Some electric devices generate some electric noise and send them to the network. This noise not only affects other devices (for instance, they can cause interference on Hi-Fi stereos) but they can also affect the quality of your X-10 network. Other devices, specially using capacity loads (e.g. UPS) absorb the X-10 signals, avoiding them to reach the destination in the best possible conditions.
The S4813M, is an in-wall module especially design to filter all noise generated from the device connected to it. It supports loads with a maximum of 5A.
For additional information about X-10 noise and interferences, please consult the X-10 Info