Mains upgrade for Virgin Media


Long-standing client Virgin Media is the first UK provider of all four broadband, TV, mobile phone and home phone services. Virgin’s cable network delivers ultrafast broadband, with speeds of up to 152Mb, to more than half of all homes here in the UK. They continue to expand their service to reach thousands more homes and businesses across the country.


Holborn was appointed to undertake a mains power upgrade for Virgin Media’s Hemel Hempstead site, where the recorded maximum site electrical load exceeded the LV Tail capacity. The decision was taken to increase the Available Supply Capacity (ASC) and restore the load threshold to considerably below 65%, to cope with demand from increasing service delivery to the public.


The upgrade required a new high voltage (HV) supply from UK Power Networks (UKPN), with the installation of a new sub-station at the rear of the site to accommodate a new GRP Enclosure, 11KV/400V transformer and LV feeder pillar.

The first step was to create the base for the sub-station and build the new enclosure. This was formed from 18mm ply core GRP encapsulated walls with timber strengthening, with a roof capable of withstanding a snow loading of 2.5kn/m² & IP55 ingress protection.

The next step was to undertake the excavation and installation of service ducts for the low-voltage (LV) cabling from the new sub-station to the main Virgin Media building. The new mains supply was installed (parallel 2 x 185mm² 4c XLPE/SWA/PVC cables) in a new trench from the intake point at the rear of the premises.

The new cables entered the building through an existing weather cable entry and were routed into the sub floor in the technical space and jointed with the existing supply using resin cable joints within the subfloor.


The critical point in our project was reached when the switch over to the new, larger capacity supply was made out of normal working hours, with the work beginning at midnight to minimise the risk of an interruption to supply.

Although the switch over would utilise the existing back-up generator capacity within the building, the decision was made to install and commission additional generating capacity until the new supply was tested and found to be working as it should.

We delivered to site a 1000kVA/800kW generator on a 40ft trailer, complete with a 4500-litre fuel tank, which would keep it running for days, should it be needed as back-up to the site embedded set.

Temporary load cables were installed from the mobile generator and connected to the existing LV panel using the mobile generator connection point, with the entire system test run, including phase sequence testing.

We never expect a problem with work like this, but we plan for every eventuality, which is why transferring the site load to the embedded generator set went smoothly, with our temporary set as back up under manual switching.


The final step was to isolate the existing UKPN LV supply service, prove it was no longer live and disconnect it from the old supply cubicle. We cut back the existing mains supply cables within the sub floor and joined them to the new final mains cables from connection box to sub floor.

The final important step was to check the phase rotation of the new supply to ensure we didn’t undo all our good work, before transferring the site load back to mains power on the new supply.

It’s our ability to cope with every aspect of a project from mechanical & electrical work to excavating trenches and laying concrete bases, whilst working safely and considerately on site, that ensures our clients keep coming back for more.

  • increase ASC and restore load threshold to below 65%
  • new sub-station on site to accommodate 11KV/400V transformer and LV feeder pillar
  • new mains supply installed with parallel 2 x 185mm² 4c XLPE/SWA/PVC cables
  • install additional generating capacity – 1000kVA/800kW generator with a 4500-litre fuel tank
  • isolate and disconnect existing UKPN LV supply service
  • check the phase rotation of the new supply

“Holborn have successfully undertaken many projects for Virgin media and always deliver the same high-quality service that instils confidence; we know when switches are flicked everything will work. They face every challenge with the same practical, pragmatic approach that ensures projects finish on time and to budget.”

– Colin Greenwood

Programme Manager, Virgin Media