Insulation

5.7.1 All Electrical Installations must be sufficiently insulated to protect against electric shock from Direct Contact by any person (clause 5.3.1). Such insulation must be capable of withstanding wear and tear during normal use of the equipment. Supplementary insulation or ‘double insulation’ may be used where additional robustness is required.

Protection

5

5.7.2 The application of paints, resins, varnishes and similar materials is not considered to satisfy the requirements of clause 5.7.1 and additional insulation, barriers or obstacles are required to prevent Direct Contact by any person.

[Note: an example may be a motor winding which is enamelled or resin coated and therefore must be guarded against Direct Contact by persons.]

5.7.3 Live conductors are required to be inaccessible without the use of a special key or tool, available only to authorised persons and only for the purpose of testing, using special equipment and procedures.

5.7.4 Uninsulated equipment may be used at voltages not exceeding 12 V a.c. or 30 V d.c. and only where supplied by a SELV source, see Regulation 9.1.

5.7.5 The insulation resistance of Circuit conductors must be measured and recorded as part of the test procedures given in Chapter 8 and must be greater than the values given in Appendix A19(f).

5.7.6 Where an Electrical Installation is supplied by underground cables, no special provisions are required for protection against over-voltages arising from atmospheric origin or from switching. Where an Electrical Installation is supplied by overhead lines, advice should be sought from the Distribution Company or the requirements of BS 7671 – Chapter 44 may be used.

6

6.1

General principles

6.1.1 Earthing of Exposed-Conductive-Parts of an Electrical Installation and of Appliances in a Premises is required and must provide the following functions of safety:

(a) allow the passage of fault current in the event of a live conductor touching an Exposed-Conductive- Part;

(b) ensure that the magnitude of fault current is sufficient to operate Protective Devices within 0.4 seconds for all parts of an Electrical Installation; and

(c) ensure that, in association with Protective Devices, a ‘high resistance’ fault to Earth does not persist so as to cause overheating or fire.

6.1.2 The necessary requirements to achieve the above functions of safety are detailed in the following sections.

6.2

Systems of Earthing

6.2.1 The following types of system Earthing are considered in these Regulations:

(a) Locally Earthed System (TT): the Owner provides

a Main Earth Terminal for the Electrical Installation, which is connected to a sufficient number of local Earth Electrodes to provide a maximum Earth Resistance measured at the Owner’s Main Earth Terminal no greater than 10 Ohms (referred to in BS 7671 as a ‘TT’ system); and

6

(b) Distribution Company Earthed System (TN-S):

the Distribution Company provides a connection to the Owner’s Main Earth Terminal, using the distribution network Earthing system, normally via the armouring or metallic sheath of the incoming connection cable (referred to in BS 7671 as a TN-S system). The Distribution Company system is Earthed at the distribution transformer and separate Earth and neutral conductors are used throughout the distribution network.

[Note: these types of Earthing systems are illustrated in Appendix A5(a) – A5(c). Earth Resistance values of less than 10 Ohms may be specified by the designer for purposes such as Functional Earthing or for specialist installations such as petrol stations, telecommunications sites, etc.]

6.2.2 The Earthing system to be used in Abu Dhabi Emirate is a combination of both TN-S and TT. The use of any other system of Earthing must be approved by the Distribution Company. The type of Earthing system must be stated on the Electrical Installation Certificate and clearly labelled at the Main Distribution Board.

6.2.3 In all cases, the neutral and Earth Conductors must be kept separate and not connected together at the MET or at any other point downstream from the Owner’s Connection Point.

6.3

Main Earth Terminal

6.3.1 The Owner’s Electrical Installation must include a Main Earth Terminal located close to or within the Main Distribution Board and must be clearly labelled.

6.3.2 The Locally Earthed System (TT) and the Distribution Company Earthed System (TN-S) shall be connected to the MET.

Ear

thing

6

6.3.3 A means of removing the connection to the Locally Earthed System (TT) and the connection to the Distribution Company’s Earthed System (TN-S) (such as by a removable metal link) must be provided at the Main Earth Terminal to facilitate testing of the Earthing conductivity within the Electrical Installation and the Locally Earthed System (TT), see Appendix A5(e).

6.3.4 The Earth Resistance of the Locally Earthed System (TT) measured at the MET must be recorded on the Electrical Installation Certificate in accordance with the procedures described in Chapter 8. Methods of measuring Earth Resistance are shown in Appendix A19(a).

6.4

Earth Electrodes

6.4.1 A Locally Earthed System (TT) must be provided at all Premises.

6.4.2 For Premises consisting of more than one building, a Locally Earthed System (TT) must be provided at each building that is at a distance of more than 30 m from the Electricity Intake. 6.4.3 Earth Electrodes will normally comprise of 15 mm diameter steel-cored copper rods driven to a minimum depth of 3 m. The top of each Earth Electrode must be housed inside a 300 mm x 300 mm x 300 mm inspection pit which is clearly labelled and accessible for routine testing, see Appendix A5(d). Alternative types of Earth Electrodes may be used with prior approval of the Distribution Company (e.g. plate or wire mesh type, or structural steelwork in a building). 6.4.4 Where more than one Earth Electrode is required to achieve

the required Earth Resistance value, these must be separated at a distance greater than their mutual resistance zone.

[Note: a minimum separation of twice the Earth Electrode depth is considered to give adequate separation of the mutual resistance zones. See BS 7430 for further advice on spacing of Earth Electrodes.]

6

6.4.5 The condition of the ground in which an Earth Electrode is placed must be taken into account to ensure its long-term performance. In particular, potential corrosion effects and ground moisture content is of critical importance. Proprietary chemical or salt materials may be used around the Earth Electrode to maintain moisture content but these must be stable and sustain their chemical properties over time.

[Note: consideration to be given to the use of stainless steel rods in coastal regions.]

6.4.6 For supplies of 500 A rating and above, at least two independent Earth Electrodes must be provided, regardless of the Earth Resistance value achieved for each Earth Electrode, and connected to the same Main Earth Terminal, see Appendix A5(k).

6.4.7 Metallic service pipes (such as water, gas, etc) must not be used as Earth Electrodes.

6.4.8 The use of structural foundations or piles as an Earth Electrode shall be in accordance with the requirement of BS 7430 and be subjected to all of the following conditions: (a) adequate precautions against the possibility of

electrolysis and consequential degradation of the metal (e.g. corrosion, continuous d.c. earth leakage current);

(b) electrical continuity between all metalwork forming part of the Earth Electrode is maintained (e.g. welding, clamping or bonding links across structural joints);

(c) provision for measuring and monitoring the Earth Resistance value at regular intervals;

(d) prior approval of the design by the Distribution Company;

(e) the consent of the Owner of the Premises; and (f) the consent of the Premises structural engineer.

Ear

thing

6

6.5

Earth Conductors

6.5.1 In these Regulations the following terminology is used to refer to Earth Conductors in an Electrical Installation, see illustration in Appendix A5(f):

(a) Main Earth Conductors (MEC): conductors connected between Earth Electrodes and Main Earth Terminals; and

(b) Circuit Earth Conductors (CEC): conductors connecting all Circuits emanating from Main Distribution Boards, Sub Main Distribution Boards, Final Distribution Boards including Circuits connecting to equipment and Appliances. Outside these Regulations, these may also be known as the Circuit Protective Conductor (CPC) or Earth Continuity Conductor (ECC).

6.5.2 All Earth Conductors must be covered with green/ yellow PVC insulation and be securely terminated with purpose- made lugs or fixings.

6.5.3 Where associated with Circuits, all Earth Conductors must be labelled at their termination points with Circuit identification numbers, see Guidance note G7(f).

6.5.4 The connection of Earth Conductors to Earth Electrodes must be made using corrosion resistant clamps.

6.5.5 Where Earth Conductors are buried below ground they must be mechanically protected (e.g. in metal duct or pipe) and, where necessary, with additional tiles or plates laid above the duct or pipe, see Appendix A5(d) and Guidance note G6.

6.5.6 The connection of Earth Conductors to the Main Earth Terminal must be made using purpose-made lugs or other fixings and the connection must be clearly labelled as shown in Appendix A5(d).

6

6.5.7 All Circuits must have a Circuit Earth Conductor sized in accordance with Appendix A5(j).

6.5.8 Circuit Earth Conductors must run alongside the associated phase and neutral conductors.

[Note: this minimises the risk of a Circuit Earth Conductor being damaged or disconnected without any damage or fault indication being detected via the phase conductors.]

6.5.9 For metal-sheathed or armoured cables, the sheath or armouring shall not be used as the Earth Conductor. However, the metal sheath or armouring shall be terminated by suitable cable glands with appropriate connection to an Earth Conductor providing a supplementary return path, see Guidance note G7(i).

6.5.10 The use of metal conduit, trunking, busbar trunking or switchgear metal enclosures as Earth Conductors is not permitted without the prior approval of the Distribution Company. In such cases, the provision of additional measures such as resistance measurements or supplementary Earth Conductors will normally be required.

[Note: metal conduit, although not to be used as an Earth Conductor, must nevertheless be effectively connected to Earth since it comprises an Exposed-Conductive-Part. See Guidance note G7(j).]

6.5.11 No switches, isolators or circuit-breakers may be installed in the electrical path of any Earth Conductor. Removable links may be installed to allow testing at the Main Earth Terminal.