I agree with Welchyboy, this is a sticky one. I don't see how BS7671 could give specific depths for specific locations or purposes. I will put my tin hat on and say that 600mm is quite often unrealistic and possibly unnecessary. If the average spark working in the domestic arena quoted for the amount of labour involved he would never get the job. You have to use your common sense, if you are putting a cable under a garden that is going to be dug, then it needs to be deep enough to avoid being hit by "normal" activity, but what's normal? There is always the chance that ANY underground cable is going to be struck, including DNO cables. I recently came across a gas pipe installed to a new build with no marker tape at a depth of about 200mm, so even the supply authorities can't do it properly. What about a cable running round the perimeter of a building in a gravelled 100mm gap between the wall and paving? Are we really supposed to get it down to 450mm minimum? I think not. Sometimes its better to leave it clipped to walls/fence posts at ground level so its visible. And what if it is protected with an RCD? Should we approach it in the same manner that cables concealed in walls are? It is armoured cable at the end of the day.
I have come across loads of situations where is has been buried barely below the surface, probably because the contractor involved thought "well I can't get it 600mm so sod it". BS7671 tells us it should be deep enough to avoid being disturbed by "normal" activity, but it doesn't say what that might be. Sometimes the best you can do is make sure it is marked by tape and get it as deep as is practically possible, however deep that may be and at reasonable cost.
