Electric Power Engineering: Testing Insulation Resistance of Conductors, Accessories and Equipment

Testing Insulation Resistance of Conductors, Accessories and Equipment

This is probably the most used. 
Affectionately known as meggering .

An insulation resistance test 

is performed in order to ensure that the insulation of conductors, accessories and equipment is in a healthy condition, and will prevent dangerous leakage currents between conductors and between conductors and earth. 
It also indicates whether any short circuits exist.

Insulation resistance, as just discussed, is the resistance measured between conductors and is made up of countless millions of resistances in parallel(Figure).

The more resistances there are in parallel, the lower the overall resistance,and in consequence, the longer a cable the lower the insulation resistance.

 Add to this the fact that almost all installation circuits are also wired in parallel, it becomes apparent that tests on large installations may give, if measured as a whole, pessimistically low values, even if there are no faults.

Under these circumstances, it is usual to break down such large installations into smaller sections, floor by floor, sub-main by sub-main etc. 
This also helps, in the case of periodic testing, to minimize disruption. 

The test procedure is as follows:-

First step:

  1. Disconnect all items of equipment such as capacitors and indicator lamps as these are likely to give misleading results.
  2. Remove any items of equipment likely to be damaged by the test, such as dimmer switches,electronic timers etc.
  3. Remove all lamps and accessories and disconnect fluorescent and discharge fittings.
  4. Ensure that the installation is disconnected from the supply, all fuses are in place, and MCBs and switches are in the on position.
  5. In some instances it may be impracticable to remove lamps etc. and in this case the local switch controlling such equipment may be left in the off position.

Second step:

  1. Join together all live conductors of the supply and test between this join and earth.
  2. Alternatively, test between each live conductor and earth inturn.

Third step:

  1. Test between phase and neutral. 
  2. For three phase systems, join together all phases and test between this join and neutral. 
  3. Then test between each of the phases. 
  4. Alternatively, test between each of the live conductors in turn. Installations incorporating two-way lighting systems should be tested twice with the two-way switches in alternative positions.
  5. Table 19.5 gives the test voltages and minimum values of insulation resistance for ELV and LV systems.

System Test Voltage Minimum insulation resistance
SELV and PELV 250V DC 0.25 M ohm
LV up to 500V 500V DC 0.5 M ohm
Over 500V 1000V DC 1 M ohm

If a value of less than 2M ohm
is recorded it may indicate a situation where a fault is developing, but as yet still complies with the minimum permissible value. 
In this case each circuit should be tested separately andeach should be above 2M ohm.
An installation comprising six circuits have individual insulation resistances of 2.5M ohm , 8M ohm , 200M ohm, 200M ohm , 200M ohm and 200M ohm
, and so the total insulation resistance will be:

  • 1/R= 1/2.5 + 1/8+ 1/200 + 1/200 + 1/200 + 1/200        = 0.4 + 0.125 + 0.005 + 0.005 + 0.005 + 0.005 = 0.545
  •  R= 10.545 = 1.83M ohm
This is clearly greater than the 0.5M ohm minimum but less than 2M ohm
, but as all circuits are greater than 2M ohm the system could be considered satisfactory

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