Ground-fault protection of medium-voltage and high-voltage systems has been applied successfully for years using ground current relays (device 51G).
Ground-fault protection of low-voltage systems is a considerable problem because of the presence and nature of low-level arcing ground faults. Leakage ground-fault current is low magnitude current (milliampere range) generally associated with portable tools and appliances.
Leakage ground-fault current presents a hazard to personnel. Personnel protection from leakage ground-fault current is provided by ground-fault circuit interrupters and are required by the NFPA 70 in certain locations. Arcing and bolted ground-fault currents are higher magnitude currents, and are associated with equipment damage rather than personnel hazard. That doesn’t mean that arcing and bolted ground-fault currents don't present a personnel hazard. They do. But, the primary concern is with the equipment hazard since arcing and bolted ground-fault currents usually occur only on supervised commercial and industrial electrical systems.
For voltages 100-400 kV the earth-fault currents required to be detected can be in range 100-400 A whereas on lower voltages the value can be as low as 50-100 A.
When power lines are not transposed the occurring unbalance current in the system will cause problems for sensitive earth-fault protection relays. Operating current should then be increased to up to 70% of maximum load.
For low-ohmic earthed systems, where system earthing is a zig-zag transformer with or without a low-ohmic resistor, or a low-ohmic resistor directly in a transformer neutral, the fault current is generated from one point only and selectivity can be achieved by grading the time settings of the earth-fault protection relays at different locations.
Required current setting is normally 10 - 30% of achieved maximum earth-fault currents and the same for all protection relays in the system. A small increase of protection settings, moving to wards the source should be used to prevent random operation due to differences in CT's and protection relays.
As a complement a stand-by earth-fault protection can be included in the zig-zag transformer neutral. This protection is set on the continuous thermal capacity current of the zig-zag transformer and on a very long time (20-30 sec.)
High-ohmic earthed systems are earthed in the same way as low-ohmic earthed systems but with a high-ohmic resistor in the neutral. The resistor is normally selected to give fault currents of 5 - 25 A.Current relays with definite time delay are used and selectivity is achieved by time-grading. Required current setting is normally 10 - 30% of maximum earth-fault current and close to equal for all relays throughout the system. Directional earth-fault relays, measuring the resistive component only, are often required due to infeed of capacitive earth-fault current from healthy objects.
Arcing ground fault 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.