On my big industrial site:-
HV - 1 metre to top of cable
LV - 600mm to top of cable
Comms - 600mm ttoc
If you're doing a spec don't forget the fine riddled sand (75mm above and below the cable), the cable tiles on the sand, then soil, then Warning tape 300mm from the surface, then more soil to ground level.
Put in your spec that you will ask them to dig two (or more)
slit trenches in positions of YOUR choosing to check the installation and if they're not correct they must dig it up and start again, If your Clerk of works is happy and fully supervises them you may not actually have to ask them to do it (BUT DON'T TELL THEM THAT).
Also, make sure your Clerk of works watches them like hawks to ensure the trench is free of obstacles etc prior to cable pulling, fine riddled sand is used (no stones in it), the correct number of rollers are used, the pull is done correctly using the correct equipment, bends are not too tight and you don't want rocks in the backfill.
Hope this helps.
Oh and don't pull if it's too cold as the insulation will crack (check temp with Cable manufacturer, but above 3 degrees C for 24hrs prior as an average or you have to keep the cable drum in a heated tent or something prior to installation).