cable & wiring presentation

7/28/2019 Cable & Wiring Presentation

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Schedule

Ref. Date Topic

A07 27 Oct 2005

Introduction and Load Assessment

A08 3 Nov 2005 Standards and Basic Equipment

A09 10 Nov 2005

Power Distribution & Final CircuitA10 17 Nov 2005 Protection & Earthing

A11 24 Nov 2005 Cable & Wiring

A12 1 Dec 2005 Standby Generator and PowerSupplies


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Introduction

Load Estimation

TerminologyBasic Equipment Codes and Standards

Power Distribution & Final Circuit

Protection & Cable Wiring

Earthing

Design of Electricity Distribution

Standby Generatorand Power Supplies


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Earthing and Design of

Electricity DistributionDate : 24 November 2005

Module Code : A11Ir. KF Cheung


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Earthed Equipotential Bonding and

Automatic DisconnectionC) Determination of Disconnection Time

3) Compare the actual Zs with the tabulated Zs(max):

The actual Zs value measured from the installation should besmaller than the Zs(max) value from IEE Tables in order to achieve

safe disconnection time. Attention is drawn on that the Zs (max)form IEE Tables shall be converted to nominal supply voltagesystem in Hong Kong before comparison.

Zs (max :220) = Zs (max : 240) in IEE Tables X 220/240


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Earthed Equipotential Bonding and

Automatic DisconnectionC) Determination of Disconnection Time

4) The earth fault current can be calculated using thefollowing formula:

If= Uo /ZsUo = Phase to earth voltage

If= earth fault current

5) By putting the calculated fault current against thecharacteristic curves of the protective device given in IEE,the actual disconnection time can be found.


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Example

A 220V circuit is protected by a 30A Type 2 MCB, thecable used is 2.5/1.5 twin with cpc PVC copperconductor, if the circuit length is 15m and Ze up to theMCB board is 0.5, what is the actual disconnection time?

From table 17, R1+R2 /m = 19.51m x 1.38

= 0.269 /m


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Time(s)

Current (A)


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A) Cable Selection

Factors to be considered in sizing of cable conductors Conductor material

Insulating material

Method of installation

Installed environment Ambient temperature

Thermal insulating enclosure

Adjacent cables

Type of protective device

Voltage drop

Minimum cross-sectional area


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Comparison between Copper

Conductor and Aluminum Conductor A) Copper Conductor

High degree of electrical conductivity

Tough, slow to tarnish

Can be jointed without any special provision to prevent electrolytic

action B) Aluminum Conductor

Lower price & light in weight

Pliable, it can be used in solid-core cables

Excellent resistance to corrosion


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Insulating Materials


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Bends of Non-flexible Cable

The minimum internal radius bend in cables for fixingwiring are shown in the following table


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Correction Factor for Conductors

Factors which affect the ability of a cable to lose heat are: Grouping (Cg or C1)

Ambient temperature (Ca or C2)

Thermal insulation (Ci or C3)

Semi-enclosed fuse to BS 3036 (0.725 or C4) Type of installation (Table 4A)


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A) Grouping factor (Cg) - 1 IEE Table 4B1 gives correction factors to be applied to te tabulated

current-carrying capacities where cables or circuits are grouped.

Where the horizontal clearance s between adjacent cables exceedtwo cable diameter (2D2), no correction factor need be applied.


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A) Grouping factor (Cg) - 2 If a cable is expected to carry not more than 30% of its grouped

rating, it may be ignored from the rest of the group.


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B) Correction Factor for Ambient Temperature (Ca) Correction factor for ambient temperature is shown in IEE Table

4C1. Where for semi-enclosed fuses are being used, see IEE Table4C2.

It In / Ca

Typical data are shown in the following table for quick reference.


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C) Correction Factor for thermal Insulation (Ci) The value of current-carrying-capacity for various sizes of

conductors shown in Tables of Appendix 4 have been taken intoaccount of cables installed in a thermally insulated wall or ceilingwhere one side of the cable is in contact with a thermally

conductive surface.

Where the cable is totally enclosed in thermal insulation, Ci=0.5shall be used in absence of more precise information.

It In / Ci

Ci shall only be applied to the open and clipped direct column ofrespective IEE Tables.


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D) factor for Semi-enlosed fuse to BS3036 (C4) When semi-enclosed fuse is used for protecting the conductor, a

derating factor of 0.726 shall be applied.


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Example


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Example

Protective device : BS 3036 fuses Ambient temperature: 30oC

Cable use :PVC twin with cpc cable

Cabling conditions at:

1) Bunched and clipped direct

2) Passed through totally enclosed thermal insulation area

3) One side in contact with thermally insulated ceiling

4) Passed through a boiler house where ambient temperature of

45o

C 5) Clipped direct

Ignore voltage drop

What cable sizes are required?


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Voltage Drop

The overall voltage drop shall not exceed the valueappropriate to the safe functioning of the equipment innormal service.

The voltage drop in any circuit from the origin of

installation to the current-using equipment should notexceed 4% of the nominal voltage.

Volt drop pre unit value in from of mv/A/m are shown onIEE tables of Appendix 4. The values are based on the

circuit conductor working at the maximum permittedoperating temperature and at unity power factor.


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Voltage Drop


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Voltage Drop


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Voltage Drop


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Voltage Drop

Voltage drop (V.D.) can be calculated as follows:V.D. = design current (Ib) x circuit length (L) x volt dropper unit (mv/A/m)


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Example

A PVC/SWA/PVC armoured cable is to be installed from anHRC 100A fuse in a distribution board to a 3-phase 380Vmotor, along with 5 other cables fixed to a perforatedmetal cable tray where the cable sheaths will be touching,

if the cable length is 100 meters and the power factor ofthe load is 0.866, what size of cable would be required tosatisfy voltage drop if the ambient temperature is 30oCand the voltage drop in the 3 phase feeder cable up to thedistribution board is 7.7V and the total voltage drop

allowed is 4%?


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Sizing Circuit Protective Conductors

If the conductor 35mm2 : Zs = Ze + R1 + R2>35mm2 : Zs = Ze + Z1 + Z2

Use the formula : S {(I2t)} / K Value of K : from IEE Tables 54B to 54F

If= Uo / Zs Value of t from IEE Fig. 1 to 8 of Appendix 3

Use Table 54G to size the minimum size of protectiveconductors.


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Thermal Constraint

To protect conductor insulation against thermal damageduring short circuit conditions.

I2 t = K2 S2

t = K2 S2/ I2

t = duration in second

S = cross-sectional area in mm2

I = effective short-circuit current in A

K = 115 for copper conductor insulated with PVC


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Thermal Constraint

Procedure To check the prospective short-circuit current at the

farthest point of the circuit from the point where thedevice is installed

To check the operation time of the device according to theshort-circuit current from the time/ current characteristicof the device

To check the adiabatic line of the conductor by

superimposing onto the characteristics of protectivedevices.


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Cable Selection Procedure

Select wiring system to be installed and type of cable Calculate the equipment current demand using Table 4A

(15 Edition)

Calculate the circuit design current (Ib) and using diversity

allowance. Determine the overcueent protective device (In) : type;

rating

Check Ib In

Determine correction factors for installation Grouping (Cg)

Ambient temperature (Ca)

Thermal insulation (Ci)

Semi-enclosed fuse (C4)


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Calculate the tabulated current carrying capacity ofconductor:

It (min) In x (1/ Cg) x (1/ Ca) x (1/ Ci) x (1/ C4)

Select cable size from Appendix 4

Check Ib In Iz

Calculate volt drop at the farthest point of circuit


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Dose device offer shock protection in accordance withtable 41B1, 41B2 & 41D for Zs (max)?

Check Zs Zs (max) from the tables

If No :

Re-select device or re-select phase conductor size

Re-select cpc size

Use alternative method as stated in Reg. 413-02-12

Checked by calculation

Obtain Ze form supply authority Calculate R1 + R2 using Table 17A & B

Determine actual Zs = Ze + (R1 + R2)


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Dose the type and size of cpc offer protection?Check : S {(I2t)} / K

If No : re-select type and/ or size of cpc

Check the adiabatic line of conductor against the

characteristic of overcurrent protective device.


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B1) Type of Conduit

B2) Sizing of Conduit

B3) Type of TrunkingB4) Sizing of Trunking

B5) Ducting

B6) Segregation of Circuit

B) Conduit & Trunking


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Steel Conduit to BS 4568 : Part 1

A) Light duty type: plain and conduits Limited to use in dry situation;

Unsuitable for bending Low degree of mechanical protection

B) Heavy duty type: screwed-end conduits Back enamel for internal use in dry situation;

Hot-dip galvanized for external use in situationsubject to dampness or water condensation;

Good mechanical strength and electrical continuity.

B1) Type of Conduit


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B1) Type of Conduit


C) Classification for protection:

Class Protection Applied Example

1 Light protection both inside & outside Priming paint

2 Medium protection both inside &outside

Stoved enamel;

Air-drying paint

3 Medium heavy protection : inside asClass 2;

Outside as Class 4

Stoved enamel inside;Sherardized outside

4 Heavy protection both inside &outside

Hot-dip zinc coating,sherardizing


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D) Heavy duty hot-dip galvanized steelconduit system is the most common usesystem for surface conduit wiring andconcealed conduit wiring. Conduit issupplied in standard lengths of 4 meters

and is manufactured in accordance withBS4568.

B1) Type of Conduit


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Plastic conduits

To BS4607 Part 1 and 2;

Characteristics : light, easily bend, lessinstallation time, no water condensation,lower cost;

Heavy duty PVC conduits can beconcealed but CPC are required.

B1) Type of Conduit


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Copper Conduits

High resistance to corrosion;

Last for long time; Higher cost;

Act as excellent circuit protective

conductor (CPC)

B1) Type of Conduit


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Aluminum Conduits

Light weight and lower cost;

Not so good in mechanical protection

Flexible Conduits

To BS731 : Part 1

Used for final connection to machinery;

CPC are required.

B1) Type of Conduit


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IEE Regulation (15th Edition) provide thefollowing tables for ease of conduit sizing:

Table A, B for 1/C PVC cables in a straight run 3m;

Table C, D for 1/C PVC cable in conduit run > 3m. The conduit size is considered satisfactory if the

conduit factor is equal to or exceeds the sum ofthe cable factors


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Type of Conductor C.S.A. of Conductor (mm2) Factor

Solid 1

1.5

2.5

22

27

39

Stranded 1.5

2.5

4

6

10

31

43

58

68

146

Table A Cable factors for short straight runs


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Conduit Diameter (mm) Factor

16 290

20 460

25 800

32 1400

Table B Conduit factors for short straight runs

) S f C


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Type of Conductor C.S.A. of Conductor(mm2)

Factor

Solid or Stranded 1 16

1.5 22

2.5 304 41

6 58

10 105

Table C Cable factors for long straight runs, or runs incorporating bends

B2) Si i f C d i


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Table D Conduit factors for runs incorporating bends

Refer to

Table A and B

B2)Si i f C d it


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Example

In a conduit installation the length of run is 10m,assuming 2 right-angle bend. What is the

conduit size to enclose four 2.5 mm2 PVC cables? From Table C, factor for one 2.5mm2 cable = 30

Therefore, four 2.5mm2 cables = 4 x 30 = 120

From Table D, suitable conduit size with a factor of

141(>120) is 20mm.[10m Vs 2 bends, cable factor : 141]


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B3) Type of Trunking

Use in conditions where a considerable no. ofcables are required in an installation or wherecables are too large for drawing into conduits.

Erection time is reduced (wiring is easier andquicker)

Multi-compartment trunking provides circuitsegregation.


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B3) Type of Trunking

Classification for protection against corrosion:

Class 1 Electroplated zinc having a minimum thicknessof zinc coating of 0.0012mm, inside and outside.

Class 2 As Class 1 but additional coating of stoved or airdrying paint, applied at least to the externalsurface.

Class 3 Hot dip zinc coated steel.


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B4) Sizing of Trunking

Type of Conductor C.S.A of Conductor (mm2) Factor

Solid 1.5

2.5

7.1

10.2

Stranded 1.5

2.5

4

6

10

8.1

11.4

15.2

22.9

36.3

Table E Cable factors for trunking


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Dimension of Trunking (mm x mm) Factor

50 x 37.5 767

50 x 50 1037

75 x 25 738

75 x 37.5 1145

75 x 50 1555

75 x 75 2371

100 x 25 993

100 x 37.5 1542100 x 50 2091

100 x 75 3189

100 x 100 4252

Table F Factors for trunking


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Example

What is the maximum no. of 10mm2 PVC cablespermitted in 50mm x 50mm trunking?

From Table E, factor of 10mm2 conductor = 36.3 From Table F, factor of 50 x 50mm trunking = 1037

Maximum no. of cable= 1037 36.3

= 28.56 (say 28)


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B5) Ducting

It provided mechanical protection for cable runin the ground or under concreted floor.

Types of ducting:

Concrete ducts Steel underfloor ducts

Fibre underfloor ducts

Maximum spacing factor is 35%.

It should be securely fixed and protected againstcorrosion and mechanical damage.


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B5) Ducting

Entries to duct must be protected against theinflow of water.

Cables installed in underground ducts shall havea metal sheath.

Underfloor trunking should be fabricated withsheet steel of not less than 12mm thickness forcompartment width up to 100mm, but at least1.6mm thickness for compartment width over100mm. The minimum thickness of 1mm shallbe used for the partitions and connectormaterial.


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B6) Segregation of Circuits

1) Suitable segregation between enclosed circuits with differentcategories shall be provided in wiring. For example, a low voltagecircuit shall be separated from an extra-low voltage circuit.

2) Types of Circuit:

Category 1 Circuit A circuit (other than a fire alarm or emergencylighting circuit) operation at low voltage andsupplied directly from a main supply system

Category 2 Circuit With the exception of firm alarm and emergencylighting circuits, ant circuit for telecommunication

(e.g. radio, telephone) which is supplied form asafety source.

Category 3 Circuit A fire alarm circuit or an emergency lightingcircuit.

Category 4 Circuit A high voltage circuit.


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3) Low Voltage circuit shall be segregated form extra-low voltagecircuit. Extra-low voltage cables shall not be drawn into the sameconduit or duct, or terminated in the same box or block as lowvoltage cables unless the former are insulated for the highestvoltage present in the low voltage circuit.

4) Cables of fire alarm and emergency lighting circuits shall not inany circumstances be drawn into the same conduit duct or ductingof other cables.


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5) Electrical services shall not be installed with pipes or tubes ofnon-electrical services (e.g. air, gas, oil, or water) in the sameconduit, ducting or trunking. This requirement does not apply wherethe various services are under common supervision and it isconfirmed that no mutual detrimental influence can occur.

6) For cables of category 1,2,3 circuits that are installed withoutenclosure or underground, a minimum separation of 50mm shouldbe provided between different category circuits or alternatively atleast 25mm separation with slabs of concrete inserted between the

circuits and the shortest path round the concrete should exceed75mm.


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7) Insulated bridge of at least 6mm thick should be used forseparation of surface wiring of Category 1,2,3 circuit running acrosseach other. The bridge should overlap the cables by at least 25mmon either side of point of crossing.

8) For cables of Category 4 circuit that are installed withoutenclosure or underground, a minimum separation of 300mm shouldbe provided between Categories or alternatively a reducedseparation with 50mm thick slabs of concrete inserted between thecircuits and the shortest path round the concrete should exceed

180mm.


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Example

Descriptions:

A flat of about 90m2 (useable area), with three bedrooms, (the masterbedroom with en-suite bathroom), a guest bathroom, a kitchen, adining room, lounge (living room) and a store room.

An air-conditioner (2 h.p., i.e. >=15A input current) is

expected to be in the dining room, and it also for the lounge.

An electric cooker of about 14A rating is expected to be installed in thekitchen.

Hot water is provided by gas heaters in bathrooms and the kitchen Battery operated door bell and clocks are expected.


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Example


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Example


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Example

To provision here is more than that of theminimum recommended requirements in the CPfor WR.

No socket outlet is provided in the bathrooms,and the switches for lighting and the ventilationfan should be installed outside the bathrooms.


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Example


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Example


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Typical Earthing Systems - 1

TT system A system having one point of the source of energy

directly earthed, the exposed-conductive parts of theinstallation being connected to earth electrodes

electrically independent of the earth electrodes of thesource.


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TN-S system

A system having one point of the source of energy directlyearthed and having separate neutral and protective conductorsthroughout the system.


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TNC-S system

A system having one point of the source of energy directlyearthed, the neutral and protective functions are combined in asingle conductor in part of the system.


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Type of Earth Electrode

The following Earth Electrode

Deep driven earth rods and/ or parallel driven earthrods

Buried tapes/ plated Welded metal reinforcement of concrete


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Q & A


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The End

cable & wiring presentation

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