The spec
ENA TS 97-1 explained
ENA TS 97-1 is the UK standard for the sand and other backfill laid around buried power cables. It is published by the Energy Networks Association, the body for the UK’s electricity network operators, and its full title is “Special backfill material for cable installations”. The trade often writes it ENATS 97-1. The current issue is Issue 3, from 2021.
Last checked 3 October 2026. Sources at the bottom of the page.
What it is for
A buried power cable makes heat, and the ground around it has to carry that heat away. Dry soil and loose sand do that badly, so the cable runs hotter and has to carry less power. Backfill that still carries heat well when it has completely dried out lets the cable carry its full rated load all year round.
The standard covers open trench work, with cables laid directly in the ground or in ducts. The backfill has to bear load and stand up to settlement, washing out, knocks and frost heave. It does not cover grouts or bentonite in ducts, or drilled crossings.
The key measure is thermal resistivity: how hard it is for heat to get through the material, in K·m/W. Lower is better. Every limit below is measured with the material fully dried out, the worst case next to a hot cable.
The three standard backfills
The standard calls these the traditional backfills. Each has a thermal resistivity limit and rules on what goes into it and how dense it has to be.
Selected sand
Also sold as cable sand or thermal sand
2.7 K·m/W
Most it can be, dried out
- Natural sand from an approved source.
- At least 95% by weight passes a 5 mm sieve.
- No clay lumps, no organic matter, no sharp stones or flints.
- Must hold together when dry: compacted, oven-dried test cylinders have to carry a 4 kg weight, two out of three.
- Dry density of at least 1.6 Mg/m³ in the lab test, and an average of at least 1.45 Mg/m³ in the trench once compacted.
Gravel and sand
Selected sand with gravel added
1.2 K·m/W
Most it can be, dried out
- Selected sand mixed with an equal weight of 10 mm single-size gravel, rounded preferred.
- Laid with about 10% water.
- Dry density of at least 1.8 Mg/m³ in the lab test, and 1.7 Mg/m³ in the trench.
Cement-bound sand (CBS)
Also called stabilised sand
1.2 K·m/W, cured
Most it can be, dried out
- Sand with at least 95% passing 5 mm, bound with cement at 14 parts sand to 1 cement by volume.
- The sand’s void ratio no more than 0.54.
- Average dry density in the trench, once cured, of at least 1.6 Mg/m³.
These figures are from Issue 2, the issue we have read in full. A test certificate issued under Issue 3 that we have checked uses the same thermal limits. If your contract names Issue 3, check the other figures against it.
Other backfills
Other materials are allowed if they pass a type test: twelve samples tested for thermal resistivity, density, how even the material is and how well it holds together. The standard’s own examples include crushed granite, limestone and whinstone, pulverised fuel ash, recycled foundry sand, other CBS mixes and fluidised thermal backfill. See thermal backfill for how they compare.
Some network operators write their own rules on top. National Grid’s distribution network in the Midlands, the South West and South Wales, for example, asks for crushed limestone or granite dust, 3 mm down to dust. We supply that too.
Testing and the paperwork
This is how Issue 2 sets it out. Your contract may add to it.
Before the job. Samples are taken from four widely spaced places in the quarry and tested for the limits above. The results go on a test sheet for that quarry, and a copy goes to the client’s engineer.
During the job. Density is checked in the trench as the work goes on: a set of three tests every 50 m where possible, and never further apart than 100 m.
Hold Points. These are stages the work cannot go past without the engineer’s written approval. There are two: handing over the test paperwork before the contract starts, and passing the density tests once the cables are blinded.
The standard itself warns that jobs get held up when the backfill supplier does not have the right test paperwork. You canask for our test certificate before you order, to check it against your spec.
Laying to the spec
Issue 2 asks for a compacted bed of 75 mm across the full width of the trench, and at least 100 mm of compacted backfill over the top cable, duct or joint. Compact by hand or with a vibrating plate, and by hand only around cables and joints. Bone-dry sand is hard to compact, so very dry sand is wetted to about 10% water first.
The network operator’s own rules for your area may ask for more. The trench bedding guide sets out what the operators ask for.
Questions
Which issue of ENA TS 97-1 is current?
Issue 3, published in 2021. It replaced Issue 2 from February 2016, which replaced Issue 1 from 1997. Check which issue your contract names, because that is the one your paperwork has to meet.
Where can I get a copy of ENA TS 97-1?
The current issue is sold by the Energy Networks Association through its document catalogue. Issue 2 has been published openly online, and we link to it in the sources at the bottom of this page.
What is thermal sand?
In the trade, thermal sand means selected sand tested to ENA TS 97-1, so you know how well it carries heat away from a cable once it has dried out. Untested sand is just sand, whatever it is called.
What thermal resistivity does cable sand need?
Selected sand has to be 2.7 K·m/W or less once fully dried out. Gravel and sand, and cement-bound sand once cured, have to be 1.2 K·m/W or less. Lower is better: heat gets away from the cable more easily.
Is ENA TS 97-1 the same as NGTS 3.05.07?
No. NGTS 3.05.07 is National Grid’s own specification, used on its transmission jobs. Some test certificates show results against both. If your job is for National Grid, check which one the contract names.
Can other materials be used instead of sand?
Yes, if they pass a type test set out in the standard: twelve samples tested for thermal resistivity, density, how even the material is and how well it holds together. Examples it gives include crushed granite, limestone and whinstone, and fluidised thermal backfill.
Sources
- Energy Networks Association document catalogue, ENA TS 97-1. Shows Issue 2 (February 2016) has been replaced by Issue 3. Where to buy the current issue.
- ENA TS 97-1 Issue 2, February 2016, Special backfill material for cable installations. The full document, published openly by Innovate UK Business Connect. Every figure on this page comes from it. A test certificate issued under Issue 3 that we have checked uses the same thermal limits.
- ENA TS 97-1 Issue 2 extract. The Energy Networks Association’s own extract: scope and contents.
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