The Major Megger® insulation and continuity testers MJ10 and BM12 are modern, portable, self-contained instruments designed to give rapid and accurate measurements. They have four d.c. insulation testing voltages 100 V, 250 V, 500 V and 1000 V, and a single insulation resistance range of up to 2000 Megaohms. Similarly, continuity is measured, with either polarity, on a single 0-4 ohm range.

Power for the MJ10 instrument is derived from a low voltage, hand-cranked generator which has been designed to be easy to turn even under full load conditions. The low voltage generator is connected to an electronic inverter to provide a very stable test voltage. Accuracy of measurement is unaffected by variations in the generator cranking speed and the test voltage is maintained at its rated value to as low as 1 Megaohms at 1000 V or 0,5 Megaohms 500 V. The BM12 has a similar specification and circuitry to the MJ10 but it is powered by internal, replaceable batteries. As a safety feature, the a.c. voltage range becomes effective as soon as the instrument is connected to the circuit under test. Thus, a warning is given that the circuit is not de-energized before the instrument is operated. Though calibrated for a.c. voltage, this range also acts as a monitor for the automatic discharge facility, so that after equipment having capacitance, e.g. a cable, has been tested, an indication can be given that the voltage has discharged to a level which is safe for removing the test leads.

Every instrument has a guard terminal which may be used to prevent the effects of surface leakage from influencing the readings. This is achieved by diverting the leakage current through the guard terminal and away from the instrument’s measuring circuit. The continuity measuring function has the facility for reversing the polarity of the test current, without altering the test leads. The operator may then eliminate the effects of standing e.m.fs. etc. by averaging the measurements made with both polarities. Each instrument is robust, lightweight and built into a strong, polycarbonate case. All readings are given directly on an analog meter with a white on black scale, an orange ‘dayglow’ pointer. The ranges are selected by a rotary switch and a ‘Test’ push-button is pressed to execute a test. On the MJ10 the button is pressed and the generator handle turned simultaneously. Batteries for the BM12 fit into a compartment accessible from the bottom of the case and a check of the battery condition can be made by operating a second push-button.

The test leads supplied have right angled connectors and, when fitted to the instrument’s recessed terminals, the carrying handle folds down over them. These features help to ensure that the leads are not accidentally pulled out of the terminals during a test.