In an industry or an electrical world. Insulation resistance test using megger is a very important test in the electrical industry. Wire/cable insulators are as important as the conductivity of the cables. We test for the conductivity of an electrical cable to know its ability to conduct electricity. Likewise, we test the insulators of the cables to know their ability to resist cable current. We are always interested in increasing the conductivity of a cable by reducing its resistance. In the same way, we are interested in increasing the resistance of the insulator in other for it to withstand the currents of the cable. The instrument that is used to carry out these functions, testing the insulator resistance is a megger meter.
What is a megger meter.
Like said before, a megger is an electrical measuring instrument used in testing an insulator resistance of electrical materials. Is similar to an ohmmeter just that megger is used to test a very high insulator. The megger generates its voltage of about 500 and 1000V in DC form and in a small current, which it injects to a conductor at the cause of testing their insulator. This voltage generated by the megger is safe working with. But can cause electric shock when the two probes come together before discharging the current.
Types of megger meter.
There are two types of megger in and electrical engineering field,
- electronic megger
- manual megger.
precautions measures to take why using the megger
For safe work on testing the insulation resistance test of a conductor using megger, the following precautions measures should be adhered to.
- The equipment under test should be discounted and isolated properly.
- make sure the connection was tight and firm.
- Megger should no be used in an explosive atmosphere.
- why carrying out this test on a cable, make sure the end earth of the cable is not in any way touching with each other.
- The cable end are should be marked properly which all the switches should be blocked out.
- personal protective equipment should be worn why carrying out the test.
How to carry out insulation test on a line/cable using a megger.
Open circuit test is just the same as continuity test on a cable. Our main aim of the test is to know if the cable has been broken inside or cut internally.
So the following steps should follow in other to correctly test for the cable.
The following precaution stated earlier should be strictly adhered to. Why carry out this particular work.
Get the megger testing set and place it on a level surface where one can easily turn it without difficulty.
Set the functions knob to an INSULATION on the megger and turn the handle why observing the pointer of the megger. The pointer will read infinite. Meaning the megger is good and ok to test for the installation test.
The fault that always occurs on our distribution or transmission cables is either a short circuit fault or an open circuit fault. So on the cable that is about to test. Disconnect the cable from the supply and separate each terminal making sure they are not contacting each other.
Connect the megger connecting probes to the megger. Connect the first probe to L (line) and the second to E (Earth) ports. Without the two probes terminals connecting or making any contact with the body or metallic objects turn the handle of the megger. While turning the handle, the pointer read infinite. Showing the connection is ok and now read to be used to test a line.
Then connect the L terminals of the megger to the line of the conductor itself while the E terminal to the Earth terminal of the conductor. And G terminal to the ground.
Then used the handle of the megger to generate emf to the line, by turning the handle at a speed greater than 120 r.p.m. and watch the pointer as it reads the installation value of the conductor.
Discharge the megger after the test by turning the knob button to discharge.
How to determine continuity of a line using a megger.
On a distribution or transmission line especially an underground line. When a fault occurs on such a line. A megger is used to determine the type of fault a line is having. So is when the type of fault is known to be either line to line,line to earth, or even line to neutral before locating the exact place the fault occurs.
The following procedure is used for checking the continuity of a using megger.
On testing for the continuity of a cable with megger, set the megger meter the same it was set during the installation test.
If the line is not a single line, that is if is a multiple core. At the other end of the cable to be check for continuity, join the core with another core. For example, a three-phase armored cable. Let say am testing for continuity of the red phase of the line. All I need to do is to join the red phase line to another line maybe the blue, yellow, or black phase of the cable.
At the other end, separate the cables and connect the megger probes to the cables. One to the red cable and the other to the second cable joined at the order end.
Then turn the megger handle with a speed greater than 120 r.p.m and observe the pointer of the meter.
When the pointer reads infinity, it means that the cable is broken inside. Is either the red cable testing or the second one is open. Meaning is open circuit. While if the pointer reads zero, it signifies the cable is ok and not an open circuit.