Quck answer
A megohmmeter, also known as an insulation tester, is a device used to measure the resistance of electrical insulation. It is commonly used in electrical maintenance and troubleshooting to ensure the integrity of insulation in motors, generators, cables, and other electrical equipment. To use a megohmmeter, follow these steps:
1. Make sure the equipment is disconnected from the power source.
2. Set the megohmmeter to the desired test voltage.
3. Connect one lead of the megohmmeter to the insulation being tested and the other lead to a suitable ground.
4. Press the test button to initiate the test.
5. Read the resistance value displayed on the megohmmeter.
6. Compare the measured resistance value with the equipment’s specifications to determine if it meets the required insulation resistance level.
A megohmmeter, also known as a megger, is a device that measures high electrical resistances. It is commonly used to test the insulation value of electrical wires and motor windings. The prefix “meg” represents a numerical value of 1,000,000, which is also described in ohms for an ohmmeter. A megohmmeter reads the resistance of a wire or motor winding in Megohms, where 1 Megohm equals 1,000,000 ohms. To measure such high resistance values, the meter must generate a very high voltage, with the smallest value being 1,000 volts. Some hand-crank meggers can even supply up to 10,000 volts or more using a small generator within the meter.
Step 1
Before conducting high-voltage tests, ensure that all power to the wires or circuit being tested is removed. The circuit must have no voltage connected to it whatsoever.
Step 2
Disconnect all wires that require testing. This means that any wire requiring insulation value checks must be completely disconnected at both ends of the circuit. The bare copper ends are sometimes covered by a wire nut or a piece of tape to protect equipment or individuals from being shocked. When testing motors, all wires feeding the motor must also be completely disconnected.
Step 3
Connect one of the megger’s leads to the electrical frame or earth ground of the electrical system. When testing motor windings, this lead will be connected to the actual metal frame of the motor.
Step 4
Attach the other lead from the meter to the bare copper end of the wire or one of the motor terminals. Ensure that the other end of the wire being tested is in free air or covered by an insulator such as a piece of tape or wire nut.
Step 5
Turn on the meter or begin to crank the generator handle. It may take 2 to 5 seconds for the high voltage to build inside the wire or motor windings.
Step 6
Read the meter. A reading of greater than 999 megohms is a near-perfect resistance reading for a wire or new motor. Resistance readings less than 1.5 megohms may present problems in old wires or used motors. In most cases, any reading in between these values may be fine, unless trouble has been experienced in these circuits.
Step 7
Continue testing other wires or motor terminals. Once the meter’s high-voltage source has ceased, the wires or motor leads are safe to handle.
FAQ
1. What is a Megohmmeter and what is it used for?
A Megohmmeter, also known as an insulation tester, is a specialized instrument used to measure the resistance of electrical insulation materials. It is commonly used to detect any potential faults or weaknesses in insulation, such as moisture, contamination, or deterioration. By applying a high voltage to the insulation material and measuring the resulting current flow, the Megohmmeter can determine the quality and integrity of the insulation.
2. How does a Megohmmeter work?
A Megohmmeter works by applying a high DC voltage to the insulation being tested and measuring the resulting insulation resistance. The instrument generates a high voltage, typically ranging from 500 volts to several thousand volts, which is then applied across the insulation material. The Megohmmeter measures the current that flows through the insulation and calculates the resistance, which indicates the quality of the insulation. The higher the resistance, the better the insulation.
3. When should I use a Megohmmeter?
A Megohmmeter should be used whenever it is necessary to assess the insulation quality of electrical equipment or installations. It is commonly used during routine maintenance and troubleshooting to identify insulation problems before they lead to equipment failure or electrical hazards. Megohmmeters are especially useful for testing high-voltage equipment, such as transformers, motors, cables, and switchgear, as these components require reliable insulation to operate safely and efficiently.
4. What are the safety precautions when using a Megohmmeter?
When using a Megohmmeter, it is important to follow certain safety precautions to prevent electrical accidents. Always ensure that the equipment under test is de-energized and properly grounded before connecting the Megohmmeter. Use appropriate personal protective equipment, such as gloves and safety glasses, to protect against any potential electrical hazards. Avoid touching exposed conductors or terminals during the testing process. Finally, carefully read and follow the manufacturer’s instructions and guidelines for safe operation of the Megohmmeter.
5. How do I interpret the results obtained from a Megohmmeter?
The interpretation of Megohmmeter results depends on the specific application and the acceptable levels of insulation resistance. Generally, higher resistance values indicate better insulation quality, while lower values may indicate potential insulation problems. The manufacturer of the equipment being tested usually provides guidelines or standards for acceptable insulation resistance values. It is important to compare the measured resistance with these guidelines and take appropriate actions if the insulation resistance is below the recommended threshold.
6. Can a Megohmmeter be used on live circuits?
No, a Megohmmeter should never be used on live circuits. It is designed to measure insulation resistance on de-energized equipment only. Testing insulation on live circuits can be extremely dangerous and may result in electrical shock or damage to the Megohmmeter. Always ensure that the equipment is properly de-energized and grounded before connecting the Megohmmeter for testing.
7. What are the different types of Megohmmeters available?
There are various types of Megohmmeters available, including handheld and benchtop models. Handheld Megohmmeters are portable and suitable for field testing, while benchtop models are larger and provide more advanced features. Megohmmeters may also differ in terms of the voltage range they can generate, the resistance range they can measure, and additional features such as data logging or Bluetooth connectivity. It is important to select a Megohmmeter that is appropriate for the specific application and meets the required testing standards.