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Transcript of 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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Correction Factor for Conductors
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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Correction Factor for Conductors
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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Correction Factor for Conductors
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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Correction Factor for Conductors
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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Correction Factor for Conductors
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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Cable Selection Procedure
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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Cable Selection Procedure
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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Cable Selection Procedure
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
Steel Conduit to BS 4568 : Part 1
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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Steel Conduit to BS 4568 : Part 1
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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B2) Sizing of Conduit
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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B2) Sizing of Conduit
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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B2) Sizing of Conduit
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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B2) Sizing of Conduit
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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B2) Sizing of Conduit
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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B2) Sizing of Conduit
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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B4) Sizing of Trunking
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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B4) Sizing of Trunking
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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B6) Segregation of Circuits
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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B6) Segregation of Circuits
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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B6) Segregation of Circuits
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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Typical Earthing Systems - 2
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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Typical Earthing Systems - 3
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