Experiment Name:Test wiring Insulation using Megger.

 

Bundwan Polytechnic

Department of Electrical Engineering

Subject: Building Electrification laboratory

Lab Manual

By- Palash Das

Lecturer in Electrical Engineering

 

Experiment Name:Test wiring Insulation using  Megger.

Test1: Measurement of Insulation Resistance Between Conductors.

Experiment Number:

Aim:   To find out the insulation resistance between conductors .

Experiment Date:

Experiment Submission Date:

Theory:

 Megger consists of a built in megaohm meter and a d.c. generator. The generator is driven by hand or by a motor.

Meggers (Megaohm meters) are available for d c. voltages of 500 V and 1000V. Generally in practice 500V Meggers are used.

0.5 ohm.

Measurement of Insulation Resistance Between Conductors ::

This test is taken up to know the standard of insulation of wires and cables used in the installation, ie, insulation should be sufficient enough to avoid any possible leakage of current to earth. The leakage of the current to earth should not exceed one/five thousand part or 0.02% of, the full load current. The test is conducted on whole installation on the main switch.

Circuit/ Image of the instrument:

 

Procedure: The line terminal  of the meggar is connected to phase terminal of the installation and the 'E' (earth) terminal of megger is connected to neutral link. The handle of meggar is turned on steady speed to generate the required testing voltage. The insulation resistance so measured on the meggar should not be more than that specified for the test to earth, i.e.,

50 MS/ No. of outlet points + switches But in any case value should not be less than 0.5 MS2 and more than 1 ΜΩ.

 

Specification:

Write down the Specification of megger.

Voltage grade- 500 Volt.

Speed:

Maker:

Table/ data value:

Specified value –

Tested value-

Precautions:Before performing insulation resistance test between conductor the position of main switch, fuses and switches should be as under :

 

(a) Main switch in off position with fuses out

 

(b) All switches in on position. (c) All lamps and other appliances should be removed.

 

(d) The fuses beyond the main switch should be in position.

Conclusion:

 

 

 

Viva question:

1.      What is megger?

2.      What are the voltage grade of generally used megger available in market?Ans:500Volt and 1000 volt

3.      What is the difference between insulation resistance and earth resistance?

Ans: Différence between insulation resistance and earth resistance :

Insulation resistance is measured between conductor and earth. It is of the order of several tens/thousands Mega Ohms or infinity for healthy insulation. It is measured by means of megger (mega-ohm meter), with built-in DC generator.

 

Earth resistance is measured for a earthed metallic body or neutral point. It is measured by means of earth resistance tester. For good effective earthing, the earth resistance should be of the order of 0.5 Ohm.

4.      What should be the minimum value of insulation resistance?Ans- 0.5MOhm

5.      What is the value of earthing resistance of a large sub-station? Ans-0.5Ohm

6.      What is the ratio of salt and charcoal mixed around the earth electrode? Ans- 1:5

 

Source:Book-EC&MEE (EIMT) by Tarlok Sing. EIMT by J.B. Gupta.