ALL YOU WANT KNOW ABOUT WIRING OF BUILDINGS - LCETED

 WIRING OF BUILDINGS

Types of Wiring

The following three types of wiring are used:

1. Sheathed wiring by surface fixing

2. Conduit installation on walls and ceilings

3. Concealed conduit wiring.

Nowadays concealed PVC conduit wiring is the fashion in most buildings. Conduit wiring can also be made on the surface. It must be remembered that PVC has a high coefficient of expansion. For concealing in concrete, the conduits are laid before concreting and in masonry, a chase is made before the final plastering. In each case, care should be taken to see that there will be no chance for water to enter these pipes from any place after they are finally laid. When laying in concrete, it is a good practice to surround the conduit in chicken mesh reinforcement.

 

Use of Flexible Cords

Flexible cords are used in many electrical appliances such as electric iron, pendant lights, etc. Some important and useful properties of flexible chords are discussed further.

 

Rating of strands. Flexible cords are made of small strands and not a single wire. The usual sizes of available flexible cords are as given in the Table below

Table: Rating of strands

Cross-sectional area (mm2)	Number and diameter (mm) of strands	Current rating (ampere)
0.5	16/0.20	3
0.75	24/0.20	6
1.0	32/0.20	10
1.25	40/0.20	13
1.5	30/0.25	15
2.5	50/0.25	20
4.0	56/0.30	25

 

From the Table below, it can be observed that for lighting and ligh duty appliances the sizes to be used are 0.5 and 0.75 mm². Typical sizes for various electric ratings can be in terms of cross-sectional areas as shown in the table below.

Table: Sizes of cords of various electrical ratings

(Note: The area can be used as from Table)

Electrical rating (kW)	Cross-sectional area (mm2)
1	0.75
2	1.0
3	1.5

 

Weight carrying capacity: The maximum weight that may be supported by twin flexible cords is as shown in the table below

Table: Weight carrying capacity of flexible cords

Area (mm2)	Maximum weight (kg)
0.5	2
0.75	3
1.0	5
1.5	5
2.5	5
4.0	5

 

When used for suspended lights, it is always preferable to employ additional steel straining wires, suitably positioned, which will take the weight away from the cords. Further, when a non-metallic outlet box of thermoplastic material (like PVC) is used for the suspension of a light fitting, care is necessary to ensure that the box temperature does not exceed 60°C. The mass suspended from the box must not exceed 3.2 kg.

 

Colour-code identification for flexible cords: Appliances with 3-core flexibles are usually identified by:

Live core - Brown, Red, yellow

Neutral core - Blue

Earth - Green/Yellow

 

Precautions are necessary in light fittings and pendants. Heat-resisting cords are necessary for most connections between the ceiling rose and lamp holder where tungsten filament lamps are to be used due to the abnormally high temperatures generated by these. Light fittings and shades, especially if flush-mounted and totally enclosed, require heat-resisting insulation suitable for the temperatures likely to be encountered. Here heat-resisting sleeves are also used. They should be fitted over the individual cores of the flexible cables in such a way that the normal insulation of the cores is not relied upon to prevent a short circuit between the conductors or an earth fault. Similar methods should be employed for accessories and appliances which are subject to such heating conditions.

Temperature ratings for flexible-cord materials. The maximum operating temperatures of insulation or sheath of flexible cords are usually rated as given in the Table below

 

Table: Temperature ratings of cables (IEE ratings)

 

Type of cord	Normal service °C	Contained within fittings and not subject to stress °C
General-purpose rubber compound	60	75
General-purpose PVC	70	75
Heat-resisting PVC	85	100
EP or butyl rubber	85	100
Silicone rubber	150	200

 

Where the insulation and sheath are of different materials, appropriate temperature limits must be observed for both. If the insulation is not good, it may become hard and brittle so that it lacks flexibility. If allowed to run hot, a twin-twisted cord can thus easily fail and cause a 'short' at the bifurcation point just above the holder, often resulting in a rupture of the flexible wire and collapse of the fitting and lamp.

Flexible-cord sheaths are suitable for contact with oil or petrol. Where a risk of fire is present or if a flexible sheath which will not support combustion is required, cords with heat-resisting, oil-resisting and flame retardant (HOFR) sheaths must be used.