Protection against Electric Shock (Note: All the mentioned tables in this course refer to, unless otherwise specified, Low Voltage Electrical Installation Handbook, by Johnny C.F. Wong, Edition 2004)

1
Protection against Electric Shock (Note All the
mentioned tables in this course refer to, unless
otherwise specified, Low Voltage Electrical
Installation Handbook, by Johnny C.F. Wong,
Edition 2004)

• (Textbook Chapter 7)

2
Introduction

• 3 Approaches
• Combined protection against both Direct
  Indirect Contact
• Protection against Direct Contact
• Protection against Indirect Contact

3
Combined Protection against Both Direct
Indirect Contact

• By Separated Extra-Low Voltage (SELV) System
• It is extra-low voltage system without connection
  to earth
• By Limitation of Discharge of Energy
• The equipment incorporates means of limiting the
  current which can pass through the body of a
  person to a value lower than that likely to cause
  danger
• However, the open circuit voltage is not limited
• E.g. equipment with power source and very
  high internal impedance

4
Protection against Direct Contact

• By Insulation of Live Parts
• By Barriers or Enclosures
• By Obstacles
• By Placing out of Reach
• Refer to Fig. 7.5 for illustration

5
Protection again Indirect Contact

• BS7671 (IEE Wiring Regulations) stipulates 5
  methods of protection against indirect contact
• Protection by earthed equipotential bonding and
  automatic disconnection of supply (EEBADS)
• Protection by Class II equipment or by insulation
  equivalent
• Protection by non-conducting location
• Protection by earth-free local equipotential
  bonding
• Protection by electrical separation
• Method 1 above is commonly adopted in HK.

6
Protection again Indirect Contact

• IEC61140 classifies methods of protection into 4
  types
• Class 0 by basic insulation only and no
  provision is made for the earthing of accessible
  conductive parts
• Class I by basic insulation and earthing of all
  accessible conductive parts
• Class II by double or reinforced insulation, and
  no provision is made for the connection of a
  protective conductor to the accessible conductive
  parts
• Class III by SELV supplies

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SELV
8
Reduced Low Voltage System
55V
55V
9
Reduced Low Voltage System
10
EEBADS

• Earthed Equipotential Bonding Automatic
  Disconnection of Supply (EEBADS)
• An established practice in Hong Kong
• Earthed Equipotential Bonding
• TT - when l.v. supply is given directly by the
  supply company
• TN-S - allowed only when the supply transformer
  is owned by the consumer
• Equipotential Bonding - to create an
  equipotential zone within reach, and all
  equipotential zones should be bonded to each
  other
• Automatic Disconnection of Supply
• Purpose - to limit duration and magnitude of the
  touch voltage (voltage that arises between
  simultaneously accessible exposed and extraneous
  conductive parts)

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EEBADS (Contd)

• Protective device can be
• overcurrent protective device (e.g. MCBs, MCCBs,
  fuses, etc)
• residual current device (in socket outlets and
  where prospective earth fault current is
  insufficient for prompt operation)
• CoP requirements differ from IEE
  Requirements(BS7671). We mainly focus our
  discussion on CoP requirements.

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Terms for Earthing and Protective Conductors
extraneous conductive parts
exposed conductive parts
A1
A2
gas pipes, water pipes, lightning down conductor,
A/C ducts, etc
B1
B2
circuit protective conductor(cpc)
supplementary equipotential bonding
main equipotential bonding
main earthing terminal
earthing conductor
earth electrode
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EEBADS (Contd)

• Exposed conductive parts Extraneous conductive
  parts (refer to Fig. No. 11(1) of CoP)
• Refer to Fig. 7.7 for installation component
  illustration
• Earth fault loop impedance, Zs Z1 Z2 ZE
• where Zs earth fault loop impedance
• Z1 phase conductor impedance
• Z2 CPC impedance
• ZE earth fault loop impedance
  external to the installation
• For max. permissible Zs, please refer to CoPs
  Tables 11(8) to 11(14) for different types of
  protective device.

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Touch Voltage, Vt

• Refer to Fig. 7.8 for symbols and illustration
• Earth fault current, Ia Uo/(Z1Z2ZE)
• ?Vt Ia Z2 Z2
  Uo/(Z1Z2ZE)
• Generally, the max. disconnection time should not
  exceed those indicated in fig. 7.4 for the
  particular Vt involved,
• E.g. Z1 0.3O, Z2 0.6O, ZE 0.5O
• Ia Uo/(Z1Z2ZE) 220 / (0.30.60.5)
  157 A
• Vt Ia Z2 Z2 Uo/(Z1Z2ZE) 157 x 0.6
  94.2 A
• From Fig. 7.4, in order to avoid danger, the
  max. disconnection time is 0.34 s

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Touch voltage

• Fig. 7.8

Extraneous Conductive parts
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General requirement for touch voltage

• However, if the circuit complies with the
  specific requirements as laid down by the IEE
  Regulations or the CoP (see the following
  discussion), the general requirements for touch
  voltage are deemed to be complied.

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EEBADS according to COP (see Table 7.14)

• Socket Outlet Circuits
• must be protected by a residual current device
• must satisfy
• Refer to Table 7.5 or CoPs Table 11(14)
• Fixed Equipment used inside Equipotential Zone
• Disconnection time in case of earth fault within
  5 sec.
• Fixed Equipment Circuits outside Equipotential
  Zone or inside Bathroom
• Disconnection time in case of earth fault within
  0.4 sec.

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EEBADS according to COP

• Installation supplied from Overhead Line System
• - must be protected by RCD and,
• Distribution circuit supplying circuits for both
  Socket Outlets and Fixed Equipment
• Equipotential bonding shall be provided at the
  distribution board connecting it to the same
  types of extraneous-conductive-parts as the man
  equipotential bonding.

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Calculation of circuit impedance

• Refer to Tables 7.15 to 7.17
• For cable sizes gt 35 mm2, reactance is taken into
  account
• An average earth fault temperature of (70160)/2
  115oC is assumed for PVC copper cables used as
  CPC

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Size of Protective Conductor

• Method 1 Refer CoPs Table 11(2), or
• Method 2 Refer CoPs Tables 11(3) to 11(7)
• Method 2 By using formula,
• where S CSA of protective conductor
• I earth fault current
• t disconnection time
• k a factor taking account of
  the resistivity, temperature coefficient and heat
  capacity of the conductor material, and the
  appropriate initial and final temperatures.
  Values of k are given in Table 7.20

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Types of Earthing SystemsTN-S System
22
Types of Earthing SystemsTT System
23
Types of Earthing SystemsCombined TT TN-S
System
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Electric Shock Protection in Locations Containing
a Bath or Shower

• Is of hazardous areas
• In case of earth fault, equipment need
  disconnecting within 0.4 s, except that supplied
  from SELV
• Local supplementary equipotential bonding is
  required for those parts simultaneously
  accessible with extraneous-conductive-parts
  and/or other exposed-conductive-parts.
• Every switch or other means of electrical control
  should be inaccessible to a person using the
  facilities.

25
Electric Shock Protection in Locations Containing
a Bath or Shower

• Lampholder within a distance of 2.5m from the
  bath or shower cubicle should be constructed of
  or shrouded in an insulating material.
• No stationary equipment having heating elements
  which can be touched should be installed within
  reach of a person using the bath or shower.
• No electrical installation or equipment should be
  installed in the interior of a bath tub or
  shower.

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Electric Shock Protection in Locations Containing
a Bath or Shower

• Divided into 4 zones, see Fig. 7.17A B
• The provision of socket outlets can be in zone 3
  location (i.e. 0.6m away from shower basin or
  bath tub) and they should be protected by a RCD
  with a residual operating current not exceeding
  30mA

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To Bond or Not to Bond

• To bond, one has to determine if the following 2
  conditions are fulfilled together
• 1. the part is an extraneously-conductive-part
  ,
• i.e. insulation to earth 22000 O ?
• 2. the part is simultaneously accessible with
  exposed- conductive-parts and/or other
  extraneously-conductive-parts,
• i.e. separation distance 2 m ?

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Extraneous Conductive Parts ?