This document provides an overview of 8 modules for the design of electrical installations in buildings according to IEE Regulations BS 7671. The modules cover: general characteristics including maximum current demand, circuit division and earthing types; overcurrent protection; cable selection; protection against electric shock; earthing and bonding; isolation and switching; special installations and locations; and inspection and testing. Each module discusses the relevant considerations and regulations for that topic.

1. Design of Electrical Installation of
Buildings
IEE Regulations BS 7671
Introduction – this brush up of practice overviews to 8
modules as following
•Assessment of General Characteristics
•Over current protection
•Cable Selection
•Protection Against Electric Shock
•Earthing and Bonding
•Isolation and Switching
•Special Installations and Locations
•Inspection and Testing

2. Assessment of General
Characteristics
To just understand the following and its
applications will be discused in the
sections
• Maximum Current Demand
• Division in to Circuits
• Earthing Types
• Parameters of Supply

3. Maximum Current Demand
• Shall be assessed with the guide given in
Regulations for the smaller installations and the
large installations with integral parts similar to
smaller category.
• In the lager environments, (Industrial and High-
rise) Engineer has the discretion to decide the
maximum demand with his diversities and
conditions.
• Load management can be a factor for designing
the maximum demand.

4. Division into circuits
• In one circuit all equipment are in parallel
• In small installations
1. 5/6/10 A circuits are generally recommended for
lighting circuits
2. 15/16/20/30/40/45/Radial Circuits are for fixed
items of power outlets
3. 30/32 A ring circuits are generally used and
becoming obsolete.
4. But generally you may design whatever the
circuit if u want to subjected to proper usage of
the components commercially available.

5. Diversity factors
• 66 % of lighting Total (or the total of the
section according to design)
íe. max demand applies to the main switch
(es) to the installation.
• 100% of the nominal Rating of the first
circuit and 40% of the nominal Rating of
the rest of the circuit of the Total
installation (or the sections according to
the design.)

6. Types of Earthing
• Tree types are TN , TT , and IT
• TT system has earth Electrode within the
installation.
• TNS and TNCS the earth conductor
supplied by the supply authority.
• IT system does not use an earth electrode
or uses very high impedance on earth
connection.
• Sri Lanka generally uses a TT system

7. Parameters of Supply
• 50 Hz frequency
• Nominal voltage 400 V phase to Phase
• 230 V phase to Neutral
• The Earth fault loop impedance depends on the type of
earthing and the Electrode and the conductor properties.
• Fusible or Tripping Devise to suit the main Incomer
rating
Then the Supplier Includes
• Maximum Prospective Short Circuit Current, and
• The supplies Requirment of the meter installation.

8. Over Current Protection and Earth
Fault Protection
• Each main Incomer to be protected at least with Over
current and short Circuit Protection
• Each circuit to be protected against over current short
circuit and earth fault
• Over current short circuit will be usually covered by one
device like MCCB or MCB
• In cases of TT system definitely the earth fault currents
are cut off by residual current devices. In certain
instances The Earth Fault currents are handled by MCBs
and MCCBs itself and on other residual currents are
required.

9. Cable Selection
• For the cable selection phase conductors and
the Circuit Protective Conductor (which may not
be the same size) shall be considered.
• Conditions like Ambient Temperature and the
Laying methods are depending factors in cable
sizes for a certain application.
• The tripping characteristics of an overload
device and the cable characteristics would have
to be always coordinated properly on every
application. (Otherwise the cable will catch fire
before breaker trips.)

10. Protection Against Electric Shock
• Protection against Electric shock by direct
contact is insulations barriers and so on.
• Protection against E/S by indirect contact
is the limitation of fault current or
disconnection on electric shock
•

11. Earthing and Bonding
• Earth Electrodes with Earth Rods or Pipes
• It can be Reinforcements and metal
structures or pipes on central heating or
water supply.
• Earthing types of TN and TT will be
understood and covered
• Circuit Protective Conductors Main
Equipotential Bonding and Functional
Eathing will be covered

12. Isolation and Switching
• Isolation for emergency
• Isolation for maintenance
• Switching
• Switching for duty Cycles
Shall be covered in this section

13. Special installations and locations
• In indoor installations bathtubs with
electric heaters and Sauna etc are special
installations.
• Outdoor swimming pools with Electric
heaters and caravan sites etc are
considered as special installations
• Further considerations will be given to
Construction sites and open baths of metal
processes etc.

14. Inspection and testing
• This section will give a comprehensive
coverage on tests after installtions and
inspection and then reporting etc.