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Abstruct: Single-line fault diagnosis in delta-delta connected distribution systems suffers due to the low fault currents associated with such faults. Simulation tests on this type of system reveals that rule based deciiion support can be used for such diagnosis.This pppa describes the development of le^ for single-line fault diagnosis utilizing simulation test results. The key parameters used are the voltage magnitude of each phase at the bus bar and the currents on the feeders including their aupnce components. mmmmSingle line fault detection, faulted feeder identifdon. phasehype classification, location and impedance estimation can be broadly lumped into single-line fault diagnosis. Single line fault diagnosis is a topic widely discussed especially when applied to high impedance faults [l-lo]. High impedance single-line fault detection has challenged many researchers in the recent past. The low currents associated with such faults makes it virtually impossible to distinguish the fault from a noisy load or a switching event. Hence these faults can go undetected and can cause much damage to property and life. Most of the attention has been focused on rht detection of such faults in wye-grounded systems.In delta-delta connected distribution systems, single-line fault diagnosis has its difficulties at any impedance level due to the ungrounW nature of the system. The only conductance path to ground is through the shunt capacitance of the distribution lines which essentially has a wye-grounded formation. The high impedance offered by this capacitance limits the fault current regardless of the fault impedance and makes it virtually indistinguishable in the presence of load currents several times the magnitude of the fault current.Ground alarm relays used in this type of system can detect and identify the faulty phase for certain types of single-line faults at the low impedance levels. These cannot however identify the faulted feeder. Some distribution systems employing both, delta-delta and wyegrounded representation, require the use of single phase voltage regulatops on each feeder. The zero sequence voltage is monitored and the voltage on one phase 0-7803-1 883-8$04.00019941EEE 111 North Hope Street P.O. Box 11 1 , L a Angeles. CA 90051 regulatcr on each feeder is chaugd If the ZQO sequence bus voltage cbanges, the fault is considered to be on that feeder. This is a manual method, and requires the use of single phase reguhm and, is applied only to certain types of single-line faults. Tbe use of directional relays sensitive to zeto sequence curreats have also been considemi. These hil at the higher fault impedance levels and are also applicable only to certain Los Angeles Depiuanent of W a t e r and Power, a utility in the W e s t Coast of the United States, has <a<a connected distribution systems at the 4.8kV voltage level. They have over 1500 feeder circuits at this voltage level. A typical distribution station has about 4-5 delta-delta connected, 35kV/4.8kV & m e r banks, each with a capacity of up to about 1...