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.
|