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 20°C.
|
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 20°C 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
|
| 85°C rubber |
1.26
|
1.04
|
| 90°C thermosetting |
1.28
|
1.04
|
|