BS 7671 is clear about the requirements for the cross-sectional area (csa) of protective conductors but says nothing directly about their length.
Take, for example, the requirements for main protective bonding conductors in Table 54.8 of
Regulation 544.1.1. Instructions are stated for the minimum csa but no requirements are given for their maximum length<sup>1</sup>.
As a general guide, main bonding conductors should be as short as possible but there are situations where they may need to be much longer than normal.
Maximum length of main bonding conductors
One way in which the maximum length can be determined is to use the figure of 0.05Ω given in
IET Guidance Note 3 (GN3), Inspection and Testing, Section 2.6.5. This gives the maximum resistance of a main protective bonding conductor measured from end-to-end when carrying out test method 2.<sup>2</sup>
The 0.05Ω figure is sometimes used as a rule of thumb but the origin of this – and if it is being used for its original purpose – is uncertain<sup>3</sup>.
Worked Example 1 shows how the 0.05Ω figure can be used to calculate a theoretical maximum length of a main bonding conductor.
Worked Example 1
Calculate the maximum length of a 10mm<sup>2</sup> main bonding conductor.
Solution
Obtain the resistance per metre of 10mm<sup>2</sup> cable. Table I1 of the On-Site Guide (OSG) – and Table B1 of GN3 – gives the resistance as 1.83 mΩ/metre (0.00183 Ω/m). The maximum length L of a 10mm<sup>2</sup> main protective bonding conductor can be found from:
0.05 Ω
L = 0.00183 Ω/m = 27.3 metres
However, if a 10mm<sup>2</sup> main protective bonding conductor is longer than 27.3 metres, it is not possible to say with certainty that it is unacceptable because, as a conductor increases in length its resistance will also increase.
Appendix B of GN 3
In order to get expected resistance values for cables and connections when using Test Method 2, GN 3 refers to the resistance data in Appendix B. Table 1 uses the data from Appendix B for some typical copper main protective bonding conductors showing the resistances for various lengths.
Comparing Table 1 with the 0.05Ω figure
It can be seen in Table 1 that the 0.05Ω figure is acceptable for some cable sizes and lengths but not for others.
The 0.05Ω figure used in Worked Example 1 resulted in a maximum length of 27.3 metres for a 10mm<sup>2</sup> copper main protective bonding conductor. By comparison, Table 1 indicates that the resistance of a 10mm<sup>2</sup> main protective bonding conductor between 25 and 30 metres in length is 0.05Ω. In this case the figure of 27.3 metres in Worked Example 1 is accurate<sup>4</sup>.
Any test of a disconnected protective bonding conductor should indicate its continuity is satisfactory and its resistance is in relation to its csa and length. This may be greater than 0.05Ω.
Conclusion
Table 1 indicates that the 0.05 Ω figure is accurate for the larger cables sizes – 16mm<sup>2</sup>, 25mm<sup>2</sup> and 35mm<sup>2</sup> – up to a length of around 45 metres.
For 10mm<sup>2</sup> conductors, it is accurate up to 30 metres but for 6mm<sup>2</sup> conductors it is accurate up to 15 metres.
It should be noted that not all test instruments can read such low values as those shown in Table 1. The accuracy of the test instrument must also be considered.
Such lengths for main protective bonding conductors are quite unusual. Shorter lengths which are within the 0.05Ω figure are much more common.
Next month we will consider how the maximum length of supplementary bonding conductors can be obtained.
Endnotes
- Table 54.8 links the size of main bonding conductors to the size of the supply neutral conductor for PME supplies. As the size of the supply neutral conductor may be difficult to obtain, this is usually taken to be the same as the size of the main tails.
- The 0.05Ω figure is mentioned again in Chapter 1 of IET Guidance Note 7 Special Locations in connection with supplementary bonding in locations with a bath or shower. It states: “In practice the resistance between bonded extraneous conductive parts and exposed conductive parts should not exceed 0.05Ω.” The key word in this statement is bonded as parallel paths will have an effect causing values to be reduced when tested.
- Some believe that the 0.05Ω figure originated as the maximum expected value for the connection of the bonding conductor to an extraneous- conductive part – that is the resistance across the connection itself rather than the resistance of the bonding conductor. Others regard it as the recommended maximum resistance of the length of the main bonding conductor.
- We might consider the use of the maximum prospective earth fault current (PEFC) as a possible means of determining the length of main protective bonding conductors but this method gives an impractically low value – at least if the PEFC is taken to be 16,000A. If the conventional maximum figure for touch voltage of 50V is divided by 16,000 amps in order to obtain the maximum resistance of the bonding conductor – in order to then obtain a maximum length – we arrive at a figure of 0.003Ω.
Comparing this with the values in Table 1 shows this value to be unrealistic.
Further reading:
IET On-Site Guide.
IET Guidance Note 3 Inspection and Testing.
IET Guidance Note 7 Special Locations.
IET Guidance Note 8 Earthing and Bonding.
Taken from the IET
.
