9 THEORY OF OPERATION
Figure 9–2: TWO-TERMINAL LINE PHASE COMPARISON
Thus, the logic shown in Figures 9–2A and 9–2B would rarely, if ever, produce a trip output on an internal fault because the
8.33 ms (which is the time of a half cycle on a 60 Hz base) requires perfect matching. In actual practice a 3 to 4 ms setting
is used rather than the 8.33 setting illustrated. This makes it much easier to trip on internal faults. It also makes it much eas-
ier to trip undesirably on external faults. However, experience has indicated that with proper settings and adjustments in the
relay such a timer setting offers an excellent compromise. This may be better appreciated if it is recognized that item (a)
above is generally minimized and item (b) is nonexistent on external faults.
As shown in Figure 9–3: Stability Angle, a stability angle setting of 3 ms for a 60 Hz system allows for about 65 electrical
degrees of blocking zone. This provides sufficient security to prevent tripping in the cases indicated above and provides
reliable tripping for all types of internal faults.
In the event that ON-OFF communication equipment were to be employed rather than frequency-shift equipment, the logic
would appear as in Figures 9–2C and 9–2D. It will be noted in these two Figures that the reference to MARK and SPACE
have been conveniently omitted since the receiver output is either present or not as against the case of the frequency-shift
equipment where it could be there in either of two states. Figure 9–2C illustrates a tripping scheme while Figure 9–2D a
blocking scheme. Here again, the integrator is, in practice, actually set for 3 to 4 ms.
L60 Line Phase Comparison System