BSE One and a half day CPD course – Advanced Electrical Design … · 2010. 11. 9. · BSE One...

BSE One and a half day CPD course – Advanced Electrical Design for the Built Environment by Prof. Chi Man Sung on 13 & 14 October 2010

Organized by the Department of Building Services Engineering (BSE), a one and a half day CPD

course in Advanced Electrical Design for the Built Environment was conducted by Prof. Anthony

Chi Man Sung on 13 and 14 October 2010. 62 participants attended the CPD course.

Prof. Sung is currently an Adjunct Professor at BSE. He has published many useful electrical

services engineering articles and papers in CIBSE Journal and BSERT Journal. He has delivered

numerous BS7671 Design Courses hosted by the Institution of Engineering and Technology (IET)

in the past 20 years.

Prof. Sung

This CPD course was aimed at practitioners and policy makers who are responsible for the

design, installation and commissioning of a low carbon, safe and energy efficient built

environment projects in developing and developed countries.

To apply the underlying design theory and principles correctly would help satisfy the functional

and safety requirements of relevant wiring rules and regulations (e.g. Supply Rules, NEC® and

BS7671, etc). In this course, the design procedures and calculations were presented. A real life

case study was provided as a workshop example to help validate the design output.

CPD course by Prof. Sung

Participants of the CPD course

Reference materials, including a copy of textbook “Advanced Electrical Services Engineering –

Vol. 1” and course notes, were distributed to participants to help them consolidate the newly

acquired design skills. The course provided participants with very useful skills and knowledge on

advanced electrical design.

Powerpoint slides for Session 1 :

Comparison of Electrical Installation Rules and proposed

changes in BS7671:2011


Overcurrent and fault calculations


Design case study

BSE News CPD20101013-14

1

13-10-2010 Slide no: 1Adjunct Prof Tony Sung

Hong Kong Polytechnic UniversityDepartment of Building Services Engineering

Dr Tony Sung CEng FCIBSE FIET

Adjunct Professor Chairman of CIBSE Electrical Services Grouphttp://www.cibse-electricalservicesgroup.co.uk

Advanced Electrical Design for

the Built Environment


Session 1

Comparison of Electrical Installation Rules

andProposed changes in BS7671:2011

2


Outline of Session 1

Overview of Rules and Regulations for Electrical Installations

IEEE Standards, NEC versus BS7671 and IEC60364

Risks

Conclusions


Movie

Danger of electricity

3


Fundamental Principles

Protection for safety

• General (design out risks)

• Protection against electric shock

• Protection against thermal effects

• Protection against overcurrent

• Protection against fault current

• Protection against voltage disturbances and EMI

• Protection against power supply interruption


Design Considerations• General (Design aim)• Characteristics of supply or supplies• Nature of demand• Electrical supply systems for safety services or standby electrical

supply systems• Environmental conditions• Cross-sectional area of conductors• Type of wiring and method of installation• Protective equipment• Emergency control• Disconnecting devices• Prevention of mutual detrimental influence• Accessibility of electrical equipment• Documentation for the electrical installation• Protective devices and switches

4


Selection Considerations

General (Item of equipment)Characteristics • Voltage• Current• Frequency• Power• Conditions of installation• Prevention of harmful effects


Installation Considerations

Installation • Good workmanship• Characteristics not impaired (Method statement)• Conductors identification• Sound electric joints - mechanically and thermally (does

not cause unacceptable excessive temperature rise)• Arcing shall be contained safely not to cause burns, fire

or ignitionNew Materials and inventionAdditions and alterationsInitial Verification (inspection, tests, certification, first

periodic inspection and subsequent schedules)

5


Terms

Statutory Instrument• Rules• Regulations• Standards

Electrical Installation rules and the Regulations and Standards referred to are non-statutory.


Statutory Instrument

Legislation - delegated

It is law made by an executive authority (e.g., a council body) under powers given to them by Primary legislation

6


Rules

Sets requirements and Regulations under certain Legislative Acts

It should be:

• Interpretable, not difficult to understand• Technically accurate, not excessive/unreasonable/unnecessary,• Encourage stakeholders to take part in the rule-making


Regulations

A Regulation normally establishes the criteria or ways to be used to satisfy the ‘rules’ that we use to meet the requirements of the statutory instrument.

7


Standards

A Standard normally establishes the consolidated results, by consensus and approved by a recognised body, that to be used for manufacture or design based on agreed science, technology and experience of the experts.


Construction (Designand Management) Regulations - CDM

Flowchart

Electricity safety laws(USA OHSA, UK Electricity at work Regulations)

Wiring Rules/Regulations(Flexibility)IEEE Standards

NEC(no flexibility)

IEC, UL and NEMA Standards

CENELEC Standards

Mandatory requirementsEngineers’ Judgements

8


CDM ProsecutionA fatal incident involved a worker falling nine metres from a platform on a flat roof.

Express Park Construction Company Limited (EPCC) pleaded guilty to breaching Section 3(1) of the Health and Safety at Work etc. Act 1974 for failing to safely manage subcontractors working for it. The architects involved, Oxford Architects Partnership, also pleaded guilty -to breaching Regulations 13 and 14, of the Construction (Design and Management) Regulations 1994, which require designers to take safety considerations into account. Speaking after the hearing, HSE Inspector Sue Adsett, said: "While it is rare for designers to be charged with breaching health and safety legislation, they must be aware they can be held responsible where bad design is an important contributory factor to a work-place fatality.

Designers must ensure that plant and equipment can be accessed safely, and that safety harnesses are only used as a last resort.” EPCC was fined £75,000 plus costs and the architects were fined £120,000 plus costs.


Proposed Changes in BS7671:2011

Some of the changes have significant impacts on design and installation specifications.

Part 1 – minor changesPart 2 – Includes new definitions mainly on Medical equipment, Surge protection and EMCPart 3 – minor changes but now include a few new rating factors for illustrative diagrams for ac, dc supplies and earthing systemsPart 4 – now include Section 444 on EMC and annex A444 design/installation guidelinesPart 5 – now include Section 534 Surge protection device with annex A, Section 559 and Chapter 56 have new informationPart 6 – minor changesPart 7 – Sections 710 (ML) and 729 (OMG) may be includedAppendices: App 6 Model forms, App 9 (MS), App 11 remove text,Volt-drop requirewents, App 15 diagram corrected

9


• Lightning protection installations should be installed to IEC 62305, BS EN62305, AS/NZS 1768, NFPA 780 or equivalent.

• The LPS and LEMPS must be risk assessed to 62305

• Consultant to give guidance on what RT

value should be followed

HK COP_E

26I Lightning Protection Installation


LPS (ref: Dehn (UK))

10


Section 443, Section 534

• Lightning Electromagnetic Pulse Protection now takes an active role in the protection of electrical and electronic equipment


Air Termination Systems

11


Lightning Electromagnetic Protection


Contrast to LPS classifications

12


SPD Selection

Flow Chart


Limit conductor lengths

Induced Voltage:

13


Induced Voltages

1.0m 1.0m


M

25mm2

Cable intheground

80AHRCfuse

Smartmeter

30mA, 40ms 50A RCD

6A 1.0mm2 stairs light/alarms

6A 1.0mm2 lighting

6A 1.0mm2 lighting

6A 1.0mm2 Ext/bathroom light

15A 2.5mm2 immersion heater

25A 6.0mm2 electric shower

32A 10.0 mm2 cooker circuit

25A 6.0mm2 surge arrester N-E

Spare way

25A 6.0mm2 surge arrester P-E

25A 4mm2 radial final circuit



OHLcable

16mm2

Meter tails

100A DPSwitch

14


Section 444 of BS7671

• The Regulations in this section are intended to provide requirements for the operational and functional safety of electrical installations in the event of voltage disturbances and electromagnetic disturbances generated for different specified reasons.


Electromagnetic Environment

15


EMI damages can be severe• Where large metal loops exist• Where different electrical wiring systems are installed

in common routes, • Power cables carrying large currents with a high rate

of change of current (dI/dt) and on the size of the loop (e.g. the starting current of lifts or currents controlled by rectifiers) can induce overvoltages in cables of information technology systems, which can influence or damage information technology equipment or similar electrical equipment.

• In or near rooms for medical use, electric or magnetic fields associated with electrical installations can interfere with medical electrical equipment.

• The requirements and recommendations given in this section can have an influence on the overall design of the building including its structural aspects.


Sources of EMI Consideration shall be given to the location of the sources

of electromagnetic disturbances relative to the positioning of other equipment. Potential sources of electromagnetic disturbances within an installation typically include:

1) Switching devices for inductive loads2) Electric motors3) Fluorescent lighting4) Welding machines5) Rectifiers6) Choppers7) Frequency converters/regulators including Variable Speed

Drives (VSDs)8) Lifts9) Transformers10) Switchgear11) Power distribution busbars.

16


To reduce the effects of EMI• (i) Metal sheaths, screens or armouring of cables shall be bonded

to the CBN unless such bonding is required to be omitted as required for safety reasons.

• (ii) Where screened signal or data cables are used, care shall be taken to limit the fault current from power systems flowing through the screens and cores of signal cables, or data cables, which are earthed at both ends. Under particular conditions, additional conductors may be necessary, e.g. a by-pass equipotential bonding conductor for screen reinforcement;

• (iii) The impedance of equipotential bonding connections shall be as low as practicable and this should be achieved by:

• (a) being as short as possible, and• (b) having a shape that results in a low inductive reactance and

impedance per metre of route, e.g. a bonding braid with a width to thickness ratio of at least five to one.


To reduce the effects of EMI

• (iv) For electrical equipment sensitive to electromagnetic disturbances, surge protection devices and/or filters should be installed to improve electromagnetic compatibility with regard to conducted electromagnetic phenomena.

• Inductive loops should be avoided by the selection of a common route for all the conductors, (live and protective conductors) of a power circuit.

• (vi) Wherever practicable power and signal cables should be kept separate and should cross each other at right-angles (see Regulation 444.6.3).

• (vii) Where a lightning protection system is installed, the recommendations of BS 6651 (BS EN 62305) should be followed.

• (viii) The sum of the currents within a wiring system should nominally be zero.

17


Avoidance of neutral conductor

currents in a bonded structure

by using an installation

forming part of a TN-S system from the origin of the public supply up to and including the final circuit

within a building


Example of equipotential

bonding networks in a

structure without a lightning

protection system

18


Parts to be equipotentially bonded

• Metallic containment, conductive screens, conductive sheaths or armouring of data transmission cables or of information technology equipment;

• Earthing conductors of antenna systems;

• Earthing conductors of the earthed pole of d.c. supply for information technology equipment;

• Functional earthing conductors

• Protective conductors.


Minimum separation distance between different cable categories

• (i) Immunity level of equipment connected to the information technology cabling system to different electromagnetic disturbances (transients, lightning pulses, bursts, ring wave, continuous waves, etc.);

• (ii) Connection of equipment to earthing systems;• (iii) Local electromagnetic environment (simultaneous appearance

of disturbances, e.g. harmonics plus bursts plus continuous wave);• (iv) Electromagnetic frequency spectrum;• (v) Distances that cables are installed in parallel routes (coupling

zone);• (vi) Types of cables;• (vii) Coupling attenuation of the cables;• (viii) Quality of the attachment between the connectors and the

cable;• (ix) Type and construction of the cable management system.

19


Cable manage

ment system


choice of material and the shape of the cable management system

• It depends on the following considerations:• (i) The strength of the electromagnetic fields along the

pathway (proximity of electromagnetic conducted and radiated disturbing sources);

• (ii) The authorised level of conducted and radiated emissions;• (iii) The type of cabling (screened, twisted, optical fibre);• (iv) The immunity of the equipment connected to the

information technology cabling system;• (v) The other environment constraints (chemical, mechanical,

climatic, fire, etc.);• (vi) Any future information technology cabling system

extension.• Non-metallic containment systems are suitable in the following

cases:• (i) Electromagnetic environments with permanently low

levels of disturbance;• (ii) The cabling system has a low emission level;• (iii) Optical fibre cabling.

20


Do’s and don’t of cable management system

Not recommended

Shading indicates screening performance

Shading indicates screening performance

Not recommended

Recommended Recommended



21




Example of an EMC matrix

22


EMC Processes flow chart


Responsibilities

• EMC Coordinator

• Design Team

• Procurement Team

• Delivery and Installation Team

• Commissioning and testing team

• Maintenance team

EMC task team

23


Avoid non-compliance – 1



24





25




Avoid non-compliance - 6

26


Conclusion• Remember the Fundamental Requirements for

Safety – governing everything• Record Risk assessment outcomes• Agree with clients, Architect and the main

contractor• If in doubt, appoint independent consultant• Apply consensus standards• Record design documentation• After T&C, provide analysis and comparison of

installed system, handover the documentation record to the client or his representative

1








Session 2

Overcurrent and

Fault Calculations

2



Overview of Requirements for Overcurrent and Fault calculations

IEEE Standards, IEC60909-0 versus BS7671 and IEC60364

Overcurrent Risks

Conclusions


Movie

Danger of electricity

3












Design Considerations

• General (Design aim)• Characteristics of supply or supplies• Nature of demand• Electrical supply systems for safety services or standby electrical


4










5


Protection against Overcurrents

Overload Current – burns and shortened life risksCurrent exceeding the designed rating of the equipment (cables/busbars/terminals/joints) resulting in high temperature. The cause of which can be excess demand (not a circuit fault).

Fault (short circuit) Current –fire/explosion/electromechanical force risksA current many times higher than the designed rating of the equipment resulting in excessively high temperature and induced electromechanical force. The cause of which is due to a negligible impedance fault between the live parts.


Induced electromagnetic force

6


Operating temperature of cables


Overcurrent design sequence

7


Basic Equation for protection against Overload current

BS7671 stipulates I2 1.45 x IZWhere I2 = actuating current ofthe circuit protection device


Rating (correction) factors - 1

8




BS7671 sizing cables with high Harmonic content

Page 139

However, BS7671method only appliesto three phasecircuits

9



Flowchart



NEC(no flexibility)


CENELEC Standards



BS EN 60287 method

Page 170

10


Principle of Heat Transfer in electric cables

Heat generatedequals

Heat loss by convectionand

Heat loss by radiation

Method of Raleigh no (page 181).


Joule loss approach

Page 177

11


Churchill – Chu equation

Page 182/3


The rating factor for grouping (Cg)

12


Grouping of different size cables


Suggestions for mixed cable groups

When a group of cable consists of large size and small size cables:1. Apply equation 5.35a to the large size cables2. Apply equation 5.35b to the smaller size cables3.Use joule-loss approach to evaluate the design temperature of the cable group

(see design example page 222 – 258)

13


Tea Break


Ohm’s Law

14


Important facts for passive elements1. An inductor acts like a short circuit to d.c.2. The voltage in a capacitor cannot change immediately3. The current in a resistor obeys Ohm’s Law at the flick of a

button.

Hence, IEEE Standards/IEC 60909-0 suggest that:a. Maximum asymmetrical short circuit current can be

evaluated by ignoring inductancesb. For most systems, the inductance is many times the

resistance, hence the worst case symmetrical short circuit circuit can be determined using just the reactance values

c. when under short circuit situations Inductance in motors can generate a short duration back emf (hence power)


15


Pre-arcing energy/Total arcing energy Let through energy

Adiabatic Equation

R R



Adiabatic Equation

R R

16


Adiabatic line of a conductor

Value of k dependson initial and finalconductor temperature,conductor and dielectricmaterials.

For a short circuit thatlast longer than 5s, useBS7454 to assess thefinal temperature reached


Product Tolerances• Equipment manufacturers now provide details of

their product tolerances. We can use them in:

17


Discrimination / Selectivity• Get the pre-arcing and total arcing I2t from the

fuse manufacturers. We can use them in table format:

560A

400A 250A

X Y


Fault Calculations

• Make/break capacity of circuit breakers

• Many software neglect motor contributions in fault calculations

• When the Breaker and switchboard are under-rated can cause disasters ……watch this movie

18


Methods of Fault Calculations

1. Ohmic Method (basic software)

2. Per Unit Method (more expensive software)

3. Fault MVA Method (Done by hand – very low cost (training budget) but much more cost effective)

(1) and (2) can be prone to errors

(3) helps engineers to be more qualified


Ohmic Method• Generally acceptable for LV calculations when the

supply is derived from no more than a 3MVA source

• For MV/LV network, only 3-phase symmetrical fault calculations can be done

• Asymmetrical fault (make/break) capacity is derived by the application of a nominal factor.

• The Ohmic method becomes tedious and prone to error when more than one voltage level has to be taken into considerations.

19


Ohmic Method Example

(to Fault X1)


Ohmic Method Example

(to Fault X1)

20


Ohmic Method Example info

(Table 8)


Ohmic Method steps

21


Per Unit Method• When more than one voltage level has to

considered, we normally will use the Per Unit method to perform fault analysis

• This is the classical method that is used in IEEE Standards and IEC 60909-0

• Both standards have their own way of taken into account the variation (tolerance) of supply voltage and motor contributions

• The result of the two calculations are within a few percent of each other


P.U. Method steps

22


P.U. Method Example

(to Fault X1)


Fault MVA Calculation Method

See Appendix 3 of the book “Advanced Electrical Services Engineering – Vol 1”. It takes into account the ‘Regulation %’of the transformers and standby generator.

23


Fault MVA Calculation

Method



Safety – govern everything• Record Risk assessment outcomes• Agree with clients, Architect and the main

contractor the control methods to be used• If in doubt, appoint independent consultant• Apply consensus standards• Record design documentation• After T&C, provide analysis and comparison of


1








Session 3

Design

Case study

2



Overview of BS7671 design requirements

Assessments

Design Sequence

Documentations

Conclusions


Electrical Services Documentation

Non-domestic buildings1. O&M manuals2. Electrical Installation Certificate3. Schedules, schematics and layouts4. Periodic Inspection CertificateHome information pack – Domestic buildings• Electrical Installation Certificate• Periodic Inspection Certificate

3












Design Considerations

• General (Design aim) – Building’s nature of use and occupants• Characteristics of supply or supplies• Nature of demand• Electrical supply systems for safety services or standby electrical


4










5


High Level Design issues

• Post Occupants’ response on similar buildings• Stakeholders’ requirements• Energy and water use, Carbon emissions

considerations• Maintenance and Periodic T&I requirements

(might need FM’s input)• Technical feasibility and Cost, Sustainability

issues (BREEAM, HKBEAM, LEED etc)• Concept and detail design but in close

consultation with other built environment team members (Architect, Structural Engineer… etc)


Technical Design

Sequence

6


Induced electromagnetic force


Operating temperature of cables

7


Overcurrent design sequence


Basic Equation for protection against Overload current

BS7671 stipulates I2 1.45 x IZWhere I2 = actuating current ofthe circuit protection device

8





9


BS7671 sizing cables with high Harmonic content

Page 139

However, BS7671method only appliesto three phasecircuits



Flowchart



NEC(no flexibility)


CENELEC Standards


10


Joule loss approach

Page 177


The rating factor for grouping (Cg)

11


Grouping of different size cables


Suggestions for mixed cable groups

When a group of cable consists of large size and small size cables:1. Apply equation 5.35a to the large size cables2. Apply equation 5.35b to the smaller size cables3.Use joule-loss approach to evaluate the design temperature of the cable group

(see design example page 222 – 258)

12


Ohm’s Law


Important facts for passive elements1. An inductor acts like a short circuit to d.c.2. The voltage in a capacitor cannot change immediately3. The current in a resistor obeys Ohm’s Law at the flick of a

button.

Hence, IEEE Standards/IEC 60909-0 suggest that:a. Maximum asymmetrical short circuit current can be

evaluated by ignoring inductancesb. For most systems, the inductance is many times the

resistance, hence the worst case symmetrical short circuit circuit can be determined using just the reactance values

c. when under short circuit situations Inductance in motors can generate a short duration back emf (hence power)

13




Adiabatic Equation

R R

14



Adiabatic Equation

R R


Adiabatic line of a conductor

Value of k dependson initial and finalconductor temperature,conductor and dielectricmaterials.

For a short circuit thatlast longer than 5s, useBS7454 to assess thefinal temperature reached

15


Product Tolerances• Equipment manufacturers now provide details of

their product tolerances. We can use them in:


Discrimination / Selectivity• Get the pre-arcing and total arcing I2t from the

fuse manufacturers. We can use them in table format:

560A

400A 250A

X Y

16


Fault Calculations

• Make/break capacity of circuit breakers

• Many software neglect motor contributions in fault calculations

• When the Breaker and switchboard are under-rated can cause disasters ……watch this movie


Methods of Fault Calculations

1. Ohmic Method (basic software)

2. Per Unit Method (more expensive software)

3. Fault MVA Method (Done by hand – very low cost (training budget) but much more cost effective)

(1) and (2) can be prone to errors

(3) helps engineers to be more qualified

17


Fault MVA Calculation Method

See Appendix 3 of the book “Advanced Electrical Services Engineering – Vol 1”. It takes into account the ‘Regulation %’of the transformers and standby generator.


Fault MVA Calculation

Method

18


Transformer fault kA


Transformer fault MVA

19


Worst case kA values


Power factor correction capacitor

When the power factor pf of the circuit is improved from cos(61.83)=0.47 to (11.5/13.39)=0.86 (see Figure A3.1b). If we assume there is an internal resistance of 1 in the supply (e.g., the internal resistance of transformer and supply cable), using the P=I2R equation, the reduction of joule (energy) losses in the supply will be:

2 2_ _ _ _

2 2

_

24.36 1 13.39 1 414.12

before PFC compensation after PFC compensationI R I R Power saved

W

Assuming this is a small data server installation, it is left for the reader to calculate the saving in cost and carbon emissions for 8760 hours a year:

(8760 x 414.12)/1000 x 0.10 = £363.

(8760 x 414.12)/1000 x 0.527=1,913kg.

20


Application of PFC

Calculate the capacitor Qc kVAr needed to improve the power factor (pf) from 0.78 to 0.92 if the apparent power S of an installation is 1500 kVA?

P = S pf = 1500 x 0.78 = 1170kW. From Table A3.2, locate cos = 0.78 in the column labeled values before compensation and go across to the column under at the cos = 0.92 desired, the cross reading gives a Kpfc = 0.38. Hence, the capacitor kVAr rating needed is equal to Qc = Kpfc P = 0.38 1170 = 444.6kVAr.


Tea Break

21


Design Brief• It is required to produce an electrical design for a detached

house in the south of England with electrical loads comprising general electrical appliances and electric lighting. The supply to the house is single phase a.c. at 50Hz with UO = 230V. The earthing system type is TNC-S. The open circuit supply voltage may be taken as UOC = 250V.


Initial Information In accordance with BS7671 Part 3, the characteristics of the

building and source of energy have been assessed as follows:

• The supply is single phase a.c. at 50Hz with UO = 230V (UOC = 250V )

• The earthing system type is TNC-S• The installation is mains fed with a 17.5 m 25mm2 90ºC

thermosetting armoured aluminum multicoreunderground cable in a cable duct

• The maximum prospective short circuit current IP is 25kA in the road

• The earth fault loop impedance that is external to the installation is 0.35 maximum

• The protective device used by the supply utility company is an 80A HRC fuse with a pre-arcing I2t = 7,800A2s

22


Initial Information • The wiring is to be 90ºC rated low smoke and fume

(LSF) thermosetting insulated copper conductors cables installed inside metallic conduit

• A separate circuit-protective-conductor will be used rather than relying on the metallic conduits. However, the electrical continuity of the metallic conduit will be ensured to act as an integral duplicate circuit-protective-conductor for controlling protective-conductor-current from high-earth-leakage equipment

• The owner has asked that the circuit-protective-devices shall be circuit breakers or RCBOs complying with relevant British standards. Additional protection against electrical shock would be required where appropriateTheassessed after diversity maximum demand (i.e., peak demand at any one time) for the property will not exceed

• The house uses gas central heating, gas cooker with electric oven. It is estimated that the kWh used per day is between 11kWh minimum and 25kWh maximum.


Initial Schematic

23


Maximum IP calculation• For maximum IP, assume the electricity supply network

capacity is infinite, i.e., the source impedance = 0. Since

• where ZPSC is mainly resistive. • The worst case maximum IP can be estimated as

• Therefore protective devices with a rated breaking capacity (ICN) at 5kA or higher should be used.

PSC Source phase neutralZ Z Z Z 0 (0.0254 0.0042) (0.0254 0.0042)

0.0592PSC

PSC

Z

Z

max

2504222.97 4223

0.0592OC

PPSC

UI A A

Z


Minimum IP calculation• For minimum IP, the electricity supply network capacity

should be taken into account. Since we been told that ‘in the road’ IP is 25kA, the source impedance (which would be mainly reactive):

where Z source is mainly reactive. • The minimum IP can be estimated as

• Since the earthing system type is TNC-S, the external earth fault loop impedance Ze is equal to ZPSC. Therefore Zearth=Zphase. Hence the External earth fault loop impedance

250 0.01

25000OC

SourceP

UZ

I

max 2 2

2303830.87

0.0592 0.01O

PPSC

UI A

Z

2 2 2 20.0592 0.01 0.06Ze r x

24


Operating temperature Constraint


Voltage Drop Constraint(BS7671:2008 App 12)

If the supply is derived from a public supply:a. Lighting circuits 3%b. Other circuits 5%

If derived from a private supply:a. Lighting circuits 6%b. Other circuits 8%

( _ ) ( / / ) ( _ ) ( _ _ )bVD I per fitting mV A m L per fitting distributed load factor

25


Overload Protection Constraint


Heat generation check

26


Final check using after applying maximum demand diversity


Shock Protection Constraint(0.4s disconnection for circuit rating less than 32A)

From these results, the designer and installer can conclude that Type C circuit breakers would be appropriate to satisfy the maximum route lengths of the proposed installation circuit arrangements (i.e., maximum route length < 30m).

27


Additional Shock Protection Constraint(Use of RCD or Local Supplementary Bonding)

It was decided that the following circuits will be installed at the RCD protected section of the consumer units:•One 6A lighting final circuit for bathroom/external lighting•One 15A immersion heater final circuit•One 25A electric shower final circuit•All 25A radial socket outlet final circuits


Short Circuit Protection Constraint

28


Surge Protection Constraint

Surge arresters checkThe purpose of the surge arresters is to divert lightning surges with very low cable impedances put in place. The original design proposed the use of 2.5mm2 cables which was too small to absorb and dissipate a 50kA 80s lightning current efficiently. After consulting the surge arrester manufacturer, the cable is upgraded to 6mm2, the 25A circuit breaker provides protection against short circuits for the cable.


Isolation Protection Constraint

Isolation and Switching checkFor safe maintenance, the installation should have in place lockable isolation devices. Therefore it is required to ensure that a consumer unit with a lockable door shall be used. Also, the appliances and lighting should have in place functional switches for on or off operations.

29


Inspection & testing ConstraintInspection and testing checkWhen the design is completed, a model schedule of the testing results and Electrical Installation model Certificate in BS7671 Appendix 6 can be used by a suitably qualified engineer to carry out inspection and testing work to ensure the whole installation complies with BS7671 before the owner takes possession of the property.

A schedule of concept design information (e.g., a copy of the single line diagram as shown in Figure 5.15) should be produced by the designer for the installer’s information. This will allow the installer to compare and check against the installed results with the concept designed values. Should there are any technical departures from the performance design specifications, the installer will have to discuss with the designer what corrective or safety control solution(s) might be needed to be put in place where appropriate.


Energy and Carbon ConstraintIt was mentioned in the assessment of general characteristics that when occupied, the property is expected to consume from 11kWh to 25 kWh a day, an estimated annual energy usage of 6570kWh can be derived from taking the average of the two values and multiplying it by 365.

Assuming the carbon content of mains electricity is 0.527kg/kWh, the estimated carbon emissions of the property is equal to 65700.527=3,462.4 kg per annum. This can be offset by installing an 8kWp rated photovoltaic system in the south of England.

30


Documentation

A design and commissioning folder comprising of design calculations, commissioning test results and all supporting information such as O&M manuals should be put together by the designer and installer jointly. It should be available for technical enquiry, auditing and record keeping purposes and a copy should be kept by the building owner. In some countries, it could also be a legal requirement for buying and selling properties.



Safety – govern everything• Record Risk assessment outcomes• Agree with clients, Architect and the main

contractor the control methods to be used• If in doubt, appoint independent consultant• Apply consensus standards• Record design documentation• After T&C, provide analysis and comparison of


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