Check the cable insulation using megger

Insulation resistance - this is the most important parameter for the cable performance, and as soon as resistance falls below a certain level, then the cable is unusable and must be recognized immediate replacement. In this article I will talk about the reasons that lead to the deterioration of the insulation, and how to check its level with megger.

Table of contents

Why insulation deteriorates.

Safety when working with Megger.

Check megaohmmeter performance.

How to understand that isolation became worthless.

Why insulation deteriorates

There are a number of factors affecting the value of the insulation resistance, namely:

1. Atmospheric conditions. If the cable is constantly surrounded by moisture, even a hairline crack in the insulating material will cause the insulation resistance will worsen. That is why in the rain appliances, connected via cable with poor insulation can just simply do not work.

2. Incorrect cabling. If when laying cables to prevent damage to the insulating material, even a new cable (the formation of moisture) can show the lowest rate of insulation resistance.

3. Obsolescence isolation. Like it or not, even the most high-quality cable with ultra reliable insulation over time will become worthless because of the constant exposure to the environment.

In time to identify problematic cable and prevent an emergency, just used to periodically check the status of such a device as a megohmmeter.

There are both mechanical and electronic measuring instruments. Next I will talk about the process of checking a mechanical cable Megger ES0202 / 2-G.

Safety when working with Megger

For safe testing in occupational safety regulations for electrical installations (as amended by Order of Ministry of Labor of Russia from 2/12/2016 number 74N) heard the following requirements:

Note. Of course, in the secondary circuits to connect and disconnect the ends by means of insulating rods, no one will, but to use insulating gloves still stands.

Check Megger performance

Before direct measurements of insulation is necessary to test the functionality of the instrument. To do this, follow these steps:

- Take the device out of the case and examine it carefully probes. They you do not detect damage to an insulating material;

- Then, insert the probes, regulators are putting as shown in the picture and turn the knob a few times and make sure that the arrow is committed to showing infinite resistance;

- The next step is closable probes with each other (via crocodiles), and also make several turns and make sure that the arrow indicates a zero value;

So, make sure the full serviceability of the measuring device, you can proceed to further action.

Check the cable insulation

1. Before checking cable is disconnected from the electrical installation on both sides and grounding it.

2. Then, attach the megger to the measured conductor and earthing circuit (or to two adjacent veins, if check the insulation resistance between the conductors), the device itself is to be installed on a horizontal surface.

Note. Depending on the position of switch Megaohmmeter ES0202 / 2-T is able to measure resistance up to 50 and up to 10,000 MW.

3. Next, remove the grounding measured lived.

4. We begin to turn the handle and monitor the performance of the device. And if we make the measurement of high-voltage cable, the voltage regulator is set to 2500 V.

If the first limit off scale readings, then translate it to a second limit and the readings will now participate upper scale.

Note. In the first limit values ​​increase from right to left, and the second redistribution from left to right.

5. Then fix the readings. And then a special jumper (normal piece of wire will do) remove the residual charge with the measured conductor (connecting it to the ground) and set the ground back.

6. All measurements of this particular vein or veins is considered ended. Measurements other end of the cable is exactly the same. But under the terms of the work of the megger break between each measurement must be equal to two minutes.

In this case, the voltage range of testing regulated PUE 7th edition n. 1.8.7

Note. If you check the wiring insulation, do not forget to disconnect the neutral conductor of the total zero bus. If you do not, then you will see the weakest isolation area and will not know the true isolation of individual wiring sections.

How to understand that isolation became worthless

According to the requirements of technical documentation lower limit after which the insulation cable replacement unavoidable equals 0.5 megohms

But for better understanding of the degree of the quality of the cable insulation, you can use the following table

This will be enough to understand the degree of deterioration of the isolation of a particular cable.

That's all I wanted to tell you about checking cable insulation with megger. If the article was helpful or interesting to you, you will appreciate it Like.

Thank you for your attention!