A perforating string insertable into a wellbore comprising: an upstream perforating gun (62 1); a downstream perforating gun (62 2) having an upstream end, an annular receptacle fitting in the upstream end, a communication line with an end electrically connected to the receptacle fitting, and a detonating cord disposed lengthwise therein so that it is positioned proximate each of shaped charges; a cartridge sub; and a cartridge assembly that selectively inserts into the cartridge sub and that comprises: an elongated body; a detonator in the body and having a detonating end adjacent to and directed towards the detonating cord; a signal line and a ground line (86, 78) connected to the detonator; an annular connector that circumscribes an end of the body that selectively inserts into electrical communicating contact with the receptacle fitting; a switch assembly in the body and in communication with the upstream perforating gun (62 1) via an inlet line, and in selective communication with the signal line and ground line (86, 78) and in selective communication with the communication line via the selective communicating contact between the connector and the receptacle fitting and an exit line, so that when an electrical signal is communicated via the inlet line to the switch assembly from the upstream perforating gun (62 1), the electrical signal is selectively communicated to the signal line to initiate the detonator and detonating cord in the downstream perforating gun (62 2), and the electrical signal is selectively communicated via the exit line to the communication line. an upstream perforating gun (62 1); a downstream perforating gun (62 2) having an upstream end, an annular receptacle fitting in the upstream end, a communication line with an end electrically connected to the receptacle fitting, and a detonating cord disposed lengthwise therein so that it is positioned proximate each of shaped charges; a cartridge sub; and a cartridge assembly that selectively inserts into the cartridge sub and that comprises: an elongated body; a detonator in the body and having a detonating end adjacent to and directed towards the detonating cord; a signal line and a ground line (86, 78) connected to the detonator; an annular connector that circumscribes an end of the body that selectively inserts into electrical communicating contact with the receptacle fitting; a switch assembly in the body and in communication with the upstream perforating gun (62 1) via an inlet line, and in selective communication with the signal line and ground line (86, 78) and in selective communication with the communication line via the selective communicating contact between the connector and the receptacle fitting and an exit line, so that when an electrical signal is communicated via the inlet line to the switch assembly from the upstream perforating gun (62 1), the electrical signal is selectively communicated to the signal line to initiate the detonator and detonating cord in the downstream perforating gun (62 2), and the electrical signal is selectively communicated via the exit line to the communication line. an elongated body; a detonator in the body and having a detonating end adjacent to and directed towards the detonating cord; a signal line and a ground line (86, 78) connected to the detonator; an annular connector that circumscribes an end of the body that selectively inserts into electrical communicating contact with the receptacle fitting; a switch assembly in the body and in communication with the upstream perforating gun (62 1) via an inlet line, and in selective communication with the signal line and ground line (86, 78) and in selective communication with the communication line via the selective communicating contact between the connector and the receptacle fitting and an exit line, so that when an electrical signal is communicated via the inlet line to the switch assembly from the upstream perforating gun (62 1), the electrical signal is selectively communicated to the signal line to initiate the detonator and detonating cord in the downstream perforating gun (62 2), and the electrical signal is selectively communicated via the exit line to the communication line.