A power distribution system including a first circuit protection device (106, 114) and a second circuit protection device, the first circuit protection device (106, 114) comprising: a trip mechanism configured to interrupt a current flowing through said first circuit protection device (106, 114); and a trip unit operatively coupled to said trip mechanism, said trip unit comprising: a zone selective interlock signal generator; a zone selective interlock output port coupled to said zone selective interlock signal generator and configured for a communicative connection to a zone selective interlock input port of the second circuit protection device; a zone selective interlock monitor coupled to said zone selective interlock output port and configured to monitor a status of the communicative connection between said zone selective interlock output port and said zone selective interlock input port of said second circuit protection device, characterized in that said zone selective interlock signal generator is configured to produce a first current, which is less than a current needed to trip the trip mechanism, between the zone selective interlock input port of the second circuit protection device and said zone selective interlock output port, said zone selective interlock monitor is configured to produce a second current between the zone selective interlock input port of the second circuit protection device and said zone selective interlock output port, and the second current has a magnitude less than the first current, whereas the second current is less than the current needed to trip the trip mechanism of the first circuit protection device (106, 114) such that the trip unit of the first circuit protection device (106, 114) is configured to operate in a restrained mode of operation to avoid the first circuit protection device (106, 114) tripping before the second circuit protection device. a trip mechanism configured to interrupt a current flowing through said first circuit protection device (106, 114); and a trip unit operatively coupled to said trip mechanism, said trip unit comprising: a zone selective interlock signal generator; a zone selective interlock output port coupled to said zone selective interlock signal generator and configured for a communicative connection to a zone selective interlock input port of the second circuit protection device; a zone selective interlock monitor coupled to said zone selective interlock output port and configured to monitor a status of the communicative connection between said zone selective interlock output port and said zone selective interlock input port of said second circuit protection device, characterized in that said zone selective interlock signal generator is configured to produce a first current, which is less than a current needed to trip the trip mechanism, between the zone selective interlock input port of the second circuit protection device and said zone selective interlock output port, said zone selective interlock monitor is configured to produce a second current between the zone selective interlock input port of the second circuit protection device and said zone selective interlock output port, and the second current has a magnitude less than the first current, whereas the second current is less than the current needed to trip the trip mechanism of the first circuit protection device (106, 114) such that the trip unit of the first circuit protection device (106, 114) is configured to operate in a restrained mode of operation to avoid the first circuit protection device (106, 114) tripping before the second circuit protection device. a zone selective interlock signal generator; a zone selective interlock output port coupled to said zone selective interlock signal generator and configured for a communicative connection to a zone selective interlock input port of the second circuit protection device; a zone selective interlock monitor coupled to said zone selective interlock output port and configured to monitor a status of the communicative connection between said zone selective interlock output port and said zone selective interlock input port of said second circuit protection device, characterized in that said zone selective interlock signal generator is configured to produce a first current, which is less than a current needed to trip the trip mechanism, between the zone selective interlock input port of the second circuit protection device and said zone selective interlock output port, said zone selective interlock monitor is configured to produce a second current between the zone selective interlock input port of the second circuit protection device and said zone selective interlock output port, and the second current has a magnitude less than the first current, whereas the second current is less than the current needed to trip the trip mechanism of the first circuit protection device (106, 114) such that the trip unit of the first circuit protection device (106, 114) is configured to operate in a restrained mode of operation to avoid the first circuit protection device (106, 114) tripping before the second circuit protection device.