Textbook chapter 7
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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). Introduction. 3 Approaches

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(Textbook Chapter 7)

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Textbook chapter 7

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)


Introduction

Introduction

  • 3 Approaches

    • Combined protection against both Direct & Indirect Contact

    • Protection against Direct Contact

    • Protection against Indirect Contact


Combined protection against both direct indirect contact

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


Protection against direct contact

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


Protection again indirect contact

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.


Protection again indirect contact1

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


Textbook chapter 7

SELV


Reduced low voltage system

Reduced Low Voltage System

55V

55V


Reduced low voltage system1

Reduced Low Voltage System


Eebads

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)


Eebads cont d

EEBADS (Cont’d)

  • 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.


Textbook chapter 7

Terms for Earthing and Protective Conductors

exposed conductive parts

extraneous 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


Eebads cont d1

EEBADS (Cont’d)

  • 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 CoP’s Tables 11(8) to 11(14) for different types of protective device.


Touch voltage v t

Touch Voltage, Vt

  • Refer to Fig. 7.8 for symbols and illustration

    Earth fault current, Ia = Uo/(Z1+Z2+ZE)

    Vt = Ia Z2 = Z2Uo/(Z1+Z2+ZE)

  • Generally, the max. disconnection time should not exceed those indicated in fig. 7.4 for the particular Vt involved,

    E.g. Z1 = 0.3Ω, Z2 = 0.6Ω, ZE = 0.5Ω

    Ia= Uo/(Z1+Z2+ZE) = 220 / (0.3+0.6+0.5) = 157 A

    Vt = Ia Z2 = Z2Uo/(Z1+Z2+ZE) = 157 x 0.6 = 94.2 A

    FromFig. 7.4, in order to avoid danger, the max. disconnection time is 0.34 s


Touch voltage

Touch voltage

  • Fig. 7.8

Extraneous

Conductive

parts


General requirement for touch voltage

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.


Eebads according to cop see table 7 14

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 CoP’s 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.


Eebads according to cop

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.


Calculation of circuit impedance

Calculation of circuit impedance

  • Refer to Tables 7.15 to 7.17

  • For cable sizes > 35 mm2, reactance is taken into account

  • An average earth fault temperature of (70+160)/2 = 115oC is assumed for PVC copper cables used as CPC


Size of protective conductor

Size of Protective Conductor

  • Method 1: Refer CoP’s Table 11(2), or

  • Method 2: Refer CoP’s 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


Types of earthing systems tn s system

Types of Earthing SystemsTN-S System


Types of earthing systems tt system

Types of Earthing SystemsTT System


Types of earthing systems combined tt tn s system

Types of Earthing SystemsCombined TT & TN-S System


Electric shock protection in locations containing a bath or shower

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.


Electric shock protection in locations containing a bath or shower1

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.


Electric shock protection in locations containing a bath or shower2

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


To bond or not to bond

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 Ω ?

    2. the part is simultaneously accessible with exposed- conductive-parts and/or other extraneously-conductive-parts,

    i.e. separation distance ≥ 2 m ?


Textbook chapter 7

Extraneous Conductive Parts ?


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