The Megger MJ359 Insulation Resistance Tester is part of a series of testers called Major Megger. The Major Megger Insulation Resistance Testers are quality crafted, with excellent test voltage regulation, direct measurement readout and an external guard terminal to eliminate surface leakage current from the measurement. Five models are available for general purpose testing which include a low-voltage model.
All are portable, self-contained instruments designed to give rapid and accurate measurements.
Four of the models feature an analog display having four selectable test voltages of 100, 250, 500 and 1000 VDC, measuring insulation resistance up to 2000 MΩ, and an ohm range to 5000Ω. The other model has four test voltages of 50, 100, 250 and 500 VDC measuring insulation resistance up to 1000 MΩ.
The only other difference between the five analog models is the type of power source. Power for models MJ159 and MJ160 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. Power for model MJ359 is provided by 120 VAC 50/60 Hz or low-voltage generator. Model MJ559 is also 120 VAC 50/60 Hz or internal rechargeable batteries and model MJ459 is powered by 6 replaceable "AA" alkaline cells.
As a safety feature, the ac voltage range becomes effective as soon as the instrument is connected to the circuit under test. Therefore, a warning is given that the circuit under test is not de-energized before the instrument is operated. Though calibrated for ac voltage, this range also monitors the automatic discharge feature so that after equipment having capacitance (i.e. a cable), has been tested, an indication can be given that the voltage has discharged to a level that is safe for removing the test leads. Each 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.
Each instrument is built into a strong ABS plastic case with a fold down carrying handle.
The Major Meggers have been designed for convenience and ease-of-use when testing complex or larger electrical installations, and commissioning, servicing or maintaining electrical equipment. They are designed for the safe testing of motors, generators. cables, switchgear, transformers, distribution networks, industrial and domestic installations, components and appliances.
The range of insulation test voltages available allows one instrument to be used for a variety of applications. For example, installations and equipment can be tested at 1000 V d.c. when this requirement is specified, also aircraft and tele-communications equipment can be tested at the relatively low 100 V d.c. and 110 V to 120 V a.c. systems can be tested using 250 V d.c.
Test leads with fused prods are available and it is recommended that these be used when checking that equipment has been isolated from the supply (by performing a voltage test), especially in high energy situations.
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