Field
The disclosed concept pertains generally to power systems and, more particularly, to power systems, such as, for example, main-tie-main power systems employing zone selective interlocking communication. 
Background Information
Zone selective interlocking (ZSI) allows for coordinated communication between circuit breakers or other overcurrent protection devices.  This allows for circuit breakers to communicate during observed fault incidences, allowing for faster response times on clearing a fault.  In order to effectively use ZSI, circuit breakers are divided into zones based on location relative to a main protective device, which is in a first zone.  By decreasing tripping delays, ZSI significantly increases the protective ability of protection system. 
For example, in a typical ZSI arrangement, if a lower order circuit breaker of the ZSI hierarchy sees an overload current, it sends an interlock signal to the next higher order device to block generation of an instantaneous trip signal by the latter and to give the former time to react.  This permits adjacent circuit breakers in the ZSI hierarchy to have their overcurrent/time trip characteristics set to achieve normal time-current coordination for faults supplied by the lower order circuit breaker, yet allow the higher order circuit breaker to trip without delay for faults between the higher order and lower order circuit breakers. 
For example, zone interlocking can be provided on both phase and ground protection, if enabled.  As a non-limiting example, ground and short delay interlocking functions can be combined on one common set of connections.  So, for example, a restraint or zone interlock output signal (ZONE_OUT or ZOUT) is enabled when a fault: (1) exceeds the ground fault setting of the circuit breaker; or (2) is greater than two times the rated current value of the electrical current of the circuit breaker. 
ZSI employs a priority process.  The main goal is to clear a fault “instantaneously” (i.e., an instantaneous trip as is understood by persons of ordinary skill in the art), regardless of where the fault is in the system, but keep power to the rest of the system, if possible.  In order to do this, the circuit breaker in the furthest downstream zone that recognizes the fault will send a restraint signal to the upstream zone and attempt to clear the fault by opening.  As a fail-safe, however, the upstream circuit breakers of the upstream zone will begin their normal time out delay, and once timed out, will attempt to clear the fault if the downstream zone fails to clear it.  However, if there is a fault on the load side of an upstream zoned circuit breaker, then that circuit breaker will not receive a restraint signal, and will trip instantaneously instead of waiting for its time out delay in order to allow it to open and clear the fault.  This significantly increases the safety of the system and decreases the damage caused by the fault. 
U.S. Pat.  No.  5,875,088 discloses three zones and ZONE_IN and ZONE_OUT signals.  A main circuit breaker of a first zone supplies two feeder circuit breakers of a second zone.  One of the feeder circuit breakers also acts as a main circuit breaker for two downstream devices of a third zone. 
ZSI for power circuit breakers in low voltage power systems greatly increases safety and decreases damage to the system caused by, for example, a ground fault.  While extremely effective, ZSI, in use, is limited in its ability to react instantly to a fault on the load side of a main circuit breaker of one side of a Main-Tie-Main power system with an open tie circuit breaker, while a simultaneous and independent fault occurs on the feeder of the other side of the tie circuit breaker.  Known ZSI communications allow for undesired communication between feeder circuit breakers or protective devices on one side of an open tie circuit breaker to cause undesired delayed tripping of the main circuit breaker or protective device on the opposite side of the open tie circuit breaker.  This decreases the effectiveness of the system in quickly clearing the fault on the load side of the main circuit breaker within the protection zone of the interlocked system. 
Referring to FIGS.  1A and 1B, a double-ended substation employs a single blocking diode D to prevent a tie circuit breaker T from “self interlocking” while allowing it to provide an interlocking signal to both main circuit breakers M1,M2.  The single diode D is electrically connected from the zone input (anode) to the zone output (cathode) of the tie circuit breaker T. However, the configuration of FIG.  1A also allows all of the feeder circuit breakers F1,F2 to communicate with the tie circuit breaker T and both main circuit breakers M1,M2 at the same time as the tie circuit breaker T. The presence of the tie circuit breaker T creates a problem since it must provide a ZOUT signal to multiple upstream devices (including the main circuit breakers M1,M2), and each of those upstream devices M1,M2 is allowed to receive ZOUT signals from downstream feeders F1,F2 (through the diode D) that should not communicate across the tie circuit breaker T (i.e., the left feeder F1 should not communicate with the right main circuit breaker M2 and the right feeder F2 should not communicate with the left main circuit breaker M1. 
There is room for improvement in power systems.