Electrical control system

An electrical control system for controlling a plurality of electrical loads each having a contactor in the electrical supply circuit thereto, said system comprising for each load a switch unit including means for producing electrical signals representative of the condition of the switch unit, a contactor control unit connected so as to control the operation of the contactor in the supply circuit to the load in response to signals received from the switch unit, and an electrical signal generating unit for producing output signals including a fixed frequency coded electrical interrogator signal, said last mentioned signals being supplied to the units of all the loads, and an electrical link conductor to which all of the units are connectable, the two units associated with each load being responsive to the same part of the period of the interrogator signal to connect the units to the electrical link conductor and during said part of the period the units of the other loads are non-responsive thereto and said units are not connected to the link conductor.