[Here insert diagram showing protective conductor current flowing back to the means of earthing]

Transcription

1 Leakage May 30 PROTECTIVE CONDUCTOR CURRENTS When energized and in normal use, some electrical equipment causes current to flow in the circuit protective conductors. Such protective conductor current (previously referred to as leakage current ) is often associated with filters and suppressors in items such as computers, fluorescent luminaires and telecommunications equipment, though it may also be associated with certain types of heating element. Risk of electric shock A circuit with a high protective conductor current (that is, greater than 3.5 ma) will present a risk of electric shock should the protective conductor become disconnected. BS 7671 recognizes this risk and stipulates particular requirements in Section 607: Earthing requirements for the installation of equipment having high protective conductor currents that supplement or modify the general safety requirements. [Here insert diagram showing protective conductor current flowing back to the means of earthing] Filters and suppressors Filters may be provided in a circuit to protect downstream equipment from the effects of transient or switching overvoltages originating from other parts of the fixed installation or from the supply distribution system (protective purpose), or to remove unwanted interference on the incoming supply that would affect the operation of the equipment (functional purpose). Suppressors may be provided in equipment to prevent high frequency noise generated in the equipment (such as from switch-mode power supplies) from being transferred into the power supply. Such filters and suppressors usually include capacitive and inductive elements between the live conductors and the circuit protective conductor, resulting in a current in the circuit protective conductor under normal operating conditions. It is important to recognize that, where earthing is provided for combined protective and functional purposes, the protective requirements must take precedence (Regulation refers). In other words, whilst functionality is important, safety is paramount. Heating elements

2 Certain heating elements may, due to their construction, exhibit earth leakage resulting in a protective conductor current. The magnitude of protective conductor current may vary with the temperature of the heating element. High frequency luminaires Many modern fluorescent luminaires operate at high frequency, such as 30 khz, to improve light output, reduce control gear losses, provide silent operation and reduce perceptible flicker. To prevent high frequency noise being impressed on the supply, these luminaires may be fitted with suppressors which produce protective conductor currents. A large office building fitted with such luminaires may have a significant current (perhaps of the order of amperes) in the earthing conductor. EQUIPMENT HAVIN A PROTECTIVE CONDUCTOR CURRENT OF LESS THAN 3.5 ma No special precautions are required where equipment having a protective conductor current less than 3.5 ma is connected (Regulation refers), unless it is known or can reasonably be expected that the total protective conductor current in a socket-outlet final circuit in normal service will exceed 10 ma. For example, a socket-outlet final circuit in an office building may have a protective conductor current exceeding 10 ma, depending on the range of computer equipment connected. Where a protective conductor current exceeding 10 ma is anticipated, special precautions are required, including the provision of a high integrity protective connection. EQUIPMENT HAVIN A PROTECTIVE CONDUCTOR CURRENT EXCEEDIN 3.5 ma BUT LESS THAN 10 ma. In accordance with Regulation , equipment which in normal service will have a protective conductor current exceeding 3.5 ma but less than 10 ma must either: be permanently connected to the fixed wiring of the installation without the use of a plug and socket, or be connected by means of a connector complying with BS EN [Here insert picture of a connector to BS EN ] Where the protective conductor current is expected to exceed 3.5 ma, Section 607 gives further requirements concerning TT and IT systems, and residual current devices. These requirements are summarized later in this article. EQUIPMENT HAVIN A PROTECTIVE CONDUCTOR CURRENT EXCEEDIN 10 ma Where an item of equipment having a protective conductor current exceeding 10 ma is to be supplied, Section 607 gives additional requirements for:

3 the means of connection of the equipment the final and distribution circuits supplying the equipment the termination of protective conductors There are also additional requirements relating to socket-outlet final circuits, the cross-sectional area (csa) of the protective conductors, additional information to be provided at the distribution board, TT and IT systems, and residual current devices. Means of connection for equipment having a protective conductor current exceeding 10 ma The means of connection must be effected by one of three permitted methods - a permanent connection, a plug and socket-outlet complying with BS EN , or by employing an earth monitoring system (Regulation refers), as follows: Permanent connection. Equipment with a protective conductor current exceeding 10 ma should preferably be permanently connected to the fixed wiring of the installation with the protective conductor meeting the high integrity requirements detailed below. It is permitted for the final connection between the item of equipment and the wiring of the installation to be made by means of a flexible cable. Plug and socket-outlet complying with BS EN Equipment with a protective conductor current exceeding 10 ma may be connected to the fixed wiring of an installation by a plug and socket-outlet complying with BS EN (as for equipment with a protective conductor current between 3.5 and 10 ma), provided that one of two permitted additional requirements relating to the cross-sectional area (csa) of the protective conductor are met. The first requirement is that the csa of the protective conductor of the associated flexible cable is to be not less than 2.5 mm 2 for plugs rated at 16 A and not less than 4 mm 2 for plugs rated above 16 A. Alternatively, the protective conductor of the associated flexible cable is to be of a csa not less than that of the phase conductor. Earth monitoring system. An earth monitoring system to BS 4444 may be employed provided that, in the event of a continuity fault in the protective conductor, automatic disconnection of the supply to the equipment occurs. The final and distribution circuits supplying equipment having a protective conductor current exceeding 10 ma The final and distribution circuits supplying an item (or items) of equipment with a protective conductor current exceeding or likely to exceed 10 ma must meet requirements centred on the provision of a high integrity protective conductor

4 arrangement, which is to be realized by one or more of the following (Regulation refers): A single protective conductor having a csa of not less than 10 mm 2 A single copper protective conductor having a csa of not less than 4 mm 2, the protective conductor being enclosed to provide additional mechanical protection. For example, a flexible conduit or a trunking system could be employed. Two individual protective conductors, each one complying with the requirements of Section 543, dealing with the csa and types of permitted protective conductors. It is permitted for the two protective conductors to be of different types, such as a metallic conduit and a protective conductor enclosed within the same conduit. The two individual protective conductors may also be incorporated in the same multicore cable providing the total csa of all the conductors, including the live conductors, is not less than 10 mm 2. One of the protective conductors may be formed by the metallic sheath, armour or wire braid screen incorporated in the cable, provided that it meets the requirements of Regulation Here insert diagram of independent termination using an armoured cable. Swa and one core form the two protective conductors. An earth monitoring system to BS 4444 configured to automatically disconnect the supply to the equipment in the event of a continuity fault in the protective conductor A double-wound transformer or equivalent in which the input and output circuits are electrically separate. In this case, the circuit protective conductor(s) is required to connect the exposed-conductive-parts of the equipment and the secondary winding of the transformer to the main earthing terminal of the installation, and the protective conductor between the equipment and the transformer is required to comply with the requirements of Regulation It is important to note that the limits on earth fault loop impedance associated with protection against indirect contact must be met, taking into account the added impedance represented by the transformer. Termination of duplicate protective conductors Where two protective conductors are employed in accordance with Regulation (iii), the ends of the conductors must be terminated independently of each other at all connection points in the circuit. This requires accessories such as socket-outlets, junction boxes, and connection units to be provided with two separate earth terminals. Distribution boards and consumer units must also be provided with sufficient connection points to permit independent termination of the ends of the protective conductors (Regulations and refer). Here insert illustration from p4 of Electrium catalogue: Rear view of 4306/D/TE twin s-o with two Earth terminals. Mention illustration courtesy of Electrium Ltd.

5 Socket-outlet final circuits A final circuit supplying socket-outlets or connection units where it is known or reasonably to be expected that the total protective conductor current in normal service will exceed 10 ma, must comply with Regulations and (Regulation refers). Such a final circuit must be provided with high integrity protective conductor connections, such as two individual protective conductors or a single protective conductor having a csa of not less than 10 mm 2 (Regulation refers). Where two separate protective conductors are used, Regulation requires socket-outlets and other accessories in the circuit to have two separate earth terminals, to enable the ends of the protective conductors to be terminated independently of each other at all connection points. The following arrangements for a final circuit are acceptable: Ring final circuit. A ring final circuit, by its particular configuration, provides duplication of the protective conductor and, provided the requirements of Section 543 are met, a high integrity protective conductor connection exists. However, for the requirements of Section 607 to be met, the ends of each section of protective conductor in the ring must be separately terminated. Socket-outlets and accessories such as connection units must be provided with two separate earth terminals. Spurs (cable branches), if provided, require a high integrity protective conductor connection and separate termination of the protective conductors. Here insert diagram of a ring final socket-outlet circuit Radial final circuit. A radial final circuit must be provided with a high integrity protective conductor connection and, if duplicate protective conductors are employed, separate termination of the protective conductor ends. The following methods may be employed. A separate duplicate protective conductor is provided to configure the protective conductor in a ring A separate protective conductor is provided at the final socket-outlet by connection to metal conduit, metal trunking or metal ducting The protective conductor of an adjacent radial circuit is employed. (Regulation (ii)(c) provides details of the requirements applicable when this method is selected). Other circuits. Other circuits must meet the requirements of Regulation , which include the provision of a high integrity protective conductor connection and the independent termination of protective conductors.

6 Cross-sectional area of protective conductors The cross-sectional area of the protective conductors must not be less than that determined in accordance with Section 607 or by Regulation , whichever is the greater. (Regulation refers). Additional information to be provided at the distribution board Where a circuit has or is likely to have a high protective conductor current, the protective conductor connection arrangements at the distribution board will, in many cases, be affected. For example, there will in some cases be duplicate protective conductors separately terminated. At the distribution board, information must be provided indicating those circuits having a high protective conductor current, and such information must be so positioned as to be visible to a person modifying or extending the circuit (Regulation refers). [Herebelow is a label] Warning - high protective conductor currents The circuits identified below may have a high protective conductor current Circuit: Modifications to these circuits must comply with the requirements of BS 7671: Requirements for Electrical Installations (as amended) including the supplementary requirements of Section 607: Earthing requirements for the installation of equipment having high protective conductor currents. CONNECTION OF EQUIPMENT HAVIN A PROTECTIVE CONDUCTOR CURRENT EXCEEDIN 3.5 ma TT Systems Where items of equipment having a protective conductor current exceeding 3.5 ma in normal service are to be supplied from an installation forming part of a TT system, the product of the total protective conductor current (in amperes) and twice the resistance of the installation earth electrodes (in ohms) must not exceed 50. Where this condition cannot be achieved, the equipment must be supplied through a double wound transformer or equivalent (Regulation refers). IT Systems Equipment with a protective conductor current exceeding 3.5 ma in normal use must not be connected directly to an IT system. Connection by a suitable double wound transformer may be suitable (Regulation refers).

7 Residual Current Devices Protective conductor current in a circuit will be detected by any residual current device (RCD) used in that circuit, which could result in the device operating during normal operation or during switching surges. An RCD must be so selected and the circuits so subdivided that any protective conductor current expected to occur during normal operation of the connected load(s) will be unlikely to cause unwanted operation of the device (Regulation refers). As a rule of thumb, the designer should take care that the anticipated total protective conductor current does not exceed 25% of the rated residual operating current of the RCD. Where, for example, several items of equipment producing protective conductor current are to be supplied, it may be necessary to provide a number of separate RCD-protected circuits, each designed to supply a limited number of items appropriate to their protective conductor current. Regulation refers.