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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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)
E.g. equipment with power source and very high internal impedance
Method 1 above is commonly adopted in HK.
exposed conductive parts
extraneous conductive parts
gas pipes, water
pipes, lightning down
conductor, A/C ducts, etc
circuit protective conductor(cpc)
supplementary equipotential bonding
main equipotential bonding
main earthing terminal
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.
Earth fault current, Ia = Uo/(Z1+Z2+ZE)
Vt = Ia Z2 = Z2Uo/(Z1+Z2+ZE)
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
- must be protected by RCD and,
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
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 ?