GB427183A - Improvements in or relating to earth-leakage protective arrangements for electric apparatus - Google Patents

Abstract

427,183. Protective cut-out arrangements. REYROLLE & CO., Ltd., A., Hebburn-on-Tyne, and LEYBURN, (formerly LEBEN), H., 212, Wingrove Road, Newcastleon-Tyne. Oct. 17, 1933, No. 28650. [Class 38 (v)] An earth-leakage protective arrangement comprises a direct earth connection from the apparatus frame, a tripping or indicating device connected between the frame and another earth or the neutral supply conductor, and means for applying a testing potential continuously to a tapping point on the coil of the device. Currents due to the applied potential normally balance each other, but the device is operated in the event of a fault in the protective or earthing circuits. In Fig. 1, apparatus having a casing C and windings C<1> is connected across a line A<2> and neutral A<3> of a supply having an earthed star-point B<1>. The casing C is earthed at E and connected through a normally-closed testing switch G and sensitive winding D<1> to the neutral A<3>, the line A2 being connected to a mid point tap D<2> through an indicator lamp F or other resistance whereby a testing potential is continuously applied while breaker D is closed. The coil D<1> may be a trip or indicator coil, or the coil of a trip or indicator - controlling relay, and normally currents flow from A<2> through its halves in such a way as to balance one another, and the continued lighting-up of F indicates that the protective circuit is sound. If any part of the earthing circuit fails, however, or if a framefault occurs in C, the balance is upset and the breaker is tripped, so giving an indication. Fig. 2 shows a form in which the coil D<1> is connected between casing C and an auxiliary earth H, and the tap D<2> is shown connected to the line A<2> on the supply side of D whereby the testing potential is applied even with D open. If the latter is a loose-handle breaker, closure will be prevented unless the protective circuit is sound, but an interlock device may be connected between A<2>, D<2> in addition to, or instead of, the lamp F to prevent closure if the coil of the device is de-energized due to a break in this connection or failure of the supply. If the connection is made to the apparatus side of D, the lamp F can replace the indicating lamp normally provided on, for instance, domestic cookers. When the resistance of E<1> is low enough for fuses J to protect against leakage, the protective circuit of the invention will give back-up protection if for any reason the fuses fail to blow, and will give an indication of a break in itself whether or not there is a fault in the apparatus. The invention can also be adapted to include in the area of protection the casing of the breaker D by including that casing in series in the earth circuit E<1> and connecting the ends of D<1> to that casing and to the auxiliary earth H, Fig. 3 (not shown). When applied to portable apparatus, a trailing cable is provided having two extra cores, one for the main earth and the other for the trip coil connection.