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Soil thermal resistivity and ambient temperature

ianlloyd

For buried cables, the soil thermal resistivity and ground ambient temperature are required to be able to calculate the CCC of the buried cable.

The Regs states 2.5 K.m/W and 20 °C respectively but with the caveat that other values may be used either where measured or the ERA figures can be used.

Table 4A2 shows example drawings of buried cable which appear to show the cable buried direct in the soil with no indication of sand/pea gravel bedding, cover etc. (How many installations just dig a trench, drop the cable in then just backfill with the soil that's been dug out with no bedding/cover?)

Chapter 16 of the Commentary on the Regs states that the 2.5 K.m/W value is "conservative" and that "...BS 7769-3-1 (IEC 60287-3-2) provides standard conditions for the UK as follows:" and lists 1.2 K.m/W and 15 °C.

1. Why doesn't the Regs use the "standard conditions for the UK"?

2. Where does sand/pea gravel bedding/cover come into this, or is the "conservative" 2.5 K.m/W meant to cover such but it just isn't mentioned anywhere?

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    ianlloyd said:
    1. Why doesn't the Regs use the "standard conditions for the UK"?

    As explained in Section 2.2 of Appendix 4 to BS 7671, the 2.5 Km/W is for cables in and around buildings, where we are looking at cables passing through building materials, and not soil in general. Outside of the vicinity of buildings, if the soil resistivity is not measured Cs can be taken as 1.2 Km/W in the UK in accordance with BS IEC 60287-3-1 Electric cables. Calculation of current rating. Part 3-1: Operating conditions - Site reference conditions.

    ianlloyd said:
    2. Where does sand/pea gravel bedding/cover come into this, or is the "conservative" 2.5 K.m/W meant to cover such but it just isn't mentioned anywhere?

    The gravel is to stop cables being immersed for long periods in water in winter. It provides drainage ... important in much of the country because soil is often laden with clay that prevents drainage. The sand bedding over the gravel helps stop the gravel damaging the sheath.

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    ianlloyd said:
    1. Why doesn't the Regs use the "standard conditions for the UK"?

    As explained in Section 2.2 of Appendix 4 to BS 7671, the 2.5 Km/W is for cables in and around buildings, where we are looking at cables passing through building materials, and not soil in general. Outside of the vicinity of buildings, if the soil resistivity is not measured Cs can be taken as 1.2 Km/W in the UK in accordance with BS IEC 60287-3-1 Electric cables. Calculation of current rating. Part 3-1: Operating conditions - Site reference conditions.

    ianlloyd said:
    2. Where does sand/pea gravel bedding/cover come into this, or is the "conservative" 2.5 K.m/W meant to cover such but it just isn't mentioned anywhere?

    The gravel is to stop cables being immersed for long periods in water in winter. It provides drainage ... important in much of the country because soil is often laden with clay that prevents drainage. The sand bedding over the gravel helps stop the gravel damaging the sheath.

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