Fig 1 Electric shock risk due of open-circuit fault in supply PEN conductor
• Non-conducting location (Regulation Group 418.1). The use of this measure is permitted only in installations under the supervision of skilled or instructed persons (Regulation 410.3.6 refers).
Secondary of distribution transformer
L1 L2 L3 PEN Source earth
Open-circuit in PEN conductor
Additional source earth
(some possible positions)
N
PME terminals
L
RCD does not offer protection against open-circuit PEN fault
Load
Consumer’s installation
Person in contact with Earth, touching exposed-conductive-part
Shock current returns via mass of Earth Red arrows denote path of shock current
supplying multiple electric vehicle charging points, provided that load control is available to prevent overloading of the circuit. Protective measures against electric shock Section 722 allows the use of any of the five protective measures against electric shock listed below in an electric vehicle installation. The two measures that seem the most likely to be used in practice are: • Automatic disconnection of supply (Section 411), subject to certain additional requirements in Section 722, mentioned later, including special requirements where PME conditions apply. • Electrical separation (Section 413), via a fixed isolating transformer with no part of the secondary winding earthed. A secondary winding may supply only one electric vehicle (Regulation 722.413.1.2 refers). The measures that seem less likely to be used are: • Double or reinforced insulation (Section 412). This measure is not suitable with charging equipment or vehicles having parts that need to be earthed. • Extra-low voltage provided by SELV or PELV (Section 414).
Protective Multiple Earthing (PME) Where the protective is automatic disconnection supply and a PME earthing facility is used as the means of earthing (TN-C-S system), special requirements of Section 722 apply if an electric vehicle charging point is: • located outdoors, or • might reasonably be expected to be used to charge a vehicle located outdoors. The special requirements are intended to protect against a risk of electric shock that can arise in the unlikely event of an open-circuit fault in the combined protective and neutral (PEN) conductor of the low voltage network supplying the installation. The fault can result in a dangerous voltage to Earth existing for long periods on the earthed metalwork of the installation and equipment connected to it (including any electric vehicle and its charging equipment), posing a danger to any person touching the metalwork whilst in contact directly with the general mass of Earth, as shown in Fig 1. An RCD offers no protection in these circumstances, as the shock current flows in both the line and neutral conductors passing through the core balance of the device and consequently there is no imbalance to cause operation of the RCD. The relevant requirement of Section 722, given in Regulation 722.411.4.1, is that – with one permitted exception (see later) – at least one of three conditions, summarised below, has to be met. (i) The charging point forms part of a three-phase installation that also supplies loads other than for electric vehicle charging, and the overall load is balanced or reasonably well balanced, meeting certain conditions described in Regulation 722.411.4.1 and Annex A722. (ii) The main earthing terminal of the installation is connected to an installation earth electrode having a resistance to Earth not exceeding a value calculated in accordance with a formula given in Annex A722, which is typically very low. (iii) Protection is provided by a special type of voltage-operated earth-leakage circuit-breaker (not an RCD) that disconnects the charging point from the live conductors of the supply and from protective earth under specified conditions. The device is not yet commercially available but a product standard for it is being developed (Regulation 722.411.4.1 refers). NICEIC Connections Winter 2013-14
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