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
Protection and Cable Wiring

• Date 13 November 2008

4
Example

• A 20-Storey residential building with total 160
  no. of flats (ADMD 6.5 kVA/ flat)
• Lift 4 Lifts _at_ 25kVA (one is a fireman lift)
• Water Pump Potable _at_ 10kVA
• Flushing _at_ 10kVA
• F.S. Pump _at_ 30kVA
• Public Lighting _at_ 5kVA
• Emergency Lighting 10
• Draw a schematic diagram

5
Protection for Safety

• Scope
• Protection From Fire Burns (To property/
  Installation
• Protection against Thermal Effect
• Protection against Overcurrent
• Protection Against Electric Shock (To persons)
• Protection against Direct Contract
• Protection against Indirect Contract.

6
Protection Against Thermal Effect

• When fixed equipment is installed having a
  surface temperature sufficient to cause a risk of
  fire or harmful effects to adjacent materials
  suitable measures such as screening by proper
  material or mounting so as to allow safe
  dissipation of heat and at a sufficient distance
  from adjacent material.

7
Protection Against Thermal Effect

• Any part of fixed installation likely to attain a
  temperature exceeding the temperature limit
  specified in IEE standard should be guarded to
  prevent accidental contact.

8
Protection Against Thermal Effect

• IEE Regulations 422-01-05 specify that adequate
  precautions shall be provided to prevent spread
  of burning liquids or flame for equipment
  containing flammable liquid in excess of 25
  litres.

9
Protection Against Overcurrent

• Definition
• Overcurrent is a current exceeding the rated
  value for a current carrying part. It may be
  either an overload current or a short-circuit
  current.
• Overload Current is an overcurrent occurring in a
  circuit which is electrically sound, due to
  loading in excess of the design current of the
  circuit.
• Short-Circuit Current is an overcurrent resulting
  from a fault of negligible impedance between live
  conductors having a difference in potential under
  normal operating conditions.

10

11
Protection Against Overcurrent

• Overcurrent Protective Devices
• Every installation and circuit shall be protected
  against overcurrent by devices which will operate
  automatically at values of current which are
  suitably related to the safe current ratings of
  the circuit. (9A1)
• The overcurrent devices shall be suitably located
  and constructed so as to prevent danger from
  overheating or arcing and to permit ready
  restoration of supply without danger.

12
Protection Against Overcurrent

• Overcurrent Protective Devices
• The means of protection against overcurrent shall
  be a device capable of breaking any overcurrent
  up to and including the prospective short circuit
  current at the point where the device is
  installed. Such device may be circuit breaker
  incorporating overload release, or fuses, or
  circuit breaker in conjunction with fuses. (9A2)
• The overcurrent devices shall be of adequate
  breaking capacity and, where appropriate, making
  capacity for protection against fault current.
  (432-04-01)

13
Protection Against Overcurrent

• Overcurrent Protective Devices
• Overcurrent protective devices should be placed
  in non-flammable enclosures. The breakers should
  be designed or installed in a way that it is not
  possible to modify the overcurrent setting
  without the use of a key or tool. 9E(a) 9E(e)
• Protective devices are required to be labelled,
  marked and arranged so that related circuits can
  be readily identified. E.g. name plate, warning
  notice, intended nominal current, circuit chart,
  etc. 9E(b)

14
Protection Against Overcurrent

• Overcurrent Protective Devices
• The overcurrent device protecting socket outlet
  circuits must be operate on an earth fault within
  0.4 seconds and device protecting fixed equipment
  circuits within 5 seconds, except for bathroom
  where this value for fixed equipment circuits is
  also 0.4 seconds. To meet this requirement, the
  measured earth fault loop impedance of circuit
  must not greater than the values givens in CoP
  Table 8,10,11.

15
Protection Against Overcurrent

• Overcurrent Protective Devices
• In TT system, the use of residual current devices
  is preferred to overcurrent protective devices
  because the value of earth fault loop impedance
  of such system is variable depending upon
  moisture levels at each earth electrode.

16
Overload Protection

• Protective device shall be provided to break any
  overload current flowing in the circuit
  conductors before such a current could cause a
  temperature rise detrimental to insulation,
  joints, terminations, or surroundings of the
  conductors. 9B (1)a

17
Overload Protection

• IEE Reg. 433-02 states the condition of
  co-ordination between conductors and protective
  devices.
• I) Ib In Iz
• II) I2 1.45 Iz
• Ib Design current of the circuit
• In Nominal current rating or setting of
  overload protective device
• Iz Current carrying capacity of the circuit
  conductor
• I2 The current ensuring effective operation of
  the protective device
• The factor of 1.45 is the fusing factor of the
  protective device. (Fusing factor I2 / In)

18
Example

• For a single phase 220V circuit , the protective
  device is a MCB (BSEN 60898 Type B). The current
  carrying capacity of the cable Iz 19A and the
  electric load is a 2.6kW electric heater.
• Estimate the MCB rating (In).

19
Overload Protection

• If the overload protective device is a fuse to BS
  88 Part 2 or MCB to BS 3871 and compliance with
  condition (I), then condition (II) will deem to
  be complied with.

20
Overload Protection

• One overload protective device dose not comply
  with the requirement of condition (II) is
  semi-enclosed fuse to BS 3036. Such kind of fuse
  has a fusing factor of 2, then
• I2 2In
• I2 1.45 Iz
• Therefore, 2In 1.45 Iz
• In 0.725 Iz

21
Overload Protection

• Position of overload protective device
• A device for protection against overload shall be
  placed at the point where a reduction occurs in
  the value of current carrying capacity of the
  conductors of the installation. 9D(2a)
• The device protecting conductors against overload
  may be placed along the run of those conductors,
  provided that no branches or outlets are
  introduced between the point of reduction and the
  device. 9D(2b)

22
Short Circuit Protection

• IEE section 434 deals only with the case of short
  circuit anticipated between conductors belonging
  to the same circuit.
• Protective devices shall be provided to break any
  short circuit current in the conductors of each
  circuit before such current could cause danger
  due to thermal and mechanical effects produced in
  conductors and connections.

23
Short Circuit Protection

• Reg. 434-03-01 states that the breaking capacity
  rating shall be not less than the prospective
  short circuit current at the point at which the
  device is installed. Where supply is taken direct
  from the local power companys transformer, the
  main switch or circuit breaker shall have a
  short-circuit breaking capacity of 24MVA (at
  three-phase low voltage of 346V)
• Isc 24 000 000 36.5KA
• v 3 x 380

24
Short Circuit Protection

• Three-phase and Single-phase Symmetrical Short
  Circuit Currents
• If we are dealing with three-phase symmetrical
  short-circuit, then we need only to know the
  impedance of one-phase conductor.
• Three-phase symmetrical short-circuit current
• Isc(3Ø) Vp / Z1
• Vp phase to neutral voltage
• Z1 impedance of phase conductor
• If we are concerned with single-phase symmetrical
  short circuit current between phase and neutral
  we have to take into consideration the impedance
  of the phase and neutral conductors.
• Single-phase symmetrical short-circuit current
• Isc(1Ø) Vp / (Z1Zn)
• Zn impedance of neutral conductor

25
Example
26
Short Circuit Protection

• Position of Devices for Short Circuit Protection
• The short-circuit protective device shall be
  placed at the point where a reduction occurs in
  value of current carrying capacity of the
  conductors or placed on the conductor that
• Not exceed 3m in length after the reduction of
  cross-section area and
• To reduce the risk of fault current to a minimum
  and
• To reduce the risk of fire or danger to persons
  to minimum.
• For MCB to be used as short circuit protective
  device, both maximum and minimum short circuit
  current should be checked. The fault current in
  the circuit must lie between Imax and Imin that
  the conductor shall be protected by the
  overcurrent device.

27
Short Circuit Protection

• The breaking capacity of the protective device
  shall capable to withstand the prospective
  short-circuit current flow through during earth
  fault. Engineers are required to check/ calculate
  the prospective short-circuit current at the
  point where the device is installed, and to
  compare the breaking capacity of the device.
  Typical breaking capacity of protective devices
  and the minimum breaking capacity of protective
  devices to be installed in respect of different
  types of supply, with or without back-up fuse.

28
Typical Breaking Capacity of Protective Devices
29
Typical Protective Short Circuit Current
30
Typical Protective Short Circuit Current
31
Protection Against Both Direct and Indirect
Contact

• A) Separate Extra Low Voltage (SELV)
• The nominal voltage should not exceed 50Vrms
• Safety sources such as Class II safety isolating
  transformer to BS 3535 with secondary winding
  isolating from earth or battery
• Live parts of the circuit shall be isolated from
  Earth, circuit of other system or extraneous
  conductive parts

32
Protection Against Both Direct and Indirect
Contact

• A) Safety Extra Low Voltage (SELV)
• If the supply voltage used dose not exceed 25
  Vrms, no precautions to prevent direct contact is
  required under normal body resistance. If the
  supply voltage used is lied between 25 Vrms and
  50 Vrms , direct contact protection shall be
  either by barrier or enclosure having IP2X, or by
  insulation which capable to withstand a test
  voltage of 500V a.c. for one minute.

33
Protection Against Both Direct and Indirect
Contact

• B) Functional Extra Low Voltage (FELV)
• If the system complies with the requirements of
  safety extra-low voltage system, except that it
  fails to isolate from earth or protective
  conductors of other circuit, the system shall be
  protection against direct contact by
• Enclosures to IP2X, and/or
• Insulation of live parts capable to withstand
  500V a.c. for one minute.

34
Protection Against Both Direct and Indirect
Contact

• B) Functional Extra Low Voltage (FELV)
• If the system generally fails to comply with the
  requirements of safety extra-low voltage system,
  then the system shall be
• Protection against Direct contact by barriers or
  enclosures and/ or insulation subjected to test
  voltage for primary circuit.
• Protection against indirect Contact by connecting
  exposed conductive parts to protective conductor
  of the primary circuit.

35
Protection Against Both Direct and Indirect
Contact

• C) Limitation of Discharge Energy
• Equipment shall comply with appropriate British
  Standards, and shall limit current lower than the
  shock current, and shall be isolated from other
  circuit.

36
Protection Against Direct Contact
1) Methods
37
Protection Against Direct Contact

• 2) Insulation should withstand the mechanical,
  electrical, thermal and chemical stresses to
  which it serves and only to be removed by
  destruction. If applied on site, the quality of
  the insulation shall be tested to that of similar
  factory-built equipment.

38
Protection Against Direct Contact

• 3) Live parts shall be inside enclosures or
  behind barriers on following conditions
• In general, at least IP2X
• At readily accessible or top surface, at least
  IP4X
• In removing the enclosures or barrier, it shall
  be
• By tool or key, and/or
• Possible only after disconnection of supply and
  restoration of supply only possible after the
  replacement of the enclosure/barrier, and/or
• Intermediate barrier to IP2X.

39
Protection Against Direct Contact

• 4) IEE Reg. 412-04-02 states that suitable
  precautions such as using obstacles to prevent
  unintentionally touching to live part for
  equipment intended for protection against direct
  contact.
• 5) A bare live part should not be placed within
  arms reach or 2.5m of any exposed/ extraneous
  conductive parts.

40
Protection Against Indirect Contact

• Methods
• Earthed equipotential bonding and automatic
  disconnection of supply
• Use of Class II equipment or equivalent
  insulation
• Non-conducting location
• Earth-free local equipotential bonding
• Electrical separation.
• The most commonly used protective measure
  against indirect contact is that of automatic
  disconnection of supply using the overcurrent
  protective device.

41
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.

42
(No Transcript)
43
What happen if electrical appliances are not
earthed?
44
Can earthing help?
45
What happen if extraneous conductive parts are
not bonded?
46
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

47
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.

48
Earthed Equipotential Bonding and Automatic
Disconnection
49
Earthed Equipotential Bonding and Automatic
Disconnection

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

50
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.

51
Earthed Equipotential Bonding and Automatic
Disconnectio
52
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)
53
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

54
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.
55
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

56
Time (s)
Current (A)
57
Provision of Residual Current Device (RCD)

• If the required disconnection time cannot be
  achieved, RCD shall be provided
• A RCD shall not be used as the sole means of
  protection against direct contact. The use of a
  RCD is recognized as reducing the risk of
  electric shock if residual operation current not
  exceeding 40ms at a residual current of 150mA in
  addition to other means of protection against
  direct contact.
• IEE Reg. 412-02-16 states that where RCD are used
  to comply with the disconnection times, the
  residual operating current times earth fault loop
  impedance must not exceed 50.
• e.g. I2 30mA, Zs 50/ 0.03 1667ohms.

58
Provision of Residual Current Device (RCD)

• RCD having a rated residual operating current not
  exceeding 30mA shall be provided for every socket
  outlet circuit in a household or similar
  installation forming part of TT system.
• For instillation supplied form overhead line
  system, the installation shall be protected
  against earth leakage by RCD.
• The sum of normal leakage current of various
  items of equipment must not grater that half of
  the value of nominal residual operating current
  of the RCD.

59
Provision of Residual Current Device (RCD)

• Leakage current for typical equipment are shown
  below for reference

60
Coordination/ Discrimination of Protective Devices

• Overcurrent protective devices shall be
  coordinated so that the energy let-through by the
  fault current protective device does not exceed
  that which can be withstood without damage by the
  overload current protective device.
• It is allowed to install a protective device with
  a breaking capacity lower than the prospective
  short-circuit current available provided that
• There is a protective device on the supply side
  of the lower rated protective device which has
  the necessary breaking capacity and
• Both protective devices are coordinated so that
  the energy let-through the supply side device
  will not damage the lower rated device or cables
  conductors protected by both devices.

61
Coordination/ Discrimination of Protective Devices

• When a consumer's main switch is connected
  directly to the power companys transformer, the
  overcurrent protection of the main switch shall
  discriminate with the power companys high
  voltage protective settings.
• Discrimination between fuse-links connected in
  cascade internal power distribution can be
  achieved if the current rating of general purpose
  fuse-links have a discrimination ratio of 1.6 1
  or based upon the pre-arcing I2t values shown
  Table VI of BS 88 Part 1 1988.
  Engineer-in-charge should check the
  characteristic curves and I2t of protective
  devices installed in series for proper
  discrimination.

62
Coordination/ Discrimination of Protective Devices

• Protective device for motor circuit shall be
  carefully selected so as to suit the maximum
  starting current. The maximum acceptable starting
  current for induction motors are tabulated as
  follows

63
Q A
64
The End