Level 2 Electrical and Electronic 3

Conductors and insulators. A material that freely allows the passage of an electrical current is called a conductor, whereas a material that resists the flow of current is called an insulator.

Conductors and insulators Factors affecting the resistance of a conductor. Length of the conductor. Cross sectional area The material the conductor is made of The temperature of the conductor

Factors affecting the resistance of a conductor.

What is the main conductor used in motor vehicles? COPPER Reasons why copper is preferred, Very low resistance to electron flow. Malleability. Describes a material that means it can be easily shaped. Ductility. The property of a material that allows it to be drawn into wires.

The main insulator used is PVC because of its, High resistance to electron flow. Resistance to water, fuel, oil and other contaminants.

Conductors and insulators

Earth return List the reasons why the vehicles chassis is used as the earth return circuit. Cheaper, less wire is required Lighter, less wire means less weight

Insulated earth return List vehicles where the vehicle frame could not be used as the earth return. Vehicles with composite bodies Fuel and chemical tankers

Cable sizes. The quoted size of a cable refers to the number of strands and the wire diameter. Example. 9/0.3 = 9 strands of wire, each being 0.3mm diameter. As a guide, a 0.3mm strand of wire will carry approximately 0.5 amps of current.

Cable sizes. 14/0.3 8.75amps General applications such as, side lights, indicators 28/0.3 17.5amps Headlamps, horn, heated windows. 65/0.3 35amps 84/0.3 42amps 97/0.3 50amps 120/0.3 60amps Charging wire from alternator to battery depending on alternator output 37/0.9 170amps Starter cable/ battery earth

List 2 reasons why it is important to select the correctly rated cable for each application. To ensure enough power is supplied to the consumer with minimal losses Over rating of the cable will increase vehicle weight and cost.

Fuses a fuse contains a metal strip which is designed to overheat and melt when subjected to a specified excessive level of current flow, breaking the circuit and stopping the excessive current flow from potentially damaging the more valuable components and wiring.

circuit breakers. Circuit breakers are not destroyed by excess current. A bimetallic strip heats up and bends, opening a set of contacts and breaking the circuit. Circuit breakers can be either manual or automatic reset types depending on the type of circuit they are protecting. Electric windows, window heaters

Blade fuse colours Current rating. Colour code. 3 Violet 4 Pink 5 Clear/beige 7.5 Brown 10 Red 15 Blue 20 Yellow 25 Neutral 30 Green

Electrical terminals and connectors What is the meant by the term connectors? Connectors join wires together What is the meant by the term terminals? Terminals are the point at which the cables connect to a component.

Electrical terminals and connectors

Relays.

Relay

Relay operation

Moving contact (Armature) Simple Relay Components Moving contact (Armature) Fixed contact 2 4 Relay switches high currents. Construction Windings Soft iron core 1 3 Windings wrapped around iron core to form electromagnet. Return spring Contains fixed and moving contacts. Spring ‘returns’ moving contact to ‘home’ position. Next >

Simple Relay Operation (one normally open contact) Operation: 12V supply is available at switch. Switch closes, magnetic field attracts armature, contacts close. Current flows across contacts. 12V Switch opens, return spring pulls contacts apart. Next >

5 Pin Relay N.C. N.O. COM Contains 3 high current terminals: N.O., N.C. and COM, and 2 coil terminals. Operation: Current flows across N.C. and COM contacts when relay not energized. Switch closes, magnetic field attracts armature, N.O. and COM contacts close. Current flows across N.O. and COM contacts. Switch opens, contacts return to home position. Commonly found in automotive circuits. Next >

Briefly describe the operation of a relay When a small current energizes the electro-magnet, it attracts an armature blade and closes contact points. Current can then flow across the points to power the main circuit.

Electrical faults. The 3 main faults to occur in motor vehicle wiring systems are; Open circuit, where the circuit is broken and no current can flow. Short circuit is where a fault has caused a wire to touch another conductor and the current uses this easier path to complete the circuit. A high resistance in part of the circuit, such as a dirty connection, which will reduce the amount of current that can flow.