Thursday, December 8, 2022

How to choose a megger according to the work to be done

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The megger It is a measuring instrument that allows to quantify, in MΩ, the degree of isolation of an electrical or electromechanical element; It is frequently used in the manufacture and maintenance of electric motors, transformers, generators, electrical insulations, cables, etc.

Your choice, when purchasing one, depends on different criteria, which we will see below.

Criteria for choosing a megger

There are various activities within the electromechanical field where it is necessary to have this measuring instrument; by case, the maintenance of electric motors, medium and high voltage transformers, the manufacture of cables, the manufacture of insulation for medium and high voltage cabling, the installation of medium voltage poles, railway cabling, and so on.

Within each of these fields there are two possibilities: that the work is within a workshop or in the field. Finally, it is important to determine the ohmic (mega-ohmic) range needed for the readings: will the same product always be measured or will we have different electrical elements?

1. Field work or laboratory work?

If the measurement work will be carried out in the field in a workshop or laboratory environment, it is preferable to use a megger battery-powered, since the task of handling it will be much more comfortable: it can be operated with one hand, while taking notes with the other. Newer models only require the press of a button, and even offer the ability to transmit measurements via Bluetooth.

The megger Battery-powered are also lighter and generally allow operation at different voltages, making them more versatile as well. On the other hand, they require recharging or changing the batteries quite frequently —because, due to their operation, they consume a lot of energy—; This disadvantage, in a work environment such as a laboratory or workshop, does not presuppose a major problem.

On the contrary, field work makes it essential to have a megahmmeter provided with crank generator. They are heavier and bulkier than megger battery operated, but can be operated indefinitely as long as electrical current is generated by means of its crank. The disadvantage is that they usually allow only one working voltage, which is why it is necessary to take the precaution of knowing in advance the working voltages of the electrical elements to be measured, in order to have the megger suitable.

2. Field of application

The megger It is a versatile instrument that allows you to measure the insulation of different elements. It will depend on them which voltage range we must choose. What does the voltage influence? Simple: it goes according to the specifications of the item to be measured. If it is a low voltage cable, it will not be necessary to test it with high voltage; On the contrary, if we are measuring the resistance of a ceramic insulator for high voltage poles, we will surely need to reach voltages as high as 5kV.

Some examples of items that are usually measured with these devices (meggers) are: low, medium and high voltage cables, transformers, electric motors, ceramic insulators and panel housings, motors, machines, etc. It is also possible to measure assemblies such as the railway hatching (to guarantee effective isolation in the assembly of track circuits), street lights (to comply with safety regulations and verify that, under no circumstances, the pole is energized), machine tools (check that no metal surface is energized).

In the photo above it is possible to see a fish plate attached to two rails. In order to build a track circuit that detects the passage of a train, it is necessary to perfectly isolate both rails by means of an insulating polymer. You can see, on both sides of the fish plate, two terminals ready to receive the medium voltage cables that analyze the voltage level and actuate a relay when the train bogies close the circuit.

For a job of this type it is necessary to have a megger that delivers at least 2 kV at the terminals. In general, the following table allows you to choose the megger suitable according to the working voltage of the device to be measured.

Working voltage (in V)
Measurement voltage (in V)
Up to 100 From 100 to 250
Between 440 and 550 From 500 to 1000
2400 From 1000 to 2500
Over 4160 From 1,000 to more than 5,000

3. Ohmic range

The megger They work by emitting a potential difference between their terminals, in the range of thousands of volts. Based on that, the equipment makes a resistance measurement. But not all equipment offers the same measurement ranges.

Usually a megger with an effective range of 5 to 1000 MΩ, although sometimes it will be necessary to be able to measure in higher ranges, such as gigaohms, (GΩ) and even teraohms (TΩ) (for example: measurement of high voltage transformers).

As a general rule, the following values ​​are taken as acceptable for any dielectric. The measurement is carried out in two stages; for example, a 10-minute long measurement is taken, followed by a one-minute measurement. With both values ​​a quotient is obtained that will be the one we will look for in this table.

Dielectric condition
Ratio 60 s / 30 s
10 m / 1 m ratio
Dangerous 0 <1
Doubtful 1 to 1.25 1 to 2
Okay 1.4 to 1.6 2 to 4
Excellent > 1.6 > 4

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