IEE Wiring Regulations

Designing an electrical installation

Assessment of general characteristicPurpose and intended use of the building and the type of constructionMaximum current demandCompatibilityMaintainability

PROTECTION AGAINST ELECTRIC SHOCKS

Direct Contact

An electric shock result from contact with a conductor which forms part of a circuit and would be expected to be live.

DIRECT CONTACTL

N

PROTECTION AGAINST ELECTRIC SHOCKS

Indirect contact

An electric shock is received from contact with something which would not normally be expected to be live but has become so as a results of a fault.

INDIRECT CONTACTL

N

E

PROTECTION AGAINST ELECTRIC SHOCKS

Protection against direct contactThe insulation of live partsThe provision of barrier, obstacles or enclosure to prevent touchingPlacing out of reach

PROTECTION AGAINST ELECTRIC SHOCKS

Protection against indirect contact

Making sure that when a fault occurs the supply being cutoff within a safe time using

An over-current protection devicesA residual current devices (Current operated trip switch)

Protection with over-current devices

L

N

Protective conductor

Fuse/MCB

Substation earth

Earth fault loop

Earth fault loop impedance

Phase conductor impedanceT/F leakage impedanceT/F earth resistance including protective conductorResistance of the earthResistance of the consumers earth electrodeProtective conductor impedanceFault impedance

Fixed and portable equipment

Portable equipmentShould disconnect the

supply within 0. 4 s

Fixed equipmentShould disconnect the

supply within 5 s

Operation criteria for a fuse

Fusing current < Fault currentFusing current

For a portable appliance fusing current for 0.4 s operation For a fixed equipment fusing current for 5 s operation

Fault current = Nominal voltage / Earth fault loop

impedance

Fusing characteristic

5 sec operating time for a 30 A fuse = 130 AMaximum allowable earth fault loop impedance = .....

Over-current

240V

0.1

0.150

A

C

B

5A wire

CALCULATENormal current in the circuitCurrent when B & C are short circuited due to a faultCurrent when there is a direct short circuit between A & C

Over-current and short circuit current

Over-current: current higher than that is intended to be present in the system

Short circuit current: Occur under fault conditions and may be very high

Protection against overload current

Overload current should be interrupted before a temperature rise cause damage to the insulation, joints etc.Devices that can be used:

Semi-enclosed fusesHigh breaking capacity fusesMiniature and moulded case circuit breakers (MCB and MCCB)

Selection criteria of the protective device for overload

Protective device .. Nominal current (In)Operating current (I2)I2 > In

Cable ... Current carrying capacity (For the smallest cable Iz)Designed current or load current (Ib)

Ib < In < IzI2 < 1.45 x Iz

Semi-enclosed fusesRated current (In) value of current which the fusing element can usually carry without meltingMinimum fusing current (I2) minimum value of the current at which the fuse element meltsFusing factor = I2/In 2In2 d3 ( d diameter of the fuse wire)

HBC fusesHBC fuses has quenching materials inside to quench the arc produced during the breaking processBreaking capacity --- 10 20 kA

Fusing element

Filling powder

Miniature circuit breaker (MCB)

MCB is fitted withMagnetic coil for very fast operation under short circuitBimetallic strip for slow operation for overload currents

Miniature circuit breaker (MCB)

100

101

102

10-3

10-2

10-1

100

101

102

103

Current as a multiple of plug setting

T

i

m

e

i

n

s

e

c

f

o

r

T

M

S

=

1

Moulded case circuit breaker (MCCB)

Rating can be adjusted

Protection against short circuit

Fault current is very highBreaking capacity of the protective device > Fault currentThe fault current protection system must operates before there is damaging increase in conductor insulation temperatureTime to damage a cable 2

2

IAt