Patent GB-599642-A (GB)
Title: Improvements in or relating to testing arrangements for portable electrical appliances
IPC: G01R31/50
Filed: 1945-09-21
Abstract: 599,642. Indicating and locating faults. ORCHARD, F. C. Sept. 21, 1945, No. 24578. [Class 37] A testing arrangement for portable electrical apparatus includes a series of sockets into which the plug of the portable appliance is successively inserted in order to test for earth wire continuity, insulation and running. For insulation tests, a socket Q, Fig. 1, for testing insulation to earth, has its two line terminals connected together and to one line terminal of a second socket P for testing insulation between the leads. A switch S1 is closed in its upper position to apply mains potential to the sockets P, Q through a resistance R2 of 2 megohms. A neon lamp Nt, connected across the terminals being tested, glows if the insulation is satisfactory. With the switch S1 in its lower position the sockets P, Q are connected to terminals El, LT1 for testing by an ohmmeter. For testing the continuity of the earth wire, the plug is inserted in a socket M having its earth terminal connected through a low resistance R1 and the secondary winding of a transformer T1 to an earth clip C, which is fastened to the metal casing. A lamp L1 connected across the resistance R1 lights to indicate that the earthing is satisfactory. The current is of the order of 45 to 50 amps. For testing the running of the machine, the plug is inserted into a socket N connected across the mains in series with a resistance R3 and an ammeter A short-circuited by a push-button switch PB1, which is opened some time after the motor has started in order to read the running current. If a fault develops while the motor is running, an alarm bell B connected across the resistance R3 rings. To test for open circuits, the switch of the appliance is put " on " and the plug is inserted into the sockets F, G, Fig. 2, the, line connections of which are reversed with respect to each other, and the switch S2 is closed. A neon lamp N2 across the line leads glows in the case of an open circuit. To test for short circuits, the switch of the appliance is opened and the plug inserted first in one socket and then in the other. Short circuits from either lead to earth, between leads, or between all three leads are indicated by lamps L2, L3 and the neon lamp N2. For break location, three sockets H, J, K are provided, a single but different terminal of each socket being connected up. The plug is inserted successively into each of these sockets and the three terminals on the portable tool are shorted together and connected to earth at E3. The socket for which the neon lamp N3 fails to glow, indicates the faulty lead. A switch S3 is then operated to connect a search coil SR to a magic eye, as shown, or to headphones, and the length of the cable is traversed by the search coil. An indication is obtained until the break is reached and then suddenly ceases. To locate a short-circuit the flexible cable FC. is disconnected from the appliance and plug and the two leads are connected to terminals LT2, LT3, LT4, LT5 of a Wheatstone bridge which includes a slide wire resistance SW. The terminals Z of the primary winding of a transformer T2 are connected to the slide wire arm and the terminal LT5, the secondary winding being connected to the magic eye or headphones. Power supply is fed to the terminals LT2, LT3 through a small resistance R4, a lamp L6 indicating when power is switched off by a switch S4.