How to check the cable jacket with a megger?

  • Dec 26, 2019

Almost any problem in a home or an industrial electrician can be prevented. Especially such unpleasant fault as a short-circuit, which occurs due to deterioration of the cable insulation. To timely identify leaky insulation layer can use a special device - megger, and this article is a detailed description of how to use it correctly.

What is a megger?

Megaohmmeter - narrow section measuring device, designed especially for determining the resistance of conductor or insulating material. Modern products, unlike issued in the last century, have a compact size, allowing electricians and linemen always keep the device at hand.

Figure 1: The Instrument of the old type
Figure 1: The Instrument of the old type

Testers are:

  • Analog. Old devices with a measuring graph and arrow, which is driven by the rotation of a small dynamo. Such devices differ from modern, high precision, but the main disadvantage lies in the need for continued rotation of the handle.
  • Digital. According Megger needs independent power supply (battery), and the charge given special pulse generators. The main advantages: small size and the ability of the test.
Figure 2: The modern model megaohmmeter
Figure 2: The modern model megaohmmeter

Additional functional modern megger varied: this may be to maintain the measurement operation in several modes of measurement or a simple backlight screen. Such devices are popular for its compact size and low weight.

How to check the cable jacket with a megger?

The Megger is used dangerous to human life and health of the voltage from 500 to 2500, so it is necessary to perform measurements only in the means of protection.

Important! Before carrying out the insulation measurements, disconnect the voltage to the measured area.

The sequence of checking the insulation of the cable trohzhilnogo using megger:

  1. Dilute the conductor cable wires to facilitate the measurements. If necessary, clean the wires on the insulation.
  2. Connect the test leads to the Megger, touch one probe and another scroll knob several times. If the arrow shows "0", it means that the device is fully functional and the conductor resistance is equal to zero. If you do not touch the probes to the conductor, the value on the Megger should go to infinity.
  3. Touch one probe to the first current-carrying conductor, and the second - to the second scroll handle several times. If the value goes to infinity, which means that the insulation is OK and does not have any damage.
  4. Holding the first probe to a first selected conductor, the second probe to move on a conductor and a third scroll handle several times. An infinite resistance indicates the serviceability of isolation between the two data conductors.
  5. Now move the first probe to the second conductor, and the second probe is left in its place and make a measurement. Icon infinity will serviceability insulation between the second and third conductor.
Figure 3: Modifications megger connection for measuring various parameters
Figure 3: Modifications megger connection for measuring various parameters

If the cable has an extra protective coating made of any alloy of metal or steel, then it should also check for a possible breakdown of one of the conductors. Checking procedure is the same as with other conductors.

When megger shows not infinity, and "0", it means contacting live parts to each other. In the same way it is possible to determine the integrity of the conductors or as often referred to this process - "ping" wire.