GB765050A - Improvements in or relating to devices for the protection of equipment connected to an electric low-voltage network, in particular to a telephone network - Google Patents

Abstract

765,050. Protective cut-out arrangements. DEMAN, A. March 23, 1955 [March 23, 1954], No. 8435/55. Class 38(5) In an arrangement for protecting equipment 2 connected to a low voltage network 1, e.g. a telephone network, from overvoltages, an overvoltage arrester 3 is combined with a thermal switch operative to isolate the equipment 2 and short circuit the arrester 3. The switch may comprise a stressed contact spring 4 mounted on the upper terminal 7 of the arrester and normally secured to a collar 6 by a fusible element 5, and a contact member 8 fixed to the lower terminal 9. Overvoltages of short duration are discharged through the arrester 3 without any further action occurring, but should an overvoltage persist, the heating of the arrester causes the melting of the element 5 to enable the spring 4 to move away from the collar 6 to interrupt the circuit to the equipment 2 and to engage the member 8 to short circuit the arrester. Flashover from the spring 4 to the collar 6 may be prevented by an insulating member which is moved by a spring along the contact spring 4 as it moves away from the collar (Fig. 3, not shown). In an alternative arrangement the spring 4 is a bimetallic member which normally abuts the collar 6. The combination, or a number of them, may be enclosed in a box having a lid which when opened opens contacts in the earth connection to cause the lighting of warning glow lamps if the overvoltage persists (Fig. 4, not shown).