1 Chapter 3 INSTALLATION OF BASIC FINAL CIRCUITS 3.1 INTRODUCTION A final circuit is one of the outgoing feeders from a distribution board (DB). It supplies a load or a group of loads without intervention of other final circuits of the DB or some other final circuits connected to other DBs. An internal electric distribution system in a building, therefore, consists of several parallel loads connected to a local DB (Fig. 3.1). All wiring cables must be covered and positioned to satisfactory protection against external damages resulted from mechanical impacts, heat, water or high humidity, corrosion and pollution For covering cables plastic or stainless-sheath conduit or trunking systems are installed which are erected vertically or horizontally or parallel to edges of the interior (Fig. 3.1). Every installation should be divided into final circuits to avoid danger in the event of a fault to
Transcript
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Chapter 3INSTALLATION OF BASIC FINAL CIRCUITS
3.1 INTRODUCTION
A final circuit is one of the outgoing feeders from a distribution board (DB). It supplies a load or a group of loads without intervention of other final circuits of the DB or some other final circuits connected to other
DBs. An internal electric distribution system in a building, therefore, consists
of several parallel loads connected to a local DB (Fig. 3.1). All wiring cables must be covered and positioned to satisfactory
protection against external damages resulted from mechanical impacts, heat, water or high humidity, corrosion and pollution For covering cables
plastic or stainless-sheath conduit or trunking systems are installed which are erected
vertically or horizontally or parallel to edges of the interior (Fig. 3.1).
Every installation should be divided into final circuits to avoid danger in the event of a fault to provide;
safe operation, inspection, testing and maintenance
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30 30
120
Socket outlets
Wall lamps
Ante
nna
Switch
Ceiling lamp
30
Figure 3.1 Some typical final circuit wiring in a room
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Types of standard final circuits (FC):- Lighting final circuits
- Socket-outlet final circuits- Radial socket-outlet circuits
- Ring socket-outlet circuits- Final circuits supplying fixed loads- Control and signaling final circuits
- Information, audio-visual and monitoring final circuits Basic final circuits serving to a domestic or a small scale industrial
premise should be connected a DB through busbars using standard protective devices (PDs, fuses or MCBs).
As a rule current rating of a PD protecting a FC should not exceed current rating of its FC cable.
3.2 LIGHTING FITTINGS
Lighting final circuits used for domestic, commercial and industrial services are simple. They are controlled from one point using
single-pole or double-pole branch switches to supply several lamps from different positions by looping in at ceiling roses. The
terminal components of all lighting circuits are lampholders.
4
1.5 mm2
Several looped in lighting FC
Radial FC of 13-A socket outlets4.0 mm2
4.0 mm2
2.5 mm2
Ring main circuit of13-A socket outlets
Spur
Current rating of PD shouldnot exceed current rating ofits FC cable
6 A
6 A
30 A
25 A
2.5 mm2
Spur
DB
IntakeDP switch
Fixedequip.
Fixed load Water heater
FC supplying fixed load
2.5 mm2
JB FC-Final circuit
Figure 3.2 Standard domestic final circuits.
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A. Branch Switches On domestic installations, switches that control a lamp or a group of
lamps are usually 5-A or 10-A single-pole and cut off the line conductor. In circuits carrying heavy currents, 10-A or 15-A switch should be
installed. They may be surface mounted or semi-recessed or flush type. They are
available as single- or multi-gang in construction with moving contacts usually doll or rocker activated (Fig. 3.3(a) and (b)).
Positioning A suitable distance to fix them is in between 1.00 m and 1.30 m (on average 1.20 m) above finished floor level and a minimum distance of 30 cm
from edge of the entry door. In damp situations as in the case of bathrooms, switches should be fixed
outside and immediately adjacent to the excess door of the room. All single-pole switches should be connected to the same phase
conductor of the supply.
B. Ceiling Roses Ceiling rose (CR) is an efficient method of connecting lighting final circuit
to flexible cord of pendant (drop cord) type lampholder and looping in to the other lighting FCs (Fig. 3.4).
Ceiling roses can be fixed direct on ceiling, conduit boxes, or insulating patresses. Ceiling-mounted types are manufactured with either
three (2-plate CR) or four terminals (3-plate CR).
Figure 3.4 (a) Ceiling roses: (a) Terminals of a four-terminal surface-mounted type CR. (b) Parts of a three-terminal surface-mounted type CR.
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B. Lampholders
They are used to hold lamps and to keep lamps in contact with the circuit conductor. Connection to lamp is made from flexible cable. There are basically three types of lampholders; (a) bayonet, (b) screw
and (c) batten.
Terminal forwiring
Cord grip
Ring for holdinglampshade
Pin to pressagainst contacts
of bulbBayonet slot
Brasscap
Porcelainbase
Phosphor-bronzescrew connection
Porcelaininsulating ring
(skirt)
Bottom viewshowing terminals
Brass outer cover
Skirt
Base
(a)
(b)
(c)
Figure 3.5 Lampholders:(a) Bayonet, (b) Screw and (c) Batten
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3.2 LIGHTING FINAL CIRCUITS Ligthing circuits used for domestic, commercial and industrial services
are simple. A lighting final circuit (FC) sets out from a local DB with a protective
device connected only on any of the phase line, and cut off by a single-pole switch connected to the same phase line conductor.
Lighting of rooms may be controlled from one point as in the case of room, bathroom, kitchen lighting, etc., but in the case of staircase or corridor lighting, it is necessary to control the lamp circuit from more
than one point using two-way and/or intermediate switches. Lighting FCs are commonly protected by 5-A or 6-A MCBs and wired
by 1.5 mm2 single-core line and neutral cables and protective earthing (PE) conductor.
(i) Simple Lighting Final Circuits The final circuit may serve one or more number of lamps controlled
from the same switch (Fig. 3.6 and 3.7). The standard color codes on cables should be observed for
connecting PD and switch to the line (L) and PE conductor in each element.
For controlling several lamps looping-in to different point of usage vıa CRs is effective using single-pole switches (Fig. 3.7).
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E
E
Lamp
Switch
LEN
From MB
Main earthingterminal(MET)
Local DB
PE
Figure 3.6 A simple lighting final circuit.
E
E
E
E
L
NEEN
L
N E
Ceilingrose
Ceilingrose
Lamp
To the nextceiling rose
Switch
NEL
From MB
METLocal DB
L
Figure 3.7 Looped-in two simple lighting fittings controlled from different positions using three-plate (four-terminal) ceiling rose.
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E
1
Two-way switch
E
Lamp
Flexiblecable
N
LE
Ceilingrose
2
E
Cord-operatedtwo-way switch
LE
NFromlocal DB
Single-corecable in conduit
(ii) Multi-position Control of Lighting Final Circuits Lighting circuits may also be required to turn on/off from different switching positions as in the case of staircases or halls etc. For two position control two two-way swıtches (Fig. 3.8). and for more than two positions two two-way switches and several intermediate switches may be required (3.9).
Figure 3.8 Wiring diagram of a lighting final circuit to control of a single lamp or a group of lamps from two-control positions (two-way switching).
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EN
L
E
Lamp
To the otherceiling rose
Flexiblecable
E
1Two-way switch
LE
NFromlocal DB
Single-corecable in conduit
2
E
E
3
1 4
2
3 2
41
Alternateposition
Intermediate switch
3
E
Cord-operatedtwo-way switch
EN
L
PE
Figure 3.9 Corridor or staircase lighting circuit controlled by two two-way and one intermediate switch at three switching positions (three-way switching).
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Some important recommendations related to applications of lighting final circuits are:
When the supply cable is 1.5 mm2, no lighting circuit shall be allowed to supply a total load exceeding 10 A (this corresponds to looped in 20 lamps each 100 W). In normal installations the number of lighting points or lamps connected to a lighting final circuit is limited to eight or
ten. Loop-in method is an effective way of supplying several lighting points
wired in conduit. This method is applicable to control a lamp or a group of lamps for both one-way and multi-way switching (Fig. 3.8 and
3.9) Joints or connections are made at switch terminals, ceiling roses, outlet boxes or lampholders. This allows to find faulty point easily.
There must be one PE conductor connected to each separate lighting circuit from DB.
Looping in PE conductor of one final circuit from another final circuit is not allowed.
While wiring groups of several lighting FCs, all cables passing through the same conduit or switch or joint boxes must be connected to the
same phase in order to avoid electrical shock during maintenance and line-to-line insulation failure of cables.
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Some important recommendations for the usage switches are:
Single-pole, two-way, intermediate switches are used to control domestic lighting points or fixed equipment although there is no objection to use of double-pole switches. Switches used to control only discharge lamps, must have a nominal
current rating of at least twice the steady current of the circuit. Outdoor switches shall be water-tight and metal-clad.
The mounting distance of local switches after the finished floor level (AFF) should be between 1.0 m and 1.3 m AFF (measured at dolly level). If several switches are mounted in two or more horizontal
rows, the lowest row shall have a mounting height of 1.0 m AFF. All switches shall be mounted inside the room on the side of the door where the catch or lock is situated and the nearest switches being approximately 30 cm from the door frame.
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3.3 SOCKETS AND PLUGS (IEE Reg. 553-01)
Socket outlets provide an easy and convenient method of connecting portable appliances to the supply via a plug and a connecting flexible
cable. The socket outlet is permanently connected to final circuit cables and
only correct plug can be inserted. The terminals of a socket outlet and plug are marked with letters L,
N and E (Fig. 3.10), and wired with appropriate colored cables to avoid mistakes. All domesticsocket outlets are shuttered type
operating single-phase a.c. or two-wire d.c. circuit operating at 250 V.
E
N L
Plug (top view)
Switch
E
LN
Brown BlueGreen/yellow
Flexible cord toequipment
13 Acartridge
fuse
Socketoutlet
Front view Bottom view
Figure 3.10
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The socket outlets and plugs having standard current ratings are:13-A socket outlet (BS 1363) with fused plug (BS 1363) is universally accepted for domestic and office installations. It is also extensively used in industrial premises. It has three non-interchangeable entries and
rectangular in cross-section. Accompanying with it 13-A fused plug used with flexible cords to serve portable appliances. Any appliance not exceeding 1.5 kW may safely be connected to 13-A socket outlet via 13-A plug (Fig. 3.12). Flexible cords
connected to plugs shall have brown (phase), blue (neutral) and yellow/green (earth). Current ratings of socket outlets and plugs used for
different applications are;- 2 A is used for fixed workshop equipment
- 5 A, 15 A and 30 A socket outlets and plugs (BS 196) are used for industrial applications with a maximum voltage of 250 V. Any plug with single- or double-pole fuses may be used with these sockets.
- 16 A, 32 A, 63 A and 125 A socket outlets and plugs (BS 4343) are heavy industrial type for single- and three- phase applications. Fuses
cannot be fitted in sockets or plugs. 16 A single- or three-phase socket outlets can be wired only radial circuits with a protective fuse
not exceeding 20 A.
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General recommendations in installing socket outlets. - - Any socket outlet adaptors (acting as spur) to be connected to a single
socket should contain fuses to prevent the socket outlet being overloaded.
- The mounting distance of a socket outlet is 30-40 cm +AFF.- All socket outlets in one room shall normally be connected to one
phase of the supply. - Neutral or earth wire should not be borrowed from the nearest socket-
outlet final circuit.- In large rooms socket outlets should be grouped in phases. No socket
outlets shall be fitted nearer than two meters to each other if they are wired on different phases as shown in Fig. 3.11.
LE
N LE
N
2 mPhase 1 Phase 2
Standard available fuse links are;for 5 A plugs 2 A and 5 Afor 15 A plugs 2 A, 5 A, 10 A and 15 Afor 30 A plugs 10 A, 15 A, 20 A and 30 A
Figure 3.11
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- Switched socket-outlets are recommendable, but not essential, in locations where frequent removal of plugs that cause wear due to
arcing across contacts. - No socket outlet final circuit shall be allowed in a bath or shower
room except at reduced voltage levels using isolating transformer. - As a general rule one socket outlet, single or twin, is required for a
wall in domestic applications.
The following recommendations can be made in installing domestic interiors:
Kitchen area-Socket-outlets may be located 30-40 cm above working surface or floor
level.-No socket-outlet should be allowed close to sinks within 2 m. -A cooker should be supplied from a separate final circuit isolated with a
30-A or 45-A double-pole switch. A socket outlet on the control unit of a cooker may lead to shock risk due to flexible cord of any appliance, therefore its length should be limited to 50 cm.
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Dining and Living roomsFor a separate dining area socket outlets are required for hot tables and for a radio or television. Therefore, recommendable number of socket outlets is 3 twin for dinning hall.. For a living area applicable number of sockets may be 2 single and 4 twin.
Bedrooms In bedrooms socket outlets are required on each side of bed. One additional socket outlet may be installed for other appliances such as heater or vacuum cleaner.The following table gives recommended number socket outlets in domestic premises:
Location Number of socket outlets (S-single, T-twin)
Socket-outlet FCs provide supply to every appliance at its closest point. For most common applications 13-A (BS1363) sockets and plugs wired in the form of radial or ring final circuits (Fig. 3.14) supplied from the main or
sub-main distribution board are commonly used. The earth terminal (E) of socket outlet should be connected to PE
conductor of the socket-outlet FC (IEE Reg 543-02-07). If the mounting box of a socket outlet is metal, it must also be connected to the earth
terminal of the socket outlet.
I - Radial Socket-outlet CircuitsAs shown in Fig 3.14, in a radial circuit each socket is fed from the
previous one. The phase, neutral and protective conductors are connected to line, neutral and earth terminals at each socket outlet. The fuse and
cable sizes are given in Table 3.1.
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■ Some of the important recommendations for installing socket-outlets are:
- The rating of protective device should not exceed 20 A (or max load of 5 kVA) if the area to be served is about 50 m2, and 30 A if the area to be served is 75 m2
- The maximum number of socket outlets (twin is counted as one) that can be fitted to a 4 mm2 radial circuit to serve 50 m2 area is six (Table 3.1), and ten if the area is 75 m2.- The rating of protective device depends on the area to be served. It is 30 A if the area exceeds 20 m2 (Fig 3.15).
MCB30 A or 32 A
Maximum area served 50 m 2
PVC 4mm 2
Maximum area served 20 m 2
PVC 2.5mm 2MCB 20 A
Figure 3.12 Radial circuits serving two different floor areas.
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II - Ring Socket-outlet CircuitsThe ring final circuits are formed similar to radial final circuits by wiring; each
socket from the previous one, but connecting the last socket back to the supply point forming a complete ring (Fig. 3.16). Two ends of the phase conductor are
connected to the terminal of protective device and two ends of the neutral conductor and also protective conductors are also brought back to respective
terminals in the DB.
FromMB
LE
N
LNE
ELN
LNE
Spursocket
Cable size: 2.5 mm2
Local DB
L
MET
PE
N
25 A
Figure 3.13 Ring socket-outlet FC
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Living room
BedroomCloak
Hall
Kitchen WM
WP
Table 3.14
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■ In domestic premises, in a ring circuit only 13-A socket outlet wired with 2.5 mm2 and protected with 30-A fuse or MCB must be used. Although the
rating of the cable is in the order of 20 A, since the circuit is split into two ways round the circuit, the cable will not be overloaded.
■ In any part of a premise, the number of socket outlets to be connected to a ring socket-outlet circuits according to the floor area covered by the circuit should not exceed that given in Table 3.1. Also, the estimated maximum demand on the circuit should not exceed the rating of the
overload protective device.
Type ofcircuit
Overcurrent protective device
Rating(A) Type
Minimum conductor size
mm2 (PVC insulated)
Maximum floor
area served, m2
Maximum number of
socket outlets
RingRadial
RadialRadial
30 or 32 Any30 or 32 Cartridge fuse
or MCB
20 Any20 Any
2.54
2.52.5
10075
50less than 30
See note 1See note 1
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Note 1: Maximum number of socket outlet to be connected is 10
Table 3.2 Fuse and cable sizes for ring and final circuits using 13-A (BS 1363)
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■ Some recommendations in installing ring socket outlet circuits. - The maximum number of sockets that can be fitted to a 2.5 mm2 ring circuit is ten
(a double socket outlet is counted as one). - The rating of protective device should not exceed 30-32 A (or max load of 7 kVA).
- Kitchen area requires a separate socket-outlet final circuit.- For small domestic premises at least two ring circuits should be installed.
- Fixed appliance are to be distributed uniformly along a ring circuit - In the same premise if two or more circuits are to be installed, the socket outlets and permanently connected equipment should be reasonably shared out among
the circuits, so that the total load is balanced.-When designing ring or radial circuits in kitchens, special attention should be given
in loading, which may require separate circuits. This is because the maximum demand of some equipment may exceed the rating of the circuit cable and of PDs.
III-Spurs to a Ring Socket-Outlet Circuit (IEE On-Site Guide)A spur is a branch cable connected to a ring final circuit. Depending on the
application in a ring circuit a spur branch may be non-fused or fused.
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Part of a ringcircuit
Part of a ringcircuit
Permanentlyconnectedequipment<1.5 kW
Part of a ringcircuit
1. Non-fused SpurThe total number of non-fused spurs must not exceed the total number of socket outlets and fixed appliances connected directly in the ring circuit.A non-fused spur must be connected either at the terminals of a socket outlet (Fig. 3.15(a)) or at a joint box (Fig. 3.15 (b)) as a part of a ring socket outlet circuit, or at the origin of the ring circuit in DB (Fig. 3.15 (c)).
Figure 3.15 Various circuit arrangements of non-fused spurs.
- A non-fused spur can only feed one single or twin socket outlet, or one fixed appliance.- The connecting cable must have a current rating not less than that of the conductors forming the ring (2.5 mm2).
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2. Fused Spurs- Also, the total number of non-fused spurs must not exceed the total number of
socket outlets and fixed appliances connected directly in the ring circuit.- A fused spur may be connected to a ring circuit through a fused connection unit, and
rating of the fuse must not exceed 13 A. - When a fused spur serves socket outlets the minimum conductor size of the
connecting cable should be 1.5 mm2, but its current rating must not exceed current rating of the spur fuse.
Various spur circuit arrangements feeding a permanently connected load are shown in the following figure.
Part of a ringcircuit
Permanentlyconnectedequipment(<1.5 kW)
JB
Part of a ringcircuit
Permanentlyconnectedequipment(<1.5 kW)
Fused connection unit (with or without switch)
Part of a ringcircuit
Permanentlyconnectedequipment(<1.5 kW)
Fused connection unit (with or without switch)
Non-fused spursocket outlet
(a) (b)
(c)
Table 3.16
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3.5 FINAL CIRCUITS SUPPLYING FIXED LOADS
■ Some of the fixed loads may be connected either (a) to ring or radial circuits feeding 13 A socket outlets, or (b) on individual final circuits from
the consumer unit.The group (a) devices include fixed small space heaters, fans, small
water heaters, hand dryers, etc.,and their power rating should not exceed 1.5 kW. These devices should be connected to the final circuit via 13-A
fused plug, and controlled by a single or a double-pole switch. The group (b) devices consist of floor standing fixed equipment and
covers water heaters, cookers, clothes dryer, water heaters etc. rated at more than 1.5 kW. Each of these items should be wired on a separate circuit, and be controlled by a double-pole fused or non-fused switch.
Switch must be fixed adjacent or at an excesible position close to equipment.
■ Double-pole switch (Fig. 3.17(a)) may sometimes acts as a terminating element to fix connecting flexible cable if the equipment is
close to the switch. If not, a separate wall-mounted flex outlet plate fitted to the adjacent equipment should be installed (Fig. 3.17 (b)).
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ON
D. P.switchcontacts
Neonindicator
Figure 3.17 (a) 30-A or 45-A double pole switch. (b) Flex outlet plate
A. WATER HEATERS
There are three methods of providing hot water to houses: 1. Instantaneous type
2. Immersion heaters or circulators3. Self-contained storage heaters
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Electrical connection for water heaters■ Any water heater having power rating more than 1.5 kW should not
be connected to ring or radial socket-outlet circuits. They should be supplied from the local distribution board with a separate circuit.
■ In the installations of water the heater must be permanently connected to the electric supply through two double-pole switches; one is
used for functional switching and the other for isolation. The heater is directly connected to the isolation (IEE Reg. 554-05-03) with a length of
heat-resistant flexible cable. If the distance between the water heater and the isolation switch is long, a wall-mounted power flex outlet should be
installed close to the heater. Generally:■ All the metal parts of the heaters in contact with the water must be
solidly and metallically connected to supply pipe, which is connected to the main earthing terminal (IEE Reg. 554-05-02).
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■ In bathrooms, the installation of water heater should follow the rules of IEE Regulation (IEE Reg. 601-08-01). - No switch or other means of control should be accessible to the person using the shower or a fixed bath tube. The switch must be fixed an a convenient position outside the bathroom, or to use a cord operated ceiling switch within the room. Wall-mounted switches should be 20-A double pole and ceiling mounted switches should be 15-A double pole with a pilot light indicator for 'on' and 'off' visual indication.-The length of the connecting flexible cable must be of the same size as that of installed cable.
1. Instantaneous water heaters. These provide instantaneous supply of hot water. These are suitable in locations where connection to the central water heater system is difficult and expensive, and often used to provide hot water for hand washing or showers in cloakrooms, workshops, etc. - They all include a thermostat with a safety cut.
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- They supply 1.5 or 3 liters of hot water per minute for electrical loading of 3 kW or 6 kW. When used for showers with a load of 4 to 7 kW.- They must have separate final circuit and controlled by a 30-A or 45-A cord operated ceiling-mounted double pole switch with pilot light.The unit should be provided with a metallic sheath connected to the local supplementary equipotential bonding (IEE Reg. 602-08-04).
2. Immersion Heaters. These are available in various types and loading and are installed in metallic cylinder or tank controlled with thermostat (IEE Reg. 554-04) (Fig. 3.18 (a)). A single element 3-kW heater is the most usual in domestic usage. Double pole switch-fused or non-fused isrequired for controlling the element, and must be fixed out of reach, and water tank must be bonded (Fig. 3.18(b)).
- Dual heaters are also available in the same housing; one of them short and the other is long. Since hot water will be available at the top of the housing, the short heater provides a small amount hot water quickly and economically, the other one is used when it is required to heat all the water in the tank. This type of water heaters is installed with special dual switches.
-These heaters are very efficient. They should be fixed as near as possible to the space of usage.
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DP switch forfunctional switching,
20 A
Thermostat andheater elements
Tankconnected
to SEB
Protection is providedby 15 A MCB or fuse
DP switch forisolation, 20 A
2.5 mm2
cable
Sorage watertank
Heat resistantflexible cord
L
EHeat resistantflexible cord
ThermostatTop view
N
Figure 3.18 Immersion water heater. (a) Water heater element with thermostat. (b) Wiring arrangement.
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Living room
Bedroom Cloak
Hall
Kitchen WM
WP
WH
Examples for the usage of double-pole switches for supplying fixed loads.
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3. Self-contained storage water heaters. They are used to supply hot water more than one tap. Various types are available; non-pressure, semi-pressure, pressure cistern and two-in-one. - - The heater elements are rated at 6 kW to 15 kW depending on the thermal capacity, and heater circuit is connected to the consumer DB and protected by 30 A, 45 A or 60 A MCB or fuse as appropriate (Fig. 3.20).
Functional DP switch, 45 A
DP switch forisolation, 30 A
Thermostat andheater elements
Tankconnected
to SEB
Protection is provided by30 A MCB or fuse
4 mm2
cableDP switch for
isolation, 30 A
4 mm2
cableHeat resistantflexible cord
Figure 3.20 Possible wiring connections of storage type water heaters.
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- The control requires special fused or non-fused double-pole switch, which also acts as a terminating element for wiring.
- Wiring to each storage heater from the isolation switch or remotely from a wall-mounted flex outlet plate must be done by heat resistant flexible cords
of adequate size- If there is more than one storage heater, each heater may have its own
separate functional switching as well as a double-pole switch for isolation.
