to the 16th Edition IEE Regulations
   
   
   
 
 

chapter 5
Earthing

chapter 6
Circuits

Inspection and Testing
  8.1 - Introduction 8.5 - Insulation tests
  8.2 - Inspection 8.6 - Earth testing
  8.3 - Testing sequence 8.7 - Test instrument requirements
8.4 - Continuity tests 8.8 - Supporting paperwork


8.4.4 - Measurement of R1 + R2

In {5.3.6} and in {Table 5.5} we considered the value of the resistance of the phase conductor plus that of the protective conductor, collectively known as (R1 + R2). This can be measured with a low resistance reading ohmmeter as described in item 1 of {8.4.1} (strictly the phase, rather than the neutral resistance is being measured here, but there should be no difference) and will be necessarily at the ambient temperature which applies at the time. Multiplication by the correction factor given in {Table 8.6} will adjust the measured resistance to its value at 20C.

Table 8.6 - Correction factors for ambient temperature
Test ambient temperature C
Correction factor
5
1.06
10
1.04
15
1.02
20
1.00
25
0.98

These will be the resistance values at 20C and must be adjusted to take account of the increase in resistance of the conductor material due to the increased temperature under normal operating conditions. This second correction factor depends on the ability of the insulation to allow the transmission of heat, and values will be found in {Table 8.7} for three of the more common types of insulating material.

An alternative method for measuring (R1 + R2) is to carry out a loop impedance test at the extremity of the final circuit and to deduct the external loop impedance for the installation (Ze). Strictly it is not correct to add and subtract impedance and resistance values, but any error should be minimal.

Table 8.7 - Temperature correction factors for insulation
Insulation type
Correction factor
-
Bunched or as a cable core
Not bunched or
sheathed cable
p.v.c.
1.20
1.04
85C rubber
1.26
1.04
90C thermosetting
1.28
1.04

 

 

 

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Extracted from The Electricians Guide Fifth Edition
by John Whitfield
Published by EPA Press Click Here to order your Copy

Click here for list of abbreviations