Wiring System

Previously in this article it was stated that if a propery developer was involved in deciding the installation requirements then the cheapest options would be selected, to ensure the maximising of their profit. That should not deter from the need to properly assess the system and offer a proper wiring system based on best practice.

The key to proper design is to select the best methods and materials to suit the situation and conditions where they are to be installed.

For the electrical system it is best to start at the consumer unit, the unit should have the number of ways determined for the design of the system plus an additional 10% spare for future use.

Where the PES meter cabinet is back to back with the consumer unit standard double insulated cables should be used without containment, where this is not the case containment should be installed between the meter cabinet and consumer unit. The maximum size of conductor that can be terminated in an electricity meter is 35mm², the minimum should be suitable for the cutout fuse fitted in the PES service head, which is normally a 100A or 80A HRC type a good choice is 25mm², which with a rating of 131A in free air is suitable for both fuse sizes. The size of the meter tails determines the main protective bonding conductors at 16mm².

Accessory boxes should have a minimum depth of 35mm where the circuit cables are up to 4mm² and 47mm deep where larger cables are used. The reason for the depths quoted is to allow adequate space for installing and terminating cables.

The biggest decision is whether the wiring system is to be surface or concealed, as previously mentioned developers use minimal cost as the main criteria for determining the installation methods so the decision may not be yours to make. Where the decision is for you then a concealed wiring system is to be preferred for all habitable rooms.

The next decision is the dwelling internal wiring system should it be twin and earth or conduit with single core cables? That is twin and earth with or without capping where the cables are to be plastered over or conduit either round or oval or a combination of both.

The cheapest option is to use twin and earth, careful siting will allow the use of twin and earth without capping, that is the cables are buried directly in the plaster. The twin and earth wiring method does not allow for rewires, a rewire will require the removal of the plaster covering the wiring and its re-instatment on the completion of the rewire.

The preferred method is to use conduit with single core cables, the installation of conduit can include both round and oval types with proprietry adaptors between the two types, oval conduit should be used where the conduit is to be plastered over as a 20mm oval conduit has a minor dimension of 10mm which can be fixed directly on the wall without chasing into it and will normally have cover of 8mm of plaster. The conduits should run from the consumer unit to accessory point to accessory point with a maximum of three bends between any two points for ease of cable installation and future rewires. Where there needs to be additional bends there will need to be inspection points to enable the cables to be installed. The type of cables to be used should be stranded LSF insulated types with coloured insulation to suit the use, that is brown for live, blue for neutral and green and yellow for the circuit protective conductor, usually called the earth. When single core cables are used the switchlines can be differentiated from the other cables by their insulation colour.

If it is accepted that a conduit system should be employed for the electrical system it follows that the same method should be employed for the other systems that are to be installed. It therefore is a question of the cable types to be used.

Computer network cables should be CAT6 unshielded 4 pair twin twisted 0.575mm solid cores each with high density polyethylene insulation each pair separated by a spline with a sheath of LSF to TIA-568-C standard. For any cables that are exposed to the elements the sheath should be of medium density polyethelyne. There is an option for the network cables to be shielded, in a domestic situation the additional expense cannot be justified. The same cable type should be used for the telephone network. Where it is decided not to use a conduit containment system great care will need to be taken with the cable installation, computer network cable with LSF insulation is hydroscopic it cannot be installed without capping on walls that are to be plastered. Whatever containment is used it is essential that the cable is handled with care and at no time should kinks be allowed to develop in the cable during installation. If computer network cables are to be run externally then the cable should be as previously described and then have an extruded bedding layer, a galvanised steel wire armouring and medium density polyethelyne sheath.

Aerial distribution cables that connect the aerial distribution amplifiers to the television aerial points should be low loss coaxial cable with a solid 1mm² conductor, solid foam dielectric, overlapping copper foil screen, copper wire braid and LSF sheath with a maximum loss per 100metres of 5.7db @100MHz, 19dB @ 1000MHz, 27dB @ 2000MHz and 32dB @ 3000MHz and an overall maximum diameter of 7.5mm. Where it is decided not to use the conduit containment system great care will need to be taken with the cable installation, aerial cable with LSF insulation is hydroscopic it cannot be installed without capping on walls that are to be plastered. Whatever containment is used it is essential that the cable is handled with care and at no time should kinks be allowed to develop in the cable during installation. Aerial cables that connect to the external aerials and satellite dish should be as the aerial distribution cables but with a 1.25mm² conductor and a medium density polyethelyne sheath, which is waterproof, the signal losses should each be better than 3dB lower for each frequency when compared with the aerial distribution cables. It cannot be over emphasized that the aerial distribution cable described above or any other cable with a LSF sheath must not be used for external cable runs either as part of the aerial distribution system or for aerial connections also deprecated for external aerial system cabling is the use of cables with a sheath of PVC it will last longer than LSF but will degrade and fail in time due to a combination of sunlight, rain and frost. That degradation will be evidenced by the loss of television channels either terestrial of satellite.

Any cables installed as part of the AV system should be as stated in the installation manual for the AV equipment, however it must be kept in mind the cable insulation properties to ensure the appropriate containment is used.

The earthing system cables for those items mentioned in the earthing system design section should be sized as required by the IET Wiring Regulations and BSEN 62305 lightning protection. Where it is determined that high frequency low impedance earthing is required then the earth cable should have a flexible conductors instead of stranded conductors. For lightning protection an earth electrode with an earth resistance of 10ohms is the minimum required a better target is 1ohm, to achieve this multiple rods should be used. The earth rods should be connected to an earth bar located adjacent to the earth rods, with a minimum 25mm² PVC insulated cable. For regulatory purposes the main earth bar should be the earth bar in the consumer unit; connections from that bar should be made to the service head earth point, the earth rod earth bar, the water and gas service pipes where they enter the dwelling and the surge arresters mentioned in the earthing system design section. Connected directly to the earth rod earth bar should be the aerial mast and satellite dish metalwork.

The wiring method of the fire alarm system should reflect the chosen installation method used for the electrical system, where a conduit system is used then the wiring of the fire alarm system should utilise 1.5mm² Str:MGT:LSF (stranded conductor mica glass tape : low smoke and fume insulation) installed in conduit; where the wiring method is twin and earth then FP200 type cable should be used. Care must be taken when installing FP 200 as the sheath is of the LSZH type which is hydroscopic, where cables are to be installed where the cable is to be buried in plaster then the cable should be protected by capping. Fire alarm wiring must be kept separate from all other wiring, which means it cannot share trunking or conduit with other wiring.

The wiring of the security system is by tradition alarm cabling that at best is 0.75mm² or more usually 0.22mm² multicore cable, the larger size is better as it is more robust. The installation system should reflect the electrical system wiring installation, where that is by wiring not in conduit that wiring that is to be plastered over must be protected by capping as any moisture penetration of the cables will affect the circuit resistance which will in turn affect the alarm system anti-tamper detection which may trigger the anti tamper alarm or intruder alarm. Where a conduit system is used then proper cables should be used.

The external wiring can be split between outbuildings such as sheds and summer houses and outdoor systems such as hot tubs and garden lights. Where the electrical system is to be installed in outbuildings then the installation should be as for the habitable rooms of dwellings, excepting that surface conduit systems may be appropriate. External cables should be of the armoured type which offer better protection against the elements than cables in conduit which due to the inspection points are more liable to water ingress. Armoured cables consist of one or more conductors each conductor has an insulating sheath, the insulated conductors are then laid up with fillers to give a circular shape and given a taped or extruded bedding for the armour wires which are, for single core cables of aluminium and for multicore cables of galvanised steel, an oversheath is then extruded over the armour layer. Extruded bedding is preferred for outdoor cables as the extruded bedding acts like a pipe, whereas a taped bedding is a tape that is wound round the conductors and has a much lower resistance to water penetration. Armoured cables for outdoor use or for underground use must have a bedding and an oversheath of PVC; under no circumstance may LSF or LSZH be used as they are hydroscopic. Great care must be taken when installing armoured cables to ensure the outer sheath is not damaged since any damage can lead to water penetration into the cable and the errosion of the armour wires. The termination of armoured cables should be by proprietry gland to BSEN 50262: 1999 or BS 6121: Pt 1: 2005. Gland manufacturers will state that armoured cable glands type CW are suitable for external use, however that type of gland only seals the oversheath which allows atmospheric moisture to access the armour wires, via the enclosure, which have no galvanising on the cut ends which leads to rusting of the armour wires; a superior gland is type E1W which seals both the bedding and oversheath thereby minimising the effects of atmospheric moisture. The gland must be fitted with a tight fitting shroud. The interface between the gland and the enclosure into which it is to be connected must be sealed with a fibre washer, where the gland is to be secured with a backnut an earth tag should be fitted to the gland internally to the enclosure and connected by suitably sized fly lead to the enclosure earthing point.