a clip - on device to protect telephones and communication apparatus , wherein clips for releasably holding a 3 - terminal gas protector are external extensions of electrically insulated spaced terminals within the housing . a heat coil in series with each communication line and a carbon block air gap protector across each half of the gas protector for redundancy , are within the housing . the heat coil protects against a sustained overcurrent condition on the line while the gas and air gap protectors ground over - voltages of short duration . utilizing heat conduction means , heat generated from sustained over - voltage melts a fusible disc allowing spring biased parts within the housing to move and short line terminals to ground .