Teleprotection
GARD 8000 SYS RFL Electronics
March 1, 2006 9-4 973.334.3100
9.2.3 DIRECTIONAL COMPARISON BLOCKING
DCB schemes locate a fault internal or external to a protected line. The results are passed between the
two ends of the protected line over a communications channel using teleprotection devices such as the
GARD 8000. If the reverse element detects a fault, the start input is keyed and a block signal is sent to
the remote end to prevent tripping for the external fault. However, if the forward looking element also
detects a fault, the stop input is keyed and the block signal is discontinued allowing the remote end to
trip for the internal fault. Both terminals must stop their block signal in order to trip because the block
signal prevails for both ends; even if one terminal transmits the block signal, the breakers will not trip.
Figure 9-4. Directional Comparison Blocking