Sim's Electrical Companion
PART 1 - Scope, Object & Fundamental Principles
The regulations for low voltage installations installation ≤1000V AC & ≤1500V DC.
It Does not cover all electrical installations as some systems have their own regulations which they have to work to.
Examples of what is not covered by BS 7671:
Internal wiring of appliances, Public distribution systems, railway signals, motor vehicles other than caravans, electric fences, lift installations, equipment of ships and aircraft, lightning protection systems, offshore installations, mining electrics covered by their own statutory regulations.
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If the installation does not come into any of the categories above it can be safely ascertained that the installation is covered by BS 7671. If you are in doubt then you should not be working on that system.
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In some cases the regulations are used alongside other regulations where there is a combination of applicable attributes.
Examples include:
Emergency lighting, Fire detection systems, Telecommunications, surface heating systems, Explosive atmospheres, open cast mines, temporary event systems, quarries, fire fighting and life safety equipment.
PART 2 - Definitions
A breakdown of the technical terminology used within the book. At the end of the definitions is a spread that identifies the algebraic symbols used within BS 7671.
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UNDER CONSTRUCTION
PART 3 - Assessment of General Characteristics
This covers the type of earthing arrangement and supply system e.g. TT, TN-S TN-C-S single or three phase and the amount of live conductors 2,3,4 wire etc. See the design section of this book for information on supply systems and earthing arrangements.
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Before any installation is put into service the supply must be determined with regards to:
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• Supply Voltage and frequency
• Ze – External fault loop impedance
• PSCC – Prospective Short Circuit Current
• PEFC – Prospective Earth Fault Current
• Maximum demand
• Main cut out – Suppliers fuse type and rating
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It is stated these may be determined by Calculation, Enquiry to the DNO (Distribution Network Operator), Measurement or Inspection. Without determining these values a new installation may not be fit to connect to the given supply system.
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Division of Installation
Every circuit should have an independent protective device. The designer and installer should take into account the potential for danger or inconvenience in the event of a fault. Ideally a fault will only take out of service the individual circuit that is affected. This consideration needs to be made in the case of RCD’s that are covering more than one circuit. If there is expected earth leakage for example a server room then this needs to be taken into account during the design stage.
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Compatibility
The installation must be designed to be compatible with other systems that are present, the environment and for the types of intended load. The installation should not interfere with any other system or be affected by any currently installed system.
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Safety Services
Continuous supply for services designed for the preservation of the building or human life must be considered. This includes fire alarms, fire detection systems, emergency lighting, sprinkler systems etc. This section acknowledges the backup supply systems that may be used to ensure continuation of safety services in the case of main supply failure. Accepted systems are – Batteries, cells, independent generator or independent additional main power supply.
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Other systems could be employed as a back up supply but may need to be identified as a departure. A departure from BS 7671 will need to be documented to ensure it meets or exceeds the equivalent British Standard. See departures in the testing section of this site.
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Maintenance
The installation must be installed in such a way as to facilitate future modifications and maintenance including periodic inspection and testing. Accessories and controls should be accessible and sufficiently lit for this purpose. This is also a requirement of Part M of the building regulations - Access to and Use of Buildings.
PART 4 - Protection for Safety
This section identifies all the accepted methods to achieve basic and fault protection in an electrical installation.
Basic protection is intended to prevent danger of coming into contact with live conductors of a healthy electrical installation. Fault protection is designed to disconnect an unhealthy circuit in the case of fault. In some cases additional protection may be specified due to external influences or the nature of the installation.
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Basic Protection
Acknowledged methods of Basic Protection are:
Insulation of live conductors able to withstand the highest voltage present in the associated installation or grouped circuits.
Containment that prevents accidental contact with live conductors top surfaces must be IP4X or IPXXD and all other surfaces must be a minimum of IPXXB or IP2X. See International Protection (Ingress Protection) in the design section for more detailed information.
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Placing out of reach & obstacles are measures that need to be supervised by a skilled/instructed person. They both require bare live parts to be a minimum of 2.5m away from there they could be reached by a person (Arms reach). Obstacles must be obvious and not able to be removed unintentionally.
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Fault Protection methods include:
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Automatic disconnection of supply
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Double or reinforced insulation (Provides both basic and fault protection)
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Electrical separation (for one item of current using equipment) e.g. a shaver socket outlet.
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Extra low Voltage (SELV or PELV) voltage is reduced to 50V or less.
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Additional protection includes: Supplementary equipotential bonding and Residual Current Devices (RCD's).
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Earth Free Equipotential Bonding requires supervision from a skilled / instructed person. It involves bonding all exposed/extraneous conductive parts to prevent differences in potential in the case of a fault. This method is used in laboratories, operating theatres and other specialist installations.
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Non Conducting Locations is also the realm of a supervised highly specialised installation. The walls and floors are essentially insulated and completely separate from earth. No earthing bonding or circuit protective conductors are allowed within the zone.
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Automatic Disconnection of Supply (ADS) is the most common method of fault protection. It relies on a combination of protective measures including earthing, bonding, circuit protective conductors and as a minimum a protective device that will operate in the case of overload or short circuit. Additional requirements that will cause automatic disconnection of supply include residual current to earth & undervoltage protection which also cause disconnection in response to earth leakage or loss of supply.
Greater detail is in the design section on earthing, bonding & protective devices.
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Protection Against Thermal Effects
When designing and installing the installation ensure it will not cause heating, scorching or fire that can negatively affect persons, property or livestock.
It also requires that the building will not cause heating effects to the wiring system e.g. thermal insulation, heating or cooking appliances.
PART 5 - Selection & Erection of Equipment
This part is composed of the requirements for selecting wiring systems, conductors, protective devices and equipment appropriate to external influences and intended use. As well as the requirements of BS 7671 manufacturers’ instructions must be adhered to comply.
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Considerations for appropriate selection of equipment include the required voltage, frequency, power and current of the intended installation loads.
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External influences
All equipment and containment must be suitable for the expected external influences. These include ambient temperature, humidity, Impact resistance/mechanical protection, presence of solid foreign bodies including liquids, flora, fauna, vibration, electromagnetic effects UV, atmospheric effects, and wind/air movement.
See the section on external influences in the design section of this site for greater detail.
All accessories and non-maintenance free connections need to be accessible for use and periodic inspection.
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Identification and Notices
All conductors need to be identified by colour, numbers or letters. If a conductor is serving a different purpose than its colour suggests then sleeving should be used to identify the conductor for example switched lines in lighting circuits served by flat PVC/PVC multi core cables.
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All switchgear and protective devices need to be identified at their point of installation. All markings should be clearly legible and affixed in a durable manner to last the expected life of the installation.
Warning notices
Voltage – If the voltage exceeds 230V where it may not be expected use a warning notice indicating what voltage is present e.g. 400V.
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Isolation
If more than one isolation device is required warning notices should be affixed for example in the case of dual supply. e.g Photo voltaic, wind turbines, batteries or generator sets.
Periodic Inspection and Testing
This should be fixed on or near each consumer unit, distribution board or at the origin of the installation indicating when it was tested and a recommended period to re test the installation to ensure it is safe for continued service.
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Residual Current Devices
If a circuit or circuits are protected by and RCD or RCBO then a notice needs to placed close to the device to advise the user of the system to press the test button twice per year. This action proves the mechanism works and the device will operate. Additionally it keeps the mechanism free from dust and greases the mechanism.
It is recommended that temporary installations are tested each time they are put into service.
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Earthing and Bonding Connections
These connections should be terminated with a BS 951 Earth clamp that is suitable for the environment it is located in. Colour coding on the labels refer to the capacity to withstand moisture or corrosion:
Plain Zinc Internal areas that are not susceptible to moisture.
Red Internal areas that are not susceptible to moisture.
Blue Internal / external areas that are susceptible to moisture.
Green Internal or external areas that are susceptible to moisture and may be corrosive (Shown above).
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These labels should be present at all connections to TT earth conductors, every bonding connection to extraneous conductive parts and the main earthing terminal if it is separate from the main switchboard.
Examples of extraneous conductive parts that may have earth potential are:
Lightning protection systems, gas pipes, water pipes, conductive cladding, structural steel work, oil supply pipes etc.
Electrical Separation & Earth Free Equipotential Bonding
If either of these methods of protection are used then a durable label should be affixed to all electrical equipment in this location or that may be brought into the location with the following warning:
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'The protective bonding conductors associated with the electrical installation
MUST NOT BE CONNECTED TO EARTH
Equipment having exposed-conductive-parts connected to earth must not be brought into this location.'
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Non Standard Colours
Installations with identification to more than one version of BS 7671 should have a label at the origin of the circuits and at junctions where both colours are present. It is common to see older single phase red & black conductors still in use with modern colours also. Three phase colours changed from Red, Yellow, Blue to Brown, Black, Grey place a notice at the origin of any installation that contains both versions. Blue used to be a phase colour and could be mistaken as a Neutral. Like wise Black used to be a neutral colour and is now L2 in a three phase installation.
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Multiple Supplies
In the case of there being more than one method of supplying a circuit this needs to be identified at or near the source of supply and at all isolation points.
The signage should read as: WARNING MULTIPLE SUPPLIES
Isolate all electrical supplies before carrying out work
Isolate the main supply at ...... ...(Give location)
Isolate the alternative Supply at ... ...(Give location)
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Examples of alternative supplies are:
More than one Line provided by the distribution network operator.
Batteries, cells or capacitors used for emergency back up supplies. Micro generation sources including photovoltaic, hydro, wind turbines, combined heat and power generators and standard fuel generators.
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Prevention of Mutual Detrimental Influence
Simply put all wiring types that are contained or run alongside each other must not have any negative affects. The negative effects may be through electro magnetic effects or differences in voltage, current or phase. All conductors should be insulated to the highest voltage present and where necessary be screened to prevent negative effects of electromagnetic disturbances.
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CHAPTER 52 - Selection & Erection of Wiring Systems
This chapter in Part 5 is dedicated to selection of cables, their supports and enclosure methods with regards to external influences. Prefabricated systems, busbar and power tracks have to comply with BS 7671 as well as their own individual regulations.
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Ferromagnetic Enclosures - Electromagnetic Effects
Any steel or iron containment will increase the magnetic fields of current carrying conductors. In turn this can cause electromagnetic disturbances and / or heating effects that can reduce the current carrying capacity of the cable and even melt its insulation. To combat this all current carrying conductors of an individual circuit must enter the same cable entrance if it is metallic.
In single phase systems Line and Neutral should be run together to cancel out the magnetic effects. Three phase systems three wire systems are run in Trefoil formation shown below or bunched with the corresponding neutral in TP-N (Three Phase & Neutral) systems.
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Additional circuit protective conductors may enter a separate cable entry. They should not be carrying current except in the case of a short duration of fault current in the case of short circuit or earth fault.
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Single core armoured cables must be armoured in a non-magnetic armour like aluminium.​
Electromechanical Stresses
Cables must be sufficiently robust to remain intact during standard expected service loading, currents and voltages. They must also be able to withstand the transmission of the maximum possible fault current long enough to disconnect without failure of the cable or its insulation.
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Cables
Non Sheathed cables have to be installed in containment excluding protective conductors that are sufficiently sized to withstand mechanical damage. Any cable that is in a corridor, access or escape way must be supported to withstand collapse in the case of fire. Firemen have been killed by becoming entrapped by collapsed wiring systems so this amendment to the regulations requires the use of metallic containment or binding with metallic or ties. This applies to cables in PVC conduit or trunking in the event of fire the PVC can burn so cables still need to be bound to the building fabric with metallic ties in these locations.
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Standard flexible cables are not fixed installation cables, they should not be used as such, unless they are of a heavy duty type e.g. NYY-J See cables in the design section for more details.
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External Influences
All wiring systems need to be resistant to expected external influences. The list of environmental conditions gives the installer an idea of what considerations need to be made.
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Ambient Temperature
Increased temperature reduces a cables current carrying capacity. Cables must not exceed their limiting temperature in normal use or in fault conditions.
Ca (Ambient temperature) is the rating factor applied to It (tabulated current rating) when calculating cable current carrying capacities. For correct application of this see Cable Calculations in the Design section. Note at 30 degrees there is no change to the current carrying capacity of any of the conductors.
External Heat Sources
The wiring system should not be adversely affected by external heat sources. Examples include plant, solar radiation, heating appliances, heat producing lighting or machinery, heating and hot water services.
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In some cases additional shielding, thermal heat shrouds, insulation or sleeving may be used to protect the wiring system. If cables are passing through luminaires they should be adequately protected and only installed in luminaires that allow this practice.
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Presence of Water (AD) or high humidity (AB)
The correct application of IP rated equipment should be used as defined in BS 60529. All accessories and wiring systems should be protected to the same degree or higher to ensure compliance and resistance to the passage of moisture.
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The second number of the IP code applies to humidity and presence of liquids:
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IPX0 Not Protected
IPX1 Protected from vertical droplets of liquid
IPX2 Protected from droplets up to 150 off Vertical
IPX3 Protected from direct sprays up to 600 off Vertical
IPX4 Protected against splashing water from any direction
IPX5 Protected against low pressure jets for any duration
IPX6 Full protection from particulates includes a vacuum seal
IPX7 Full immersion 0.15-1 meter depth for up to 30 minutes
IPX8 Full immersion and pressure - see manufacturers specification for level of protection afforded.
IPX9 Protection against high pressure, high temperature jets or steam cleaning
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Many of the special locations listed in Part 7 require the application of minimum IP ratings for example where there are spas, pools, fountains, bathrooms, outdoor lighting, marinas, agricultural applications etc.
Presence of Solid Foreign Bodies (AE)
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As per presence of water solid foreign bodies are classified in BS 60529 IP degrees of protection. A wiring system should not be detrimentally affected by the expected presence of solid foreign bodies be it a childs finger or dust.
If the build up of dust is expected to be great then the insulating effect of this should be considered when specifying the wiring system. The first number of the IP rating refers to the level of protection afforded and the higher the number the greater the level of protection.
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IP1X Protected against objects ≥50mm e.g. a hand
IP2X Protected against objects ≥12.5mm wide and 80mm long e.g. a finger
IP3X Protected against objects ≥2.5mm wide e.g. a screwdriver
IP4X Protected against objects ≥1mm wide e.g. a wire
IP5X Protected against intermittent dust that may enter the enclosure but not hinder its performance 2-8 hours
IP6X Dust tight - No ingress of dust under prolonged exposure 2 - 8 hours
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Note all wiring system enclosures and containment should have an IP rating of minimum IP2X (IPXXB) for bottom and side surfaces. The top or horizontal surfaces should be afforded a level of protection of IP4X or IPXXD.
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Presence of Corrosive or Polluting Substances (AF)
All parts of the wiring system needs to be protected from corrosion or damage from corrosive / polluting substances. Examples of where this may be applicable is galvanised trunking, tray, ladder, basket and conduit systems that are external or in areas prone to moisture. All cuts and threads and damage to the galvanised coating during construction should be sprayed with galvanising spray to maintain its level of protection. Black enamel conduit requires cuts and threads to be painted to maintain its level of protection. Plastic conduit can be used in some environments to protect against moisture or chemicals.
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In some cases dissimilar metals can cause an electrolytic effect that causes corrosion of these connection. In this case consideration should be made to separate the two or apply a coating that is non reactive.
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The Anodic Index can be used to determine how reactive two dissimilar metals will be. The further away they are on the scale the greater the difference in charge and the higher the rate of degradation. When this is occurring in a system it is common for the most reactive element to be attacked first. This is the reason for sacrificial anodes to protect keels, rudders and props on boats or the use of magnesium in the bottom of water storage tanks.
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Most Cathodic
Gold, Rhodium, Platinum, Silver plated Copper, Silver,
High Nickel Copper alloys, Copper, Silver Solder,
Low Brass/Bronze alloys, Nickel Chrome Alloys,
High Brasses & Bronzes, Chromium plated Steels,
Tin plating and Tin solders, Lead and High lead alloys,
Wrought Aluminium, Iron, Low Carbon Steel (LCS),
Cast Aluminium, Cadmium, Galvanised Steel,
Most Anodic
Zinc, Magnesium, Beryllium.
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Impact (AG)
Cables concealed in walls, floor or above ceilings need to be protected from accidental damage. This section gives the electrician guidance on where it is accepted to install cables. Where cables are above ceilings or below floors it is recommended that the cable be ≥50mm from the top / bottom of the joist. If this cannot be achieved the cable may be installed horizontally or vertically from any visible accessory. Cables may also be installed in the top 150mm of a wall or 150mm from the joining of two walls as shown in green in the illustration below.
If the installation cables are not mechanically protected against penetration by screws or nails it must have additional protection of a ≤30mA RCD (Residual Current Device). To negate the requirement for an RCD or to be installed out of the zones the cables may be protected by earthed metallic conduit, trunking or ducting or be BAND I Separated or Protected Extra Low Voltage (SELV/PELV) ≤50V.
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Vibration (AH)
Vibration can cause early failure of electrical systems if it has not been accounted for. Solid core conductors will suffer from repeated vibration and will eventually break. For this reason multi-fine wire cabling is recommended like tri-rated or conduit cabling. Larger Fine wire cables may include some additional mechanical protection e.g. SY Control Cable. This is a consideration on house boats, caravans, marinas, mobile transportable units, fairgrounds, shows and connections to fixed equipment that causes vibration like fans or motors.
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Note Multi-fine wire cables should have crimped ferrules or lugs for screw
type onnections.They must not be tinned (soldered) or terminated bare.
Other Mechanical Stresses
Cables must be supported throughout their run and must not suffer undue stresses that may damage them. Cables may be buried but must have an earthed armour or sheath unless they are installed in a duct. If a cable is routed across an expansion joint then it too should be so installed to allow the expansion and contraction without damage. Conduit systems should be fully erected before cables are drawn in.
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Cable Supports
Minimum internal bending radii of cables
Supports for Trunking and Conduit Sytems
Presence of Fauna (AL) and or Mould Growth (AK)
The wiring system should be protected against any possible mechanical damage that may be caused. In the case of mould growth the wiring system should be easily cleaned to prevent growth on the installation. Recommended methods include PVC or stainless steel conduit with distance or hospital saddles that facilitate cleaning access behind the conduit. Additional enclosures may be used to provide mechanical protection to accessories or installed equipment that is vulnerable to damage.
Solar Radiation or other source of Ultraviolet radiation (AN)
Ultraviolet light can damage many of the types of plastic used in electrical enclosures and cable insulation. Cables and enclosures must be UV resistant to be used outdoors. UV light can cause serious degradation of many plastic polymers including PVC.
PVC is the most common insulating material and subject to prolonged exposure to UV can cause discolouration, cracking, flaking and reduced impact resistance. Check manufacturers’ specifications to ensure correct materials are used when subject to UV exposure. Coatings like Carbon Black or additional enclosures may be used to protect UV susceptible materials.
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Seismic Effects (AP) & Air Movement (AR)
As per vibration cables subject to air movement or seismic effect should be installed with flexible cores not solid core types. Containment and cable supports should be appropriate and fixed in such a way as to allow for flexibility where required.
Nature of Processed or Stored Materials (BE)
If the electrical installation is in an environment where there is a greater risk of fire extra measures must be employed due to the nature of the stored materials. The installation should not be a risk of heating, scorching or arcing or allowing the spread of fire. All penetrations into walls ceilings and floors must have the same or higher fire resistance as the building fabric. Ducting and trunking will need to employ fire pillows to seal where fire compartments have been breached by the wiring system.
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Building Design (CB)
If the building is subject to movement or is designed to move then the wiring system must be installed in adequately protected flexible cables.
PART 5 - Volt Drop
Voltage Drop is dependant upon the length of the circuit, the conductor csa, length, material, design current, frequency and the installations environmental conditions. It is prudent to calculate what cable size is appropriate based on the Volt Drop calculation (shown below) and comparing it with the tables in BS 7671: Appendix 4.
The full tables are too numerous to print in this book. A snippet of some common cables are reproduced in the cable calculations in the design section
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For single phase applications:
Maximum volt drop for lighting applications = 3% = 230 x 0.03 = 6.9V
Maximum volt drop for power or other circuits = 5% = 230 x 0.05 = 11.9V
Some equipment may state the maximum volt drop in the manufacturers' instructions.
The Tables in BS 7671 take into account cable type, grouping, reference method, AC/DC, ambient temperature, single / three phase and operating temperature.
Example:
A PVC thermoplastic non-armoured single phase cable 2.5mm2 Clipped direct, not bunched has a volt drop of 18mV/A/m if the installation was lighting with a design current of 6A the maximum length of the circuit would be:
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Rearrange the equation for length:
L = (Max volt drop x 1000) / (max mv/a/m x design current)
L = (6.9 x 1000) / (18 x 6) ≈ 64 meters
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Maximum length is 64 meters beyond which the circuit would not comply with the regulations on volt drop. More detail on calculating and applying volt drop is in the design section.
PART 5 - Connections
All electrical connections should be durable, designed for the intended cable type and cross sectional area and number of intended conductors. The connections should be designed to withstand the expected operating temperature. If the connection is intended for use where it may be subject to vibration then it should be able to lock in position.
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All screw type terminals need to be accessible for periodic inspection. The only connections that are not required to be accessible should be either labelled maintenance free of be encapsulated or filled or buried in the ground as per manufacturer’s instructions.
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Some connections are labelled to indicate what type of cable they are designed for as follows:
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S / (Sol) = Solid cores only
r = Rigid conductors only
f = Flexible cores only
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If there is no marking then it should be suitable for all conductor core types although fine wire connections should be terminated in a crimped lug / ferrule as pictured.
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PART 5 -
Under Construction