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How a megohm meter works

time:2020/5/6   source:华天电力  reading:329 time

A megohm meter is an instrument used to measure very high resistances (such as the insulation resistance of a cable) that require a high voltage source to pass a measurable current through these resistances. Thus, a megohm meter is essentially an ohmmeter with sensitive deflection instruments and a high voltage source.


As in the case of low-resistance ohmmeters, the megohm scale represents infinity (∞) when measuring an open circuit, zero when measuring a short circuit, and a half-scale when the unknown resistance is equal to the standard resistance inside the megohm. Elsewhere in the scale, the deflection is proportional to the ratio of the unknown resistance to the standard resistance. The range of the instrument can be changed by switching different values of the standard resistor into the circuit.


Insulation Resistance Tester.png


Also can use a battery-powered megohmmeter, they ohmmeter is essentially a very high resistance, battery voltage (through the electronic circuit) usually increases to the level of 1000 v, so that produces measurable electrical current through the unknown resistance, hold down the power button, can be measured, this operation can minimize the drain current on the battery.


Application of megohm meter


Megohm meters are also used to detect defective insulation in motors and transformers. This is achieved by inducing high voltage in the windings of these electrical components. The introduction of a large voltage will cause the insulation to weaken. Most likely to cause motor failure or transformer short circuit. The voltages used in the megohm insulation test range from 50 V to 5000V. By inducing high voltage in the windings of the motor or transformer, you will be able to detect whether the insulation is deteriorating.


How to use megohm meter


1. The equipment to be tested is isolated from all power supply circuits.


2. Connect wires to appropriate terminals for insulation testing.


3. Set the function switch so that the instrument will sense the required voltage of the electrical components.


4.before proceeding, it is important to contact the manufacturer for insulation testing and electrical component ratings. Too high voltage will lead to warranty invalidation, shortened life or damage to the motor or transformer under test.


Connect the tip of the test wire to the device being tested. Most meters will issue some kind of warning if a voltage is present.


5. Follow the equipment manual and start testing.


When tested between the winding and the ground, the result should be zero resistance. If there is some kind of resistance between the winding and the ground, the ground fault will be caused, so it is important to replace the unit.


When tested between two separate windings, the result should be close to zero. If there is some kind of resistance between the two separate windings, it indicates that the insulation layer has been broken down, which is important for planning replacement equipment.


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