What is a Megohmmeter?

Megohmmeters, more commonly known as “Megger Meters” are used to measure high insulation resistance.  Megger has become the generic description for a high voltage, low current insulation tester.  Ohmmeters and multimeters seem to be capable of similar measurements; however, only a Megger type instrument can test the quality of the insulation at or above its operating voltage.  Two basic tests are possible; insulation to ground and insulation between conductors.

How Do Megohmmeters Work?

The megger consists of a DC generator and a direct reading ohm meter.  The moving element of the ohm meter has two coils (A& B) which are firmly mounted to a pivoted central shaft and free to rotate over a C-shaped core.  Flexible leads connect these coils.  When current provided by the hand generator flows through Coil B, the coil will set itself at right angles to the field of the permanent magnet.  When the test terminals open and an infinite resistance is given, no current flows in Coil A which causes Coil B to govern the motion of the rotating element, causing it to move in the extreme counter-clockwise position, which is marked as infinite resistance.  The choice of test voltage is usually determined by the operating voltage of the circuit under test.

What are Safety Precautions When Using a Megohmmeter?

* Never touch the test leads when the handle is being cranked

* Always deenergize and discharge the circuit completely prior to connecting the meter

* Use megohmmeters for high-resistance measurements only

* When possible, disconnect what is being checked from other circuitry before using the meter.

What Level of Voltage can a Megohmmeter Test?

The megohmmeter will test 100, 250, 500, 1000 or 2,500 Vdc, depending on the model.

What is the Difference between Analog and Digital Megohmmeters?

The difference lies in display and interface technologies.  Analog meters display values on a dial, normally using a needle or pointer.  Digital meters display results with a numeric readout.

What are Common Features of Megohommeters?

Megohmmeters can have remote controls to permit control by computers or other instruments.  Some have built in calibration, allowing operators to calibrate test instruments without having to remove them from the testing location.  Most meters have warning indicator lights that work with audible buzzers and alarms to indicate a pass or fail.  Megohmmeters are often interfaced with programmable logic controllers.  Output frequencies are also commonly available, and rapid cutoff circuitry to protect the device from extremely high levels of voltage or current.

What are Applications for Megohmmeters?

A variety of applications exist for this useful tool.  Many meters are used to test the insulation resistance of wires, cables, transformers, and electrical motors.  They can also be used to verify devices such as aircraft fuel pumps meet manufacturer IR specifications.  Other electrical components can be tested as well, including cable reels, rectifiers and solid-state diodes.

Megohm Meters are devices that are typically used to perform insulation resistance tests, otherwise known as Megger tests.   These meters are scaled to read in megohms or millions of ohms.  Tests performed with megohm meters are aimed at determining the effectiveness of the insulation in resisting the flow of electrical current.  The voltage will typically range from 1000V to 5000V.  The megohm meter can be used to check the quality of the insulation both when the product is being used, or when it is being manufactured.

When using a megohm meter, the equipment under testing is usually connected to the meter, and the voltage is ramped from zero to the final value desired.  Once the selected voltage value is reached, it is maintained at that value for a prescribed period, usually for a few seconds before the resistance test is measured.  A higher insulation resistance reading indicates a better insulation.

Megohm meters are classified as two types; the handheld and the bench top models.  The handheld version is widely used in field and maintenance applications.  The bench top model is primarily used in research and production environments.  Aeorspace, defense, industrial, automotive, electronics, and medical are other applications.