TECHNICAL: FOUNDATION EARTH ELECTRODES
Working with the Ogilvie Group, DEHN UK and Megger Ltd, SELECT have been helping an IET working group investigate the benefits of foundation earth electrodes
Foundation earth electrodes (FEE) are being considered as a possible solution to prevent the potentially dangerous consequences of a break in an incoming supply protective earth and neutral (PEN) conductor.
FEE may be installed to provide an additional connection to earth to the PEN conductor of TN-C-S (PME) earthing systems provided by the distributor.
In BS 7671 there is already a note to Regulation 411.4.2 which advises that for TN systems, an additional connection to Earth is an option that may be considered by the installation designer for these systems at the point of entry of the supply into the building.
Regulation 411.4.2 requires that the neutral point of the power supply system shall be earthed and exposed-conductive-parts of the installation are connected by a protective conductor to the main earthing terminal of the installation, which is connected to the earthed point of the power supply.
In protective multiple earthing (PME) systems, the neutral and protective functions are combined in a single PEN conductor in part of the system, i.e. in the incoming low voltage distribution network cable. Thereafter, the functions are provided in separate conductors within the installation.
If a break in a distributor’s PEN conductor occurs, this could result in danger to persons located within that installation due to the significant potential that can appear between exposed conductive parts of the electrical installation and the general mass of Earth.
In single phase installations this is caused by load current returning through the protective conductors of the installation to the general mass of Earth, rather than through the neutral conductor which is broken. In three-phase installations this is due to imbalance between loads on the different phases of the supply downstream of the break in the neutral.
However, if main protective bonding is provided connecting any extraneous-conductive-parts that exist within the installation – e.g. incoming metal water pipes or gas pipes – to exposed-conductive-parts of the installation, the potential between these will be reduced to a safer touch voltage and danger avoided.
Where exposed-conductive-parts or extraneous-conductive-parts are located outside – e.g. a metal outside light or a metal pipe connecting a water tap – the main protective bonding will not reduce touch voltages between metalwork connected to the main earth terminal and Earth outside the building. It may therefore be appropriate to provide additional measures such as providing an additional connection to Earth at the consumer’s earth terminal. This is where FEE can be of particular benefit.
This may also be an option in situations where extraneous-conductive-parts do not exist within an installation, e.g. due to the use of plastic pipes for incoming water and gas services.
There are a number of options given in Chapter 54 of BS 7671 for earth electrodes, e.g. earth rods or pipes, earth tapes or wires etc. However, it is often difficult to obtain reliable low resistances to earth when installing these and a FEE is more likely to provide a greater area of influence around the exterior of the building than a single rod electrode.
This may be one of the advantages that installing a FEE provides.
Work to date
To date, the Institution of Engineering and Technology (IET) JPEL/64 working group research has included a number of foundation earth electrode installations carried out around the UK, including several in Scotland.
The work in Scotland has been kindly facilitated by Ogilvie Homes – part of the Ogilvie Group of companies located in Stirling – with FEE installed at their housing developments in Irvine, Ayrshire, and Bridge of Earn, Perthshire.
The FEE installed at these sites comprises of 8mm (CSA of approximately 50mm2) diameter galvanised steel bar, installed at a depth of 500mm in the concrete strip foundations, with the work being carried out by DEHN UK Ltd, who manufacture surge protection, lightning protection and earthing products.
The subsequent testing carried out by SELECT made use of a range of test equipment provided by Megger Limited, including their latest DET2/3 high resolution earth tester and MFT 1741 multi-function tester together with an ETK professional earth test kit.
So far, test results obtained from these installations have been encouraging and produced some useful information. These will be further considered and discussed by the JPEL/64 working group as they continue their research into this interesting project.
SELECT will continue to provide assistance on this research and will keep Members advised on how it may impact electrical installation work in the future as it develops.
Bob Cairney I Eng MIET
Technical Standards Adviser, SELECT