GB1176983A - Tell-Tale Apparatus for Use in Conjunction with Electrical Appliances - Google Patents

Abstract

1,176,983. Protective arrangements. H. B. SANDERS. 28 March, 1967 [28 March, 1966], No. 13968/67. Heading H2K. [Also in Division G1] Tell-tale apparatus for use with or in an electrical appliance operating on line and neutral supply and having a supposedly earthed casing, in order to determine the adequacy of the earthing of the casing, includes a pair of indicators mounted on a support and connected in series, the series pair being adapted for connection across the appliance supply, and a contact member in connection with the interconnected sides of the indicators and adapted to be connected in use to the appliance casing either by direct contact or by a probe, whereby the indicators may indicate the adequacy or otherwise of the earthing of the casing. The apparatus may be incorporated in a switched plug socket to which the appliance will be connected. Two differently coloured (green and red) neon lamps 2 and 3 are connected in series between live and neutral terminals, while a lead 9 connects their common junction to an external terminal. Initially, the correct connection of the supply may be tested. With the switch off, the external terminal is connected to earth. Should lamp 3 glow, either the line and neutral connectors are reversed or the earth is live. If, however, no lamp glows the appliance may be connected and the switch turned on with terminal 11 disconnected, whereupon both lamps should glow to show that live and neutral sides of the supply are present at the socket. If terminal 11 is now connected to the appliance casing only lamp 2 should remain on. If both remain on, faulty earthing of the casing is indicated, while if only lamp 3 remains on, the casing is live. The socket arrangement may be embodied in a portable plug (Fig. 5, not shown). In other arrangements, the lead 9 may be kept permannently connected to the casing of the apparatus under test, so that a single (green) lamp normally glows, but either both lamps glow if the earthing of the casing becomes faulty or the other (red) lamp alone glows if the casing becomes live (Figs. 4 and 6, not shown). An audible alarm may replace the red lamp, and a circuit breaker may operate in response to a detected fault condition to interrupt the supply.