GB429503A - Improvements in or relating to earthing arrangements for electric apparatus - Google Patents

Abstract

429,503. Protective cut-out arrangements. REYROLLE & CO., Ltd., A., Hebburn-on-Tyne, LEYBURN, (formerly LEBEN), H., 212, Wingrave Road, and CLOTHIER, G. D., 87, Jesmond Park West, both in Newcastle-on-Tyne. Nov. 23, 1933, No. 32724. [Class 38 (v)] [See also Group XXXVI] Switches, latching and unlatching.-In electric apparatus having its frame or other metalclad part solidly earthed, an indicating, tripping, or lock-out device has its operating circuit connected in series with a source of E.M.F., and a testing switch or other control means is provided for connecting this circuit between the frame and a point on the supply system at earth potential, so that a current flows through the main earth of the supply and the direct earth connection of the apparatus to cause operation of the device. Fig. 1 shows a portable equipment for testing the earth connection G' of the apparatusframe G by means of a flexible cable F having a pin F<1>. By means of a press-button switch A<3>, the neutral conductor C' of the supply to the apparatus can be connected to the secondary A<1> of a transformer, the other side of which is connected over a relay-coil D and a scraping earth contact B<4> on the plug B to the socket earth C<3>. If this circuit is in order, an indication is given at D<2> and relay contacts D' close and make the circuit of an indicator E over the circuit F<1>, G, G<1>, main earth of the supply, C', A<3>, A'. Failure of the indicator to operate shows that the connection G' is faulty. Lamps D<3>, D<4> under control of a switch D<5> serve to indicate abnormal voltage on the neutral C<1> and correct connection of the plug to the socket and a fuse A<2> serves as a protection against incorrect coupling, but the switch may be dispensed with so that an automatic indication is given immediately on plugging-in. Modifications of the above are as follows. The winding A<1> may be connected between B<4> and the contacts D', the coil D being connected between A<2> and the unearthed side of A<1>. The lamps and switch A<3> can be omitted. The indicator may be replaced, or supplemented, by an ammeter in the plug box or free end of F, and the latter may also be adapted to mount the lamps. The preliminary test on C<3> may be omitted and the relay D be accordingly dispensed with, in which case the indicator E is connected directly to A<1>. In Fig. 2, the apparatus G<2> is connected over a switch H to the neutral J earthed at J<5> and line J<1>, and switches K, K' connect the primary A and secondary A<1> of a transformer respectively across J, J<1> and between J and the coil E<1> to earth G<1>. This arrangement, shown structurally in Fig. 3, automatically tests the earthing circuit every time the handle H<3> is raised to close H. The coil E<1> is energized by the momentary bridging of contacts K, K<1> and so holds up its armature M<1> while the gap H<4> in the hub H<2> is beneath the arm M<3>. If the earth circuit is faulty however, the coil E<1> is not energized and a spring M<5> pushes the arm M<3> through the gap H<4> and so brings an indicator M<4> into view through a window L<3>. The handle H<3> may either be allowed to complete the closing movement, or it may be prevented from such further movement by forming the hub so that it is obstructed by M<3>. A hand-operated switch may be substituted for the automatic switch K, K<1>, Fig. 4 (not shown), and the coil E<1> may be either an indicator coil or a trip coil for the main switch, in which latter case the manual switch may be employed for normal tripping and at the same time demonstrate the soundness of the earthing. Fig. 5 shows the invention applied to a core-balance transformer, the secondary P<1> of which, when energized by leakage occurring, energizes the trip coil H<5>. A tapping R<1> of a resistance R connected across the supply is connected over a manual switch N<3> to a further winding E<3> and thence via the bonding lead G<3> to the earthed frame G, so that when N<3> is depressed and the earthing is sound, a voltage is impressed on E<3> which energizes the trip coil H<5>. Fig. 7 shows an arrangement wherein a portable apparatus is prevented from being plugged-in unless the earth circuit is sound, in which case a coil E', which is energized by partial application of the plug S<2> to the socket S<1> withdraws a bolt S and so allows complete insertion. The earth lead is brought to the socket S<1>, thence by scraping earth connection to the plug S<2> whence a lead O<1> in the cable O is connected to the apparatus frame G. A return lead O<4> therefrom is taken through cable O<5> to pin S<3>, through coil E' and the secondary of a testing transformer to the neutral conductor J. In a simpler arrangement, Fig. 6, (not shown), which does not include the apparatus G itself within the tested zone, has a single testing lead in the apparatus supply cable connected to the casing of a plug, testing being effected by applying the plug casing to S<3> before it is connected to the apparatus. A further modification, dispensing with the cable O<5> and pin S<3>, Fig. 8, (not shown), has the return lead 04 connected to an insulated scraping contact on the plug S<2>, which co-operates with an insulated, armature-carrying contact on the socket S<1> at partial insertion and so energizes the coil E<4> which retracts the contact to allow complete insertion of the plug. Proper earthing plug-and-socket contacts of appropriate length may replace the scraping contacts. In all cases the testing voltage is so arranged that if the earthing is imperfect, the frame potential to earth cannot exceed 30 to 40 volts. The apparatus may comprise several independent elements the frames of which are bonded together. The Provisional Specification states also that where the apparatus, e.g. a cooker, already has an indicator lamp, the lamp may be adapted to the invention by providing a manual switch which in normal position connects the lamp across the supply. In a first operative position a dimming resistance is placed in series with the lamp and in a second operative position the resistance is shunted by the connection to the earthing circuit, which, if sound, restores the normal brilliancy of the lamp.