9328900990 Rev L BE1-951 Protection and Control 4-27
Figure 4-20. Sequence Components for an A-B Fault
The phase-to-phase fault is made up of both positive and negative-sequence components as shown in
Figure 4-21. For a phase-to-phase fault, the magnitude of the negative-sequence component is 1/√3
(58%) of the magnitude of the total phase current. When these two factors (√3/2 and 1/√3) are combined,
the 3 factors cancel, which leaves the one-half factor.
Figure 4-21. Phase-to-Phase Fault Magnitude
Negative-Sequence Coordination Settings
The 51Q settings should be checked for coordination with phase-only sensing devices such as
downstream fuses and reclosers and/or ground relays. To plot the negative-sequence time current
characteristics on the same plot for the phase devices, you need to multiply the negative-sequence
element pickup value by the correct multiplier (see
Table 4-15). The multiplier is the ratio of phase current
to negative-sequence current for the fault type for which you are interested. To plot the negative-
sequence time current characteristics on the same plot for the ground devices, you need to multiply the
pickup value by the multiplier for phase-to-ground faults.
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