Abstract: A control system for diverting/equalizing electrical potentials between two objects, for example the shell (1) of a storage tank installation and the body (10) of a tank truck/vessel, when loading flammable/explosive fluids, wherein an electrical equalizer connection (5,6,14) connects the body (10) of the tank truck via a terminal (10a) to the storage tank (1) via a terminal (1a). The terminals (1a and 10a) are part of respective control circuits comprising a low voltage source (U.sub.1 ; U.sub.2), a resistor (4; 13) and an indicator (8,9) for registering current flow and thereby controlling/blocking the connection/disconnection of a fluid pump. To check that electrical connection has in fact been established at the terminals (1a,10a), each of these is connected to a respective control circuit via separate terminals, namely a terminal (1b; 10b) on the respective bodies (1; 10) and a terminal (6a; 6b) on a busbar (6) in the equalizer connection ( 5,6,14).