base of transistor 70 will no longer exist during negative half cycles of the applied ac supply voltage capacitor 92 will then charge positively until transistor 82 commences to conduct rendering transistors 62 and 64 conductive once transistors 62 and 64 commence conducting the alarm condition is established as described above with the buzzer sounding continuously once an alarm condition has been established either as a result of a low resistance condition between the conductor 49 and ground or as a result of an open circuit between the conductor 49 and ground the only way to restore the non alarm status is to turn off the ac supply voltage provided from the trunk until capacitors 72 and 92 have discharged if the ac supply voltage is restored with the capacitors 72 and 92 discharged and the proper resistance is provided between the conductor and ground transistor 70 will prevent the charging of capacitor 92 as described above if the proper resistance is not present between conductor 49 and ground the circuit will return to the alarm condition it will be apparent from the foregoing that an important feature of the invention is the use of trunk lines not only to provide tv signals to television sets connected to the trunk line but also to provide a path for the flow of low frequency ac current through the trunk line which does not interfere with the operation of the television sets connected to the trunk line operation of the system is dependent upon detection of changes in the low frequency ac current flowing through the trunk lines both for the purpose of detecting an alarm condition in.