A most uncommon object.
Manufactured by GEC, this is probably from the 1940s. It has never been used.
ELF Circuit Breaker Cat No. X8359
30 Amps 250 Volts AC
Test Button When Pressed a Leakage Fault is Introduced
Breaker Should Operate
Leakage Trip GEC Patent Applied For
Made In England by The General Electric Co Ltd. of England
The resistor at the top is connected in series with the trip coil when the test button is pressed.
The resistor measures 3400 ohms, although the coloured bands indicate it should be 3000 ohms (Orange, Black, Red). The silver band indicates 10% tolerance.
The two larger terminals top right and bottom left are the incoming and outgoing line.
The smaller terminals would be connected to the earth electrode and installation earth. In normal operation, the trip coil is connected between these terminals. The test button disconnects the top left terminal and connects it to the incoming supply via the test resistor to simulate a fault.
The main contact is visible at the top - this is a single pole circuit breaker.
The test button is just a cylindrical rod which directly presses onto the sprung metal contact.
The breaker is tripped by the hinged metal bar just above the coil - when magnetism attracts the bar towards the coil, a tiny lever releases the main switch, opening the contacts.
The complete device is about 75mm diameter (3 inches).
The device is connected to a voltage source to see if it will trip or not. View this video on Youtube.