Many homes have a 2 phase 220V power supply which is split into 2 110V legs within the house. Sometimes the X10 signal will not reliably get from one leg of the 110V wiring to the other leg of the 110V wiring. The signal bridge is installed at the breaker box and provides a a clear X10 signal path from one leg to the other. This device is only necessary on installations where X10 control is required on multiple circuits and the some of the circuits are on the opposite phase.