Stroma Certification’s technical team offer their expert advice on earthing systems in electrical installations.
There are three common types of earthing systems in use for electrical installations, including TNS, TN-C-S and TT. Where the distributor has supplied the means of earthing with their supply, it will either be TNS or TN-C-S. If the distributor hasn’t provided a means of earthing, an earth electrode will need to be provided in the consumer’s premises to form a TT earthing system.
Up until 1966, a metallic water pipe from a public supply was commonly used as a means of earthing and contractors may occasionally find public water pipes still in use for this purpose. Whilst metallic water pipes are required to be bonded to earth, the Wiring Regulations no longer permit them to be used as a means of earthing. If contractors find this method of earthing still in use they will need to request the Distribution Network Operator (DNO) to supply an earth. Or, if this is declined, then an earth rod will need to be installed to form a TT earthing system.
Contractors aren’t permitted to interfere with the distributor’s equipment and they shouldn’t attempt to apply clamps to incoming cables or make a connection to the supply neutral inside the head. Applying BS951 clamps to old lead sheathed cables could be very dangerous as the lead sheath is very thin and the conductors inside will be enclosed in oiled paper. Tightening a clamp on the cable could cause a short between the internal conductors and expose the contractor to serious flash burns. Before any additions and alterations to the install are made, contractors need to verify that the means of earthing is satisfactory and the necessary protective bonding is in place to comply with Regulation 132.16.
Contractors will find older installations supplied from Paper Insulated Lead Covered (PILC) service cables, with the lead sheath providing a TNS earthing system; the earthing conductor being soldered to the lead sheath or clamped to the lead with a special clamp. These cables may have been in the ground for many decades and may have decayed with age and water ingress where the service cable is jointed underground to the supply in the street.
It’s important that an earth loop test, to confirm the value of Ze, and earth continuity, is performed with the installation isolated and the earthing conductor disconnected from the Main Earth Terminal (MET). If a Ze test is performed with the earthing conductor connected to the MET, an apparent satisfactory test result may be obtained due to parallel paths to earth provided by the protective bonding of service pipes.
PILCs and DNOs
Often an earth loop test may result in 10s of ohms or an open circuit due to the failure of the PILC cable joint underground. If this is the case, the DNO will need to be contacted to undertake a repair to the cable, or to supply a new cable, or they may convert the installation to TN-C-S. It is not unknown for DNOs to say they’ve never provided a means of earthing to a given premises and refuse to supply an earth. In this case, the installation will need to be converted to a TT earthing system.
From the 1970s onwards, most DNOs ceased to use PILC cables and have provided PVC covered service cables, mostly with a combined Protective Earth and Neutral (PEN) conductor. The supply neutral and earth are combined in one conductor in the service cable and then separated out at the origin of the consumer’s premises. This means of earthing is TN-C-S and is known a Combined Neutral and Earth (CNE), which is generally only used where a single consumer’s installation is supplied directly from a transformer.
Where multiple consumers’ installations are supplied from the same cables with PEN conductors, the cable will have additional connections to earth electrodes installed at joints along the cable run or at the bases of overhead supply poles. This system of earthing is TN-C-S and additionally Protective Multiple Earthing (PME).
In some areas and for some particular installations, such as farms, DNOs may supply a PVC covered service cable that has separate neutral and earth conductors inside the cable; these cables are known as Separate Neutral and Earth (SNE).
Contractors looking at a supply head and seeing an earthing conductor emerging from the side or top of the head may incorrectly conclude that the means of earthing is TN-C-S, believing that the conductor is connected to the supply neutral. Supply heads may have solid brass blocks for termination of a CNE neutral. For connection of the earthing conductor, some may have separate blocks that may have a link connected or not connected for termination of an SNE cable.
If the DNO has applied a ‘PME’ label to the supply head it can be assumed that the earthing system is TN-C-S. As a contractor isn’t permitted to interfere with the DNOs’ equipment, the means of earthing can’t be deduced by simple visual inspection. Only the DNO will be able to confirm the actual earthing system in use.
Failed PILC cables
Since the 1970s, where service cables and joints on PILC cables have failed underground, DNOs have often repaired and jointed these with CNE cables. Where these PILC transition joints are made, cable jointers will connect the supply neutral to an earth electrode adjacent to the joint. So, with the passage of time, some DNOs’ networks have become TN-C and PME. One DNO, UK Power Networks, has now declared their entire network to be PME.
A contractor seeing a connection of the earthing conductor to the lead sheath of a PILC service cable may conclude that it is TN-S when in fact it may be TN-C-S. Again where the DNO has supplied a means of earthing, only the DNO will be able to confirm the actual earthing system in use as it can’t be determined by visual inspection.
DNOs and means of earthing
Whilst DNOs are under a statutory duty to confirm the means of earthing to consumer’s installations, this may be difficult and time-consuming to verify. Where the DNO has provided a means of earthing, unless confirmed to be otherwise, contractors should assume it to be PME and ensure the protective bonding conductors are sized in accordance with Table 54.8 of the Wiring Regulations and the earthing conductor is also suitably sized.
In addition, contractors should be mindful that PME earthing is not permitted or recommended in some Special Installations or locations set out in Part 7 of the Wiring Regulations.