As the cables would be buried in the ground they'd need to be armoured types (if not in steel conduit or the like) and suitable for the (possibly wet) conditions. Most manufacturers don't recommend embedding cables in concrete (some chemical reaction between the cement and plastic as the concrete cures I suspect) so you may need to duct the last bit through the slab in any event.

Are you cutting through an existing slab or this this a new build? Either way some thought might need to be given the the damp proof membrane (if any) - punching a hole through it for a cable might not be the easiest thing to make water-tight reliably (let alone cutting a trench through an existing one) - again water proofing sealant chemicals (e.g. bitumen based) might not be the best things to come into direct contact with cable sheaths.

Ducting the lot would also avoid possible damage to the cable while the concrete is being poured or other heavy building work (and indeed risk of copper theft while the building is still open to the elements and so less secure).

   - Andy.

I agree with Andy and would further suggest asking what would happen should there be a cable fault, or indeed a future change in spec... it sounds like excavating to re-lay would not be easy.

There is nothing stopping you, regs wise, from burying a cable suited for direct burial, back filling with soft sand and all the rest, and then laying  tarmac or concrete or indeed building a wall over the top - DNOs do this all the time, but they expect their cables to outlast the buildings above, and while after a fashion it is "accessible with tools", this may - as others have alluded to above, mean some serious demolition, or more likely an abandon and re-route if there is ever a problem.

Where cables emerge through the floor slab will need some sort of hockey stick shape of tubing to protect them during pour, and maybe a solid protective enclosure if the floor is the kind laid by cement mixer and 'elephants trunk', at least until the final skim and making good.

Possible, and perfectly fine so long as that risk is accepted and understood by the folk who want to do it that way.

Are there rebars in the floor and are they being earthed ?

Mike

mapj1 said:

There is nothing stopping you, regs wise, from burying a cable suited for direct burial, back filling with soft sand and all the rest,

Except, as others have pointed out, Regulation 522.8.10 of BS 7671 requires a cable buried in the ground to either have an earthed metallic armour or similar, suitable for use as a protective conductor, or alternatively use metallic containment, or be installed in a a conduit or duct with appropriate mechanical protection.

mapj1 said:

DNOs do this all the time

Agreed, but that cabling is not within the scope of BS 7671.

Steven Gibson said:

We are looking at a heavy industrial unit

I have run SWA (so armoured as Graham points out) from an intake in a garage on the boundary across the garden (so soft sand as Mike points out) and then under the kitchen floor, but this sounds rather different.

I suppose that compliance is compliance irrespective of the load, but how heavy is this heavy industrial unit? What size are the cables?

Chris Pearson said:

but this sounds rather different

In terms of someone accidentally digging it up, yes ... but thinking about the hazards ... it's not like you could excavate to find the cable if there's a need to core and bring another service up through the slab, is it?

I'm still on the side of needing 522.8.10 provisions ?

To mole under the finished building may be possible - though that rather depends on the peripheral foundations. In the event of a problem I still thing it will most likely be a case of abandon and re-route.

Ducting at least going to a manhole at the outside edge of the building would make far more sense.

Mike.

mapj1 said:

Ducting at least going to a manhole at the outside edge of the building would make far more sense.

Agreed ... but then perhaps causes the problem (if the ducts can fill with water) of PVC being more susceptible to water-logging?