Ground-fault sensing and relaying equipment is intended for use in power distribution systems rated at 600V maximum and are considered to be equipment protection devices, not personal protection devices. Ground-fault protection devices function at a predetermined value of ground fault in accordance with Section 230.95 of the National Electrical Code (NEC). Determining the setting for these devices requires knowledge of the system and familiarity with pertinent information for the actual installation.
Ground-fault protection of equipment (GFPE) is defined in the NEC in Article 100 as “a system intended to provide protection of equipment from damaging line-to-ground-fault currents by operating to cause a disconnecting means to open all ungrounded conductors of the faulted circuit. This protection is provided at current levels less than those required to protect conductors from damage through the operation of a supply circuit overcurrent protection device.”
Section 230.95 requires solidly grounded Wye electrical services with main disconnecting means rated at 1,000A or more to be provided with GFPE where the voltage is more than 150V to ground but does not exceed 600V phase-to-phase. The setting of the GFPE must not be greater than 1,200A and the maximum time delay before operation shall not exceed one second (60 cycles) for ground-fault currents of 3,000A or greater.
In determining the setting for the ground-fault protection system, it is necessary to determine the permissible amount of damage to the electrical system. The higher the Ampere setting of the ground-fault device or the longer the ground fault is permitted to continue, the more damage to the system.
Since these devices are set with both maximum ampere and time settings, the maximum allowable energy that is permitted to flow before tripping the device is normally measured in kilowatt-cycles, which can then be converted into Ampere-cycles. Some engineering books recommend approximately 20,000 kilowatt-cycles as the maximum allowable energy that should be permitted to flow in an arcing ground fault.
Electrical systems operating at more than 150 Volts-to-ground have arcing voltages that vary from 70 to 140V, so an average value of 100V can be used for converting the kilowatt cycles to Ampere-cycles. Assuming a 20,000kW-cycle energy fault, the following is the conversion from kilowatt-cycles to Ampere-cycles using the formula wattage equals voltage times amperage [Wattage = (amperage) ¥ (voltage)], so solving for amperage = wattage ? voltage:
20,000W-cycles ¥ 1,000 (for kilo conversion) ? 100V (for arcing voltage) = 200,000A-cycles.
The next step in establishing the setting of the GFPE is to determine the maximum phase-to-phase fault current that is available at the main disconnect for the service. Either check with the local utility company for their fault values or figure an approximate value in accordance with the following: where a single transformer is used to supply the service, an approximate value of fault current can be established by dividing the secondary amperage of the transformer by the impedance of the transformer. For example, using a 1,000kVA transformer with 3 percent impedance and a secondary voltage of 277/480V, the maximum secondary current of the transformer is determined as follows: 1,000,000VA ? (480V ¥ square root of 3) or 1,000,000VA ? 831V = 1203A. Dividing 1,203A by the 3 percent impedance of the transformer will provide an approximate value of available phase-to-phase fault current as follows: 1,203A ? 3 percent = 40,100A. The maximum available ground-fault current is approximately one-third the value of the phase-to-phase fault current so ground-fault current is approximately 40,100A phase-to-phase ? 3 = 13,367A of available ground fault.
The maximum time delay in cycles can now be determined. Using 200,000A-cycles as the maximum recommended fault energy (converted into Ampere cycles) and dividing that value by the maximum ground fault available in the system will provide the maximum time delay setting for the GFPE device. So 200,000A-cycles ? 13,367A of ground fault = 14.96 cycles or approximately 15 cycles. Since the maximum setting permitted by Section 230.95 is 1 second (60 cycles), the time setting for this example should not be in excess of a quarter of a second.
The maximum ampere setting is normally figured at about 10 percent of the size of the main disconnecting means. This 10 percent value will provide a good compromise between protection of the equipment and any leakage current that may be available in the system. This setting should be high enough that devices downstream are permitted to open first in a ground fault before the main starts to open, unless using a series rated and coordinated system.
Remember, these values are approximate and each individual situation should be considered in determining the setting of the ground-fault protective device. EC
ODE is a staff engineering associate at Underwriters Laboratories Inc., in Research Triangle Park, N.C. He can be reached at 919.549.1726 or at email@example.com.