Schedule

1
Schedule
2
Introduction
Load Estimation
Terminology
Basic Equipment
Codes and Standards
Power Distribution Final Circuit
Standby Generator and Power Supplies
Protection Cable Wiring
Earthing
Design of Electricity Distribution
3
Earthing and Design of Electricity Distribution

• Date 20 November 2008
• Ir. KF Cheung

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Earthed Equipotential Bonding and Automatic
Disconnection - 1

• A) Principle
• To bond all the exposed and extraneous conductive
  parts to earth in order to create a zone at
  earthed potential so that the potential
  difference (touch voltage) between those parts
  are minimized in the event of an earth fault
  inside the zone (touch voltage would be reduced
  by bonding), and then to cut the supply within
  the maximum safe time duration.

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What will happen if electrical appliances are not
earthed?
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Can earthing help?
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What will happen if extraneous conductive parts
are not bonded?
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Earthed Equipotential Bonding and Automatic
Disconnection

• B) Disconnection of Supply under Earth Fault
• A protection device shall disconnect the supply
  during an earth fault so as not to cause danger.
• Maximum disconnection times are shown as follows

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Earthed Equipotential Bonding and Automatic
Disconnection

• B) Disconnection of Supply under Earth Fault
• Maximum value of earth loop impedance achieving
  the above disconnection time for various
  protective devices are shown in Table 41B1, Table
  41B2 and Table 41D of IEE Regulations (for 240V).
  The values shall be multiplied by 0.916 (220V/
  240V) for nominal supply voltage of 220V.

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Earthed Equipotential Bonding and Automatic
Disconnection
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Earthed Equipotential Bonding and Automatic
Disconnection

• B) Disconnection of Supply under Earth Fault
• The breaking capacity of the protective device
  shall be capable to withstand the prospective
  earth fault current.
• Local supplementary bonding shall be required
  where the disconnection time cannot be achieved

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Earthed Equipotential Bonding and Automatic
Disconnection

• B) Disconnection of Supply under Earth Fault
• The principle is to limit the resistance of the
  protective conductor so that the voltage
  appearing on exposed conductor parts under fault
  conditions is limited to 50 volts or if the
  voltage is a higher value the circuit would
  disconnect faster.
• For ring circuits the impedance of the protective
  conductor is calculated between its two ends
  before final connection is made and shall not
  exceed four times the value given in table 41C.

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Earthed Equipotential Bonding and Automatic
Disconnectio
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Earthed Equipotential Bonding and Automatic
Disconnection
C) Determination of Disconnection Time 1) Maximum
values of earth loop impedance for various
overcurrent protective devices are shown in table
41B1, Table 41B2 Table 41D 2) Actual earth loop
impedance can be calculated as follows Zs Ze
Z1 Z2 Ze Earth loop impedance at the
source Z1 Impedance of phase conductor Z2
Impedance of circuit protective conductor (cpc)
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Earthed Equipotential Bonding and Automatic
Disconnection

• C) Determination of Disconnection Time
• 3) Compare the actual Zs with the tabulated
  Zs(max)
• The actual Zs value measured from the
  installation should be smaller 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 voltage system 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
Disconnection
C) Determination of Disconnection Time 4) The
earth fault current can be calculated using the
following formula If Uo /Zs Uo Phase to
earth voltage If earth fault current 5) By
putting the calculated fault current against the
characteristic 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,
  the cable used is 2.5/1.5 twin with cpc PVC
  copper conductor, if the circuit length is 15m
  and Ze up to the MCB board is 0.5O, what is the
  actual disconnection time?
• From table 17, R1R2 /m 19.51mO x 1.38
• 0.269
  O/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
  fixing wiring 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 exceed two 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 Table 4C2.
• 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 into account of cables
  installed in a thermally insulated wall or
  ceiling where one side of the cable is in contact
  with a thermally conductive surface.
• Where the cable is totally enclosed in thermal
  insulation, Ci0.5 shall be used in absence of
  more precise information.
• It In / Ci
• Ci shall only be applied to the open and clipped
  direct column of respective 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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Correction Factor for Conductors

• E) General Formula for Correction Factors Applied
  to Cable Sizing
• It In / Cg x Ca x Ci x C4

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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 45oC
• 5) Clipped direct
• Ignore voltage drop
• Select the appropriate size of cables?

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Voltage Drop

• The overall voltage drop shall not exceed the
  value appropriate to the safe functioning of the
  equipment in normal service.
• The voltage drop in any circuit from the origin
  of installation to the current-using equipment
  should not exceed 4 of the nominal voltage.
• Volt drop pre unit value in from of mv/A/m are
  shown on IEE tables of Appendix 4. The values are
  based on the circuit conductor working at the
  maximum permitted operating 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 drop per unit (mv/A/m)

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Example

• A PVC/SWA/PVC armoured cable is to be installed
  from an HRC 100A fuse in a distribution board to
  a 3-phase 380V motor, along with 5 other cables
  fixed to a perforated metal cable tray where the
  cable sheaths will be touching, if the cable
  length is 125 meters and the power factor of the
  load is 0.89, what size of cable would be
  required to satisfy voltage drop if the ambient
  temperature is 30oC and the voltage drop in the 3
  phase feeder cable up to the distribution board
  is 3.7V and the total voltage drop allowed is 4?

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• Cg 0.74 (from the Table)
• It 100/0.74 135.14A
  
  From the Table
• Iz 163 A (for 50mm2 cable)
• V.D. allowances for the circuit
• (380V x 4) 3.7V 11.5V
• From the Table, z 0.81 mV/A/m
  Actual
  V.D. circuit 125m x 100A x 0.81 x 10-3
• 10.125V
• V.D. from Table for 50mm2 r 0.8, x 0.14
  Consider the power
  factor
• cos Power factor is 0.89
• sin power factor is 0.46
• V.D. ((0.89 x 0.8 0.14 x 0.46) x 125m x 100A)
  / 1000
• 9.705V lt 11.5V

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Sizing Circuit Protective Conductors

• If the conductor 35mm2 Zs Ze R1 R2
• gt35mm2 Zs Ze
  Z1 Z2
• Use the formula S v(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
  protective conductors.

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Thermal Constraint

• To protect conductor insulation against thermal
  damage during 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 the device is installed
• To check the operation time of the device
  according to the short-circuit current from the
  time/ current characteristic of the device
• To check the adiabatic line of the conductor by
  superimposing onto the characteristics of
  protective devices.

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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
• 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
  of conductor
• 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

• Does it offer shock protection in accordance with
  table 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

• Does the type and size of cpc offer protection?
• Check S v(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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Q A
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The End