Chapter 2ELECTRICAL POWER SUPPLY AND DISTRIBUTION IN BUILDINGS

2.1 TRANSMISSION AND DISTRIBUTION OF ELECTRICITY

Power network (grid): Combination of power transmission and distributionsystems 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.

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(iii) Secondary TransmissionFrom 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 DistributionThe 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 DistributionClose 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.

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Underground lateral distribution(415 V/240 V

Local garage(415 V)

Houses(240 V)

Shops(240 V)

Local (secondary) distributionS/S (11 kV/415 V, 240 V)

Small Industrialestate (11 kV)

Heavy industrial estate(11 kV/ 2400 V, 415 V )

Primary distribution S/S33 kV D/11 kY Y

Secondary transmissionline (66 kV)

Overhead transmisssion line (220 kV or 132 kV)

Step-up S/S(11 kV/220 kV)

Generating Station (11 kV)

Primary transmission S/S (220 kV/66 kV)

Primary distribution (33 kV)

Secondary distribution (11 kV)

S/S-Substation

Secondary transmissionS/S (66 kV/33 kV)

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

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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.

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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.

Secondarydistribution

S/S

Low-voltage distributioncabinet or junction

point

Underground 11-kV cable

Underground230 / 400 V supply cable

SecondaryDistributionS/S

SecondaryDistributionS/S

Functional building

Industrial buildingResidential buildings

Figure 2.2 Distribution system for residential, functional and industrial loads.

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Secondary Distribution SubstationThe 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.

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Secondary distributionsubstation

Load levelSupply level

Undergroundsupply cable

Localgarage

HousesShops

Meter and distributionboards (TPN)Meter and distribution

boards (TPN or SPN)

HV cable system

LV distribution with cable systemLV Installation with cable system

Secondary distributionsubstation panel (switchboard)

JB-Joint box

Underground service cable

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Most of domestic or small scale commercial consumers with power requirements up to 200 kW - 300 kW is usually supplied from local distribution system of the electric authority as

- SPN (Single-phase and neutral) or - TPN (Three-phase and neutral)

Power suppliedfrom 11 kV

primarydistribution

network

R

Y

B

N

N

CB

11 kV/415 V, 240 Vdelta/wye connected (local) transformer

Protective and switchingdevices for secondary loads

E

Protective earthing (PE)

Secondaryloads:residential,commercialetc

All connections to (E) terminalsin DB via armour (PE) of cables.

Secondary distribution sub-station (room) chamber

Figure 2.3 Simplified local secondary distribution substation with /Y connected 11 kV/415 V, 240 V transformer supplying residential buildings

Secondary Distribution Substation

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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.

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2.3 DISTRIBUTION SYSTEMS IN BUILDINGS

Distribution in a large scale residential buildings (Fig.2.4).

Secondary distributionpower station

DB

HeatingLightingLift andWater pump

Socketoutlets

WashingmachineCooker

Lighting

Conventional installationsor final circuits in

residential buildings

Loadlevel

Maindistribution

level

Power supplylevel

Distribution boards toflats or other

building services

Main distribution board (cubicle)

Busbar trunking system or cable

Busbar trunking system or cableInstallation with cable system

PowerDB

Final circuits

415 V /240 V

11 kV

11 kV / 415 Vpower transformer

Substationdistribution panel

Figure 2.4 Supply arrangement in a residential building.

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Secondary distributionpower station

Conventional installationsin service buildings

Load levelMaindistribution

level

Power supply level

Distribution boards, busbartrunking, cable installations tosystem services

Main distribution panel(cubicle)

Sub-distribution level

Busbar trunking system or cable/line systems

Busbar trunking system or cable systemsInstallation with cable systems

Installation with cable systems

Main distributionboard

Sub-distributionboard

11 kV

415 V / 240 V

Figure 2.5 Supply arrangement in an industrial or functional building.

Distribution in an industrial or functional building (Fig.2.5).

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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.

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

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DistributionBoard

Sub-distributionBoard

Secondary Distribution Substation

SubstationPanel

11 kV / 415 VTransformer

Main DistributionBoard (Block 1)

Distribution Board

Trunking System

RaisingMain

Main DistributionBoard (Block 2)

Figure 2.6 Supply arrangement in an industrial building

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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)

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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:

- Protecting persons and property

- Protecting cables, lines and loads

- Safety disconnection for maintenance and repair

- Monitoring, signalling and controlling

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Meter board (MB)

Underground supply cable system in duct

Customer distribution board (DB)

Power meter

Final circuitsfeedingsocket-outlets,air-conditioningsystemswater heaters,heating systems,etc.

Consumer'sEquipment

Supplier'scut -out box

Supply authorityequipment (sealed)

Protectivedevices

Main supplycable

Four-corePVC cable

R

Y

B

N

Isolatorswitch

Final circuitsfeedingsocket-outlets,air-conditioningsystemswater heaters,heating systems,etc.

Protectivedevices

MCCB

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

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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.

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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.

Protectivedevices

N

N

NIsolatorswitch

ERCCB

GR-2: Final circuitsserving lighting loads,all lamps, small fansetc.

GR-1: Final circuits servingfor power loads: socket-outletcircuits, air-conditioner,motors, etc.

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

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Radial circuit of 13-Asocket outlets

Looped in lighting final circuits

4.0 mm2

2.5 mm2

2.5 mm2

Spur

6 A

30 A

20 A

30 A

2.5 mm2

Spur

DB

Two-wayswitching

Double-poleswitch WH W ater heater as

single-point loadMain supplycable from MB.

Ring circuit of 13-Asocket outlets

1.5 mm2

13-A socket outlets

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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.

22Figure 2.12 Meter and distribution boards in mult-metering residents.

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).

kWh kWh kWhkWh kWhkWhkWh

Meter Board

Servicecutout box

Ground

1st

2nd

Main supplycables to flatsinstalled throughconduits

DB for commonservices

Costomer DB

MCB or fuse

Floor levels

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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.