Saturday 10 March 2012

BASIC ELECTRICITY

[COMPETENCY 1][COMPETENCY 2][COMPETENCY 3][COMPETENCY 4][COMPETENCY 5] [COMPETENCY 6][COMPETENCY 7][COMPETENCY 8]

Wednesday 14 September 2011

Competency 2

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Competency 2: Apply the Requirements of Fundamental Principles as Regards Safety

2.1State for whose protection the requirements of fundamentals principles as regard to safety apply

The regulations as regards to safety are designed to protect:

1. Persons

2. Property

3. Livestock in locations intended for them against hazards arising from an electrical installation used with reasonable care having regard to the purpose for which the installation is intended

The requirements relate to protection against:

Electric shock

Fire

Burns

Injury from mechanical movement of electrically actuated equipment

2.2 Requirements for protection against:

(i) Electric shock

Ø Insulation of live parts

Ø A proper earthing system.

Ø All electrical appliances & equipment connected to earth.

Ø Use of 30mA sensitivity Residual Current Device (RCD 30mA).

(ii) Fire

RCDs are very effective devices to provide protection against fire risk due to insulation fault. This type of fault current is actually too low to be detected by the other protection (over-current, reverse time).
For systems in which leakage current can appear, the use of 300mA sensitivity RCDs provides a good protection against fire risk due to this type of fault.

Overheating can also be due to the modification of the initial method of installation (addition of cables on the same support).

BURNS

· Common Causes of electrical burns

Ø From biting on electrical cords or sticking fingers or objects in electrical outlets, etc.

Ø Heating due to resistance can cause extensive and deep burns.

Protection against injury

Protective guards are provided to protect

against injury from mechanical movement of electrically actuated

equipment.

2.3 Precautions to be observed against electric shock.

Avoid touching bare electrical conductors before being sure that it is “DEAD” and not “LIVE”.

Avoid inserting metal objects in power/ socket outlets

Make a visual check for any damage to insulation of portable machines or electrical appliances before use.

Before performing any electrical work, make sure that there is no electric supply to the part where the work is to be done.

2.4 Precautions to be observed against thermal effects.

Ø Avoid getting in contact with electrical parts

Ø Use appropriate insulated tools

Ø Use safety equipment when working on lines

2.5 Devices used to protect an installation against:

Over-current

Definition

Over-current (Overload) is a condition in an electrical circuit when the current (amperage) in the circuit exceeds the rated amperage capacity of that circuit or of the connected equipment on that circuit.

Over-current may be caused by a short circuit, loose connection, ground fault or surge power drawn when a motor starts up.

Over-current (Overload) protection devices are used to protect conductors from excessive current flow. These protective devices are designed to keep the flow of current in a circuit at a safe level to prevent the circuit conductors from overheating.

Over-current (Overload) protection is a term given to circuit breakers or fuses and sometimes GFCI outlets.

Definition:
The Circuit Breaker is found in an electrical service panel and is an electrical device used to protect the electrical wiring from an overloaded (over-current) condition when exposed to more electrical current than it is designed to handle.

Similar in function to an electrical fuse which will blow when overloaded, the circuit breaker will turn off or "trip" when in an over-current condition. However, unlike the fuse which is rendered useless when it blows, the circuit breaker is not damaged when it trips, and can be reset.

§ Fault Current (Leakage current)

Level I

Ø EARTHING

Level II

Ø RCD (Residual Current Device) or ELCB (Earth Leakage Circuit Breaker)(30mA)

Ø GFCI (Ground Fault Circuit Interrupter--5 mA)

Protection against Overvoltage

Electronic and electrical devices are designed to operate at a certain maximum supply voltage, and considerable damage can be caused by voltage that is higher than that for which the devices are rated.

For example, an electric bulb has a wire in it that at a given rated voltage will carry just large enough for the wire to get very hot ( giving off light and heat), but not enough for it to melt.

Ø Surge protector

Ø Voltage stabiliser

2.6 Reasons for inspection and testing after completion or addition or alteration of an electrical installation.

Ø To check whether all the wiring and connections are done as per standards.

Ø To check for any loose connections or bad insulation that can cause fire.

Ø Before delivering a house, the electrician must ensure that there is no problem in the circuit and it is safe to use.

2.7 Requirements for isolation and switching.

(1) Isolation

Deprive a particular circuit from electricity supply, to allow work on or near live parts.

Ø Done by : Main Isolator Switch

: Circuit breaker

(2) Switching off for mechanical maintenance

Switching off equipment like motors, generators, air conditioning unit for maintenance.

Ø   Done by : Circuit breaker

                      : Main switch



                 (3) Emergency switching

Rapid switching off of supply in case of accidents, fire, or any emergency.

Ø Done by : Main Isolator Switch

                                           : Emergency stop



2.8 Sketch of how a person gets an electric shock.

2.9 Precautions to be taken when removing a person in contact with a live conductor/faulty equipment.

Ø Try to turn off the source of the current as fast as possible.

Ø If not, the person should be pulled or pushed away by his clothing or any insulating medium (e.g. a stick, a plastic broom)

Ø Never touch the skin of the person who is in contact with an electric supply- you will get electric shock too.

Competency 4

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Measure value of Carbon/Ceramic resistances, A.C and D.C voltages with analogue and/or digital multimeter.

4.1 Definition of current and its unit.

Electric Current à Rate of flow of electrons.

Unit: Ampere (A)

Instrument to measure current: Ammeter

Symbol: I

4.2 State the difference between the terms e.m.f and potential difference.

12 V Battery

E.m.f (Electromotive Force) → EMF can be considered as electrical pressure that a battery exerts. It exists whether there is a circuit or not. Measured in Volts (V)

12v

Voltage or Potential difference → The voltage between two points is a short name for the electrical driving force that could determine an electric current between those points. Measured in Volts (V)

4.3 State Ohm’s law.

Ohm's law states that the current through a conductor between two points is directly proportional to the potential difference or voltage across the two points, and inversely proportional to the resistance between them.

Small resistance > Large current

Large resistance > Small current

Current (I) depends on two parameters:

1. Voltage (V)

2. Resistance (R)

Current is directly proportional to voltage.

Ohm's Law

Resistors in series

Resistors in Parallel

4.5 Difference between A.C and D.C

In alternating current (AC, also ac) the movement of electric charge periodically reverses direction. In direct current (DC), the flow of electric charge is only in one direction.

4.9 Purpose of a multimeter

Multimeter is an instrument used to measure current, voltage and electric resistance

e.g.; in circuits

Ø Current measured in Ampere(A)

Ø Voltage measured in Volt(V)

Ø Resistance measured in Ohm(W)

Use: To measure electrical quantities in an electrical machine, electrical appliance and a circuit.

4.10 Types of multimeter.

Multimeter

A multimeter or a multitester, also known as a volt/ohm meter or VOM, is an electronic measuring instrument that combines several measurement functions in one unit. A typical multimeter may include features such as the ability to measure voltage, current and resistance.

Multimeters may use analog or digital circuits—analog multimeters and digital multimeters (often abbreviated DMM or DVOM.)

Analog instruments are usually based on a microammeter whose pointer moves over a scale calibration for all the different measurements that can be made.

Digital instruments usually display digits, but may display a bar of a length proportional to the quantity measured.

i. Analog multimeter

Value indicated by a pointer.

Value to be read on a scale.

ii. Digital multimeter

Value given on a display (digital).
No need to read on a scale.
Value given with precision.

4.11 Parts of a digital multimeter

A multimeter has :

1. a display

2. terminals

3. probes

4. leads

5. a dial(selector knob or function switch) to select various measurement ranges

A digital multimeter has a numeric (digital) display, while an analog multimeter has a dial display

4.12 State precautions to be observed.

Precautions

Ø It is important that voltage and current levels do not exceed the specified maximum level. Overloading can destroy the meter, and is potentially fatal.

Ø Always check the insulation around the probes and wires for any breaks, before using the multimeter.

Ø Never measure voltage on a current based setting because the low resistance will act as a short, if the fuse does not blow first.

Set the function switch on the correct mode.