Cables buried direct in ground...

Chris Pearson: 
 

Martynduerden: 
Not too sure on Auckland 60 & 30 year old cables at the time, the older lot failed stressing the ‘newer’ ones I believe, I thought they decided it was poor maintenance of cables that ought to have long been replaced?

It is self-evident that if one of a pair of cables in parallel fails, the other will be overloaded.

I sincerely hope that cables last more than 30 years. Our service cable is at least 39 years old; the transformer dates from 1959; and there is a map of the cables in the Hampshire Archives which is pre-war.

Agree, I would expect that, baring accidents, that DNO street mains, and large fixed cables with a building, should last AT LEAST 100 years. Large cables fixed in place within the building structure should be expected to last the life of the building.

For smaller sub-circuit cables I would expect cables to last at least 50 years, replacement after that time might be required in any case due to changing needs.

For these reasons I feel it to be poor practice to push cables right to the limit on a regular basis, as so doing may result in a calculated life of only a few decades.

Martynduerden: 
 

Hi Folks,

I have been giving a bit of thought to direct buried cables and was looking around for the ‘Best’ method for installation in reference to current carrying capacity especially with the rise in copper prices. 96A vs. 152A on something as small as a 25mm 4c or 365A Vs 592A on a 300mm 4c.

The differences between ERA & BS7671 figures are significant and they seem to come down to actual installed conditions, for example ERA have an assumed thermal resistivity of 1.2k.W/m vs. that of BS7671 which uses 2.5k.W/m.

I got to thinking is there an installation method that could be more readily assured to be better in terms of thermal resistivity?

Damp compacted sand would seem to be the best for decreasing thermal resistivity, but how much? would a liner in the base of the trench be of any use to reduce drying or migration of the sand or might this just add to the overall thermal resistivity?

I read this earlier https://ictinternational.com/casestudies/underground-power-cable-installations-soil-thermal-resistivity/ 

And then what about the ‘New’ Conducrete? could encasing in such material have significant benefits or might this risk damage with settlement?

Thoughts?

(Attempt #3…)

Firstly both BS7671 and ERA 69-30 provide methods for derating (or up-rating) the cables where the STR differs from the default. Directly comparing tabulated values will unspurprisingly give large differences; after applying a suitable factor the ratings are closer.

ERA 69-30 is consistent with many cable manufacturers (but not all - always read the datasheet) using 1.2Km/W as the baseline. This corresponds conveniently with the UK nominal summer STR given by BS IEC 60287-3-1. Meanwhile I've not checked but BS7671 likely takes its tables directly from the EN / IEC parent standards, and in any case 2.5Km/W for well-drained sand is not far off worst case for normal projects.

Note that specific sites and installation conditions may vary considerably!

Direct measurement is preferred but rarely practicable for LV projects. ERA 69-30, as well as BICC as referred to in Amtech for those who use it, does give some reference values for various soil types and load profiles.

Where assurance is needed, ENA 97-1 provides serveral methods for providing a cable with backfill of reliable known STR. Cement Bound Sand (CBS) is readily available, if expensive, from various suppliers, while the sand and gravel needed for other methods can be obtained from some, but not all, quarries. CBS allows you to assume 1.2Km/W and is also good for laying up cables in specific arrangements. Note that certification of conformity with ENA 97-1 should be obtained. However, for smaller installations it's probably cheaper to be more conservative with cable selection.

Conducrete is, if I understand correctly, designed for earthing systems, and reduces the soil electrical resistivity; I don't think it makes any claim toward thermal resistivity, just like Marconite.