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Case Study

“Sub R” Ring Main Unit Replacement

Project “Sub R” Ring Main Unit Replacement
Location Natural History Museum, London
Completed December 2017

RJ Power Networks carried out the safe removal and replacement of 2x Brush Falcon ring main units (RMU’s) in the ‘R’ Substation at the Natural History Museum.

In November 2017 RJ Power Networks were invited to quote for the removal and replacement of 2x Brush Falcon ring main units in the ‘R’ Substation at the Natural History Museum. This was primarily due to SOP 1996/0167/00 issued on 09 Jul 1996, stating that ‘all ring switches shall only be operated when dead’, rendering the switchgear inoperative for live switching. The job also had to be carried out in an extremely tight time frame of 1 single shutdown visit with an estimated working window of 10 hours from start to finish.

As part of the competitive tendering process, RJ Power Networks were not the only company to have been invited to quote for the works but as always the Networks team provided a cost effective quote with an outstanding technical solution.

Project Deliverables

Following the successful tender, the task was handed over to the RJ Power Networks Projects team. The team then instantly began the meticulous planning of the works and the replacement switchgear was ordered almost immediately to ensure the lead time from the supplier met the client’s expectations.

Due to the tight time scale of the job, continuous communication was required between RJ Power Networks and the client to ensure all aspects of the planning and co-ordination was agreed well in advance of the work. The site was surveyed and a programme of works was produced and agreed between both RJ Power Project Managers and the Client.

The projects team compiled the risk assessment and method statement, which covered all aspects of the works including; the switchgear removal and replacement works, the associated HV cable jointing and terminations, the removal of the old cables and replacement with new cables including the new containment throughout, and the testing and commissioning that went with all the above activities.

All materials and switchgear were delivered to our depot in good time to allow for site acceptance testing of the switchgear and for the equipment to be methodically checked prior to the works.  Due to the programme of work taking place over one single Sunday night shift, there was no room for error as replacement parts would not be readily available outside of normal working hours.

The current RMU’s were close coupled to Midel filled Transformers, so samples were taken prior to works commencing to obtain a condition analysis of the Midel.  In order to remove the RMU’s, the transformer lids were removed and the Midel level was reduced to a workable level below the Internal HV connections, which allowed for the removal of the RMU and associated throat box. Once the works were completed and the Midel was refilled to the correct level, a second sample of each Transformer was obtained and sent off for independent analysis to confirm that the Midel handling techniques used did not degrade the condition of the insulating medium.

All RJ Power networks engineers in attendance on the job held a meeting prior to the works where they were allocated specific roles and responsibilities for the works.  This ensured that all materials and plant required was onsite and that everyone was clear of what was expected of them as soon as the works begun. This enabled the works to progress smoothly and ensured that there was minimal/no lost time.

Challenges and Solutions

The only challenge encountered onsite was the incorrect selection of HV cable straight joint kits collected from our stores and taken to site, as the kits needed were almost identical to 2 other straight joints that were in the stores for a job later that week. Whilst carrying out a requisition of materials onsite, the mistake was highlighted by one of team and the issue was overcome by one of our engineers making a trip back to our depot to collect the correct HV cable straight joint kits. The quick identification of the issue and decision making process meant that minimal time was lost on site and work could resume.

RJ Power successfully replaced both RMU’s, all cabling associated, tested and commissioned the system before re-energising within the given timeframe agreed with site. The old redundant switchgear was removed from site and disposed of safely being mindful of the SF6 gas.

Why RJ Power Networks

RJ Power Networks undertakes a range of installation and maintenance works, from control cabling and relay works through to the installation and commission of distribution switchgear and transformers in addition to provision of all aspects of high voltage and low voltage maintenance in private distribution networks, making us the ideal company for this project.

We continually strive to offer a consistent, reliable and flexible service as well as improving and adapting our methods, works, equipment and services to maintain a cutting edge.


“CBRE were tasked with a complex and challenging project on behalf of our client at the Natural History Museum. From conception, we understood that we were facing a challenge with a time restricted project coupled with high-levels of risk that required extensive management. From our initial meeting with RJ Power they quickly and reassuringly started to simplify the complex for us. RJ Power during the tender process proved not only to be cost effective but also to be a company that went the extra mile. They were exceptionally responsive to the projects needs by providing important design and cost input during the initial stages of the process while consistently meeting our projects requirements throughout. The real value was acknowledged when the project commenced and we watched the team in action, we then witnessed the professionalism of their service first hand. We were extremely grateful for RJ Power’s service and look forward to working together again in the future.”

Graham Syder
Head of Projects
CBRE Managed Services Ltd


RJ Power Networks Limited
Polhill Business Centre,
London Road,
Kent, TN14 7AA
0345 034 1480
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