Cable and Circuit Types

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Cable and

Circuit Types

Learner Work Book

Name:

Group:

Tutor:

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Foreword

In this section we will look deeper into the different circuits used within everyday installations. Different types of mains voltage circuits use the same type of cable but will most likely be a different size to safely cope with the demand of that circuit. Some circuits need specific cable types to function correctly. Circuits operate and function in dependence upon what information the electrician knows about the circuit and how he connects them up. Circuit diagrams, if used correctly, ensure that circuits will function as they are designed to do.

BS7671 dictates how all our everyday lighting and power circuits are installed but the emergency systems that we come into contact with everyday in the training centre, in a public place are governed by their own British Standard.

This workbook is to be accompanied by PowerPoint “Cable and Circuit Types”

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Cables and Circuit Types Unit Overview

Practical Skills

To achieve the learning outcome the candidate must be able to:

Complete the practical tasks in the electrical unit ensuring correct cables are selected

Select correct sizes conductors and use correct installation methods Complete wiring diagrams for various types of lighting circuit

Complete design current calculations for various types of lighting and power circuit

Select the correct circuit components for lighting and power circuits Knowledge Requirements

To achieve the learning outcome the candidate must know:

The different cable types used within industry for the common types of circuit What BS7671 says about circuits and their parts

How circuits are categorised with regard to their duty The standard ratings of circuit types

About lighting types and how lighting circuits operate

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Cable Types

Insulation properties are one of the most important things to consider when selecting a cable for a circuit. If the insulation is not rated for the environment the circuit is installed in it can have a number of detrimental effects on its operation and safety.

Mechanical protection for a cable is probably as equally as important as its insulation. If a circuit with limited mechanical strength is installed in an environment where there is a risk of it coming into contact with heavy objects that circuit will most likely

become unsafe through being damaged.

What can happen to a circuit if the insulation is incorrectly rated? List the factors below

Name the type of cables you have heard of in the box below.

So you can improve your understanding of cables as we progress through the power point note down the common names of the cables next to their picture

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Light duty cables

6242Y Flat twin and earth

Available sizes: 1.0mm²; 1.5mm²; 2.5mm²; 4.0mm²; 6.0mm²; 10.0mm²; 16.0mm²

(CPC is one size less)

Description: PVC insulated and Grey or white PVC sheathed. Solid copper

conductor. Approved to BS 6004

Uses: Domestic fixed wiring for any type of circuit; buried in building fabric or clipped

direct; Commercial wiring if above a ceiling or installed on tray or in trunking

6242YH Flat twin brown and earth

Available sizes: 1.0mm²; 1.5mm²

Uses: Domestic fixed wiring for switch wires only; buried in building fabric or clipped

direct; Commercial wiring if above a ceiling or installed on tray or in trunking

6243Y Flat three core and earth

Uses: Domestic fixed wiring for two way strapping wires or circuits that require

permanent live and switched live conductors such as extractor fans or automatic security lighting; buried in building fabric or clipped direct; Commercial wiring if above a ceiling or installed on tray or in trunking

With all flat twin and three core cables the circuit protective conductor is smaller than the line and neutral conductors. A 1.5mm² cable has a 1.0mm²

cpc. A 2.5mm² has a 1.5mm² cpc and so on. This bare copper conductor needs to be covered with green and yellow sleeving to protect it from live

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3182Y Two core round flex

Available sizes: 0.5mm²; 0.75mm²; 1.0mm²; 1.5mm²

Description: PVC insulated and White or Black sheathed. Stranded copper

Uses: Double insulated appliances that do not require earthing; domestic lighting

cabinet display etc. push switches, table and standard lamps, radios, and lighting pendants etc

3183Y Three core round flex

Available sizes: 0.5mm²; 0.75mm²; 1.0mm²; 1.5mm²; 2.5mm²

Uses: Domestic lighting cabinet display etc; kettles, toasters etc; earthed equipment;

extension leads.

3184Y Four core round flex

Available sizes: 0.75mm²; 1.0mm²; 1.5mm²

Uses: Domestic security lighting where permanent and switch live conductors are

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3185Y Five core round flex

Uses: Domestic and commercial heating and ventilation controllers; timers.

3183TQ Three core rubber flex

Description: Heat resistant, rubber insulated and White or Black sheathed. Stranded

copper conductor. Approved to BS 6500

Uses: Domestic and commercial heating supplies; equipment in high temperature

zones up to 85°C.

6242BH Flat twin and earth

Available sizes: 1.0mm²; 1.5mm²; 2.5mm²; 4.0mm²; 6.0mm²; 10.0mm²; 16.0mm² Description: Low smoke and fume twin and three core & earth. To BS 7211. Solid

copper conductor

Uses: Domestic fixed wiring for any type of circuit where reduced risk of smoke

hazard is required such as in schools; buried in building fabric or clipped direct; Commercial wiring if above a ceiling or installed on tray or in trunking

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6181Y Double insulated meter tails

Available sizes: 10.0mm²; 16.0mm², 25.0mm²; 35.0mm²

Description: PVC insulated Grey PVC sheathed. Stranded copper conductor.

Approved to BS 6004

Uses: Double insulated surface wiring cable for meter tails for connection to

consumer units and switchfuses.

6491X Single core PVC insulated and 6491B Single core LSF insulated

Available sizes: 1.0mm², 1.5mm², 2.5mm², 4.0mm², 6.0mm², 10.0mm², 16.0mm²,

25.0mm² and higher

Description: PVC insulated single core. Approved to BS 6004 or LSF insulated

single core. Approved to BS 7211; Solid or Stranded copper conductor

Uses: Fixed wiring within conduit and trunking; earthing conductors; main

equipotential and supplementary bonding

Bell wire / speaker wire

Available sizes: Solid 0.2mm² / 13, 42, 79 strands

Description: Twin stranded plain annealed copper cores laid side by side in a flat

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CAT5E Network cable

Available sizes: 4TP (four twisted pairs) 0.2mm²

Description: Solid plain annealed copper. Category 5 cables offer extended

transmission distance over frequency ranges up to 350Mhz and data speeds up to 1000 Mbps

Uses: Computer networking.

Standard co-axial / Satellite cable

Available sizes: 75 Ohm (6.7mm OD)

Description: Solid plain annealed copper. Air spaced polythene insulation with

copper wire braiding

Uses: Television and radio down leads for aerials; Linking satellite dishes to

receivers

Security alarm cable

Available sizes: 0.5mm²

Description: 4, 6 or 8 core tinned annealed copper conductors, PVC insulated and

sheathed

Uses: Flexible cables normally used for the wiring of burglar alarm and other low

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Medium duty cables

Medium duty cables are selected due to their mechanical strength. Typical

environments where they would be used are construction sites, business premises, warehouses and small factories. Any of the light duty cables seen previously can be used in these installations as long as they are sufficiently protected.

3183AG Three core arctic flex

Description: Exterior use with tools, lighting and equipment. Stays flexible at -30ºC.

Generally to BS 6500. Stranded copper conductor

Uses: Construction site temporary lighting; power tools; outdoor equipment;

extension leads (Yellow for 110v, Blue for 230v).

YY Control flex

10.0mm², 16.0mm²,

Description: Three, four, five core up to 25 core PVC / PVC insulated multi core.

Approved to BS 6500. Stranded, number coded copper conductor

Uses: Used as connecting cable, as measuring, checking and control cable in

machine tool manufacturing, plant engineering and on assembly lines and production lines. Suitable for fixed installation (tray work) or flexible applications without

exposure to tensile load.

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SY Control flex

10.0mm², 16.0mm²,

Description: Three, four, five core up to 25 core PVC / PVC insulated, galvanised

steel wire braided multi core. Approved to BS 6500. Stranded, number coded copper conductor

exposure to tensile load. Due to the galvanised steel wire braiding, these cables can even be used under adverse operating conditions or when exposed to high

mechanical strain.

CY Control flex

10.0mm², 16.0mm²,

Description: PVC / PVC insulated, copper wire braided multi core. Approved to BS

6500. Stranded, number coded copper conductor

exposure to tensile load. These cables with copper screening are ideally suitable for interference free data and signal transmission in measuring and control technology

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Heavy duty cables

6943X, 6944X, 694*X - XLPE Steel wire armoured

25.0mm² and larger

Description: 3-core (6943X) and 4-core (6944X); *up to 48 core galvanised Steel

Wire Armoured Cable. Black XLPE sheathed, PVC insulated. Approved to BS 5467. Available in copper or aluminium (above 16mm² only

Uses: General circuit wiring (on tray work or ladder or clipped direct) where there is

an increased risk of mechanical damage; underground cable routes

Mineral insulated copper conductor

25.0mm² and larger

Description: Single core, two, three, four, seven, twelve and nineteen core. Orange (general use), red (fire alarms) or white (emergency lighting) LSF sheathed, mineral (magnesium oxide) insulated. Solid copper conductor. Approved to BS 6207

Uses: General circuit wiring (on tray work or ladder or clipped direct) where there is

an increased risk of mechanical damage; underground cable routes

BS5308 Instrumentation cable

Available sizes: 0.5mm², 0.75mm², 1.0mm², 1.5mm²;.

Description: 1, 2, 5, 10, 20, 30 and 50 pair. XLPE or PVC Blue sheath for I.S; Black

sheath for general purpose. Stranded copper conductor. Approved to BS 5308. Each cable combines one part and one type. For example:

Pt1 = XLPE sheathed Pt2 = PVC sheathed; Ty1 = Un-armoured Ty2 = Armoured A Pt1 Ty2 cable is Armoured Collectively Screened or individually & collectively screened with XLPE sheathing

Uses: Used to carry voice and data services, they also serve as the interconnection

between electrical equipment and instruments, particularly in and around process plants, where signals are transmitted through to panels, controllers and other devices

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1. A supply to a garage, 10 metres from a house where an overhead route is not an option.

2. An office building heating system is controlled via thermostats carrying low voltage digital signals. All wiring is above the ceiling.

3. A supply is required for a lift shaft lighting circuit. The supply will be used at various points to ensure illumination levels are sufficient on all floors. This is a high risk

mechanical protection environment 4. An extra socket is required in a

kitchen. It will not be surface mounted so the cable will have to be buried in the building fabric.

5. A new classroom is being built in a school and there needs to be a reduced risk of hazardous fumes in the event of a fire. The installation needs to be easily rewireable for future alterations.

6. An evacuation / fire alarm speaker needs to be installed in a boiler house where there is very loud machinery and an ambient temperature of 50ºC.

7. A new industrial plant requires data to be transmitted from instrument to panel on tray work

In pairs complete the exercise below. You are asked to select a cable type and wiring system and state the reasons why for some different situations. You will need to state the installation method and number given in BS7671. You will need your Tables from BS7671.

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Circuits and BS7671

Division of the installation

Section 314 of BS7671 requires an installation to be divided into circuits, as

necessary. Dividing an installation into circuits helps to avoid hazards, facilitates safe inspection, testing and maintenance and minimises the inconvenience in the event of a fault.

It is imperative that careful consideration is given to how many circuits are required for a given electrical installation. It is also important to select the most appropriate overcurrent and fault protection devices and to position them to ensure, as far as possible, that the only circuits to be disconnected are the ones where a fault has occurred. Separate final circuits provide separately controlled parts of an installation so each final circuit must connect to a separate way in the consumer unit.

Typical domestic installation Typical commercial installation

BS7671 states:

Every installation shall be divided into circuits, as necessary, to:

i. Avoid hazards and minimise inconvenience in the event of a fault ii. Facilitate safe inspection, testing and maintenance (see also 537) iii. Take account of danger that may arise from the failure of a single

circuit such as a lighting circuit

iv. Reduce the possibility of nuisance tripping of RCDs due to

excessive protective conductor currents produced by equipment in normal operation

v. Mitigate the effects of electromagnetic interferences (EMI)

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Accessories

BS7671 defines an accessory as:

“A device, other than current-using equipment, associated with such equipment or with the wiring of an installation.”

They are the items that are used to control or utilise current using

equipment on an installation. Before any work is undertaken or regarding the installation of any new accessories time needs to be spent reading any instructions for the new parts. If the work involves the replacement of accessories in an existing installation you must make sure that the new parts are fully compatible with those on the existing electrical installation.

BS 7671 does not give specific heights for accessories but the Building Regulations 2002 require the following:

Accessories must be mounted between 450mm and 1200mm from the finished floor level in habitable rooms in new dwellings

The centre of a socket outlet should be a minimum of 150mm above the kitchen work surface

Accessories must be installed a minimum of 300mm from the edge of cooker spaces, kitchen sinks and draining boards

Accessories should not be mounted so that it is necessary to lean or reach over a cooker to operate them

Accessories should be mounted on the building fabric and not on kitchen furniture

Socket-outlets supplying washing machines and dishwashers should be installed so that water that may drip from plumbing equipment is unlikely to affect the socket-outlet or plug top

BS7671 makes the following reference to accessories:

• All electrical equipment must be accessible for operation,

inspection & testing, maintenance and repair. (Reg:132.12)

• An accessory is an item of electrical equipment that does not use

any current e.g. a switch or a socket-outlet. (Definitions)

• Every termination, connection or joint between live conductors

including the neutral conductor must be made in a suitable accessory or enclosure. (Reg: 421.7 and 526.5)

• All accessories must be fitted to an appropriate mounting box.

(Reg: 530.4.2)

• A wall mounted socket outlet must be mounted high enough

above a floor or work surface to prevent damage to the flexible cord of a plug top.(Reg: 553.1.6)

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Electrical equipment

BS7671 defines electrical equipment as:

“Any item for such purposes as generation, conversion,

transmission, distribution or utilisation of electrical energy, such as machines, transformers, apparatus, measuring instruments, protective devices, wiring systems, appliances and luminaires”

Class 1, 2 & 3 equipment

Class I Equipment has exposed conductive parts connected to a protective

conductor

Class II Equipment has both basic and supplementary insulation with no

exposed metalwork connected to a protective conductor

Class III Equipment is SELV equipment

Connections of such equipment are very important. The cable sheath should enter the enclosure, the c.p.c should be sleeved and no more insulation removed from the conductors than is necessary to make a proper connection.

Any alterations or additions to an electrical installation may have an effect circuit’s maximum demand. If the circuit length is increased this will have an effect on the disconnection time for the circuit. These changes could mean that the overcurrent protective device and the circuit conductors are no longer suitable and would need to be re-assessed in terms of voltage drop, shock protection and thermal constraints.

BS7671 makes the following reference to electrical equipment:

inspection & testing, maintenance and repair. (Reg: 132.12)

• All items of current using equipment must be provided with a

functional switching device. (Reg: 537.5.1.3)

• All electrical equipment must conform to all relevant standards

and be suitable for its location and use. (Reg: 133.1)

• Electrical equipment must not create a fire hazard to nearby

materials. (Reg: 421.1)

• A ring final circuit must not supply an immersion heater, storage

heaters or a cooker rated more than 2kw. (Reg: 433.1.5 and App.15)

• For all relevant BS or EN numbers see Appendix 1.

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Isolation and switching

Isolation and switching serves the following purposes as stated in BS7671: Isolation of electrical energy

Isolation for mechanical maintenance Functional switching

Emergency switching

BS7671 defines isolation as:

“A function intended to cut off for reasons of safety the supply from all, or a discrete section, of the installation by separating the installation or section from every source of electrical energy.”

BS7671 makes the following reference to isolation:

• Isolation is used to remove the supply to all or part of an

installation for safety reasons. (Definitions)

• When an isolating device is installed remotely from the

equipment it is meant to isolate, the device must be able to be locked in the OFF position. (Reg: 537.2.1.5)

• A main switch must be provided to cut off the voltage to an

installation. (Reg: 132.15.1)

• A double pole main switch or linked circuit breaker must be

installed as close as possible to the incoming supply at the origin of the installation. (Reg: 537.1.4)

• Isolation devices should preferably be double pole. (Reg:

537.2.2.5)

• An isolation device should be labelled if the equipment it

isolates is not obvious by its position. (Reg: 537.2.2.6)

• For more detailed information on devices suitable for Isolation

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BS7671 defines mechanical maintenance as:

“The replacement, refurbishment or cleaning of lamps and non-electrical parts of equipment, plant and machinery.”

Equipment is not always isolated for electrical purposes.

Mechanical maintenance tasks such as pump repairs or machinery

guard’s replacements all require electrical isolations to ensure that it is safe to carry out the work.

BS7671 makes the following reference to mechanical maintenance:

• Mechanical maintenance is the repair, replacement or

cleaning of non electrical parts of electrical equipment. (definitions)

• Electrical equipment that may cause injury during

mechanical maintenance i.e. fans, must be provided with a means to switch off the supply. (Reg: 537.3.1.1)

• A double pole switch, circuit breaker and plug top & socket

outlet may be used to switch off the supply during mechanical maintenance. (Reg: 537.3.2.1)

• A double pole main switch or linked circuit breaker must be

installed as close as possible to the incoming supply at the origin of the installation. (Reg: 537.1.4)

• In a TN-S or TN-C-S system the neutral conductor is not

required to be switched or isolated when it is known that the supply neutral and earth conductors are connected. (Reg: 537.1.2 and Reg: 537.2.1.1)

• A single pole switch must only be used with a line

conductor. (Reg: 132.14.1)

• Only a linked switch that breaks all related line conductors

can be used with an earthed neutral conductor. (Reg: 132.14.2)

• A neutral conductor must not be independently fused or

switched. (Reg: 530.3.2)

• A main switch must be provided to cut off the voltage to an

installation. (Reg: 132.15.1)

• Extractor fans must be provided with an easily accessible

means of switching off the supply.(Reg: 132.15.2)

• Unless its purpose is obvious, all items of switchgear must

be labelled. (Reg: 514.1.1)

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BS7671 defines functional switching as:

“An operation intended to switch ‘on’ or ‘off’ or vary the supply of electrical energy to all or part of an installation for normal

operating purposes”

Functional switching is not isolation or emergency switching or switching for mechanical maintenance

BS7671 defines emergency switching as:

“An operation intended to remove, as quickly as possible, danger, which may have occurred unexpectedly.”

Emergency switching is usually part of an under-voltage protection circuit such as motor control

As you can see by the definitions above there are very distinct differences between isolations and switching. It is very important to know the difference between them so there is no confusion over whether a circuit is isolated or not.

Other circuit information

A durable notice giving details of all the circuits fed is required to be posted in or near each distribution board. The information required is the equipment served by each circuit, its rating, its design current and its overcurrent device breaking capacity. When the occupancy of the premises changes the new occupier must be provided with full details of the installation. This data must always be kept up to date.

BS7671 makes the following reference to functional switching:

• Functional Switching is used to switch on or off the electrical

supply to an installation or any part of an installation under normal operating conditions. (Definitions)

functional switching device. (Reg: 537.5.1.3)

• Fuses and unswitched fused connection units (unswitched

spurs) must NOT be used for Functional Switching. For more detailed information on devices suitable for Functional Switching (see Table 53.2 p117)

BS7671 makes the following reference to emergency switching:

• Emergency Switching is intended to remove any unexpected

danger as rapidly as possible. (Definitions)

• Fuses, unswitched fused connection units (unswitched spurs)

and plug top & socket outlets must NOT be used for Emergency Switching. For more detailed information on devices suitable for Emergency Switching (see Table 53.2 p117)

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Now answer the questions below

1. Explain why we need to divide an installation into circuits

2. In your own words, define what an accessories is

3. State three references to accessories in BS7671

4. Explain why you think it is not permissible to mount accessories onto kitchen units?

5. In your own words define what electrical equipment is

6. State three references to electrical equipment in BS7671

7. In your own words define isolation and switching

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Circuit Categories

The regulations state that circuits from different categories cannot be mixed in an installation. This is due to the detrimental interference between each circuit type. Magnetic fields generated by mains cables can interfere with the transmission of data signals and produce harmful or incorrect values on a data network. When we refer to different categories of circuit we mean:

One exception to this rule is that as long as each circuit’s insulation is rated to the highest circuit voltage they can be installed together without any segregation. On site a typical installation will have several routes for carrying the different circuit types. There may be three cable trays, side by side, each designated to carry only the circuits form each category. In an office installation there may be multi

compartment trunking or a single tray work with sufficient space between them (approx 300mm) so no interference takes place.

Category 1

– Circuits operating at low voltages (50 to 600 volts AC) and supplied from the electrical mains.

Category 2

– Any data, telecommunication, intruder alarm systems and circuits operating at extra low voltage. (not exceeding 50 volts AC and 120 volts DC)

Category 3

– Any fire detection system, emergency lighting or alarm system.

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Category 1 Circuits

Circuits operating at low voltages (50 to 600 volts AC) and supplied from the electrical mains.

Lighting circuits

All lighting circuits and their control wiring are included in category 1. One way; two way; intermediate; contactor controlled, timer controlled, automatically controlled are all examples of Category 1 circuits. Lighting that operates at less than 50 volts from a direct supply is NOT included.

Power circuits

All power circuits and their control wiring are included in category 1. Ring main and radial socket circuits; cooking appliances; fixed equipment; motors are all examples of Category 1 circuits. Control circuits that operate at voltages less than 50 volts are not included.

Heating circuits

All heating circuits and their control wiring are included in category 1. Dual-element immersion heaters; cistern-type water heaters; non-pressure water heaters;

Instantaneous water heater; controllers; timers; thermostats; pumps; boilers are all examples of Category 1 circuits.

Environmental control circuits

All circuits that provide cooled/heated air to a building are included in category 1. Air conditioning units; controlled; pumps; electronic shutters; valves; heating elements are all examples of Category 1 circuits.

Standby power supplies

All circuits that provide back up supplies to an electrical installation are included in category 1. Generators; temporary power leads; fixed back up battery units are all examples of Category 1 circuits.

The electrical supply in this country is very reliable and secure. However, as with all systems there are occasional interruptions that for some installations would be dangerous as well as inconvenient. Hospitals, air-traffic control and the petrochemical industry are just a few installations that could not tolerate an interruption to the mains supply, so a standby system needs to be

available.

Large installations need a standby generating system, whereby a large combustion engine cuts in automatically and drives a generator capable of supplying the load needed to continue working safely

Smaller establishments such as small offices cannot afford complex standby generation systems, but nevertheless they may have computer systems that cannot afford to be off or, worse still, risk losing data. In this situation

standby power systems known as Uninterruptible Power Supplies (UPS) are used, which consist of a battery supply that is charged up via the mains when not in use. When the mains supply is lost the UPS automatically cuts in and, via the electronics contained in it, converts the D.C battery supply to a mains supply capable of powering several computers.

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Any data, telecommunication, intruder alarm systems and circuits operating at extra low voltage. (Not exceeding 50 volts AC and 120 volts DC)

Data Transmission

Fibre-optic cables

This cable is used for digital transmissions used by equipment such as telephones and computers. They are made from optical-quality plastic (the same as spectacles) where digital pulses of laser light are passed along the cable from one end to another with no loss or interference from mains cables (assuming the insulation is sufficiently rated). They look like SWA cables but of course they are much lighter and contain either one core or many dozens of cores.

Tight radius bends in this type of cable should be avoided, as should ‘kinks’, as the cable will break. Jointing of these cables requires specialist tools and equipment. Never look into the ends of the cable as the laser light could damage your eyes. The applications of optical fibre

communications have increased at a rapid rate since the first commercial installation of a fibre-optic system in 1977. Telephone companies began early on replacing their old copper-wire systems with optical-fibre lines. Today’s telephone companies use optical fibre throughout their system as the backbone architecture and as the long-distance connection between city phone systems.

Fibre-optic cables are also used in Local Area Networks (LAN). These collective groups of computers, or computer systems, connected to each other, allow for shared program software or databases. Colleges, universities, office buildings and industrial plants, just to name a few, all make use of fibre-optic cables within their LAN systems. Power companies are emerging as big users of fibre optics in their communication systems. Most power utilities already have fibre-optic

communication systems in use for monitoring their power grid systems.

If you wanted to see down a dark corridor, you might shine a torch down it. But what if the corridor had a bend in it? You could probably put a mirror in just the right place at just the right angle and shine the light round the corner. But what if the corner had lots of bends? Well, what if I made the entire corridor walls out of mirrors, then I wouldn’t need to put them in just the right place or angle. The light would be able to bounce around all the mirrors along the walls.

Believe it or not, that’s the theory behind fibre-optics, as the glass core is essentially a mirror wound into a thin tube. Some 10 billion digital bits can be transmitted per second along an optical fibre link in a commercial network, enough to carry tens of thousands of telephone calls. The hair-thin fibres consist of two concentric layers of high-purity silica glass, the core and the cladding, which are enclosed by a protective sheath.

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Co-axial cable

UTP (CAT5) cable

Cat (Category) 5 cable

This cable is used extensively for data transfer in computer networks and telephone systems. It has four pairs of wires that transmit and receive data along them at very high frequencies; typically 350 MHz. Special termination ends are required for these cables.

There are three basic types of cabling used in data systems: coaxial, fibre-optic and Unshielded Twisted Pair (UTP). Coaxial is widely installed in older networks but is not recommended for new network installations. Fibre is used for high-speed networks and to connect networking devices separated by large distances. But UTP is currently the most common and recommended cabling type. UTP is inexpensive, flexible and can transmit data at high speeds. Most new installations are currently installed with Cat-5 UTP cabling and

components.

Intruder / Security Alarms

Alarm systems are classified as Category 2. All cabling that connects to keypads, remote sensors, sounders, beacons or door contacts all need to be kept separate.

Closed Circuit Television (CCTV)

Camera equipment uses a co-axial cable that transmits (and sometimes powers) cameras on an installation.

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MICC Cable

Any fire detection system, emergency lighting or alarm system

Fire Alarms

A correctly installed fire-alarm system installation is of paramount importance and can be compared to any other electrical undertaking, as life could be lost and property damaged as a result of carelessly or incorrectly connected fire-detection and alarm equipment.

Fire alarms typically use red FP200 (or equivalent) or MICC cable.

Emergency Lighting

Emergency lighting is not required in private homes because the occupants are familiar with their surroundings. However, in public buildings, people are in unfamiliar surroundings and in an emergency they will require a well-illuminated and easily identified exit route. Emergency lighting should be planned, installed and maintained to the highest standards of reliability and integrity, so that it will operate satisfactorily when called into action.

Emergency lighting typically use white FP200 (or equivalent) or MICC cable. When they are installed as an integral part of a lighting circuit they use the general circuit wiring (i.e. PVC cable).

Also included in category 3 are any circuits that are used as an evacuation system. This could be a system that is a person-operated loudspeaker system or a computer or panel operated one.

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Check your learning by answering the questions below

1. State the voltage range of a category 1 circuit

2. What type of lighting circuits are not included in category 1?

3. Give two examples of a power circuit.

4. What is a UPS and how does it function?

5. What type of circuits are included in category 2?

6. Explain the basic theory behind fibre optics.

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Standard Circuit Ratings

The table below shows most of the standard circuits found in domestic premises and the ratings of the overcurrent devices that may be used to protect them.

TYPE OF CIRCUIT RATING (A) TYPICAL CONDUCTOR

SIZES (mm²)

Lighting 5, 6, 10 1.0, 1.5

Socket outlets (BS1363) 20, 30, 32 2.5, 4

Socket outlets (BSEN60309) 20 2.5

Immersion Heater (3-4kW) 15, 16, 20 2.5, 4

Cooker, electric shower (6-10kW) 30, 32, 40,45 4, 6, 10

There may seem little difference between a 5A and 6A protective device and indeed, in terms of performance, there is little difference. This reflects the changes in

standards over time. Rewirable fuses to BS3036 and cartridge fuses to BS1361 are made in ratings 5, 15 20, 30 A. Miniature circuit breakers and (mcb’s) to BS EN60898 are made in sizes 6, 10, 16, 20 and 32A.

The third column on the table shows the cable sizes that are typically used in these circuits. The list is not an inclusive list as unique installation conditions may require a larger cable. The list is for thermoplastic (p.v.c) cable. If m.i.c.c were being used then it would be found that a cable one size smaller would usually suffice. This is because the mineral insulation can withstand higher temperatures than p.v.c and so the current ratings of m.i.c.c are higher as a result.

RCDs are required to protect users of appliances and equipment where sockets rated up to 20 amps are used. The highest risk is outside of the equipotential zone. This is due to the fault return path not being sufficient

Class discussion

Dave, an electrical apprentice needs to isolate a lighting circuit so he can fit a dimmer switch to an upstairs bed room for his parents. Explain the step by step process he will go through to ensure he does the isolation correctly.

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Lighting Circuits

Lighting is a vast and varied subject and beyond the scope of these notes. However we will see some of the basic requirements, the different lighting circuits used and the types of lamps associated with standard installations.

BS7671 makes the following reference to lighting circuits:

inspection & testing, maintenance and repair. (Reg: 132.12)

• Where an installation has only one lighting circuit, the

circuit may need to be divided into two circuits to minimize the danger that may arise in the event of a fault.(Reg: 314.1 (iii) )

• Light fittings must be installed so that any radiated heat

does not cause any damage. (Reg: 559.5.1)

• The fixing supporting a light fitting must be able to support

a weight of at least 5kg. (Reg: 559.6.1.5)

• Domestic lighting circuits should not be rated at more than

16A. (Reg: 559.6.1.6)

• Edison screw lampholders (excluding types E14, E27)

should have the outer contact connected to the neutral conductor. (Reg: 559.6.1.8)

• Lighting circuits must be controlled by the appropriate

number of switches. (Reg: 559.6.1.9)

• Through wiring is only permitted in a light fitting where the

light fitting is designed for such wiring. (Reg: 559.6.2.1)

• Through wiring in a light fitting must be suitable for the

temperature generated inside the fitting. (Reg: 559.6.2.2)

• The lamp inside an outdoor light fitting mounted less than

2.8m above the ground must only be accessible after removing an enclosure or barrier with the use of a tool. (Reg: 559.10.3.1)

• Electrical equipment outdoors must be rated at least IP33.

(Reg: 559.10.5.2)

• Switchlines must be marked brown at their terminations.

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Lighting points

For each fixed lighting point one of the following must be used.

1. Ceiling rose 2. Device for connecting a

luminaire (DCL)

3. Batten lamp holder 4. Luminaire designed to be connected directly to the circuit wiring

BS7671 states that the maximum rating of overcurrent protective devices of circuits must not exceed the lamp holder rating. These are:

Small Bayonet Cap B15 Max Rating 6 Amps

Bayonet Cap B22 Max. Rating 16 Amps

Small Edison Screw E14 Max. Rating 6 Amps

Edison Screw E22 Max. Rating 16 Amps

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Lamp types

Incandescent filament lamps (GLS and reflector)

Filament lamps are used mainly for domestic and display lighting. There are many types of filament lamp, the most common being general lighting service (gls) and decorative.

Their finish – clear, diffuse/pearl or coloured – is a significant factor in their application. Reflector lamps are similar but have an

envelope with an internal reflective coating.

Advantages of filament lamps include low initial cost, simple operation (no control gear required) and good colour rendering. Disadvantages of filament lamps are low efficacy (measure of the energy efficiency of a light source, ie lumens per watt) and a relatively short life. Certain extended life filament lamps have only about half the efficacy of standard lamps. The light output of filament lamps is particularly sensitive to voltage variations.

Halogen filled filament lamps (tungsten halogen)

The main reason for filling a tungsten filament lamp with a halogen gas is to prevent evaporated tungsten

from blackening the envelope. Tungsten halogen lamps also have an increased light output and/or an extended life compared with standard filament lamps. The envelope is of small dimensions and made of quartz or hard glass. Some mains voltage lamps have an outer protective envelope.

Lamps that are suitable for use in luminaries without a safety screen should be so marked. Otherwise, tungsten halogen lamps should only be used in suitably enclosed luminaries.

Extra low voltage (elv) lamps are, in general, more compact than their mains voltage counterparts and the small filament size can improve the optical efficiency of integral or external reflectors (generally dichroic). Elv reflector lamps make it possible to use compact luminaires for display lighting.

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Low pressure mercury fluorescent tubes

The light output from a tubular fluorescent lamp comes from phosphors that convert energy from a low pressure gas discharge into visible light. The colour temperature and colour rendering are determined by the phosphor mix coated on the inside of the tube.

The argon-filled t12 (38mm diameter) tubes are

being discontinued. The modern range of krypton-filled triphosphor t8 (26mm) diameter tubes should be the first choice for switchstart, quick start and high

frequency luminaries. Such lamps have a higher efficacy, longer life, improved lumen maintenance and better colour rendering than earlier types of tube.

Triphosphor (or multi-phosphor) tubes offer a wide range of colour temperatures from very warm (2700k), warm (3000k) and intermediate (3500k) through to cold white (4000k), daylight (5000-5500k) and northlight (6000-6500k).

Compact fluorescent lamps

A compact fluorescent lamp (cfl) has the characteristics and advantages of linear fluorescent lamps but its compact size is achieved by folding the discharge path, retaining high efficacy. The two main groups of cfls are those with external control gear and those with internal control gear.

High frequency control gear is now available

integrated into the cfl lampholder, making lamp conversion from gls to cfl very simple. Many modern fluorescent lamps are operated at high frequency (typically at or above 30 khz) which results in a reduction of energy losses both in the lamp and the control gear. The control gear size and weight are often less, the efficacy higher, dimming where required is easier, and operation is silent.

Incandescent Wattage CFL Wattage 25 50 60 75 100 120 150 5 9 15 20 25 28 39

Wattage comparison chart

The main tube lengths and their respective power ratings are: 600mm = 18w

1200mm = 36w 1500mm = 58w 1800mm = 70w

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High pressure sodium lamps

Light is generated by an electrical discharge in a gas containing sodium and mercury (sodium amalgam) contained in a sintered alumina arc-tube.

High pressure sodium lamps are used for road lighting, floodlighting and industrial interior lighting.

Low pressure sodium lamps

Low pressure sodium lamps consist of a u tube containing the discharge, and an outer heat reflecting glass jacket. The monochromatic light is concentrated in the yellow part of the visible spectrum which is close to the maximum sensitivity of the human eye at normal lighting levels. The efficacy is the highest of all lamp types, but with very poor colour rendering. Low pressure sodium lamps are used mainly for exterior applications such as road and security lighting (but are not suitable for repeated on/off (operation).

Metal halide discharge lamps

Metal halide lamps have quartz or sintered alumina (ceramic) arc tubes, generally with an outer glass envelope. Light output is from mercury and other metallic elements introduced in the form of halides. Metal halide lamps of the ‘protected’ type are now available for operation in luminaries without safety

screens. According to the mix of elements, there is a wide range of efficacy and/or colour appearance, but colour rendering is generally good.

Metal halide lamps are generally used in commercial interiors, industry and floodlighting, and (in smaller ratings) for retail lighting.

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High pressure mercury discharge lamps

The high pressure mercury discharge operates in a quartz envelope. Mercury lamps were used for illuminating road signs and industrial lighting but have largely been replaced by the more efficient lamps now available. Such lamps offer low cost

discharge lighting where high efficacy is not important. They often incorporate a third electrode for starting and in such cases the control gear required generally consists only of a ballast and a power-factor corrected capacitor.

Induction lamps

Induction is a process whereby electrical power is passed from one circuit to another without the use of physical electrical conductors. It enables lamps to be constructed without the need for wire connections to pass through the glass or quartz envelope. Induction lamps are available as low pressure mercury lamps, using the same triphosphor coating of the inner envelope surface as the familiar fluorescent tubes. The commercially available range of induction lamps is limited.

Light emitting diodes (leds)

Light emitting diodes have been used for indicating purposes for several decades and recent developments have created larger diodes and extended the range of colours including white.

A dramatic increase in efficacy is predicted in the near future. Leds have an extremely long life and are likely to be built into the luminaire and will not be a consumable item as far as the end user is concerned.

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Lighting design current calculations

Example 1

A house with ten rooms requires a light in each room. Calculate the design current. i) Select the correct formula (basic resistive lighting)

ii) Input the data into the formula and work it out to two decimal places and be sure to add the unit (A)

Example 2

A warehouse office has two rooms with two, four tube light fittings rated at 36 watts per tube. Calculate the assumed design current for the 230v circuit.

i) Select the correct formula (discharge lighting). Three line circuits use a

similar formula but the V (230v) is replaced by √3 x 400

ii) Input the data into the formula and work it out to two decimal places and be sure to add the unit (A)

Assumptions

1. A lighting outlet shall be considered to have a connected load of minimum 100 W. (Standard lighting points with lamps)

2. Discharge lighting calculations needs to take into to account harmonic currents and control gear losses. Where the exact manufacturer’s information for gear losses is not available the maximum demand shall be assumed to be 1.8 x the lamp rating

Note: If the manufacturer’s discharge lamp information is obtainable from a catalogue this would be used to gain a more accurate value of design current.

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1. A 230v lighting circuit supplying twelve rooms with one standard light point in each.

2. A 230v lighting circuit feeding twelve rooms has one centre light in each room and two wall lights in two rooms. The wall lights are twin 40w maximum type

3. A 230v discharge lighting circuit of four flood lights with a total load per fitting of 250 watts. Manufacturers lamp detail not known

4. An open plan office has twelve 4x36w light fittings all one line. Manufacturer’s lamp information reveals each lamp to be 39 watts (accounting for control gear losses).

5. A car show requires display lighting using 21 x 250 watt metal halide (discharge) lamps. The supply will be triple pole and neutral (TP&N).

6. How many lighting points are permissible to be supplied on one single line 10 amp circuit breaker?

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Lighting control

Lighting accessories or luminaries must be controlled by a switch or switches to BS 3676 or BS 5518 and must be suitable, where necessary, for the control of discharge lighting circuits.

Light switches can be one way, two way or intermediate. The number of switches per accessory are called 'gangs'. They are generally rated to carry 6 amps but some are available that can safely switch 20 amp circuits.

The type of light switch finish depends on the installation (bathroom, kitchen, outdoor etc), the number of points of switching (one way, two way, intermediate) and the number of lamps to be switched from that location.

One-way switch control

This indicates the user has the ability to switch the lighting circuit from one point only. The switch wire, which is usually blue, should be identified at both ends as a line conductor either by line colour or the letter 'L'.

Two-way switch control

This indicates the user has the ability to switch the lighting circuit from two points. Often used in bedrooms, on stairs, halls and rooms that may contain more than one point of entry.

More than two switching points

Commonly known as 'intermediate switching', the circuit contains a two-way switching position at each extremity and extra “intermediate” switching positions within the circuit (as many as required). These are used in long rooms, stairs or corridors that require many switching points, where all it may be necessary to avoid the user walking long distances in the dark to operate the circuit.

1 Gang or 1G White Plastic Switch

The extra wires that are used to connect the two light switches together are known as strapping wires or ‘strappers’. All strappers should be identified at both ends as

line conductors either by colour or the letter 'L'

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Contactor control

A contactor is a switching device that contains an electromagnet. When a voltage is applied to the electromagnet (or coil) it generates a magnetic field around it and forces contacts within the contactor to close. The full load current circuit connections are electrically separate from the coil so it can be operated with a different voltage. Contactors can be employed in lighting control circuits to switch loads that exceed the rating of switching devices. This means that standard 6 Amp switches would be used to control the supply to the coil and the entire lighting load would be taken by the contactor.

Single or three line banks of lighting, power loads including motors can be controlled in this way. Due to the current that energises the contactor being negligible, the associated overcurrent device and circuit wiring can be sized accordingly. This means that smaller rating devices and conductors can be used for the control circuit. Below is a typical example of how a contactor might be used.

Typical contactor controlled lighting circuit

Other information

• A minimum of one fixed lighting point is

required in each room, on hallways and on stairways/ landings

• Each light has to be controlled by a

switch

• When determining the position of the

lighting point and switch positions how each room is to be used must be taken into consideration

• Additional lighting that may be required

in certain rooms, such as lounges, kitchens, bedrooms and bathrooms

• Switching from more than one point

should be considered for a room with 2 or more points of entry, in bedrooms, long passages and halls as well as landings and stairs

• The height of the light switches is set out

in Regulation 553-1-6

Three gang switch terminals Centre switch terminals

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Lighting circuit wiring

Loop in Method (or Three Plate Method)

The loop-in method is the most common wiring system employed in lighting installations using PVC twin and earth, steel wire armoured or MICC cables. No joints or connections are to be made anywhere except at the recognised

termination points of the circuit, i.e. at switch terminals, ceiling roses or lamp holders. Each ceiling rose or lighting point requires three banks of terminals or plates. They are labelled Live, Loop and Neutral (earth is taken as always being present).

The main supply cable runs from the circuit protective device at the fuse board to the first loop in ceiling rose then on to the next ceiling rose in the next room and so on. From each ceiling rose a cable is run down to the light switch, this acts as the supply conductor and the switched return conductor. The normally blue, switched conductor is required to be marked as a line conductor; both at the light switch and at the ceiling rose.

Pendant connections

Three plate ceiling rose connections

Conventional Method (or Two Plate Method)

The conventional method was the only way to wire lighting circuits many years ago as manufacturers had not created a three plate accessory. Junction boxes would have been used to save on cable costs and lighting points would only have the facility for live and neutral terminations.

Today, the same accessories can be used in this method but the loop terminal is not used. To achieve correct switching principles the feed from the fuse board must terminate at the switch. This means that the supply neutral conductor must terminate into a separate connector block inside the switch box. A switched live and neutral would then carry the supply on to the lighting point.

Mixing the two methods is totally acceptable and in some cases may be the most practical. Especially where cable routes are considered.

The disadvantages of the two plate method:

• There is only ever a switched live at any one lighting point making

alterations or additions to lighting circuits more difficult

• Where junction boxes are used this presents “hidden” fault

locations and laborious fault finding by moving furniture and carpets to lift floor boards to access the jb’s

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One way switch connections using single core or multi-core

Two way switch connections

Or

+

Or

+

1G1W Switch connections wired in conduit (Single core) 1G2W Switch connections wired in conduit (Single core) 1G2W Switch connections wired in conduit (Single core) 1G1W Switch connections wired in 6242Y (Multi core) 1G2W Switch connections (first switch) wired in

multi-core

1G2W Switch connections (second switch) wired in

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Intermediate switch connections

+

1G2W Switch connections wired in conduit 1G2W Switch connections wired in conduit Intermediate switch connections wired in conduit

1G2W Switch connections (first switch) wired in

multi-core

1G2W Switch connections (second switch) wired in

multi-core Intermediate switch

connections wired in multi-core

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Lighting circuits exercise

1 Gang 1 Way, 1 Light, 3 Plate method using multi-core

Complete the exercises on the next pages once you understand the completed diagram below. Follow the supply to the loop, to the switch and then back to the light. This is the path the current takes when it operates the light.

L

LOOP

N

L1

C

LIVE

NEUTRAL

Switch

wire

Circuit

feed

6A

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1 Gang 1 Way, 2 Lights, 3 Plate method using multi-core

L LOOP N

L C

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1 Gang 2 Way, 1 Light, 2 Plate Conduit method using single core

L LOOP N C L 1 L 2 C L 1 L 2

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Truth tables

A truth table can be very useful if you forget how to connect up a lighting circuit. Draw the circuit and try it out before you wire it up by “operating” one switch at a time. If it doesn’t work on paper then it won’t work in real life!

Example to be completed with the trainer

Switch A

LEFT RIGHT

Switch B

LEFT LEFT

Light – on / off

1 Gang 2 Way, 1 Light, 3 Plate Method using multi-core L LOOP N C L 1 L 2 C L 1 L₂

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Switch A L L Switch B S S S S X X Switch C L R Light – on / off

1 Gang Intermediate, 1 Light, 2 Plate Conduit Method using single core

Complete the truth chart for the lighting circuit on this page by switching one switch at a time. L LOOP N C L 1 L 2 C L 1 L 2 L1 L2 L2 L1

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1 Gang Intermediate, 1 Light, 3 Plate Method using multi-core

L LOOP N C L 1 L 2 C L 1 L 2 L1 L2 L2 L1

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Now complete the next two circuits in the space below ensuring safe and effective switching.

1. A lighting circuit contains two lights. One light is switched from one location; the other is switched from two locations. The circuit is to be wired in multi-core cable using the three plate method.

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2. You are required to wire and terminate a lighting circuit on part of an office block. There are four rooms and a long corridor. Three of the offices have one door and the fourth has two. The long corridor has an entrance at both ends with the office entrances along its length. The corridor length is 30 metres long. The circuit is to be wired in PVC singles in conduit. Assume one light per circuit part for simplicity.

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Power circuits

A power circuit is generally anything that is not a dedicated lighting circuit.

Cooking appliances

A cooker is regarded as a piece of fixed equipment unless it is a small table-mounted type fed from a plug by a flexible cord. Such equipment must be under the control of a local switch, usually in the form of a cooker control unit. This switch may control two cookers, provided both are within 2 m of it. In many cases this control unit incorporates a socket outlet, although often such a socket is not in the safest position for use to supply portable appliances, whose flexible cords may be burned by the hotplates. It is often considered safer to control the cooker with a switch and to provide a separate socket circuit.

The diversity applicable to the current demand for a cooker is 10 A plus 30% of the remainder of the total connected

load, plus 5A if the control unit includes a socket outlet. A little thought will show that whilst this calculation will give satisfactory results under most circumstances, there is a danger of triggering the protective device under some circumstances. For example, at Christmas it is quite likely that grill and oven, all four hotplates and a 3 kW kettle could he simultaneously connected. Just imagine the chaos which a blown fuse would cause! This alone is a very good reason for being generous with cable and protective ratings.

These include:

• Cooking appliances • Motors

• Water-heaters (instantaneous type)

• Water-heaters (thermostatically controlled) • Space storage and floor heating installations • Standard arrangements of final circuits

o Final circuits using socket-outlets complying with

BS1363-2 (Standard household sockets)

o Fused connection units complying with BS 1363-4 o Final radial circuits using socket-outlets complying with

BS EN 60309-2

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Cooker types

Hobs

On electric cookers there are two main types of hob: the hotplate and the ceramic hob.

Hotplates can be broken down into two main types; Radiant rings and sealed plated.

Radiant rings is coiled metal, often the cheapest due to the fact they take the longest amount of time to heat up and cool down and are often hard to clean.

Sealed plate hobs are thin iron discs covering heating elements and sometimes have thermostats to prevent overheating.

They are also quite slow to heat up and cool down but are easier to clean and are very durable.

Ceramic hobs have halogen or semi-halogen heating elements under a heat resistant glass.

Halogen is a bulb with a tungsten element and halogen gas. Halogen hobs are much the same as radiant the main difference being it has a faster response time offering better heat control. Semi-halogen is a halogen bulb surrounded by a

radiant element

Ovens

Ovens type are normally broken down into two types; coventional and fan assisted. In a conventional oven the thermostat controls the heat in the middle of the oven; the oven will be a slightly hotter in the middle.

Fan ovens work differently by using a fan to circulate heat around the oven. This creates a temperature throughout that is even also meaning the oven heats up very quickly, reducing cooking times and saving energy.

Double ovens

Double ovens mean you can set each oven differently, as well as offering more capacity. Often the main oven will be fan assisted with the smaller second oven being conventional. The second oven generally has a primary function to act as a grill.

BS7671 makes the following reference to cooker circuits:

• All 13A socket outlets must be 30ma Rcd protected. Reg:

411.3.3

• All final circuits must be wired separately from all other final

circuits. Reg: 314.4

functional switching device. Reg: 537.5.1.3

• The Building Regulations require that accessories should not

be mounted so that it is necessary to lean or reach over a cooker to operate them

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Cooker control

Cookers must have a functional switching device for convenience. This can take a number of forms but is generally achieved via a double pole switch. Units with power ratings of less than 3kW can be supplied by a dedicated socket outlet circuit or via a switched fused spur unit.

A double pole switch makes and breaks both the live and the neutral conductors

Or

32A or 45 Amp double pole switch with or without neon indicator and socket

6.0mm2 or 10.0mm2 Supply cable 32A or 40A over

current protection (with RCD if supplying a switch with a socket outlet)

45A connection unit for easy removal and reconnection of the oven (usually mounted directly behind the oven)

Cable and Circuit Types

Cable and

Circuit Types

Learner Work Book

Name:

Group:

Tutor:

Table of Contents

Foreword

Cables and Circuit Types Unit Overview

Cable Types

Circuits and BS7671

Circuit Categories

Category 1

Category 2

Category 3

Standard Circuit Ratings

Lighting Circuits

V

P

I =

A

I

4

.

35

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100

10

=

×

=

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)

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4

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Or

+

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+

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+

L

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N

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LIVE

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Switch

wire

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feed

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