Serving to loads
Electrical power systems must be designed to serve a variety of loads safely and reliably. Effective control of short-circuit current, or fault current as it is commonly called, is a major consideration when designing coordinated power system protection.
In order to fully understand the nature of fault current as it is applied to electrical power system design, it is necessary to make distinctions among the various types of current available, normal as well as abnormal.
It is also important to differentiate between the paths which the various types of current will take.
Both current type and current path, as well as current magnitude, will affect the selection and application of overcurrent protective devices.
Normal, or load, current may be defined as the current specifically designed to be drawn by a load under normal, operating conditions. Depending upon the nature of the load, the value of normal current may vary from a low level to a full-load level. Motors offer a good example. Normal motor current varies from low values (under light loading) to medium values (under medium loading) to maximum values (under maximum loading). Maximum load current is called full load current and is included on the motor nameplate as FLA (Full-Load Amperes).
Normal current, therefore, may vary from low values to FLA values. Additionally, normal current flows only in the normal circuit path. The normal circuit path includes the phase and neutral conductors. It does not include equipment grounding conductors.
Overload current is greater in magnitude than full-load current and flows only in the normal circuit path. It is commonly caused by overloaded equipment, single-phasing, or low line voltage, and thus is considered to be an abnormal current. Some overload currents, such as motor starting currents, are only temporary, however, and are treated as normal currents. Motor starting current is a function of the motor design and may be as much as twenty times full-load current in extreme cases.
Motor starting current is called locked-rotor current and is included on the motor nameplate as LRA (Locked-Rotor Amperes). Overload current, then, is greater in magnitude than full-load amperes but less than locked-rotor amperes and flows only in the normal circuit path.
Short-circuit current is greater than locked-rotor current and may range upwards of thousands of amperes. The maximum value is limited by the maximum short-circuit current available on the system at the fault point. Short-circuit current may be further classified as bolted or arcing.
- Bolted short-circuit current
Bolted short-circuit current results from phase conductors becoming solidly connected together. This may occur from improper connections or metal objects becoming lodged between phases. Obviously, large amounts of short-circuit current will flow into a bolted fault.
- Arcing short-circuit current
Arcing short-circuit current results from phase conductors making less than solid contact. This condition may result from loose connections or insulation failure. When this happens, an arc is necessary to sustain current flow through the loose connection. Since the arc presents an impedance to the flow of current, smaller amounts of current will flow into an arcing fault than will flow into a bolted fault.
- Failure classifications
Short-circuit currents, whether bolted or arcing, will involve two or more phase conductors. Line-to-line faults involve two-phase conductors (A-B, B-C, C-A) while three-phase faults involve all three phases (A-B-C). Although three-phase bolted short-circuits rarely occur in practice, short-circuit studies have traditionally been based upon the calculation of three-phase, bolted short-circuit current. Modern personal computers and associated software have made the calculation of all types of fault currents easier to accomplish.
Ground-fault current consists of any current which flows outside the normal circuit path. A ground-fault condition then, results in current flow in the equipment grounding conductor for low-voltage systems. In medium- and high-voltage systems, ground-fault current may return to the source through the earth. Ground-fault protection of medium-voltage and high-voltage systems has been applied successfully for years using ground current relays. Ground-fault protection of low-voltage systems is a considerable problem because of the presence and nature of low-level arcing ground faults.
Ground-fault current on low-voltage systems may be classified as leakage, bolted, or arcing.
- Leakage ground-fault current.
Leakage ground-fault current is the low magnitude current (milliampere range) associated with portable tools and appliances. It is caused by insulation failure, and is a serious shock hazard. Personnel protection is accomplished by using ground-fault circuit interrupters (GFCI) in the form of GFCI receptacles or GFCI-circuit-breakers.
- Bolted ground-fault current
Bolted groundfault current results when phase conductors become solidly connected to ground (i.e., the equipment grounding conductor or to a grounded metallic object). Bolted ground-fault current may equal or even exceed three-phase, bolted short-circuit current if the system is solidly grounded. Equipment protection is accomplished by using standard phase and ground overcurrent devices depending upon system voltage levels.
- Arcing ground-fault current
Arcing groundfault current results from a less than solid connection between phase conductors and ground. Because an arc is necessary to sustain current flow through the connection, the magnitude of arcing ground-fault current will be less than that of bolted ground-fault current. Depending upon the arc impedance, arcing ground-fault current may be as low as several amperes (low-level) or as high as 20-38 percent of three-phase, bolted short-circuit current (high level) on a 480V system.
Considerable research has been conducted in the area of arcing ground-fault current magnitudes on low voltage systems. Some designers use the 38 percent value while others use the 20 percent figure.
NEMA PB2.2 applies ground-fault damage curves instead of performing a calculation. Equipment protection is accomplished by using ground-fault protective (GFP) devices. Due to ionization of the air, arcing ground faults may escalate into phaseto-
phase or three-phase faults.