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MCS: NIC1396
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Mains Supply & Earthing Systems
Location & Accessability
The Electricity Safety, Quality and Continuity Regulations 2002 requires the electricity distributor to install the cut out and meter in a safe location, where they are mechanically protected and can be safely maintained.
In accordance with this requirement the risk of flooding may be required to be taken into account. (see 'Preparing for Floods (ODPM, 2003)'. Distributors equipment and the installation consumer unit/fuse board should be above the flood level. Upstairs power and lighting circuits and downstairs lighting circuits should be installed above the flood level. Upstairs and downstairs circuits should have seperate overcurrent devices (fuses or circuit breakers).
Consumer units should not be installed where young children might interfere with them.
In accordance with these regulations and the contract for a mains supply, proposals for new installations or significant alterations to existing ones such as the introduction of PV must be agreed with the electricity distributor.
Supply Systems
The Electricity Safety, Quality and Continuity Regulations 2002 requires the electricity distributor (Regulation 27) to advise of:
- The number of Phases
- The Frequency; and
- The Voltage
On request the electricity distributor (Regulation 28) should provide the following information:
- The maximum prospective short circuit current at the supply terminals.
- For low voltage connections, the maximum earth loop impedence of the earth fault path outside the installation.
- The type and rating of the distributor's protective device or devices nearest to the supply terminals.
- The type of earthing system applicable to the connection.
Protective Multiple Earthing (PME) (TN-C-S system)
Almost all new supplies to dwellings will be from PME distribution systems. With a TN-C-S system earthing for the installation is provided from the distributor's fused cut out where it is common with the PEN or Neutral conductor.
Except in city centres the conditions assumed for a TN-C-S system are:
- The maximum external earth fault loop impedence Ze is 0.35 ohms.
- The maximum prospective fault current is 16 kA
See: Earthing: Your Questions Answered (IEE, 2005) for further details and diagrams.
Cable Sheath Earth (TN-S system)
The earth connection is the responsibility of the distributor and is made by connecting the earth connection to the sheath of the incoming supply cable. The connection should be secured with soldering or brazing.
A maximum fault level of 16 kA may be assumed and a maximum external earth loop impedence of 0.8 ohms.
See: Earthing: Your Questions Answered (IEE, 2005) for further details and diagrams.
No Earth Provided (TT system)
TT installations may be found in rural locations with overhead supplies or where a distributor may be unwilling to provide an earthing terminal such as that for a swimming pool, farm or building site.
It is necessary to install an earth electrode with an impedence to true earth of the electrode not exceeding 200 ohms. This can be checked by carrying out an earth impedence test when the supply has been connected.
Metal gas, metal water or other metal service pipes are not to be used as the earth electrode. A seperate earth electrode must be installed with any present gas, water and other metal service pipes being bonded to the new Main Earthing Terminal.
See: Earthing: Your Questions Answered (IEE, 2005) for further details and diagrams.
Main Protective Bonding of Metal Services
In every installation, main protective bonding conductors are required to connect to the main earthing terminal for every extraneous-conductive part; including:
- Water service pipes
- Gas installation pipes
- Other service pipes and ducting
- Central heating and air conditioning systems
- Exposed metallic structural parts of the building
- Lightning protection systems.
When an installation serves more than one building, the above requirement must be applied to each building. In certain special locations and in installations of increased shock risk supplementary bonding is required.
Earthing and main protective bonding conductor cross-sectional areas
Except where PME Conditions apply the minimum cross-sectional area (csa) of the main protective bonding conductors is not to be less than half that of the earthing conductor.
For a 100a TN installation, the earthing conductor csa is normally 16mm2 and that of the main earthing conductors 10mm2 where the size of the supply neutral conductor is not more than 35mm2.
| Protective conductor sizing for TN-C-S (PME) systems |
| Csa of supply neutral conductor | mm2 | 4 | 6 | 10 | 16 | 25 | 35 | 50 | 70 |
| Csa of non buried(1) earthing conductor (2) | mm2 | 10 | 10 | 10 | 16 | 16 | 16 | 25 | 35 |
| Csa of main protective bonding conductors (3) | mm2 | 10 | 10 | 10 | 10 | 10 | 10 | 16 | 25 |
(1) In the event that the earthing conductor is buried (see Table 54.1 of BS7671) it must have a csa of at least:
- 25mm2 copper of 50mm2 steel if not protected against corrosion by a sheath
- 16mm2 copper or 16mm2 steel if protected against corrosion by a sheath but not protected against mechanical damage.
(2) PME conditions apply, hence the csa of the earthing conductor must comply with the requirements of Section 543 and Section 544 of BS7671. In the table above, the csa of the earthing conductor has been selected in accordance with table 54.7 and where applicable Table 54.8.
When installing earthing and main protective bonding conductors:
- Only copper conductors should be used, other types of conductor (i.e. copper covered aluminium or steel) should only be used if special precautions are taken.
- Bonding connections to incoming metal services should be as near as possible to the point of entry of the services to the premisis but on the consumers side of any insulating section.
- The connection to the gas, water, oil etc service should be within 600mm of the service meter, or at the point of entry to the building if the service meter is external, and must be on the cosumer's side before any branch piperwork and after any insulating section in the service. The connection must be made to hard pipe, not to soft or flexible meter connections.
- The connections must be made using suitable clamps (to BS 951) which will not be subject to corrosion at the point of contact.
- If incoming gas and water services are of plastic, main protective bonding connections, if required, are to be made to metal installation pipes only.
| Protective conductor sizing for TN-S & TT systems |
| Csa of supply neutral conductor | mm2 | 4 | 6 | 10 | 16 | 25 | 35 | 50 | 70 |
| Csa of non buried(1) earthing conductor (2) | mm2 | 4 | 6 | 10 | 16 | 16 | 16 | 25 | 35 |
| Csa of main protective bonding conductors (3) | mm2 | 6 | 6 | 6 | 10 | 10 | 10 | 16 | 25 |
| Supplementary bonding conductor sizes |
| Exposed-conductive-part to extraneous-conductive-part | Exposed-conductive-part to exposed-conductive-part | Extraneous-conductive-part to extraneous-conductive-part* |
| Size of protective conductor (mm2) | Mechanically protected (mm2) | Not mechanically protected (mm2) | Mechanically Protected (mm2) | Not mechanically protected (mm2) | Mechanically protected (mm2) | Not mechanically protected (mm2) |
| 1.0 | 1.0 | 4.0 | 1.0 | 4.0 | 2.5 | 4.0 |
| 1.5 | 1.0 | 4.0 | 1.5 | 4.0 | 2.5 | 4.0 |
| 2.5 | 1.5 | 4.0 | 2.5 | 4.0 | 2.5 | 4.0 |
| 4.0 | 2.5 | 4.0 | 4.0 | 4.0 | 2.5 | 4.0 |
| 6.0 | 4.0 | 4.0 | 6.0 | 4.0 | 2.5 | 4.0 |
| 10.0 | 6.0 | 6.0 | 10.0 | 6.0 | 2.5 | 4.0 |
| 16.0 | 10.0 | 10.0 | 16.0 | 10.0 | 2.5 | 4.0 |
Labelling
Earthing clamps - Bondings to metal pipes are made with earthing clamps to BS951. A permanant label is required to be fixed into position at or near:
- The point of connection of every earthing conductor to an earth electrode or other means or earthing, and
- To the point of connection of every bonding conductor to an extraneous-conductive-part, and
- At the main earthing terminal where it is seperate from the main switchgear.
Switchgear and controlgear - Unless there is no possibility of confusion, a label indicating the purpose of each item must be fixed on or adjacent to the switchgear or controlgear. It may be necessary to label the item controlled as well as the controlgear.
Distribution Boards & Consumer Units - Each protective device e.g. a fuse or circuit breaker must be arranged and labelled so that the circuit protected can be easily identified.
Isolators - As well as being clearly identified switches used as isolators must also indicate the circuit or circuits that they switch.
Equipment supplied from more than one source - Certain equipment may require the operation of more than one switch in order to make it safe. In such a case a durable warning notice must be permanently fixed in a visible position to identify the appropriate devices.
Periodic Inspection & Testing - A notice of durable material indelibly marked with the words as below must be fixed in a prominant position at or near the origin of the installation.
Diagrams - A diagram or schedule must be provided showing:
- The number of points, size and type of cable for each circuit
- The method used to provide protection from indirect contact.
- The information necessary for the identification of each device performing the functions of protection, isolation and switching and it's location.
- Any circuit vulnerable to an insulation test.
Circuits that may be vulnerable to an insulation test would be those that have solid state devices such as burgalar alarms, fire alarms, central heating controllers, solid state transformers etc.
Residual Current Devices (RCDs) - Where an installation incorporates an RCD a notice must be fixed in a prominant position at or near the installation as follows:
Warning notice: Non standard colours - If alterations or additions are made to an installation so that some of the wiring complies with the harmonised colours and there is also wiring in the old colours, a warning notice must be affixed at or near the appropriate distribution board with the following wording:
Unexpected presence of nominal voltage exceeding 230 V - Where the nominal voltage exceeds 230 V to earth or 400 V between lines a warning label stating the maximum voltage present must be provided where it can be seen before gaining access to live parts.
Nominal voltage exceeding 230 V (U or Uo) between simultaneously accessible equipment - For simultaneously accessible equipment with terminals or other fixed live parts having a nominal voltage (U or Uo) exceeding 230 V between them, e.g. 400 V line to line, a warning label must be provided where it can be seen before gaining access to live parts.
Presence of different nominal voltages in the same equipment - Where equipment contains different nominal voltages, a warning label stating the voltages present must be provided so that it can be seen before gaining access to live parts.
Installing Residual Current Devices (RCDs)
RCDs (Residual Current Devices)and RCBOs (Residual Current Circuit Breakers with integral overcurrent protection) are required:
- Where the earth fault loop impedence is too high to provide the required disconnection time, eg where the distributor does not provide an earth - TT system.
- For socket outlet circuits
- For bathroom and shower room circuits
- For circuits supplying portable equipment for use outdoors by means of a flexible cable.
- For cables without earthed metal covering installed in walls or partitions at a depth of less than 50mm and not protected by earthed steel conduit or similar.
- For cables without earthed metal covering installed in walls or parttions with metal parts (not including screws and nails) and not protected by earthed steel conduit or similar.
Installations are required to be divided into circuits to avoid danger and minimise inconvenience in the event of a fault or to take account of hazards that might arise from the failure of a single circuit e.g. a lighting circuit. The use of RCBOs will minimise inconvenience in the event of a fault and is applicable to all situations.
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