Newnes Electrical Pocket Book

E A Reeves, Martin Heathcote

• Publisher: Routledge
• Year: 2013
• Print ISBN:
  9780367578558
  9780750647588
• Edition: 23
• eBook ISBN:
  9781136376443
• Topic: Technology & Engineering
• Subtopic: Construction & Architectural Engineering
• Index: Technology & Engineering

Newnes Electrical Pocket Book is the ideal daily reference source for electrical engineers, electricians and students. First published in 1932 this classic has been fully updated in line with the latest technical developments, regulations and industry best practice. Providing both in-depth knowledge and a broad overview of the field this pocket book is an invaluable tool of the trade. A handy source of essential information and data on the practice and principles of electrical engineering and installation. The 23rd edition has been updated by engineering author and consultant electrical engineer, Martin Heathcote. Major revisions have been made to the sections on semiconductors, power generation, transformers, building automation systems, electric vehicles, electrical equipment for use in hazardous areas, and electrical installation (reflecting the changes introduced to the IEE Wiring Regulations BS7671: 2001).

1 Fundamentals and theory

Fundamentals

Current. The term ‘current’ is used to denote the rate at which electricity flows. In the case of a steady flow the current is given by the quantity of electricity which passes a given point in one second. (Although since 1948 the unit of current has been officially defined in terms of the electromagnetic force that it produces, see below – since this force can be most conveniently measured.) The magnitude of the current depends not only upon the electromotive force but also upon the nature and dimensions of the path through which it circulates.

Ohm's law. Ohm's law states that the current in a direct current (d.c.) circuit varies in direct proportion to the voltage and is inversely proportional to the resistance of the circuit. By choosing suitable units this law may be written

The commercial units for these quantities are

Current – the ampere	(A)
Electromotive force – the volt	(V)
Resistance – the ohm	(Ω)

Using the symbols I, V and R to represent the above quantities in the order given, Ohm's law can be written

The law not only holds for a complete circuit, but can be applied to any part of a circuit provided care is taken to use the correct values for that part of the circuit.

Resistivity. The resistivity of any material is the resistance of a piece of the material having unit length and unit sectional area. The symbol is ρ and the unit is the ohm metre. The resistivity of a material is not usually constant but depends on its temperature. Table 1.1 shows the resistivity (with its reciprocal, conductivity) of the more usual metals and alloys.

Resistance of a conductor. The resistance of a uniform conductor with sectional area A and length l is given by

The units used must be millimetres and square millimetres if ρ is in ohm millimetre units.

Figure 1.1 Graphical symbols – BS 3939

Temperature coefficient. The resistance of a conductor at any temperature can be found as follows:

R_t = R₀(1 + αt)

R_t = resistance at temperature t °C

R₀ = resistance at temperature 0°C

The coefficient α is called the temperature coefficient and it can be described as the ratio of the increase in resistance per degree C rise in temperature compared with the actual resistance at 0°C. The coefficient for copper may be taken as 0.004. The increase in resistance for rise of temperature is important, and for many calculations this factor must be taken into account.

Table 1.1 Resistivities at 20° C

Material	Resistivty Ohm metres		Conductivity Siemens per metre
Silver	1.64	× 10^−8	6.10	× 10^7
Copper (annealed)	1.72	× 10^−8	5.8	× 10^7
Gold	2.4	× 10^−8	4.17	× 10^7
Aluminium (hard)	2.82	× 10^−8	3.55	× 10^7
Tungsten	5.0	× 10^−8	2.00	× 10^7
Zinc	5.95	× 10^−8	1.68	× 10^7
Brass	6.6	× 10^−8	1.52	× 10^7
Nickel	6.9	× 10^−8	1.45	× 10^7
Platinum	11.0	× 10^−8	9.09	× 10^6
Tin	11.5	× 10^−8	8.70	× 10^6
Iron	10.15	× 10^−8	9.85	× 10^6
Steel	19.9	× 10^−8	5.03	× 10^6
German Silver	16–40	× 10^−8	6.3–2.5	× 10^6
Platinoid	34.4	× 10^−8	2.91	× 10^6
Manganin	44.0	× 10^−8	2.27	× 10^6
Gas carbon	0.005		200
Silicon	0.06		16.7
Gutta-percha	2	× 10^7	5	× 10^−8
Glass (soda-lime)	5	× 10^9	2	× 10^−10
Ebonite	2	× 10^13	5	× 10^−14
Porcelain	2	× 10^13	5	× 10^−14
Sulphur	4	× 10^13	2.5	× 10^−14
Mica	9	× 10^13	1.1	× 10^−14
Paraffin-wax	3	× 10^16	3.3	× 10^−17

Power. Power is defined as the rate of doing work. The electrical unit of power (P) is the watt (abbreviation W), and taking a steady current as w...

Table of contents

• Front Cover
• Half Title
• Title Page
• Copyright
• Contents
• Preface
• Acknowledgements
• Introduction
• 1 Fundamentals And Theory
• 2 Properties Of Materialsm
• 3 Plastics And Rubber In Electrical Engineering
• 4 Semiconductors And Semiconductor Devices
• 5 Rectifiers And Converters
• 6 Computers And Programmable Controllers
• 7 Electricity Generation
• 8 Transmission And Distribution
• 9 Cables
• 10 Transformers And Tapchangers
• 11 Tariffs And Power Factor
• 12 Requirements For Electrical Installations (BS 7671)
• 13 Lighting
• 14 Motors And Control Gear
• 15 Switchgear And Protection
• 16 Heating And Refrigeration
• 17 Building Automation Systems
• 18 Instruments And Meters
• 19 Electric Welding
• 20 Battery Electric Vehicles
• 21 Battery Systems
• 22 Cable Management Systems
• 23 Hazardous Area Electrical Work
• Index