Elizabeth Line Earthing Electrode Testing Lessons Learned

Abstract

As part of the design, construction and commissioning of the Elizabeth Line earthing system, measurements for soil resistivity and earth electrode impedance have been undertaken. Much has been learned on the design and construction of the earth electrode systems for the Elizabeth Line about the importance of undertaking the measurements at the correct point in the programme, whilst elements are electrically independent. Some of the difficulty experienced in undertaking the earth electrode resistance testing could have been resolved with some minor modifications to the civil design at pre-construction stage and should be considered for future projects. The paper includes suggestions for provision of test points at key locations in future projects to facilitate the tests. Knowledge has been gained about which test methods are appropriate for a given earth mat installation type/location and different points in the construction programme. To achieve a consistent set of assurance records the types of tests, the points in the programme where the test is to be undertaken plus the contents of reports and certificates needs to be specified. It has also been found that the choice of test instrument is important to enable a consistent reliable result. The paper also considers on-going maintenance earth mat testing for statutory compliance and makes clear recommendations based on the experience from Crossrail; additionally it will explore the issue that the standards were not written for integrated structures of the magnitude of the Elizabeth Line and explain an approach that resolves the issue for future maintenance managers.