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:

1. Protection by earthed equipotential bonding and automatic disconnection of supply (EEBADS)

2. Protection by Class II equipment or by insulation equivalent

3. Protection by non-conducting location4. Protection by earth-free local equipotential bonding5. 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 co

nsumer• 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 (volta

ge that arises between simultaneously accessible exposed and extraneous conductive parts)

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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.

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Terms for Earthing and Protective Conductors

A1 A2

B1 B2

main equipotential bonding

extraneous conductive partsexposed conductive parts

main earthing terminal

circuit protective conductor(cpc)

earth electrode

earthing conductor

supplementary equipotential bonding

gas pipes, waterpipes, lightning downconductor, A/C ducts, etc

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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.

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Touch Voltage, Vt

Refer to Fig. 7.8 for symbols and illustration

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

Vt = Ia Z2 = Z2 Uo/(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 = Z2 Uo/(Z1+Z2+ZE) = 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. 7.80

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2 UZZZ

ZV

Et

ExtraneousConductiveparts

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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 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 o

r inside Bathroom– Disconnection time in case of earth fault within 0.4 sec.

VZI sn 50

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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 b

oard connecting it to the same types of extraneous-conductive-parts as the man equipotential bonding.

VZI sn 50

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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

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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, temp

erature coefficient and heat capacity of the conductor material, and the appropriate initial and final temperatures. Values of k are given in Table 7.20

2

2

K

tIS

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Types of Earthing SystemsTN-S System

22

Types of Earthing SystemsTT System

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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 wi

thin 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.

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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 Ω ?

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 ?