More essential technical advice from the team at ELECSA.
As part of their normal operation many electronic items, such as computers, printers, games consoles and the like, can cause a current to flow in the protective conductor. This is often referred to as earth leakage current. Whilst the level of leakage current in a typical domestic premises is unlikely to be an issue, other types of installations such as those comprised of large numbers of electronic devices the earth leakage currents could be of a magnitude to present a risk of electric shock should the circuit protective conductor become disconnected. Suitable measures to prevent and limit such a risk during the design stage of any circuit must be considered.
Whilst protective conductor currents are generally not sufficient to cause protective devices to operate they can pose a risk of electric shock. Therefore, where the risk of protective conductor currents exist, the relevant requirements of Section 543.7 of BS 7671, as discussed in this article, should be taken into consideration to ensure the integrity of the protective conductor is maintained to reduce the risk of shock.
Section 543.7 of BS 7671 outlines the general requirements for high protective conductor currents in circuits due to the connected load:
- Any equipment having a protective conductor current greater than 3.5 mA but not exceeding 10 mA shall be either permanently connected to the fixed wiring of the installation without the use of a plug and socket-outlet or connected by a suitable plug and socketoutlet complying with BS EN 60309-2.
- Equipment having a protective conductor current exceeding 10 mA shall be connected to the supply by one of the methods (i), (ii) or (iii) as stated in Regulation 543.7.1.202.
Circuits
In an installation where a final or distribution circuit is expected to supply one or more items of equipment and where the protective conductor current is likely to exceed 10 mA a high integrity protective connection shall be provided by one of the following methods stated in Regulation 543.7.1.203:
- a single protective conductor having a csa of not less than 10 mm2, complying with the requirements of Regulation 543.2 and 543.3,
- a single copper protective conductor having additional protection against mechanical damage and having a csa of not less than 4 mm2 and complying with the requirements of Regulation 543.2 and 543.3,
- two individual protective conductors, each complying with the requirements of Section 543. Additionally the protective conductors may be of different types for example, the armouring of a SWA cable combined with an integral cpc.
Radial circuits
A standard radial circuit has a single protective conductor. Therefore any loss of the cpc, say as a result of a loose termination at a socket-outlet, where a high protective conductor current is present could result in a risk of electric shock unless an alternative path is provided for such current to flow back to the MET.
Consequently, where it is known or reasonably expected that the total protective conductor current in normal service will exceed 10 mA, the circuit shall be provided with a high integrity protective conductor. One way to achieve this for a radial circuit is to install an additional protective conductor from the final socket outlet directly back to the distribution board as shown in Fig 2.
These precautions increase the reliability of the connections of protective conductors to equipment and to the means of earthing further reducing the risk of shock. In the event of a loss of a cpc at a socket-outlet both conductors will remain connected to the earthing terminal at the distribution board.
Note: To minimise the effects from interference any separate protective conductor should be run within close proximity to the other conductors of the circuit.
Ring final circuits
For ring final circuits, the risk of electric shock from high protective conductor currents is minimised by the parallel paths provided by the circuit. Where such a ring circuit is installed in a metallic conduit or trunking, the conduit or trunking may provide one of the protective conductor paths. Compliance with the general requirements of Section 543 regarding the appropriate cross-sectional area of cpcs must also be met.
Connections
For the purpose of maintaining the integrity of the cpc, all accessories such as socket-outlets and connection units that are designed for high protective conductor currents must have available two separate earthing terminals to accommodate the individual connections (Regulation 543.7.1.204).
Where a radial circuit includes two individual protective conductors both incorporated in a multi-core cable for example, an SWA; the total csa of all conductors including the live conductors shall be not less than 10 mm2. This increase in size is intended to provide mechanical strength for reliability rather than protection against the insignificant thermal effects from protective conductor currents.
Labelling
Where two protective conductors are used for a single circuit, Regulation 543.7.1.205 requires the conductors to be terminated separately at each accessory and at the distribution board. This will often mean that the cpcs are required to share the same terminals as with other separate circuits, as shown in Fig 2. To avoid confusion a durable and secure warning notice and/or sign shall be provided indicating those circuits having a high protective conductor currents; this shall be positioned at the distribution board and clearly visible to any person modifying or extending the circuit.
Summary
Where high protective conductor currents are likely to be present within an installation, Section 543.7 of BS 7671 requires suitable precautions to be taken to minimise the risk of electric shock.
All accessories shall be suitably selected for circuits with high protective conductor currents and are required to have dual connections for providing a reliable connection to earth.
Where high protective conductor currents are present within a circuit suitable identification shall be provided and clearly visible highlighting such risk.
