Download Testing of Low Voltage Installations

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)
1
Introduction

3 Approaches
– Combined protection against both Direct & Indirect Contact
– Protection against Direct Contact
– Protection against Indirect Contact
2
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
3
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
4
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 location
4. Protection by earth-free local equipotential bonding
5. Protection by electrical separation
Method 1 above is commonly adopted in HK.
5
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
6
SELV
7
Reduced Low Voltage System
55V
55V
8
Reduced Low Voltage System
9
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)
10
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.
11
Terms for Earthing and Protective Conductors
exposed conductive parts
A1
circuit
protective
conductor
(cpc)
main
earthing
terminal
extraneous conductive parts
A2
supplementary
equipotential
bonding
B1
B2
gas pipes, water
pipes, lightning down
conductor, A/C ducts, etc
main
equipotential
bonding
earthing conductor
earth electrode
12
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.
13
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
14
Touch voltage

Fig. 7.8
Vt 
Z2
U0
Z1  Z 2  Z E
Extraneous
Conductive
parts
15
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.
16
EEBADS according to COP (see
Table 7.14)

Socket Outlet Circuits
– must be protected by a residual current device
I n Z s  50V
– 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.
17
EEBADS according to COP

Installation supplied from Overhead Line System
- must be protected by RCD and, I n Z s  50V

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 extraneousconductive-parts as the man equipotential bonding.
18
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
19
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,
I 2t
S
K2
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
20
Types of Earthing Systems
TN-S System
21
Types of Earthing Systems
TT System
22
Types of Earthing Systems
Combined TT & TN-S System
23
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 exposedconductive-parts.
Every switch or other means of electrical control
should be inaccessible to a person using the facilities.
24
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.
25
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
26
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 exposedconductive-parts and/or other extraneously-conductiveparts,
i.e. separation distance ≥ 2 m ?
27
Extraneous Conductive Parts ?
28