How do you perform an insulation test on cables?
First, you need to know why we need to run this test. And when will we do this test?
The cable consists of either a single core (conductor) or a multi-core (conductors).
Each conductor has its own insulation, and the cable has an outer insulation surrounding all the conductors.
After pulling the cables and before energizing them, we need to ensure that no fault will happen after the energizing.
This fault can occur immediately after energization, or it may take some time.
It depends on the quality of the conductors’ insulation.
So, we must check and test the insulation quality of the conductors before energizing.
Now, I will walk you through how to perform a cable insulation test, step by step.
First, we will perform this test using an insulation tester.
This device is a portable tool, like an ohmmeter, with a built-in generator that produces high values of DC voltages.
Based on the cable’s manufacturing withstand voltage, we will select the test voltage.
Usually, the applied voltage to the low-voltage cables is 500 to 1000 volts for 60 seconds.
You need to know that in many projects, they will call this test a megger test, but this is not the correct name of the test, as the proper name of this test is the insulation test.
Megger is a brand name (manufacturer) that produces this device to measure the resistance of a cable’s conductors.
And because it was widely used in the projects, people called it the megger test. However, other famous companies are also manufacturing the insulation test devices, like Fluke, Kyoritsu, etc.
The insulation test of low-voltage cables’ main concept is to apply a specific voltage across the two conductors of the cable for a specific time, then measure the resistance between them.
The time of the test is usually from 30 seconds to 60 seconds.
Since the device’s applied voltage is known, it measures the current in the conductor and calculates the resistance.
Usually, the reading will be in values of hundreds of megaohms or Giga ohms or can even reach Tera ohms.
It depends on the quality of the insulation.
You need to know that good insulation has high resistance, while poor insulation has relatively low resistance.
The actual resistance values of the cable can be higher or lower,
Depending upon such factors as the temperature or moisture content of the insulation (resistance decreases with temperature or moisture).
As you can see in this photo, we have a multi-core cable consisting of 4 conductors.
The colors of the cable’s conductors are Red, Yellow, Blue, and Black.
The maximum withstand voltage for the cable’s conductors is 1 kV or 1000V.
Note: After this value, the insulation of the cable’s conductors will start to fail.
Before starting this test, you need to be aware of the following points:
1- You need to ensure removing the cable’s terminals from any panel or breaker.
This is only for the cable you need to test.
2- You need to ensure each cable has a tag or label labeled from both ends.
This is to avoid any mistakes, especially if you have multiple cables pulled at the same location.
3- You need to proceed with the continuity test first on the cable to ensure no touching between the same cable’s conductors.
4- There’s a separation between the cable’s conductors at both ends.
5- At the location where you will use the insulation test and apply the voltage to a cable’s end, make sure the cable’s outer sheath and the conductors’ insulation are removed.
This is to allow connecting the insulation tester’s leads to the conductors for the test.
6- You need to ensure that the insulation tester is calibrated and has a valid calibration certificate.
You need to attach this certificate to the test results during the inspection for the consultant.
The importance of this calibration certificate is to ensure the readings are correct.
7- Before starting the test, you need to ensure that you have adjusted the insulation tester for the required applied voltage and time on the cable.
Now, we will know the practical procedure for applying the test to a low-voltage cable:
A- So, let us say we will apply a DC voltage with a value of 1000V on only two conductors, Red & Yellow.
B- We will apply this voltage for 60 seconds and then record the resistance between these two conductors, Red and Yellow.
C- Once we record the reading in a sheet, we will remove the insulation tester’s leads and connect them to the other two conductors.
We will follow the same steps to check the resistance between the following conductors:
Red with Blue.
Red with Black.
Yellow with Blue.
Yellow with Black.
Blue with Black.
So we will record all the readings and put them on a table in a form that is ready-made for this test.
After that, the quality control will submit an inspection request to the consultant to come to the site and physically check the results.
Once the consultant finds the submitted results almost identical to the results obtained during his presence, he will approve the test.
Now, you are ready to energize the cables.
Follow the steps above for your project’s low-voltage cables.
Expert Advice
Whenever you’re ready to learn about the different MEP systems in a project.
So, you need to enroll now in our Bestseller MEP Package of Online Courses
By purchasing now this MEP Package of Online Courses, you will instant access to +500 lectures in the MEP field.
You can also read:
EMT Conduit Installation & Bending – Your Best Guide in 7 Minutes
Heat Shrink tubing and its importance in Panels – Top Best Practical Guide in 5 Minutes
Emergency Lighting Fixtures – What you need to know in 5 minutes
Electrical Panel Schedule in Projects – 23 Important Notes you need to know
Knockout Hole Punch – How to use it in 5 easy steps
Torque Wrench – Your Best Guide in 4 Minutes
Electrical Panels – No.1 Guide in their Practical Installations On-Site
Top 10 important tools with the electrician in Infrastructure Works
The Common 7 MEP Mistakes & Solutions in the Construction Projects- Your Best Guide
Your Easy BMS Guide “Building Management System” in Projects – No.1 Guide
Your Easy Guide in Understanding the Fire Alarm System from A to Z, 100% you will be satisfied
The Most Important 10 PPE in Construction Projects
Your No.1 Guide for better understanding MEP Infrastructure Networks in construction projects
No.1 Easy Guide about VRF & VRV Systems
