Patent GB-233431-A (GB)
Title: Improved means for testing electric circuits
IPC: G01R31/02
Filed: 1924-02-08
Abstract: 233,431. Firley, W. Feb. 8, 1924. Detecting, indicating, and locating faults. -A double line 2a, 2<b>, Fig. 1, to be tested for short-circuits or breaks is connected as a loop in series or, as shown, in parallel with the winding of an electromagnet 1 having an armature 3 which is acted on in the reverse direction by an electromagnet 6, or by a spring. The magnets 1, 6 are connected, in series with a relay 15, resistance 10, and lamp 8 across the mains 7. The line 2b is disconnected from the mains 7 by the removal of a fuse 18, and is connected through the tongue 14 of the relay 15 to one side of the magnet 1, the other side of which is connected to the line 2a through the fuse-block 17. A short-circuit between the lines 2<a>, 2<b> short-circuits the magnet 1 which therefore does not attract the armature 3. Removal of the short-circuit is indicated by the closing of a contact 5 and operation of an alarm 9. The attraction on the armature 3 due to the magnet 6 is adjusted to such a value that the contact 5 is closed when the electromagnet 1 is shunted bv the normal resistance due to 'the lamps &c. in the contact 2a, 2b. This adjustment may be effected by varying the position of the armature 3 with respect to the electromagnets, 1, 6, for example by a screw 24, Fig. 2, operated by a knob 25, the adjustment being indicated by a pointer moving over a scale graduated in ohms representing the normal resistance between the lines 2<a>, 2b, or in amperes representing the normal current. The relay 15 connects the leads 12, 13 only if the leads 12, 16 are correctly connected, the lamp 8 indicating the correctness of the connection. Instead of moving the armature 3 for adjustment, the electromagnet 6 may be variably shunted. Alternatively an adjustable spring may be employed instead of the electromagnet 6 to provide the opposing force on the armature 3. In the modification shown in Fig. 4 the armature 3 of the electromagnet 1 is in the form of a twoarmed lever pivoted at 3a and attracted at its opposite end by an electromagnet 6 which forms a single two-limber body with the magnet 1. In order to vary the attraction of the electromagnet 6 on the armature 3, the core 6<a> is adjustable axially by means of a cam 33 engaging a pin 34 on the core 6<a>, which pin is pressed against the cam by a spring 35. A pointer 36 moves over a scale graduated in resistance or current. The electromagnet 6 may be replaced by a spring, in which case the core of the electromagnet 1 may be adjustable, or the pull of either of the electromagnets 1, 6 may be varied by moving an iron plate of varying cross-section between the pole face and the armature. The armature 3 is polarized either by a permanent magnet or by an electromagnet 38. When the apparatus is used with alternating current a condenser is preferably connected in parallel with each magnet winding in order to tune the circuits so that the maximum current flows. A two-way switch 41 enables the alarm 9 to be connected in series with a second contact 5<a> in order to test for interruptions in the closed loop 2<a>, 2b. On the interruption being repaired the electromagnet 1 is de-energized, causing the contact 5a to be closed and the alarm sounded. Resistances, measuring. - A resistance to be measured is connected to terminals 40 as a shunt to the electromagnet 1 in place of the loop 2a, 2b, the leads 7 being connected to a test current supply. The cam 3 is adjusted until the armature 3 just operates and connects the alarm 9. The reading of the pointer 3 then indicates either the resistance directly or the current, from which the resistance can be calculated.