A device which is designed to shield cable modems from power surges and lightning.
They are also meant to prevent the surges going back into the cable network. However, if you
removed the isolator and transferred 240 volts into the network, an isolator wouldn't
provide much protection and would most likely fail -- a few isolators previously
installed have been found to be ineffective, however anyone with common
sense would turn their computer and cable modem off during a storm.