ELECTRICAL WORKS IN BUILDINGS - METHODOLOGY

A.            METHOD OF CONDUITING, BOX FIXING & INTERNAL WIRING

1.            Carry out the work as per the latest approved drawings confirmed by the concerned Project Engineer. All electrical conductors shall be of sufficient size and current rating for the purposes for which they are to be used.

2.            Use medium /heavy gauge rigid PVC Conduit: 20 /25 /32 mm diameter, 2mm thick black colour and of ISI approved make, as indicated in drawing. 

3.            Conduits should be laid between the reinforcement of slab and properly fastened to the reinforcement with binding wires.

4.            Care should be taken to see that the conduits are held in position and the cement grout does not enter the conduits while concreting and no conduits get damaged due to movement of laborers on the slab before pouring and during pouring for inspection of conduits & joints. For the floor conduiting care should be taken that conduiting is done only just before the flooring works starts.

5.            Vertical conduits in wall chase to be properly fixed in position at the location indicated in the drawing. Horizontal runs of conduits in wall to be avoided as far as possible. In hollow blocks, horizontal chases are not permitted.

6.            Bends in conduit pipes should not be sharp. In embedded conduiting bends made with bending spring with large radius should be used. The maximum bending radius allowed in any conduit is 2.5 times the diameter of the conduit.  The use of readymade bends should be avoided as far as possible.

7.            Switch boxes to be recessed in the wall in such a way that it should be projecting 1-3 mm from wall plaster. The thickness of plaster shall be ascertained with button or bull marks and confirmed by site engineer. The boxes may project a maximum of 1-3 mm. Further, the wall finish such as lime rendering or punning, Glazed Tiles / Marble / Granite, etc. are to be confirmed with concerned Project Engineer. The finish will determine the projection. The idea is to ensure the box projects 0.5mm beyond the painted surface and the edges do not break while the wire is drawn.

8.            Cover all the boxes with thermocol and masking tape / dummy plates to avoid damages during plastering. These covers should be removed only at the time of wiring.

9.            Pass GI pull wires of 16-gauge inside the conduits with additional loop length of 15cm as draw wire.  

10.         The wiring cables to be of ISI approved make, PVC insulated, stranded copper and 1100V grade. Use following cross section for wiring.

                                6 Sqmm         -     Sub mains (or as per drawing)

                                4 Sqmm               -              Power Circuit

                                2.5 Sqmm           -              Lighting Circuit 

                                                                                        Earthing for Sub mains

               1.5 Sqmm            -              Light point wiring, 5A socket outlets & fan circuits

                                                                                        Earthing for Power Circuits

                1 Sqmm         -      Earthing for light point wiring, 5A socket outlets & Fan Circuits.

11.         Red, Yellow, and Blue for phases, Black for Neutral and Green for earth – colour coding for wires must be followed.

12.         Expansion coupler shall be used when conduit passes through building expansion joint. One end of the expansion coupler shall not be glued.

13.          Earthing

At every light point earthing terminal should be provided and connected to the earth continuity conductor. The resistance of any point of the earth continuity system to the main earth electrode should not be more than 0.5 ohm.

Every electrical circuit and sub-circuit shall be protected against excess current by circuit breakers. No circuit breaker other than a linked circuit breaker shall be inserted in a conductor connected with earth.

Where metal works other than current carrying conductors is liable to become charged with electricity, the metal works shall be earthed in such a manner as to ensure immediate electrical discharge without danger. The neutral conductor shall be insulated throughout and at no point it should be connected to the earthing system. 

All earth connections shall be visible for inspection and shall be carefully made; if they are poorly made or inadequate for the purpose for which they are intended, loss of life and property or serious personal injury may result.

Joints in the earthing should be invariably done by means of welding except in such locations where intermittent disconnection of joints is necessitated for testing purpose or for the earthing connection to the body of the equipment etc.

Use fully insulated tools while handling electric wires and equipment.

14.         Mounting Height of Electrical Accessories

Light switch                                        -              1250 mm above Finished Floor Level (F.F.L.)

Bell Push                                              -              1250 mm above Finished Floor Level (F.F.L.)

Socket Outlet                                     -              300 mm above Finished Floor Level (F.F.L.)

TV/Telephone Outlets                     -              300 mm above Finished Floor Level (F.F.L.)

Bedside Accessories                        -              600 mm above Finished Floor Level (F.F.L.)

Socket Outlets in the Kitchen-     1140 mm above Finished Floor Level (F.F.L.) and (+300mm from Counter Top)

Cooker Control unit         1140 mm above Finished Floor Level (F.F.L.) and (+300mm from Counter Top)

Wall Lights                                           -              2200 mm above Finished Floor Level (F.F.L.)

15 A in Utility for Washing Machine          900 mm above Finished Floor Level (F.F.L.)

Connection Unit for Exhaust Fan                2300 mm above Finished Floor Level (F.F.L.)

EXTERNAL CABLE LAYING

Cable laying involves two steps

Ø            Trenching.

Ø            Cable Pulling / Laying.

TRENCHING

Trenches are dug as per requirement of cable sizes and other atmospheric conditions before digging route has to be finalized. The areas to be avoided for cable laying are near sewage line, Ground-containing ashes, Slag, Clinker, Fertilized Solid & Electrolytic Corrosion, which will affect the life of cables.

Standard Depth of Trenches:

Ø            Up to 1.1 KV = 450 to 750mm.

Ø            Up to 11KV   =750 to 900mm.

CABLE LAYING

Before laying the direction in which the cable is to be pulled should first be decided. If it is a new cable no special precautions are necessary, but if it is laying parallel to the existing cable, the core sequence to be synchronized. Pulling force for Copper & Aluminium cables with steel tape armour when pulled is as follows:

Ø            Copper                 - 7 Kg /Sqmm of total conductor area

Ø            Aluminium          - 2 Kg /sqmm of total conductor area

Also bending radius should be taken in to consideration.

D - Overall diameter of the cable

Where the cables have to be laid below ground level a trench shall be made and a layer of sand be provided at the bottom for the cable to lie on. One more layer is provided after laying the cable and a cable marking tape should be provided over it before doing the back filling.

A minimum horizontal clearance of 150mm should be provided between the cables whenever number of cables is more than one. Where cables pass through holes or metal works precaution shall be taken to prevent abrasions of the cables on any sharp edges.

High Voltage HV/ Extra High Voltage EHV DISTANCE CLEARANCE: 

It is desirable that Extra High Voltage (EHV) lines do not pass above or adjacent to any building or part of building. If this cannot be avoided, it shall have on the basis of maximum sag, vertical clearance of not less than 

For HV line up to and including 33 KV -   3.7m

For EHV lines                                                      -              3.7+0.3m for every additional 33 KV 

The horizontal clearances between the nearest conductor and any part of such building shall on the basis of maximum deflection due to wind pressure be not less than …

For HV lines up to and including 11 KV    - 1.2m

For HV lines above 11 KV and up to 33 KV             - 2.0m

For EHV lines                                                                      - 2.0 + 0.3m for every additional 33 KV or

TEMPORARY INSTALLATIONS ON BUILDING SITES

For temporary electricity supplies, voltage operated Earth Leakage Circuit Breaker shall be installed. Only double insulated cables and industrial type weatherproof socket outlets should be used for the distribution of temporary power.

TOOLS TO BE USED BY TRADESMEN

Required tools must be available at site to ensure correct work. Basic tools of the electrical personnel:

INSPECTION METHODOLOGY FOR QUALITY ASSURANCE

Ensure:

1.            All junction boxes, fan boxes, and drop points pertain to drawing location

2.            Check Inner diameter, wall thickness and allowable tolerance for conduits has to be done by means of Slide Caliper.

3.            Binding of all conduits to steel done properly before slab concreting

4.            Solvent cement for all joints applied properly

5.            All openings/ open ends are sealed

6.            Conduits running parallelly are not touching each other i.e. there are no voids in the concrete

7.            Test lamp method – ensure that all electrical points are charged

8.            Check of ELCB (Earth Leakage Circuit Breaker)/ Residual Current Circuit Breaker by random trip of electrical circuit

9.            Check with multimeter -  

v            Phase to neutral – 230V

v            Neutral to earth – less than 2V

v            Phase to Earth – 230V

10.         In contractual projects where lighting fixtures are part of the scope of work, check the illumination levels with a Lux meter to conform to client specifications

11.         Electrical switch plates should be flush with finished wall level

TOOLS TO BE USED FOR QUALITY INSPECTION

1.            Multimeter

2.            Lux meter

3.            Test lamps

4.            Vernier Calipers

5.            Spirit levels 1-2m

6.            Measuring tape