A feed line connecting circuit for selectively connecting first, second and third feed line connecting terminals under a predetermined current feeding condition comprises a first current detector connected between one of the first and second feed line connecting terminals and a central node for detecting a current over a predetermined value; a first switch connected between the one of the first and second feed line connecting terminals and the central node for connecting said one of the first and second feed line connecting terminals to the central node under an unpowered condition; a selective switch controlled by the first current detector, the selective switch having first and second contacts and a common contact connected to the third feed line connecting terminal, the common contact being connected to the first selective contact when the first current detector does not detect a current over the predetermined value, and to the second selective contact when the first current detector detects a current over the predetermined value; and a second current detector connected between the first selective contact of the selective switch and the central node to change the first switch so as to disconnect said one of the first and second feed line connecting terminals from the central node when detecting a current above the predetermined value.