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3 February 2020 heralded the publication of the first amendment to BS 7671:2018 Requirements for electrical installations (IET Wiring Regulations 18th Edition). Graham Kenyon discusses an important aspect of this update.
The changes will support the rollout of electric vehicle charging equipment infrastructure.
Use of a PME earthing terminal
The principal change introduced in Amendment 1 relates to Regulation 722.411.4.1. This makes it possible to use the emerging technology of open-PEN detection devices, which were not available when BS 7671:2018 was published. This change should be well-received in the industry, as it is not always practicable, or safe, to derive a TT earthing arrangement, where other parts of the installation have a different earthing system such as TN-C-S (PME) or TN-S.
The PME earthing terminal may be used for installations with a single-phase supply (to the whole installation) where:
- An additional earth electrode is provided that meets the requirements of 722.411.4.1 (ii). It may be impractical to achieve a low enough earth electrode resistance in many single-phase installations.
- A single-phase open-PEN detection device is used, that complies with 722.411.4.1 (iii), (iv) or (v). A device complying with (iii) will require a measurement earth electrode in single-phase installations. A device to (iii) or its equivalent in (v) may be installed in or near the charging equipment, or upstream of the charging equipment. A device to (iv) or its equivalent to (v) is intended to be installed in or near the charging equipment.
The PME earthing terminal may be used for installations with three-phase supplies (to the whole installation) where:
- The three-phase installation is sufficiently well balanced so that it is unlikely that the touch voltage will exceed 70 V rms if the PME supply neutral breaks, according to 722.411.4.1 (i).
- An additional earth electrode is provided that meets the requirements of 722.411.4.1 (ii). Depending on the anticipated worst-case phase unbalance, it may be possible to achieve a low enough earth electrode resistance in three-phase installations.
- An open-PEN detection device is used, that complies with 722.411.4.1 (iii) or (v). The device may be installed in or near the charging equipment, or upstream of the charging equipment. If the device is upstream in a three-phase portion of the network, it may protect single-phase charging equipment downstream. Where a device to (iii) requires an earth electrode, it may be installed on single-phase circuits or in/near single-phase charging equipment.
The relevant distribution network operator (DNO) should be consulted regarding the use of open-PEN detection devices for on-street installations. Street lighting supplies are often effected through three-phase distribution mains and therefore would typically be classed as three-phase supplies for the purposes of Regulation 722.411.4.1.
What is the problem with deriving a TT earthing system for the charging equipment as an alternative for PME supplies?
The problems with deriving a TT earthing system from a PME supply in a particular installation may include:
- Ensuring sufficient separation below ground from buried metalwork connected to the PME earthing system. If too little separation is provided, the vehicle is, for all practical purposes, connected to the PME earthing system.
- Simultaneous contact assessments are required, between the charging equipment/vehicles and extraneous-conductive-parts and exposed-conductive-parts connected to the original installation earthing system.
- The possibility of returning a touch voltage in PME broken neutral situations, for someone standing next to the vehicle on charge, if they are standing above buried metalwork or extraneous-conductive-parts connected to the PME earthing system. This may be just as hazardous as the original PME risk, under certain ground/soil conditions.
- The possibility of striking buried services if driving rods or excavating to install earth electrodes.
The above situations are more likely to be found in small curtilage properties, such as many dwellings.
Further guidance – 4th Edition of the IET Code of practice for EV charging equipment installation
The 4th Edition of the IET Code of practice for EV charging equipment installation is due to be published very soon. This is a major update with extensive guidance on Amendment 1:2020, and the topics discussed in the article.
Improvements include information on:
- Use of open-PEN detection devices.
- Issues with TT systems, including recommended separation distances underground.
- Improved risk assessment forms.
- Phase unbalance in three-phase systems, including a table and rule-of-thumb which can be used to avoid complex calculations.