Electrical installations and wiring systems
353
The resistance the long test lead subtracted from these readings to
give the resistance value the CPC.
This test can done using low-voltage transformer and suitably con-
nected ammeters and voltmeters, but number commercial instruments
are available such the Clare tester, which give direct reading ohms. For satisfactory test result, the resist-
ance the protective conductor should consistent with those values
calculated for line conductor similar length and cross-sectional area.85
Trunking 0.14
32 0.1 Chapter 12).25
25 1.05 less (IEE Guidance Note 3).5 times the design current the circuit, but not greater than 25A.
Values resistance per metre for copper and aluminium conductors are
shown Table 12.526
100 100 0.
Because fault currents will flow around the earth fault loop path, the
measured resistance values must low enough allow the overcurrent
protective device operate quickly. The result recorded installa-
tion schedule such that given Appendix the IEE Regulations.
Example
The CPC for ring final circuit formed 1.5mm2
copper conductor 50m approximate
length.
.
If the inspecting engineer has grounds question the soundness and
quality these joints then the phase–earth loop impedance test described
later this chapter should carried out.
Where steel conduit trunking forms the protective conductor, the stan-
dard test described above may used, but additionally the enclosure must
be visually checked along its length verify the integrity all the joints.337
. Ω
e
erefore,
the resistance mm
50 10
0 605
3
. voltage not
greater than 50V the frequency the installation and current approach-
ing 1.c. Ω
Table 14. From Table (shown Table 12.
If, after carrying out this further test, the inspecting engineer still questions
the quality and soundness the protective conductor formed the metallic
conduit trunking then further test can done using a.12.
Resistance/metre for a
mm Copper conductor /m
Th
1 10
2
.12 Resistance Values Some Metallic Containers
Metallic sheath Size (mm) Resistance 20°C (mΩ/m)
Conduit 1.1.949
75 0. The resistances some other metallic containers are
given Table 14. Determine satisfactory continuity test value for the CPC using the value given in
Table the Site Guide.
A satisfactory test result for the bonding conductors will the order of
0
