Dr. Asrul Izam Azmi Faculty of Electrical Engineering Universiti Teknologi Malaysia

Dr. Asrul Izam AzmiFaculty of Electrical Engineering

Universiti Teknologi Malaysia

Mechanical and Electrical Systems SKAA 2032

Electrical Wiring

Announcement

Assignment 1Solution will be given this week

Assignment 2: Questions will be given this week

Mid-Term Exam: Next Week (Monday, 22 Oct), 9am-10amTutorial Topic

Outline

Electrical Distribution and Wiring• Wiring system, Types and size of cables• Protections and Grounding • Substation, Switchboard and Distribution Board• Electrical Load (Estimation)• Symbols and Single line diagram

Focus on receiving end

Component Elements of Electrical Cables

Selecting a cable for an electrical installation is very important; consideration must be given to the following criteria in order to ensure the correct type of cable is chosen:• conductor material• conductor size• insulation• environmental conditions.

Conductor material

Copper and Aluminium• The choice generally is between copper and

aluminium. Copper has better• Conductivity for a given cross-sectional area and is

preferable, but its cost has risen over the years.• Aluminium conductors are now sometimes preferred

for the medium and larger range of cables. • All cables smaller than 16mm2 cross-sectional area

must have copper conductors.

Conductor material

Conductor material

Other conductor materials• Cadmium copper: has a greater tensile strength for

use with overhead lines.• Steel reinforced aluminium: for very long spans on

overhead lines.• Silver: used where extremely good conductivity is

required. However, it is extremely expensive.• Copperclad (copper-sheathed aluminium): cables

that have some of the advantages of both copper and aluminium but are difficult to terminate.

Conductor size

There are many factors that affect the choice of size of conductor

Load and future developmentThe current the cable is expected to carry can be found from the load, taking into account its possible future development, i.e. change in use of premises, extensions or additions.

Ambient temperatureThe hotter the surrounding area, the less current the cable is permitted to carry.

Conductor size

GroupingIf a cable is run with other cables then its current carrying capacity must be reduced.

Type of protectionSpecial factors must be used when BS 3036 (semi-enclosed) fuses are employed.

Voltage dropThe length of circuit, the current it carries and the cross-sectional area of the conductor will affect the voltage drop.

Insulation and sheathing

Listed below are some of the working properties of the more common types of cable insulation:• PVC• synthetic rubbers• silicon rubber• magnesium oxide• phenol-formaldehyde

Insulation and sheathing

PVCThis is a good insulator: it is tough, flexible and cheap. It is easy to work with and easy to install. However, thermoplastic polymers such as PVC do not stand up to extremes of heat and cold, ordinary PVC cables should not constantly be used in temperatures above 60°C or below 0°C. Care should be taken when burning off this type of insulation (to salvage the copper) because the fumes produced are toxic.

Synthetic rubbersThese insulators, such as Vulcanised Butyl Rubber, will withstand high temperatures much better than PVC and are therefore used for the connection of such things as immersion heaters, storage heaters and boiler-house equipment.

Insulation and sheathing

Silicon rubberFP 200 cable using silicon rubber insulation and with an extruded aluminium oversheath foil is becoming more popular for wiring such things as fire-alarm systems. This is due largely to the fact that silicon rubber retains its insulation properties after being heated up or burned and is somewhat cheaper than mineral-insulated metal-sheathed cables.

Insulation and sheathing

Many factors affect cable selection. Some will bedecided by environmental factors:• risk of excessive ambient temperature• effect of any surrounding moisture• risk of electrolytic action• proximity to corrosive substances• risk of damage by animals• effect of exposure to direct sunlight• risk of mechanical stress• risk of mechanical damage.

Color coded

conductor

Switching Circuit

One-way switching

Two-way switching

3 – Pin Plug

• the blue wire is connected to the neutral terminal;• the brown wire is connected via a fuse to the live terminal;• the green & yellow wire (when fitted) is connected to the earth terminal (the two colours are

used for the benefit of people who are colour-blind);• the cable should be secured in the plug by the cable grip;• a fuse of the correct value (rating) should be in place.• the pins are made from brass, which is a good conductor;• the plastic or rubber case, cable grip and fuse are for safety.

Errors in Wiring Plugs

1. Bare wires showing2. Proper fuse not installed3. Earth wire not connected4. Live and neutral swapped

5. Loose cable grip

Plug and Socket

Japanese unearthed sockets with a grounding post for a washing machine.

A North American grounded (earthed) plug.

British Standard (BS) 1363 plug

CEE 7/16 plug and socket(Europe countries) M Plug (south Africa)

23-16/VII with socket(Italy)

A type M (15 A version of type D) travel adapter

Terminology

Terms Meaning

Main power House electrical power supplyEarth connection Safety connection to the earth or ground

Live connection Active connection

Neutral connection Return connection

Flex/mains leadMain wire/wiring

Flexible electrical cable from plug to appliance

Socket, electrical wall outlet, power point Female part of an electrical connection or electrical fitting in a wall outlet

Pin, plug Male part of an electrical connection

Potential Hazards

Electrocution. • The least you can expect from touching a live wire is a severe belt.

If you are sweaty or wet, you can get a severe burn or even be killed. The current does not need to be very large to cause a a severe burn or death, especially if you are sweaty or wet

Electrical fires. • A short circuit or other electrical fault in an appliance can make

too big a current flow. This can melt and set fire to wires, sockets etc. When too big a current flows through the wiring they get hot, just like toaster elements, and ignite building materials such as timber, insulation, etc. Electrical fires can be very dangerous - especially in timber-framed buildings, because they often start inside a wall. They can burn there for quite a while without being noticed. The main job of the fuse or circuit breaker is to stop too large a current from starting a fire.

Shock Current ( mA)

Effect

> 25 DEATH

10 - 25 PAIN. Can’t let go

0-10 TINGLE

Resistance – 4 KΩ (moist skin) to 24 KΩ (dry skin)Safe current (through chest) – less than 20 milliamps

V = 240 VAC R = 4 KΩ I = 60mAI = 30 milliamps - NOT SAFE

The Sources of Electrical Hazards

There are various electrical hazards within the home/office : -• Long or frayed cables• Cables in contact with something hot or wet• Water near sockets• Shoving things into sockets• Damaged plugs • Connection of too many plugs within a socket• Lighting sockets without bulbs in• Appliances without covers• Short circuit

Electrical Control Measures/Circuit Protection

1. Insulation2. Earthing/Grounding3. Fuses 4. Circuit breakers

1. Insulation

Insulation

Outer sleeve (jacket)(a) Cables have a flexible plastic outer sleeve and the wires inside also have their own flexible plastic sleeves. These sleeves act as insulation layers which stop the copper core of the live wire from contacting the other wires, any metal part, or the skin of anybody that is touches the wire.

Insulation

(b) Plug casings Plug casing are made of plastic or rubber. Molded plugs attached at the factory are molded to the cable and are even safer than the plugs that you can take apart.

Insulation

(c) Double insulation• An electrical appliance which is double insulated does

not have an earth wire fitted • The appliance that not only has insulated wires inside,

but also has a casing made of plastic, so that they provide two level of protection against electrical shock.

• Common double insulated appliances are hair dryers, radios and cassette players.

Symbol

2. Earthing/Grounding

Earthing/Grounding

• The earth wire is a safety feature which prevents the metal casing of an appliance becoming dangerous to touch when a fault occurs.

• Many electrical appliances have metal cases, including cookers, washing machines and refrigerators – the earth wire creates a safe route for the current to flow through if the live wire touches the casing.

• You will get an electric shock if the live wire inside an appliance, such as a cooker, comes loose and touches the metal casing.

Earthing/Grounding

• The earth terminal is connected to the metal casing so that the current goes through the earth wire instead of causing an electric shock

• A strong current surges through the earth wire because it has a very low resistance – this breaks the fuse and disconnects the appliance

Earthing/Grounding

Earth Electrodes

• Variety of types of earth electrode including earth rods, plates electrodes, wire electrodes ground ring electrodes and underground structural metal work

• There are certain requirements such as the soil resistivity, depth of grounding and types and dimension of electrodes

RodWire electrodes

Plate electrodes

Earthing/Grounding System

• Major types of earthing system:– TN-S– TN-C– TT

• T: Earth• N: Neutral• S: Separate• C: Combined

TN-S Earthing System

• A TN-S system, has the neutral of the source of energy connected with earth at one point only, at or as near as is reasonably practicable to the source, and the consumer's earthing terminal is typically connected to the metallic sheath or armour of the distributor's service cable into the premises.

TN-C-S Earthing System

• The supply neutral conductor of a distribution main connected with earth at source and at intervals along its run. This is usually referred to as protective multiple earthing (PME). With this arrangement the distributor's neutral conductor is also used to return earth fault currents arising in the consumer's installation safely to the source. To achieve this, the distributor will provide a consumer's earthing terminal which is linked to the incoming neutral conductor.

TT Earthing System

• The neutral of the source of energy connected as for TN-S, but no facility is provided by the distributor for the consumer's earthing. With TT, the consumer must provide their own connection to earth, i.e. by installing a suitable earth electrode local to the installation. This type of earthing arrangement is commonly found in rural areas.

3. Fuses

Fuses

• An electric fuse is a device which is used to limit the current in an electric circuit.

• The fuse protects the circuit and the electrical appliances from being damaged – when there is too much current, the fuse melt.

• Fuses in plugs are made in standard ratings (3A, 5A, 13A etc…)

symbol

Fuses

• The fuse wire is generally an alloy of lead and tin. It has a low melting point and breaks the circuit if the current exceeds a safe value.

• The thickness and length of the fuse wire depends on the maximum current allowed through the circuit.

• It is connected in series in the beginning of the electric circuits.

• Short circuit, overloading, mismatched loads or device failure are the prime reasons for excessive current.

Fuses

• When the circuit current exceeds a specified value due to voltage fluctuations or short-circuiting, the fuse wire gets heated and melts.

• Thus it breaks the connection as shown in the figure and no current flows. This prevents damage to the appliance.

Fuses

Fuses

• To choose the correct fuse for an electrical device, always choose the one with the closest rating that is greater than the operating current of the device.

• Example, if a kettle operates with an electrical current of 10A, what fuse should it be fitted with (from 3A, 5A and 13A)? Ans: 13A

• Other example, If a device operates at 3A, choose 5A fuse.

• Different appliances require different sized fuses: – A cooker - 30 Amp, A lighting circuit - 5A, A table lamp - 3A, An electric

kettle, 13A

4. Circuit breakers

Circuit Breaker

• A circuit breaker (CB) is an automatically operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit.

• The basic function is to detect a fault condition and, by interrupting continuity, to immediately discontinue electrical flow.

• Unlike a fuse, which operates once and then must be replaced, a circuit breaker can be reset (either manually or automatically) to resume normal operation.

• Circuit breakers are made in varying sizes, from small devices that protect an individual household appliance up to large switchgear designed to protect high voltage circuits feeding an entire city.

Circuit Breaker

1. Actuator lever2. Actuator mechanism 3. Contacts 4.Terminals 5. Bimetallic strip 6. Calibration screw7.Solenoid 8. Arc divider / extinguisher

Circuit Breaker

Switching mechanisms used in circuit breaker• Low voltage circuit breakers• Magnetic circuit breakers• Thermal magnetic circuit breakers• Common trip breakers• Medium-voltage circuit breakers

Circuit Breaker

Types of circuit breaker• MCB (Miniature Circuit Breaker)• MCCB (Moulded Case Circuit Breaker)• Air Circuit Breaker• Vacuum Circuit Breaker• RCD (Residual Current Device) / RCCB( Residual

Current Circuit Breaker)• ELCB (Earth Leakage Circuit Breaker)• RCBO (Residual Circuit Breaker with Overload)

Circuit Breaker

MCB (Miniature Circuit Breaker)– Rated current not more than 100 A.– Trip characteristics normally not adjustable.– Thermal or thermal-magnetic operation.

• MCCB (Moulded Case Circuit Breaker):– Rated current up to 1000 A.– Trip current may be adjustable.– Thermal or thermal-magnetic operation.

Circuit Breaker

Vacuum Circuit Breaker:– With rated current up to 3000 A,– These breakers interrupt the arc in a vacuum bottle.– These can also be applied at up to 35,000 V. Vacuum

breakers tend to have longer life expectancies between overhaul than do air circuit breakers.

ELCB (Earth Leakage Circuit Breaker)– Phase (line), Neutral and Earth wire connected through

ELCB.– ELCB is working based on Earth leakage current.

Circuit Breaker

RCD (Residual Current Device / RCCB(Residual Current Circuit Breaker)• Phase (line) and Neutral both wires connected through

RCD• It trips the circuit when there is earth fault current.• The amount of current flows through the phase (line)

should return through neutral .• It detects by RCD. any mismatch between two currents

flowing through phase and neutral detect by -RCD and trip the circuit within 30 miliseconed.

• RCDs are an extremely effective form of shock protection

Magnetic Circuit Breaker

Circuit breakers work quicker than fuses.( saves lives)

Magnetic Circuit Breaker

Question

a) What is the main purpose of a wire fuse or circuit breaker?b) Explain how a wire fuse protects an appliance if it develops a fault.c) Explain how a wire fuse and an earth connection stop you getting an electric shock if an appliance develops a fault.

Question

d) In a modern home, each wiring circuit is protected by a circuit breaker rather than a wire fuse. Give two advantages of a circuit breaker compared with a wire fuse.

– Circuit breaker can be reset when tripped, while fuse need to be replaced.

– Safer to reset, flick a switch. No human error because there is no replacing of fuse wire manually.

– Longer life

Typical wiring system

Typical wiring system

Why are sockets wired in parallel with each other ?

Consumer Unit / Distribution Board

• A consumer unit is a type of distribution board (a component of an electrical power system within which an electrical power feed provides supply to subsidiary circuits).

• Consists of fuses, circuit breakers and residual current operated devices.

Dual RCD split load board