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Melksham Substation Works 400kV

Excalon recently carried out 400kV high-voltage cable installation works for Envolve Infrastructure at Melksham 400kV Substation in support of National Grid’s upgrade of the site. The works formed part of a wider project to install a new Shunt Reactor (Shunt Reactor 2), which is a key component in National Grid’s efforts to manage voltage stability across the transmission network.

This complex installation took place within a live high-voltage environment and required
careful planning, collaboration with multiple specialist contractors, and the implementation of strict safety controls. We delivered the full scope of cable installation and termination works.

Scope of Works

Excalon’s scope included the installation of 400kV cables into pre-installed cable troughs and ducts. Excalon was responsible for the cable pulling, laying, backfilling with CBS (Cement Bound Sand), and preparation for final termination works.

Additional responsibilities included the coordination of scaffolding for two Cable Sealing End (CSE) structures, management of termination interfaces, and the coordination of cable testing.

Safe Working Practices & Voltage Management

Given the high-voltage environment, impressed voltage management was a critical part of
the methodology. Field earthing was directed by the National Grid SAP and carried out by
competent persons from Principal Contractor. Excalon ensured that all pulling equipment
was appropriately earthed, and cable jointers followed insulated working practices
throughout.

Each phase of the cable was installed individually, with temporary earth bonds applied
after cutting and prior to disconnection from the winch. This approach ensured that the
risks associated with impressed voltage were fully controlled throughout the works.

Cable Installation Methodology

The cable installation process began with the transportation of cable drums to site. The drums were positioned carefully in accordance with the duct alignment. Prior to pulling, bell mouths and rollers were installed to protect the cables from abrasion and to ensure a smooth entry into the ducting.

Cable pulling was carried out using a monitored winch system that recorded all pulling tensions. Each duct was identified and phase-marked in accordance with the approved phasing diagrams.

Upon completion of the duct pulls, the cable was laid into pre-installed troughs using a combination of manual handling and controlled plant operation. Operatives worked in coordination with excavator and tractor operators to guide the cable into position, maintaining the correct cross-sectional profile and avoiding any damage during handling.

Backfilling & CBS Installation

Before cable installation, a 100mm CBS bed was laid within the troughs. CBS was delivered to site using a 6-tonne swivel dumper and distributed along the trench using hand tools and compacted with remote-controlled rollers and whacker plates to reduce the risk of HAVS (Hand Arm Vibration Syndrome).

After the cable was installed, the troughs were backfilled with CBS to the approved cross-section design, allowing for the installation of trough lids by others.

Cable Termination & Final Installation

Once the cables were fully installed and coiled safely away from traffic routes, the next phase involved lifting the cables up into the CSE structures. This process began with cable measurement and cutting in the presence of the client, followed by the capping of cable ends and preparation for lifting.

Certified lifting equipment, including slings and an excavator, was used to position the cables. The cables were raised one at a time using a hand winch and temporarily supported in black ducting to prevent sagging. Once correctly aligned, they were secured to the sealing end structures using Ellis cleats, ensuring they were stable and correctly positioned for jointing.

A final inspection was carried out to confirm correct alignment, tightness of cleats, and compliance with all safety standards. Once the works were deemed satisfactory, the installation was handed over to the client and then we proceeded testing and commissioning.

Testing & Commissioning

Following the completion of works, high-voltage and low-voltage testing was carried out in accordance with National Grid specifications. A high-voltage AC withstand test was performed on one circuit at 260kV for a duration of one hour. Partial discharge measurements were also taken on the same circuit to verify insulation integrity. Additional low-voltage testing included measurement of conductor resistance, verification of circuit impedances, and an over-sheath voltage withstand test at 10 kV DC.

The integrity of the cable sheath system was assessed through checks on single-point bonding, cross-bonding arrangements, and contact resistances. Surge Voltage Limiters (SVLs) were inspected and tested to ensure correct operating voltage and insulation resistance. All tests were completed successfully, with results demonstrating full compliance with specification requirements and confirming the system’s readiness for energisation.

Conclusion

This project demonstrated Excalon’s ability to safely and efficiently deliver high-voltage cable installation services within a complex, live substation environment. Our team worked closely with Envolve Infrastructure, National Grid SAPs, and specialist subcontractors to complete the installation in line with technical requirements and under strict safety protocols.

The successful execution of this work supports National Grid’s continued efforts to enhance the reliability and flexibility of the UK’s transmission network.

Download Case Study – 400kV Melksham