineering practices. System protection and sectionalization should be designed to localize the circuit at the fault location. Time intervals should be maintained between upstream and downstream devices so that selectivity is maximized. Exercising both will ensure proper protection and coordination within the system.
Arc Flash Evaluation Study Manager
PTW Arc Flash Evaluation calculates the incident energy and arc flash boundary for each study- defined location in the power system. The study imports the short circuit duty reports at each bus and through each protective device for evaluation. The arcing fault current through the protective devices is then calculated from the bolted fault value and used to automatically find the time duration of the arc from the time current coordination (TCC) curves. Incident energy and arc flash boundaries are calculated based on the bus 3-phase fault current and arcing duration. Clothing requirements are specified per the personal protective equipment table provided in Table 7.3.9-1 which references NFPA 70E table 3-3.9.2.
