GB521730A - Improvements in or relating to control and protective systems for electrical apparatus - Google Patents

Abstract

521,730. Protective cut-out arrangements. REYROLLE & CO., Ltd., and TIPLADY, J. Nov. 25, 1938, No. 34351. [Class 38 (v)] A control unit comprising a circuit-breaker A<3> and trip coil A<4>, supplies apparatus J, for example machinery employed in mines or oil - refinery works, over separate gate-end switches P having retaining coils P<4> energized from relays R in a pilot circuit, provided the latter is sound and on voltage. The pilot circuit (comprising, for example, a conductor B<3> and sheath of the main cable, and also used for earthing the apparatus J) contains rectifiers T<1>, G' at the apparatus and control-ends, respectively, for permitting energization of relays R by alternate half-waves of current from transformer E, as described in Specification 521,807, the circuit being established at push-buttons T at the apparatus, or by a drum switch combined with a holding relay in one of the power leads, Fig. 2 (not shown), the latter arrangement preventing automatic restarting if a momentary failure of the supply or pilot circuit occurs. If an earth fault occurs on the pilot circuit, sufficient unidirectional current flows through rectifier G<2> and the relay G, which has a low current-setting, to make the circuit of trip coil A<4>. If an open circuit occurs in a branch pilot circuit, the appropriate relay R, which is of low resistance, will open, or prevent closure of, the associated switch P, and an open circuit on the common pilot circuit de-energizes both relays R. The pilot circuit can also be opened by overload devices in the main and branch power conductors, and an earth-leakage device in the main power conductor trips A<3> directly. Relays G, R are shunted by condensers to prevent sparking due to inductive surges on fault in the pilot circuit. In Fig. 2 (not shown), the coils P' are connected in the branch circuit between the switch P and isolator, the latter having backcontacts in the pilot circuit. The trip coil A<4> is energized from transformer E, the primary of which is on the supply side of the breakercontacts. In both arrangements, the supply may be controlled from the gate-end switch or from a third point, the rectifier-position being suitably modified.