Section 11 electrical_installation_practice_1

PowerPoint

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Section 11

Electrical installation Practice 1

Copyright ©2011 Pearson Australia (a division of Pearson Australia Group Pty Ltd) – 9781442523258/Hampson/Electrotechnology Practice/2nd edition

Earthing systems and fundamental requirements

Without exception the earth is the most important conductor of any electrical installation.

An effective earth ensures personnel safety and equipment protection particularly during fault conditions.

The relevant AS/NZS 300:2007 Wiring rules standard is Section 5, which deals comprehensively with earthing arrangements and earthing conductors.



Earth

The term ‘earth’ refers to the general mass of ground or soil. The term ‘earthed’ (see Clause 1.4.43) refers to a conductor or metal within the electrical installation which is purposely connected to earth.


Earthing systems and fundamental requirements The general mass of ground/soil (earth) is chosen

as the zero reference point since it is accessible everywhere.

Earth is simply an electrical reference point for the system.

When you are standing on the ground, your body is approximately at the voltage potential of the earth. In an installation, if a conductor or all accessible metal are connected to the general mass of earth then the conductor and all accessible metal are approximately at ground potential.



The neutral point of the low-voltage system of the distribution transformer is solidly connected to the general mass of earth via an earth electrode to achieve an earth resistance not exceeding one ohm.



Advantages of an earth star distribution system are:

common earth reference

voltage stability

low-voltage supply

lower insulation requirements

phase-to-earth faults



The earthing system in an installation has the following purposes (see Clause 5.1.2):

To minimise the voltage rise (see touch voltage) of any earthed metal that may become energised due to the passage of fault currents.

To provide low-impedance path to allow circuit protection to operate when required to clear faults resulting from an insulation failure to earth.



The earthing systems adopted by the majority of power authorities in Australia are the multiple earthed neutral (MEN) system and the common multiple earth system (CMEN).



Multiple earthed neutral

The MEN system of earthing (see Clause 1.4.66) is one in which the distribution neutral conductor is used as a low-resistance return path for earth fault currents and where its potential rise is kept low by having it connected to earth at a number of locations along its length.



The ground serves to cancel out the effects of leaking currents in an installation by earthing out, or reducing to a zero potential, the voltages which are picked up by earth conductors.

When an installation is earthed according to the AS/NZS 3000:2007 Wiring rules, protection is being provided for persons and animals against the danger of electric shock.





Touch voltage (see Clause 1.4.95) is a term that is used to describe the voltage difference that occurs across a person or animal by their touching an exposed grounding conductor and the soil when fault current is flowing.

Touch voltage is defined as the potential difference between a person’s outstretched hand, touching an earthed structure, and their foot.

A person’s maximum reach is assumed to be one metre.





The MEN system was designed to ensure that if a fault occurs where the neutral conductor is disconnected for any reason and the active conductor contacted exposed metal, the fault current could flow through the earth conductor as a return path to the switchboard, allowing the protective device to open-circuit, thereby making the circuit safe.



A reverse polarity (active and neutral transposed at the mains) can cause the earthing system to be up to 230 V potential.

To test for this condition use a voltmeter and an independent earth electrode and measure the voltage between the electrode and the neutral link.

A reading above 0 V indicates reverse polarity and the installation has to be made safe.

















Once the installation has been tested, the MEN connection needs to be rechecked again.

There have been serious electrical accidents where the MEN link was missing, and subsequent faults occurred in the installation.



Even in a soundly earthed MEN system, neutral elevation can occur due to:

ineffective earth bonding connections

voltage drop in the neutral conductor due to single-phase loads

voltage drop in the neutral conductor resulting from harmonic currents: back emf from single-phase motors after being switched across the supply mains.







Main earth resistance

The resistance of the main earthing conductor (see Clause 5.5.1.4) is measured between the main earthing terminal/connection and bar and the earth electrode, including the connection to the earth electrode.

The measured resistance between these two points must not be greater than 0.5 Ω.



Earth electrodes

The purpose of the earth electrode is to connect the neutral of the electrical installation to the general mass of earth (see Clause 5.5.1.2).

The principle of earthing is to regard the general mass of earth as a reference (zero) potential.





Some additional examples of earth electrodes are:

An approved earth stake driven to a depth of at least 1.2 m into the ground.

A copper strap of at least 25 mm × 1.5 mm × 3 m buried at a depth of at least 0.5 m in a horizontal trench.

Bare copper cable of 35 mm2 × 3 m buried at a depth of at least 0.5 m in a horizontal trench.









Earthing resistance of an electrode

The earthing resistance of an electrode is made up of three important elements.

resistance of the electrode

contact resistance between the electrode and the soil

resistivity of the soil






Earthing systems and fundamental requirements The PEC can take many forms (see Clause

5.3.2.2) such as:

A separate conductor which must be green/yellow insulated

An earth conductor included in a sheathed cable with other conductors

Metallic sheaths, armours and screens of cables and Conductive framework (see Clause 5.3.2.3c)

Conducting cable enclosures such as conduit, tube, pipe, trunking, ducting and similar wiring enclosures (see Clause 5.6.2.4)















Supplementary bonding conductors

Supplementary bonding conductors (not less than 4 mm2 copper) connect together extraneous conductive parts, that is, exposed conductive materials which are not associated with the electrical installation but which may provide a conducting path if not bonded, giving rise to shock.





The single-wire earth return (SWER) system

The SWER system uses one metal conductor (in most cases a bare conductor) to provide power and the general mass of earth (ground) as a return current conductor.



The SWER system is a low-cost and low-maintenance method of supplying power to rural (isolated) areas where the base loads (100 kW to 200 kW) and the population is low.





An SWER system consists of a separate high-voltage earthing system and a low-voltage earthing system.

Therefore for each installation there will be a low-voltage earth electrode system at the SWER transformer and an earth electrode at the installation’s distribution board earthing the low-voltage neutral.





A disadvantage of an SWER system is that electrical equipment used in cold rooms, air-conditioners and irrigation pumps may not operate as efficiently as they could on three-phase power, and their output may be restricted.



A SWER line must be installed at a safe height and if near stockyards, irrigation areas (use of spray rig booms) or other areas where breaching the exclusion zone is possible the SWER line must be insulated and warnings of its presence and potential danger must be provided.

In South Australia Stobie pole (composite concrete and steel pole) or SURELINE® poles are used to carry SWER lines.





Earth fault loop impedance

Each circuit in an electrical installation must be designed to protect people, livestock and property from harmful effects (see Clause 1.6.1).

The method of protection is achieved by a system of equipotential bonding and automatic disconnection of the supply (see Clause 1.5.5.3c).



The earth fault current flowing through the impedance of the protective earth system causes a voltage to appear on the earthed metal.

This voltage is called the prospective touch voltage (see Clause 1.4.95).




Earthing systems and fundamental requirement


Earthing systems and fundamental requirement


Core balance earth leakage (CBEL)

This mechanism is also called a residual current device (RCD) or a safety switch and was created to detect very small earth leakage currents (10 mA).

Its purpose is to protect portable appliances in earthed situations.

Because of this ability it is the only protection device that affords protection to people as well.





RCD characteristics

RCDs are set to trip the circuit protection instantaneously when a current of a pre-set magnitude or more leaks from an active to earth or earthed person or equipment.





It is essential that the RCD detects current that is below the minimum anticipated current through the body and in fact the recommended tripping current for shock protection is a maximum of 30 mA, which will provide effective protection for the vast majority of the population.



Classification of RCDs

There are four different types of RCDs: Type I to Type IV. The main differences are the trip times (measured in milliseconds) and the rated residual trip currents (I∆n) measured in milliamps.

Types I and II are used to protect people and animals from electrocution and range between 10 mA and 30 mA with different trip times.

Types III and IV are mainly used in situations where nuisance trips may be a problem.



Type testing of RCDs

All RCDs must be tested by a systems electrician prior to their initial introduction to service and before return to service after a repair or servicing, which could have affected the electrical operation of the device.










Consumer’s mains

In Australia, reticulation to existing metropolitan areas is based on a three-phase low-voltage network with a phase to neutral voltage of 230 V.

A 400 V three-phase supply is available if required.


Consumer’s mains


Consumer’s mains

There is no minimum conductor size for consumer’s mains in AS/NZS 3000:2007 Wiring rules.

However, refer to the local utility service and installation rules for their requirements as some indicate 6 mm2 for copper conductors.

The final connection to the consumer from the mains in the road verge is made via a lead-in service cable or service line.


Consumer’s mains

The term ‘consumer’s mains’ describes a cable connected between the point of supply and the main switchboard in an installation (refer to AS/NZS 3000:2007 Wiring rules, consumer’s mains).

The consumer’s mains connection can occur at underground POSs or at overhead service line POSs.


Consumer’s mains Underground consumer’s mains can originate

from service pillars known as green buoys which are an above-ground means of connecting to the mains and in which service fuses may be located.

The service fuse is designed to blow if a protective device on the switchboard has not operated.

For a service not exceeding 70 A the protective device at the POS will be either a 100 A HRC service fuse or a miniature circuit breaker (refer to the local utility service and installation rules for variations).


Consumer’s mains


Consumer’s mains

Cables specified for underground consumer’s mains wiring are generally elastomer or thermoplastic insulated with elastomer or thermoplastic sheathing (double insulated) complying with the relevant Australian standard for underground cables.

Consumer’s mains are usually enclosed in heavy-duty rigid UPVC conduit suitable for a category A underground wiring system and buried at a depth of 500 mm.

Note: Local regulations will determine the depth of the underground consumer’s mains.


Consumer’s mains


Consumer’s mains


Consumer’s mains


Consumer’s mains


Consumer’s mains


Consumer’s mains


Consumer’s mains

If consumer’s mains pass under concrete then the minimum depth of consumer’s mains can vary.

In these circumstances the consumer’s mains can be 300 mm below a continuous concrete paved area such as a driveway of minimum 75 mm thickness; however, it will depend on the method of installation (refer to AS/NZS 3000:2007 Wiring rules, underground wiring systems).


Consumer’s mains


Consumer’s mains

Overhead point of supply

An overhead distribution system uses various types of conductors called aerials.

Aerials are either bare or insulated conductors, exposed to the environment and suspended under tension above ground between support structures.


Consumer’s mains


Consumer’s mains

With low-voltage overhead (aerials) distribution areas the installing electrician must choose a suitable POS for the attachment of the utility’s service line to the consumer’s mains.

The POS must be able to withstand the mechanical forces placed on it from the service line and the environment.


Consumer’s mains


Consumer’s mains

If the building to be connected to the utility service is set back from the property boundary an insulated aerial is used for the consumer’s mains.

The minimum clearance height of the service line or consumer’s mains aerial above ground level must be 3.0 m or higher and not closer than 600 mm to any other overhead service such as a telephone or cable TV line.


Consumer’s mains


Consumer’s mains


Maximum demand

The purpose of determining maximum demand in consumer’s mains is for the selection of the conductor’s CSA. The methods used are:

calculation (common method used)

assessment (used for large installations)

measurement (used by utilities via recording over time or a maximum demand indicator)

limitation (may be the sum of all current ratings of circuit breakers protecting final sub-circuits and any sub-mains)


Maximum demand

The maximum demand in an installation supplied by the active conductor of the consumer’s mains is the sum of the calculated loads including any ‘diversity allowance’.

Some local utilities stipulate that the maximum total load for a single-phase supply can be up to 100 A.


Switchboards

AS/NZS 3000:2008, Section 2.9, ‘Switchboards’, and AS/NZS 3018:2001 Electrical installations—Domestic installations, Section 4, specifies requirements for location, construction and mounting of switchboards.


Switchboards

Switchboards within an installation must:

Comply with the special requirements of the relevant supply authority.

Be readily accessible in well-ventilated areas.

Be weatherproof.

Be constructed with materials able to withstand the environmental constraints.


Switchboards

Switchboards within an installation must:

Be safely and securely mounted .

Have a means to prevent strain/damage to cables.

Have no exposed live parts if the switchboard is domestic.

Include protective and metering devices.


Switchboards

All circuit breakers are mounted so that the ON/OFF and current rating indications are clearly visible with covers or escutcheons in position.

All final sub-circuits on switchboards are to be protected by residual current devices (RCDs) with a maximum rated residual current of 30 mA.

All mains and sub-mains must be supported or tied within 200 mm of their terminations. This support or tie must be substantial due to stresses that occur under short-circuit conditions.


Switchboards

In order to limit the possible spread of fire (see Clause 2.9.7) where a busway or cable is brought into a switchboard, the insulated busway/cable entry plate must have a close tolerance cut-out to fit the busbars/cable.


Switchboards

Switchboards must have a lid/door which:

can be opened without damaging any cables or equipment

is capable of being locked

can remain open while a systems electrician works on the switchboard.


Switchboards

For general light and power distribution boards, provide schedule cards of a minimum size of 200 × 150 mm, with typewritten text showing the following as-installed information:

Sub-main designation, rating and short-circuit protective device.

Light and power circuit numbers and current ratings, cable sizes and type and areas supplied.


Switchboards

Single-line diagram for the location of underground mains/sub-mains.

Mounting: mount schedule cards in a holder fixed to the inside of the switchboard or door, next to the distribution circuit switches. Protect with hard plastic transparent covers.


Switchboards


Switchboards


Switchboards

Restricted locations for switchboards

Clause 2.9.2.5 sets out the restricted locations of switchboards. These are/or near:

water containers and cooking appliances

showers/baths, swimming pools, paddling pools and spas, fountains and water features

refrigeration rooms, fire hose reels

automatic fire sprinklers and in cupboards


Switchboards


Switchboards

Fixings

Floor-mounted switchboards should be fixed to the floor or other structure with M12 hot-dipped galvanised bolts through the mounting holes provided in the base.

Wall-mounted switchboards should be installed using a sufficient number of fixings. However, not less than four fixings per switchboard should be used.


Switchboards

It is suggested that fixings should be:

To timber:

Proprietary assemblies—hot-dipped galvanised steel No.12 wood screws of suitable length.

Custom-built assemblies—10 diameter coach screws of suitable length with a full washer under the head.


Switchboards

To masonry and concrete:

Proprietary assemblies—stainless steel 6 mm diameter expanding sleeve masonry anchors of the projecting stud type each fitted with a full washer and nut.

Custom-built assemblies—stainless steel 10 mm diameter expanding sleeve masonry anchors of the projecting stud type each fitted with a full washer and nut.


Switchboards

Checking switchboards in the field

Isolate switchboard to be worked on.

Inspect installed low-voltage switchboards for fixing, alignment, earthing and damage.

Clean interiors to remove debris and dirt.

Check tightness of all accessible mechanical and electrical connections. Re-torque busbar connections.


Switchboards

Checking switchboards in the field (cont.)

Adjust access doors and operating handles for free mechanical operation.

Adjust circuit breaker trip and time delay settings to values specified.

Test operation of RCDs.

Repaint scratched or marred exterior surfaces to match original finish.


Switchboards


Switchboards


Switchboards


Switchboards


Switchboards

Sub-mains

A sub-main circuit can be defined as a circuit connected directly from the main low-voltage switchboard to any other sub-main distribution board or from one sub-main switchboard to another.

The CSA of any sub-main cable must be able to carry the maximum demand of the installation it is supplying.


Switchboards

Each sub-main circuit is required to have its own protection device at the main switchboard.

In addition, each sub-main circuit may have an earthing conductor connected to the main switchboard earthing bar and taken to the earthing bar on the sub-mains distribution.


Switchboards

Final sub-circuits

The electrical distribution within an installation ends with final sub-circuits.

A final sub-circuit is that section of a wiring system that extends beyond the final circuit protection device.


Switchboards

Final sub-circuits make up the greatest part of the wiring of an electrical installation and are divided into four general groups.

Fixed-wiring lighting circuits

Fixed-wiring socket outlet circuits

Fixed-wiring appliance circuits

Fixed-wiring circuits (range, ovens, cooktops, water heater)


Switchboards

Switchboard design and arrangement of equipment on the panel board depends upon several factors:

the load demand of the installation

the number and type of circuits required

the prospective fault current (with a maximum transformer capacity of 315 kVA the maximum prospective fault current at the POS for a 70 ampere two-wire service is 6.0 kA if the POS is greater than 50 m from the supply transformer)


Switchboards

fault current rating of the switchboard (residential switchboards should have a minimum fault rating of 6 kA)

accessibility, location and environmental conditions


Switchboards


Switchboards


Switchboards

Metalwork and switchboards

A high standard of metalwork is required. The following points concerning the metalwork should be noted.

The thickness of the metal used for the cabinet will depend on the size of the board.

For large free-standing switchboards the minimum metal size for major components is 2 mm thick furniture quality bright steel sheet.


Switchboards

All hinged panels must be suitably stiffened and fitted with lift-off hinges.

Large lift-off panels are not favoured, but if unavoidable they must be equipped with a means of handling them such as fitted D handles. Such panels must have a means of support, such as studs or a supporting ledge, for use while the fixing screws are being installed.

Escutcheon plates and hinged panels should be fixed in place with a fixing that can be undone without the use of tools.


Switchboards


Fire detection systems

Reliable fire detection system installation is required of all systems electricians, therefore it is essential that the systems electrician has an understanding of the types of fire detection technologies available and their limitations.



The prime function of a fire detection system is to identify one or more physical changes in the protected environment which indicate the development of a fire condition. These physical changes can be:

invisible products released after a fire has started

temperature

smoke

flames.



The choice of a fire alarm system depends on the risk of the fire to be detected, type of building structure, the purpose and use of the building, legislation and the local Fire and Rescue Service recommendations.

Most fires in a domestic building originate in the living room, followed by the bedrooms, kitchen and other areas in that order.





Alarms used to alert persons to a fire condition must have a noise level of not less than 5 decibels above normal sound level for the protected area (95 decibels).

In bedrooms a minimum level in the order of 65 decibels to 75 decibels is necessary to wake sleeping persons.

It is suggested that an alarm should be placed near the bedhead.

Flashing lights are also used to warn people of a fire condition.



Heat detectors

Heat detectors can be classified as:

fixed temperature

rate of rise

rate compensation

combination

electronic thermal





Fixed-temperature heat detector

A fixed-temperature heat detector activates an alarm when the temperature of the sensor element reaches a pre-set level, usually between 50 °C and 60 °C.

These detectors provide protection from slow-developing fires.



Fixed-temperature heat detectors have a thermal lag characteristic. This means that the temperature of the environment is higher than the temperature setting of the sensor element when the alarm sounds.

Fixed-temperature detectors can provide protection in areas subject to high ambient temperatures such as elevator shafts, switchboard or switchgear enclosures, or in zoned areas such as corridors and stairwells, so that only detectors in the immediate fire area operate.



Rate-of-rise detectors

Rate-of-rise detectors are designed to detect sudden increases in ambient temperature.

This feature allows these detectors to respond to a quickly developing fire. A mechanical pneumatic element senses the change in temperature and engages with electrical contacts.



Rate-compensation detectors

Rate-compensation detectors use a design method which allows the detector to ignore momentarily a rush of warm air in the protected environment to eliminate false alarms that may occur with rate-of-rise detectors.



Combination detectors

A fixed-temperature and a rate-of-rise heat detector are on a par when it comes to protection.

However, a combination heat detector provides a higher level of protection because this detector offers the responsiveness of both a fixed-temperature and a rate-of-rise heat detector.



Electronic thermal detectors

An electronic thermal detector uses a thermistor as the sensing element and is either a fixed-temperature or a rate-of-rise electronic detector.

The principle of operation is based on the thermoelectric effect.

Positive temperature thermistors develop a change in resistance when exposed to increasing changes in temperature.



Smoke detectors

As the name implies, these devices are designed to identify a developing fire while in its smoldering or early flame stages.

There are two basic types of smoke detectors in use today and they operate on either an ionisation or photoelectric principle, with each type having advantages in different applications.



Most smoke detectors have a small visible indicator which should be seen flashing at roughly minute intervals in normal operation.

These detectors also begin to make a pulsing ‘beep’ when the battery is nearing the end of its life.

It is important to check that all smoke detectors are working from time to time.

This is carried out by pushing the detector’s small test button to apply a short-circuit which sounds the alarm.



Smoke alarms can be installed either as spot detectors or interconnected. Interconnected smoke alarms provide warning to all persons occupying a building at the same time should any one alarm activate.

Because children have deeper sleep patterns than adults smoke alarms in children’s bedrooms should be interconnected to parents’ rooms.



Ionisation smoke detectors

These smoke detectors use an open-air ionisation chamber and a few micrograms of americium 241, which is a radioactive material.

As the radioactive material decays it produces a

stream of alpha particles.







Photoelectric smoke detectors

Photoelectric smoke detectors use a pulsating LED to produce a light beam and a photoelectric cell.

Photoelectric smoke detectors are based on the principle that any level of smoke will scatter light beams.

The LED is located in a tube and emits light through the air that circulates from the protected area into the smoke alarm.





Smoke detectors operating on the photoelectric principle respond faster than ionisation detectors to the smoke generated by low-energy or smoldering-type developing fires, as these fires generally produce visible large smoke particles.

Photoelectric smoke alarms are suitable for areas likely to be affected by cooking or heating appliances.



Aspirating detectors are smoke detectors which use a small pump to draw samples of the protected environment air through a tube into a detection chamber.

The tubes can be located at different height levels within the protected area.

This feature enables the detector to pick up on incipient smoke (early smoldering).

Incipient smoke does not contain enough heat energy to lift it to the ceiling.



Installing and positioning smoke alarms

Smoke alarm laws require owners of all homes and units (Class 1a and sole occupancy units in Class 2 buildings) to have installed and to maintain smoke alarms.



Class 1a means a single dwelling that is:

(a) a detached house; or

(b) one of a group of two or more attached dwellings, each being a building, separated by a fire-resisting wall, including a row house, terrace house, town house or villa unit and cabins in caravan parks.



Class 2 means a building containing two or more sole occupancy units each being a separate dwelling.

A sole occupancy unit means a room or other part of a building for occupation by one owner or joint owners, lessee, tenant or other occupier to the exclusion of all other persons and includes a dwelling.

A dwelling must have a kitchen sink and facilities for the preparation and cooking of food, a bath or shower and a toilet and washbasin.





Electrical fires

The risk of an electrical fire in a building depends upon the risk of damage that can occur to the cables and the points of termination.

Fire with TPS cable is unlikely unless mechanical damage has occurred (at the time of installation or attack by vermin).

However, terminations will in time have dust and/or moisture build-up which can cause arcing between terminals.



Special attention should always be given to all luminaires and their terminations.

Because of their location on the ceiling a fire can rapidly spread unnoticed through the roof cavity (refer to AS/NZS 3000:2007 Wiring rules, ‘Lighting equipment and accessories’).





Flame size affects the response of the detectors and the flame needs to be proportionally greater at maximum distance for reliable detection. For example, a 0.1 m2 flame at 25 m will be seen if it is within the cone of vision. A flame in the yellow area would need to be 0.4 m2 in order to be seen.

When radiant energy indicative of a flaming condition is seen by the detector it sends a signal to a fire alarm panel. If the detector cannot see the whole area to be protected, additional detectors are required.



A flame detection system must be designed for the type of flame that will be produced.

For example, some fuels such as hydrogen, alcohol and methane burn with no visible flame.

A flame detector tuned to the water-band emission frequency of invisible flames will see the fire.



Heat- and fire-resistant cables

Heat- and fire-resistant cables are designed to withstand long-term heat, or in some cases heat and fire or fire only.

The term ‘fire resistant’, in relation to a cable, means that the cable is capable of performing its intended functions under the heat and other conditions likely to be experienced due to a fire at its particular location.



Commonly used types for heat/fire-resistant cables are:

V-105 PVC cable, temperature rating 105 °C (heat resistant only), PVC-based cables using halide-based chemicals

Glass fibre cables, temperature rating 150 °C (heat and fire resistant), Mineral-insulated metal-shielded (MIMS) cables, temperature rating 250 °C (heat and fire resistant)

Radox ® cable (fire resistant only), Pyrolex™ Ceramifiable® cable





Fire integrity

Refer to AS/NZS 3000:2007 Wiring rules, ‘Fire integrity’.

With the introduction of the BCA, building designers now have a system for defining the ability of a wall to resist the devastating effects of fire.

Section C of the BCA defines the type and class of buildings and uses three building attributes to express fire resistance levels (FRLs) as a triple rating in terms of minutes.



A one-hour fire wall is defined as having a fire resistance level of 60/60/60 where each number represents the number of minutes the building attribute can resist the fire and maintain its function.



The building attributes are:

Structural adequacy—This is the ability of a wall, floor or ceiling to continue to perform its structural function for the fire resistance period.

Integrity—This is the ability of a wall, floor or ceiling to maintain its continuity and prevent the passage of flames and hot gases through cracks in the wall during the fire resistance period.



Insulation—This is the ability of the wall, floor or ceiling to provide sufficient insulation such that the side of the wall away from the fire does not exceed a predefined temperature during the rated period. At this temperature (a rise of 140 °C over the ambient temperature or a maximum of 180 °C) surface finishes and furnishings in contact with or near the wall may combust.


Security systems

In many buildings some form of security system is required.

The increase of crime in the community together with the increased use of expensive equipment and systems within buildings mean that the contents and building envelope must be adequately protected during both day and night.


Security systems

Whatever the system to be installed the systems all have some specific requirements in common.

Cabling is normally extra-low voltage and must be correctly segregated from 230 V circuits and service mains.

The extra-low voltage cabling is a ‘weak link’ in a security system and so must be properly protected against mechanical damage.


Security systems

A security system will provide an environment where people, buildings and objects may be safe and secure. Security systems are defined by the technical features they offer.

There are metallic tape, electromagnetic, photoelectric and motion-detecting systems, passive infrared systems and acoustic systems. There are systems with security cameras and those that communicate and operate via microprocessors, telephones or wireless.


Security systems

Intruder detection systems

Intruder detection systems rely on a network of metallic tape, electromagnetic detectors, infrared detectors, microwave detectors, ultrasonic detectors and associated alarm equipment.

The detectors are installed in specific locations throughout a building.

The detection devices are all interlinked via multi-core wiring to a control panel.


Security systems

Metallic tape

Some security systems in use today use metallic tape or foil on window glass and doors.

The tape forms part of the electrical conductor for the security system.

If the metallic tape becomes open-circuited it trips an alarm.


Security systems

Electromagnetic

Electromagnetic systems are often used on doors and windows to secure one area from another.

The device used consists of a magnet in a sealed container and a magnetically operated reed switch in another sealed container.


Security systems

Motion detection

Motion-detection systems sense movement in areas protected by the motion sensors.

Some motion-detection systems use photoelectric or laser beams.

The photoelectric system uses an invisible light beam which is projected across a doorway, corridor or a room space.

If the light beam is broken an audible siren sounds.


Security systems

Infrared

Passive infrared security systems are used to detect the infrared energy emitted by hot bodies within a sensor’s viewing area.

The sensors compare the temperature of the background with that of anything moving across that background zone.


Security systems

Acoustic

Acoustic sensors detect the energy wave produced by any kind of sound, including breaking glass.

They are usually placed on glass windows or doors with a suction cup device or an adhesive.


Security systems

Access control systems

Electromagnetic locks

Access control systems are usually installed at entrances and exits to a building.

These systems consist of electromagnetic door-locking mechanisms which can be linked to a central control panel.


Security systems

Closed-circuit TV (CCTV)

A closed-circuit TV security system collects digital images from one or more strategically positioned remote cameras and subsequently transmits the image to a monitor or a personal computer for real-time and time-lapse video recording.


Security systems

Optical-fibre-based security systems

This system works by passing optical fibre cable through mechanical tripping devices, which are installed at intervals along a boundary line such as a fence.

A cut in or tug on the cable triggers the nearest tripping device.


END

Education

Section 11 electrical_installation_practice_1