You may have heard of the word ‘mega’ in the measurement of various engineering parameters. One of those parameters is resistance, when it is in the range of millions of ohms it is called megaohms is its simplest definition a megger is a mega-ohmmeter, that is an instrument used to measure high resistance values. As technology grows more functionalities have been given to this device, it can measure insulation resistance, absorption ratio, polarization index, voltage, and continuity of the circuit. Megger as is commonly known is actually an insulation resistance meter going by its primary function.
Electrical conduction determines parts of circuits are either live or the dead. In a generation, transmission, distribution, and load equipment, there are designated paths of electrical conduction and all other paths need to be non-conductive. Given that materials are either good, poor, or semiconductors, every electrical equipment ought to consider this factor in design. A poor conductor is used to shield or enclose a good conductor that is live to prevent making other conductive parts (which should not be live) energized. A good example is the metal casing of a motor, which should be at zero potential. Insulators are commonly used in underground cables, building installed cables, overhead cables close to conduction parts, and equipment internal circuitry among other areas. The integrity of insulation is critical because when metallic parts that should not be charged touch a live conductor there results in a short circuit fault, earth fault, damage of equipment, personnel injury, and at times fire. Insulation failure is usually safeguarded by appropriate protection gears.
There are three main parts of this meter whether it is analog or digital technology. These are
- Direct current high voltage generation- megger requires a high voltage supply for it to be able to measure resistance in high electrical stress conductors. This is the main difference from other meters such as a digital multimeter. The idea is that insulation level depends on electrical stress which depends on the applied voltage. To generate the voltage, the following methods are used; a hand-cranked generator, a step-up transformer to boost voltage an oscillating transistor circuit, and a battery.
- Measurement loop – an analog meter has two coils deflected at an angle of 60 degrees. One coil is connected to the meter’s generated voltage and a current flow. The second coil is connected and one end to the source voltage and the other end to the conductor being measured. Digital meters use electronics such as voltage bridges and measurement bridges to achieve the same function.
- Display- The two coils control the deflection of the head pointer on a marked scale to indicate the insulation resistance. Analog to digital converters and microcontrollers are used to process signals. The display of readings is on an LCD or LED screen in numeric figures.
The resistance is measured between a conductor that carries current and a ground point of the equipment. Measurement is always done of a dead circuit. When properly connected the deflected off the pointer shows the reading on an analog scale and numeric display for a digital meter. The higher the reading the better. Low resistance shows the high current is flowing between the live part and the ground which can result in an earth fault- undesirable. High resistance reduces the current that can flow between line and neutral parts and the equipment is working well and is safe to be operated by personnel. To take readings follow the procedure;
- Switch off power
- Discharge the equipment
- Short-circuit ground
- Clean surface of metallic parts to reduce contact resistance
- Work away from live high voltage conductors to reduce induced signals
- Check the calibration of your meters – should be 0 when test leads are shorted and infinite when open.
- Set the appropriate voltage depending on the voltage that the equipment handles.
- Connect L terminal to line terminal of equipment and E to earth.
- Discharge the equipment when the test is done.
Factors affecting the accuracy of reading
Several factors affect the reading of the megger registers. Some of them include;
- The leakage current on the surface of the metallic part (earth terminal) and the insulator itself.
- Capacitance charging current when the equipment is connected- time is being given for reading to stabilize that is when charging is full.
It provides enough electrical stress to the insulator and therefore is able to detect weak points on the insulator during its normal operation. This is what a low voltage digital multimeter could not achieve.
- The meter has been enriched in terms of functionality, and other parameters such as PV insulation, resistance, and voltage can be made.
- It has various ranges of supply voltage from 25V to 1000V for different measurement needs.
- It offers non-destructive tests on the equipment.
- Enables to detect insulation weak points and offers preventive maintenance where other meters cannot detect.
- It is costly
- The high voltage handling is dangerous to technicians and extra caution needs to be in place.
- One has to keep charging the batteries as they drain over time.
A megger is a critical instrument in the electrical world. Needless to say, it is a must-have for all electrical engineers and technicians. in the maintenance of equipment, detecting the fault is 90% of clearing the fault. Meters help in testing and detecting faults as well as controlling the performance of systems. I believe the article has given you a green light on this wonderful, ingenious and critical meter. Go ahead and make practical use of it. Bravo