Basic Electrical Theory 2! Cy Dodimead!

Transcription

1 Basic Electrical Theory 2 Cy Dodimead Earthing: The direct physical connection to earth The thing to remember is that the earth (ground we stand on) is an even better conductor of electricity than the wires used for transporting it around the house. Given the chance of a better conductor and a shorter route, electricity will take it. This means that if you place yourself between the two, it will travel through you to get there. The whole of the world may be considered as a vast conductor which is at reference (Zero) potential. (potential here being the difference between two voltages) People are usually more or less in contact with earth, so if other parts which are open to touch become charged at a different voltage from earth a shock hazard exists. When we have a voltage of potential difference of 230V, we have raised the voltage to 230V above zero. When do we earth? At all times (except in very exceptional circumstances). Why do we earth? In order to provide the shortest path to earth for the current in the event of a fault. Also the BS th edition states a circuit protective conductor shall be run to and terminated at each point in the wiring. Earthing arrangements: Internal standard IEC distinguishes three families of earthing arrangements, using the two-letter codes. TN, TT and IT. The driest letter indicates the connection between earth and the power supply equipment (generator to transformer) T = direct connecting of a point with earth (Latin: Terra) I = no point is connecting with earth (isolation), exempt perhaps via a high impedance. the second letter indicates the connecting between earth and the electrical device being supplies: T = direct connecting of a point with earth N = direct connection to neutral at the origin of installation, which is connected to the earth.

2 TN-S / TN-C-S / TT T stands for Terra which is earth in latin. N stands for direct connection to neutral at the origin or installation, which is connected to the earth 2nd T stands for direct connection of a point of earth C stands for combined in relation to the neutral and the earth S stands for separate in relation to the neutral and the earth For a 3 phase intake you will usually have a TN-C-S system. On a start connection this is known as a 3 phase 4 wire. Reasons for bonding/earthing: CPC = Circuit protective conductor (Earth) 1. Sufficient current should flow under and earth fault so that the protective device (fuse or a circuit breaker) operates within a pre-defined time. 2. While the fault exists metal work could rise to a high potential 3. Bonding occurs to enable exposed and extraneous metal work to be linked together. This gives us an equipotential zone. PAT test: Portable Appliance Test An item that has passed the PAT test means the item is electrically safe to use. Class 1 appliance: These appliances must have their chassis connected to an electrical earth by an earth conductor (CPC). A fault in the appliance which causes a live conductor to contact the casing will cause a cut tent to flow in the earth conductor. This cut tent should trip an over current device i.e fuse. Class 2 appliance: A class 2 or double insulated electrical appliance is one which has been designed in such a sway that it does not requite a softy connection to electrical earth. The basic requirement is that no single failure can result in dangerous voltage becoming exposed so that it might cause electric shock and the is achieved without trying on an earthed metal casing. This is usually achieved at least in part by having who layers of insulating material surrounding live parts of by using reinforced insulation. In Europe, a double insulated appliance must be labelled Class 2, double insulated or bear the double insulation symbol a square inside another square) Class 3 appliance: A Class 3 appliance is designed to be supplies dorm a SELV (Separated Extra-Low Voltage power source. The voltage from a SELV supply is low enough that under normal conditions a person can safely come into contact with it with ought risk of electrical shock. The extra safety features built into Class 1 and Class 2 appliances are therefor not required.

3 Earthing and Audio: Mains cables carry relatively high alternating currents that can be induced into audio cables running alongside them, even if the audio cable is well screened. The longer this distance the cables run alongside each other, the greater the amount of interface (usually hum) that will be induced int the signal cable. When audio and mains cables must cross, try to arrange the crossing to be at right angles as this is the angle of minimum coupling. Ground Loop: in an electrical system, a ground loop usually refers to a current, generally unwanted, in a conductor connecting two points that are supposed to be at the same potential, often ground (Earth), but are actually at different potential. A group loop in the power or video signal occurs when some components in the same system are receiving its power from a different ground than other components, or the ground potential between two pieces of equipment is not identical. They can also create an electric shock hazard, since ostensibly Grounded parts of the equipment, which are often accessible to users, are not at ground potential. Balancing 3 phase loads: A balanced three phase load will have the same current on al phases and therefor cancel each other out. If there is an unbalanced load then the remaining current will have to pass down the neutral conductor. Example: L1 = 100A L2 = 50A L3 = 130A kva is Apparent Power kva = Kilo Volt amps kva x 1000 / 400 / 1.73 = Amps 3 phase kva x 1000 / 230 = Amps 1 phase or google kva calculator. Basic protection: The neutral conductor is taking the excess current 70A BS 7671 says Protection against electric shock under fault-free conditions This is the enclosure of electrical parts to stop the exposure of live parts.

4 Fault protection: BS 7671 says Protection against electric shock under single-fault conditions Fuses: - BS 3036 Rewireable fuses - BS 1361/1362 cartridge fuse - BS 88 HBC fuse (High Breaking Capacity) MCB s: Miniature Circuit Breaker The sizes come in: 6A, 10A, 16A, 20A, 32A, 40A, 50A, 63A Types are B, C and D Type B trips between 3 and 5 time full load current. Type C trips between 5 and 10 times full load current. Type D trips between 10 and 20 times full load current. RCD s: All MCB s types use a magnetic fault protection, which trips the MCB. RCD s operate by measuring the current balance between two conductors using a differential current transformer. This measures the difference between the current flowing out of the live conductor and that returning through the neutral conductor. If these do not sum to Zero, there is a leakage of current to somewhere else. (to earth, or to another circuit), and the device will open its concats. Cables: H07RN-F 1. Fine stranded cu-conductor 2. Rubber insulation 3. Filler 4. Rubber outer sheath H05RN-F 1. Fine stranded cu-conductor 2. Rubber insulation 3. Rubber outer sheath IP - Ingress Protection: The IP Code or Ingress Protection Rating consist of lettert IP followed by two digits and an optional letter. As defined in inter nation standard IEC 60529, it classifies the degrees of protection provided against the intrusion of solid objects (including body parts like hands and fingers, disk, accidental contact and water in electrical enclosures.

5 The standard aims to provide users more details information than vague marketing turns such as waterproof. Levels: 0 = No protection 1 = >50mm any large surface of the body 2 = >12.5mm fingers similar objects 3 = 2.5mm tools thick wires 4 = >1mm most wires, screws etc. 5 = dust 6 = dust tight. Water protection: 0 = not protected 1 = dripping water 2 = dripping water rille up to 15 3 = spraying water 4 = splashing water 5 = water jets 6 = powerful water jets 7 = immersion up to 1m 8 = immersion beyond 1m