Iv'e read all these posts with interest and would comment thus.
BS7671 is not a Statutory Instrument, but as we all know in the event of a court case (Civil or Criminal) would be used as a bench-mark to show compliance/non-compliance to, or with 'best practice'. In my opinion the courts would also take into account the Guidance Notes, Onsite guides etc and it would be reasonable to expect a spark to reference this material in making a judgement/decision.
Someone mentioned not having GN1 because they only do certain types of work and they were not required to show they had the document for their assessment (I presume they were referring to Part P). In my opinion this does only show the bare minimum level of compliance and commitment when one is actively working in the industry? You could picture the court case... (Legal advisor) 'well Mr Electrician why didn't you have/reference document XYZ'? (sparky) 'well your honour my scam provider didn't say I had to have it to 'pass' my Part P assessment'. (Legal Advisor) 'So there are or were other documents available that could have helped you in making your decision about the cable depth, but you chose not to bother"? (Sparky) - Looks at feet and shuffles awkwardly...
I assume that when BS7671 was written, the writers took into account ESQC Regs 2002. The relevant section states:
Excavations and depth of underground cables
14.—(1) Every underground cable shall be kept at such depth or be otherwise protected so as to avoid, so far as is reasonably practicable, any damage or danger by reason of such uses of the land which can be reasonably expected.
(2) In addition to satisfying the requirements of paragraph (1), an underground cable containing conductors not connected with earth shall be protected, marked or otherwise indicated so as to ensure, so far as is reasonably practicable, that any person excavating the land above the cable will be given sufficient warning of its presence.
(3) The protection, marking or indication required by paragraph (2) shall be made by placing the cable in a pipe or duct or by overlaying the cable at a suitable distance with protective tiles or warning tape or by the provision of such other protective or warning device, mark or indication, or by a suitable combination of such measures, as will be likely to provide an appropriate warning.
Whilst neither BS7671 or ESQCR specifics a minimum depths for this instance other sections of the Wiring Regs do indicate cable depths, but normally for a reason, e.g. 600mm for caravan pitches (presumably to avoid tent peg strikes) etc. We could look at other electrical documents that it would be reasonable for an electrician to possess or at least have access to. GN1 has already been mentioned and suggests a depth of between 450mm and 600mm depending upon location. The On-site guide to the Building Regs on page 68 suggests a minimum depth of 500mm 'in all locations and where double digging is likely to occur, 600mm'. The NICEIC Snags and Solutions book, Snag 25 suggests a minimum depth of 600mm.
Now I appreciate that this reference material are not Statutory Regulations but the combined 'weight of evidence' does allude to the sort of depths cables should be laid at? All of this helps me to conclude that 300mm is certainly an insufficient depth for a buried cable and I for one would not be content with a cable that shallow. It's great that the OP? mentioned risk assessing the job about depth - but 450mm/500mm should be a minimum bench-mark depth that the risk assessment takes into account and to be honest, if I were digging or having a trench dug to 500mm you may as well go down to 600mm for good measure; which would also allow plenty of scope when back filling?
Well, that's my informed view on the matter
