The Office of Nuclear Regulation will need to give clearance for the start of the critical work package, after deeming the major milestone a safety-critical stage in the project.
The release of the first bulk MEH hold point by the regulator is expected before on-site work is programmed to start around October this year.
This would be two years since permission for first Nuclear Island concrete was given.
At this point, civil construction work on Unit 1 nuclear island will have reached the point where several individual rooms are structurally complete and ready to hand over from the main civil contractor to the MEH teams.
Balfour Beatty, NG Bailey, Altrad, Cavendish Nuclear and Doosan Babcock are all linked up in the MEH alliance to co-ordinate delivery of the complex installation of cabling and pipework.
Once started, the first bulk MEH installation across both reactor units will take place over a period of around three years, although detailed MEH system completions will continue well beyond that.
The MEH programme involves electrical and mechanical installations encompassing around 4,000 rooms, employing 3,500 workers.
Works will involve 380km of pipework, 20,000 valves, with thousands of welds and associated radiography, and 9000 km of cabling.