GB551059A - Improvements in earth leakage electric protective arrangements - Google Patents

Abstract

551,059. Protective cut-out arrangements, BRITISH THOMSON-HOUSTON CO., Ltd., TRENCHAM, H., and MORTLOCK, J. R. Oct. 30, 1941, No. 13986. [Class 38 (v)] In a protective arrangement for metal-clad switchgear (which may be insulated or not from earth) the framework 1 is connected to earth at 6 over the primary winding of a current transformer 7, the secondary of which energizes the current winding 8 of a power-directional relay so as to respond only to earth-leakage current from the framework to earth. To avoid operation by leakage in the cables, the cable-entry boxes 4 are lightly insulated from the rest of the gear and a connection 11 taken therefrom to the point A on the conductor 6 below the current transformer, so that if any current flows from 16 through the transformer 6 to the framework and thence to earth (supposing the framework to be earthed), it will be in the non- tripping direction. The polarizing winding 10 of the relay 9 may be located in, or transformerconnected to, Fig. 2 (not shown), a further earth connection. Winding 10 may alternatively be energized by the current in the neutral-earth connection of a generator or transformer supplying the gear, Fig. 3 (not shown), or from conductor 6 between the point A and earth, Fig. 4, the conductor 6 being therein divided and connected to two switchgear-sections and including two transformers 7, 7<SP>1</SP> feeding separate coils 8, 8<SP>1</SP>. The directional-relay contacts 12 may be in series with contacts of a leakage relay in the neutral-earth connection of a generator.