# Chapter 2 ELECTRICAL POWER SUPPLY AND DISTRIBUTION IN BUILDINGS

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Chapter 2 ELECTRICAL POWER SUPPLY AND DISTRIBUTION IN BUILDINGS
2.1 TRANSMISSION AND DISTRIBUTION OF ELECTRICITY Power network (grid): Combination of power transmission and distribution systems at various stages at various standard voltage levels. Stages of a Power network : (i) Generation Electrical energy is generated from power stations generally at 11 kV or 15 kV three phase and at standard frequency. (ii) Primary Transmission Power enters to step-up substation, where the voltage is stepped up to higher voltages for transmission at 132 kV or 220 kV to a complex network comprising transmission lines or cables known as national grid.

(iii) Secondary Transmission From the primary transmission substation the power is transmitted at 66 kV or 33 kV through sub-transmission lines to different load centres. The sub-transmission lines terminate at the secondary transmission substations to step down the voltage from 66 kV to 33 kV. (iv) Primary Distribution The power is distributed at 33 kV to main load centres inside a city through either primary distribution cables or lines, then further stepped down to 11 kV at primary distribution substations. (v) Secondary Distribution Close to consumers the voltage stepped down to 415 V and 240 V at local secondary distribution substations and finally delivered to domestic, commercial and small-scale industrial customers through either lines or underground cables.

Figure 2.1 Simplified diagram of a power system from power station to consumer
services.

The reason for transmitting power at high voltages is due to
conductor resistance. If the transmission would be at low voltages, transmission line I2R power losses become high because of high current. (Vtr is low → I is high → IV is high → I2R is high) If the transmission would be at high voltages, transmission line I2R power losses become low because of reduced current. (Vtr is high → I is low → IV is low → I2R is low) and ● conductor diameter would be reduced and ● efficiency of transmission will get increase. At various stages of a power system power equipment and voltages are standardized. This allows to reduce; ● construction cost, ● running cost and ● maintenance cost, These overall effectively reduce turning-off time of equipment in use and hence, of the system.

2.2 ELECTRICAL DISTRIBUTION IN BUILDINGS
The purpose of electrical installation in buildings is to ●supply and ● distribute electric power according to the requirements of the building. Figure 2.2 Distribution system for residential, functional and industrial loads.

Secondary Distribution Substation
The local secondary power distribution substations are either - indoor or outdoor type - located along roadside blocks close to the buildings or - within buildings in substation chambers and - wired to the distribution boards via underground - cables and - busbar trunks.  In every secondary distribution substation voltage transformation at various stages of the power system network is performed by step- down transformers, which are equipped with; - Connecting and coupling busbars - Low-voltage protective devices - Circuit breakers integrated to relaying systems.  All these equipment are installed in substation distribution boards (or panels), which are located closed to the secondary distribution transformer and within the substation (room) chamber. Figure 2.3 Simplified low-voltage secondary distribution system to buildings from a local secondary distribution transformer.

Secondary Distribution Substation
 Most of domestic or small scale commercial consumers with power requirements up to 200 kW kW is usually supplied from local distribution system of the electric authority as - SPN (Single-phase and neutral) or - TPN (Three-phase and neutral) Figure 2.3 Simplified local secondary distribution substation with /Y connected 11 kV/415 V, 240 V transformer supplying residential buildings

The reason for connecting the secondary side of the transformer in wye (Y) form because of the need for a neutral for single-phase distribution at load levels. To achieve this, the neutral point of the secondary winding is solidly connected to ground via ‘earth electrode’. The neutral point of the transformer is connected to neutral terminal (N) together with line (L) conductor in customer DB to provide power to customers. The neutral point of the transformer is also connected to earth terminal (E) in customer DB via protective earth (PE) conductor. The supply to premises is taken by underground cables or overhead lines terminating at service cut-out within a premise. - Underground cables are usually brought through ducts below floor level and - Overhead lines by towers.

2.3 DISTRIBUTION SYSTEMS IN BUILDINGS
 Distribution in a large scale residential buildings (Fig.2.4). Figure 2.4 Supply arrangement in a residential building.

 Distribution in an industrial or functional building (Fig.2.5).
Figure 2.5 Supply arrangement in an industrial or functional building.

 At all levels of the distribution in buildings solid connections to various
service cables or busbar trunking systems are established by main and sub-main distribution boards. These provide; - easy installation - easy maintenance and - safe operation at every levels of installation systems.  At main distribution level (Figs.2.4 and 2.5) main distribution boards are used for; - Safety disconnection - Coupling busbar sections - Protecting busbars  At sub-distribution level (Fig.2.4), subdistribution boards are used for: - Safety disconnection for maintenance - Switching lighting and power loads - Protecting all electrical cables and busbars and loads. - Control, metering and measuring purposes.

 Both distribution boards are equipped with;
- Circuit breakers and tie circuit breakers - Isolators - MCCBs - Fuses

Figure 2.6 Supply arrangement in an industrial building

2.4 INTAKE TO SMALL SCALE BUILDINGS Basic elements of TPN distribution board to a small scale customer:  Power supply authority provides low-voltage supply to premises via underground cables in populated areas and via overhead lines at remote sites or in rural area from a local 11 kV- 415 V and 240 V secondary distribution substation.)  Customer intake boards: ▪ Meter Board (MB) equipped with: - kWh meter - Protective devices PD to protect main supply cable to customer DB ▪ Distribution Board (DB) equipped with: - Isolators - RCCB (Residual Current Circuit Breaker) - MCB (Miniature Circuit Breakers)

are used for: - Protecting persons and property - Protecting cables, lines and loads - Safety disconnection for maintenance and repair - Monitoring, signalling and controlling ▪ Customer intake boards are used for:

Figure 2.7 Supply intake and connections to a small-scale residential or commercial premise.

 Meter board (MB): The supply is taken from the service cut-out to the consumer's distribution board through meter board installed with; - service cut-out fuse box - power (kWh) meter and - overcurrent protective device (PD). The service cut-out box may sometimes be located within MB. The power meter and the service cut-box belong to the supply authority and are also sealed to prevent unauthorised changes to be made on elements and connections  Overcurrent protective device: It is an optional device, and is installed just after the meter in order to protect supply cable to DB. It is a circuit breaker enclosed in a moulded plastic case, known as moulded case circuit breaker (MCCB). Under any type fault conditions or maintenance beyond MB, it interrupts the consumer's circuit and avoids unnecessary operation of cut-out fuses. (iii) Consumer distribution board (DB) In domestic and small scale industrial premises; - DB supplies final circuits to serve various types of loads, e.i., it distributes power to final circuits.

 DB is equipped with; - The isolating switch is double-pole for single phase and three-pole for three-phase installations. - One or two residual-current circuit breakers (RCCB) and - Minature circuit breakers (MCB) or fuses to protect consumer's load and final circuits. Figure 2.8 General layout of a consumer distribution board.  The supply intake to a premise should be arranged in such away that the cut-out box and meter should be outside the premise for - easy replacement of fuses and meter reading without entering the premise

POWER INTAKE TO A MULTI-METERED BUILDING
 In small-scale multi-floor residential distribution systems positioning MB;  MB of each flat are usually installed at the main entrance of the building (in main hall or stairway) enclosed in a cabinet or board.  MB should not be installed along escapes routes and be preferable to be concealed. Adequate spacing should be allowed for ventilation and for maintaining the devices.  It is advisable to reserve adequate space for additional meter installations.  To install cables serving DBs to each flat at each floor,  Location of MBs should be at a central position of the building (Fig. 2.12).  MBs must be positioned to facilite easy reading at a height between 1.0m and 1.80 m above the finished floor level. It should be protected with a lockable door.  A separate MB is required installed next to customer MBs for common services such as general lighting in stairways, corridor, halls, and intercome services etc.  The cables supplying costumer DBs from MBs should always be TPN.

In multifloor high rise office buildings subjected to single meter reading, raising-main busbar duct-system is the convenient method to supply consumers DBs (see Chapter 13). Figure Meter and distribution boards in mult-metering residents.

Safety rules applied to installation of MBs and DBs.
The minimum clearance from any fitting related to plumbing such as water tabs, basins, sinks, metal drainage boards etc., shall be 2 m.  No water sprinkler system should be installed in the close vicinity of MBs.  MBs must have an identification symbol for the space they serve in the case of multi-meter buildings. They must have an identification schedule for final circuits.