Current Affairs: Motor Starter Basics — Part 3 (December 1999)
Short Circuits, Ground FaultsShort circuits and ground faults occur when conductors unintentionally contact one another, or a grounded surface due to insulation failure, equipment damage, or human error. When this happens, the resulting connection provides a path for current to return to the source without passing through the load. With the load impedance “shorted out,” the magnitude of the resulting current is limited only by the impedance of the power source, and the conductors and can reach values of thousands of amperes in a typical motor circuit.
A short circuit often involves electric arcing at the point of contact. Resulting burning and expulsion of molten metal poses a serious fire and personnel safety hazard, as well as significant potential for equipment damage. Arcing due to motor winding insulation failure, if allowed to persist, will damage the steel laminations, increasing the extent and cost of work required to repair the motor.
In addition to damage at the fault location, the entire power circuit through which the fault current flows is exposed to rapid heating and mechanical forces caused by the high current magnitude. Protection of the motor starter as well as the motor requires rapid current interruption.
Protective DevicesProtection is provided by fuses, molded-case circuit breakers (MCCBs), or motor circuit protectors (MCPs) installed on the line side of the motor controller, either as part of a combination starter or separately-mounted. Fuses must be the dual-element type, which have time delay that will allow the fuse to carry locked rotor current long enough to start the motor.
MCCBs include both time-delayed and instantaneous tripping functions and must be sized large enough to prevent the instantaneous element from tripping on starting current. MCPs are circuit breakers with only an instantaneous function, designed specifically for use in motor starters and limited by their UL listing to this application.
All devices designed to interrupt fault currents carry an interrupting rating which is specified in Root-Mean-Square Symmetrical (RMS SYM) Amperes or kilo-Amperes (kA). The interrupting rating of any device must exceed the maximum short circuit current available at the point in the system where it is applied. Interrupting ratings range from 10 kA for the simplest fuses and circuit breakers to 200 kA for current-limiting devices.
Application of equipment with inadequate interrupting ratings can result in fire or personnel hazard in addition to equipment damage. Protective devices which attempt to interrupt faults above their ratings can fail catastrophically, creating a secondary fault location.
Both fuses and MCCBs are available with current-limiting characteristics, which cause the interruption of high currents within the first quarter cycle, greatly reducing both thermal and mechanical effects on equipment. Standard MCCBs and MCPs may require one to two full cycles to interrupt current, making current-limiting devices a better choice for high-fault-current applications. Current-limiting circuit breakers carry a price premium of approximately 200% over standard breakers, while standard 240-V and 480-V fuses available for use in motor circuits are current-limiting. For this reason, fuses are more common for high-fault-current applications.
Equipment SpecificationAssemblies that include protective devices and other components, such as motor starters, carry a short circuit withstand rating. This rating reflects both the interrupting capacity of the device and its ability to protect the components from catastrophic failure for downstream faults. If the withstand rating of a motor starter is exceeded, components such as the contactor and overload relay could fail in such a way as to burst the enclosure and present a hazard to personnel, even if the current is successfully interrupted by the protective device.
When specifying mechanical equipment which includes motor starters, the required short circuit current withstand rating must be included as a specification item. This value should be available from the electrical engineer, who must calculate the available short circuit currents for the electrical system to properly specify equipment such as panelboards and MCCs. Note that adequate withstand ratings ensure only that the equipment will not fail catastrophically; damage to the contactor or overload relay may still occur, and they should be inspected following any motor or circuit fault that causes the protective device to operate.