PROJECT OUTCOMESOne of the main functions of an earthing system is toensure personnel safety during earth fault situations. When an earth fault occurs, the earth return faultcurrent, in conjunction with the resistance to earth ofthe earthing system results in an Earth Potential Rise(EPR) at the site. This EPR reflects the voltage rise onthe earthing system where hazardous touch, reach-touch and step voltages can arise due to the voltagedifferences between the items of plant and thesurrounding soil. It is essential that each item of plantis sufficiently earthed and the below ground conductorsare effectively arranged so that safety to personnel isensured.The project involved designing the earthing system forboth the contestable (DNO Metering Substation) andprivate installations. The diesel generators are locatedoutside where the HV cabling is installed in protectivecable trays.Additional earthing in the form of both strandedconductor and rods were installed in the incoming HVcable trench to help redtesl the site resistance and theoverall site EPR.Once constructed TESL measured the site resistanceby means of a Fall of Potential (FOP) test to comparethe site resistance against the design.

PERFORMANCEInitially testing was performed on site to gather the sitesoil resistivity measurements via the Wenner method.The results were then modelled in the CDEGS-RESAPsoftware suite to determine the site soil resistivitymodel.The site was designed to ensure touch and stepvoltages are safe surrounding the DNO substation for

when it is both in isolation and bonded to the privateearthing system in the CDEGS-MALZ software suite. Tohelp redtesl the site resistance the incoming HV cabletrench was utilised, where both stranded conductor androds were installed, saving on installation costs. Since the site POC is supplied via an overheadconnection all of the earth fault current will return to thesource substation via the earth (since there is nometallic connection back to the source via an overheadearth wire, cable screens etc.). This resulted in a Hot”site classification, where underground utility searcheswere performed to confirm that no third party assetswould be affected during fault conditions. TESLprodtesld a hot zone contour drawing based on themodelled site EPR so that it could be provided toOpenReach.The FOP confirmed that the installed site resistance islower than the design, therefore confirming the touchand step voltage are safe across the site as designedand reducing the extents of the Hot Zone Contours.

SPECIFIC REQUIREMENTSWhen the site POC is provided via an overhead lineextra precautions are required to excessive EPRs arenot present during fault conditions. Care must be exercised to prevent hazardous voltagesbeing transferred to third party assets. A Hot ZoneContour drawing should be prodtesld so thatOpenReach can include this in their records. Sizing of the earthing conductors for the DNO andPrivate electrode were based on separaterequirements. This meant that the earthing conductorsfor the two earthing systems needed to be sizedaccordingly

TWIN OAKS GAS STOR EARTHING DESIGNTESL CASE STUDY

THE PROJECTTwin Oaks short term operating reserve(STOR) is a new gas powered, 5MWgeneration project located at Twin Oaksfarm, Lower Street, Doveridge, DE6 5JW. TheTwin Oaks STOR is connected into theWestern Power Distribution (WPD) 33kVdistribution network via a teed undergroundcable to an overhead line Point ofConnection (POC).

TESL INVOLVEMENTWe were contracted to undertake theearthing system design for the whole TwinOaks Gas STOR.

PROJECT DATESProject Start Date: Jul 2017Submission Date: Sep 2017

CLIENTEthical Power Ltd

On-site soilresistivity test

route

THE EARTHINGSPECIALISTSLIMITED

Cardiff HouseCardiff RoadVale of GlamorganCF63 2AW

07376 375 629www.tesl-ltd.co.uk

If you have any issues with your installed earthingsystem or require a new earthing design, please get intouch with our experts at TESL.