SET Chapter 7.pdf

Learning Outcomes Explain the electrical hazards caused by overcurrent and earth fault Understand the need to protect people and properties against electrical hazards Explain the importance of earthing to avoid the risk of electric shock Describe the application and selection of dif...

Learning Outcomes Explain the electrical hazards caused by overcurrent and earth fault Understand the need to protect people and properties against electrical hazards Explain the importance of earthing to avoid the risk of electric shock Describe the application and selection of different electrical protective devices for residential use (fuses, circuit breakers, residual current circuit breakers) Explain how protective devices can protect against electrical hazards 1.7.1 Introduction All electrical installations (i.e., the conductors and apparatus) must be protected against overcurrent and earth fault (leakage). This is because if a current in excess of the rating of the circuit flows indefinitely, fire and damage to the circuit are likely to happen. In addition, if there is an earth fault or leakage of current to the metal casing of the device, anyone who touches the metal casing that has become live will be electrocuted. Protective devices are thus installed to disconnect the faulty section of the device from the supply before damage to the devices and electric shock occur. Unit 1.7 | Electrical Hazards and Protection 49  1.7.2 Earthing The general mass of earth is considered a large conductor at zero potential. In electronics and electrical engineering, it is by convention we define a point in a circuit as a reference point. The general mass of the earth is taken as a reference point here and the potential is defined as zero and it carries a voltage of 0 V. Having earth to be at 0 V facilitates current to flow to earth to prevent electric shock when there is a fault. The neutral at the supply transformer, shown in Fig. 1.7-1, is connected to the earth, via the earth electrode. This is done by connecting a conductor from the neutral at the supply origin to a rod driven into the ground. This is known as earthing. Fig. 1.7-1: Installation without earthing In Fig. 1.7-1, the exposed metallic part of the device at the consumer’s installation is not earthed. A phase-to-earth fault occurs when the live (phase) terminal touches the exposed metallic part of the appliance. This causes the metallic part to become live, and anyone who comes into contact with the live part will become part of the earth fault current path and will thus receive an electric shock. Unit 1.7 | Electrical Hazards and Protection 50 In Fig. 1.7-2, the exposed metallic part of the apparatus at the consumer’s installation is earthed. Fig. 1.7-2: Installation with earthing When a phase-to-earth fault occurs in the electrical equipment, most of the earth fault current will flow through the circuit protective conductor (CPC). Anyone who comes into contact with the live part will receive a negligible earth fault current, as the human body’s resistance is comparatively much higher than the resistance of the CPC. If the earth leakage current is high enough, it will trip the residual current circuit breaker (RCCB) inserted between the phase and neutral and thus disconnect the circuit from the power source. In order to avoid the risk of electric shock, it is important to: provide a path for the earth leakage current to operate the circuit protection effectively and rapidly; and maintain all metalwork at the same potential to ensure no potential difference can exists under fault conditions. 1.7.3 Overcurrent An overcurrent is any current that exceeds the current capacity of the circuit. There are two main types: An overload current is the result of adding too many electrical loads to a circuit, causing a current higher than what the circuit is designed to handle. A fault current is the result of two or more live conductors touching one another (short circuit), or of a phase conductor touching the protective conductor directly or indirectly (earth fault). Unit 1.7 | Electrical Hazards and Protection 51  1.7.4 Types of Protective Devices Protective devices may be classified as: (a) overcurrent protective devices; or (b) residual current protective devices. (a) Overcurrent Protective Devices Overcurrent protection in a circuit can be by means of: (i) a fuse (e.g., cartridge fuse, high-rupturing capacity fuse); or (ii) an excess current circuit breaker (e.g., miniature circuit breaker). Fig. 1.7-3 shows some examples of overcurrent protective devices. Cartridge fuse High-rupturing capacity fuse Miniature circuit breaker Fig. 1.7-3: Overcurrent protective devices Unit 1.7 | Electrical Hazards and Protection 52  (i) Fuse A fuse should be installed on the live (phase) wire so that current from the supply to the user is cut off, due to blowing or melting of the fuse, when there is a fault. See Fig. 1.7-4a. If the fuse is installed on the neutral wire as shown in Fig. 1.7-4b, the current from the supply to the user is still not cut off even though the fuse has blown due to a fault and the person will get an electric shock. Hence, the neutral must be constructed with a solid (conductor) link to ensure discrimination of the fuse (i.e., the fuse must cut off the current supply to protect the user when there is a fault). Fig. 1.7-4a: Correct installation of fuse Fig. 1.7-4b: Wrong installation of fuse on live (phase) wire to create an open as current can still reach the person to circuit so that current does not flow to give an electric shock even though the the person fuse has blown on neutral wire A fuse consists of a thin wire connected in series with the circuit to be protected. The wire is thick enough to carry a normal current without overheating. However, if the current exceeds its nominal value, the fuse wire will melt, breaking the circuit. A fuse consists of three main parts: a fuse element, which is designed to melt when the fuse operates; a fuse carrier, which is made of incombustible material, such as porcelain or moulded plastic, and has two contacts for connecting the fuse element or fuse link with screws or clips; and a fuse base, which is made of incombustible material and encloses the fixed contacts to which the incoming and outgoing cables are connected. The three main types of fuse are: (1) rewireable or semi-enclosed fuse (2) cartridge fuse (3) high-rupturing or high-breaking capacity fuse Unit 1.7 | Electrical Hazards and Protection 53  (1) Rewireable Fuse A rewireable fuse, sometimes called a semi-enclosed fuse, shown in Fig. 1.7-5, consists of a fuse link or fuse element and a fuse base. The fuse link has two sets of contacts, which can be fitted onto the base. The fuse element, which is usually made of tinned copper wire, is connected between the terminals of the fuse link. An asbestos tube or pad is fitted to reduce the effects of arcing when the fuse element melts. However, the fuse element can be too easily replaced with one of the wrong sizes and, compared to other fuses, the rewireable fuse takes the longest to cut off a faulty circuit. Rewireable fuses are thus not used in Singapore today. Fig. 1.7-5: Rewireable fuse (2) Cartridge Fuse Cartridge has the meaning of a container. A cartridge fuse, shown in Fig. 1.7-6, consists of a sealed tube with metal end caps. The fuse element passes through the tube from cap to cap, and is welded or soldered to the inside of the cap. A more expensive fuse may have an external indicator to allow people to check if the fuse has blown (i.e., fuse element open- circuited). When the fuse is blown, the whole cartridge must be replaced. It is commonly used in the 13A plug and in electronic equipment. Fig. 1.7-6: Cartridge fuse Unit 1.7 | Electrical Hazards and Protection 54  (3) High-Rupturing Capacity Fuse A high-rupturing capacity (HRC) fuse (Fig. 1.7-7), sometimes called a high-breaking capacity (HBC) fuse, is used extensively in industrial and commercial installations. It is designed to protect a circuit against heavy overload. It is also capable of opening a circuit under both overload and short-circuit conditions. This type of fuse can carry a large, short circuit current for some time. During this time, if the fault gets removed, it does not blow. Otherwise, it blows or melts. Fig. 1.7-7: HRC fuse Unit 1.7 | Electrical Hazards and Protection 55  (ii) Excess Current Circuit Breaker Like fuses, excess current circuit breakers need to be installed on the live (phase) wires of electrical systems like supply transformers or mains supply. An excess current circuit breaker is a device for making and breaking a circuit under normal and abnormal conditions respectively. It is an automatic switch that opens in the event of an excess current. The switch can be closed again when the current returns to normal, as the device is not damaged during normal operation. The two common types of excess current circuit breaker are: (1) miniature circuit breaker (2) moulded-case circuit breaker (1) Miniature Circuit Breaker A miniature circuit breaker (MCB), shown in Fig. 1.7-8, is used as an alternative to the fuse. It gives both overload and short-circuit protection. The MCB is fitted with a thermal device for overload protection and a magnetic device for speedy short-circuit protection. It is commonly used to protect lighting and power circuits in residential premises. Fig. 1.7-8: Miniature circuit breaker (2) Moulded-Case Circuit Breaker A moulded-case circuit breaker (MCCB), shown in Fig. 1.7-9, is commonly used as an alternative to the HRC fuse. The MCCB has high breaking capacity (i.e., able to break high current), which makes it popular for use in commercial and industrial installations. Fig. 1.7-9: Moulded-case circuit breakers Unit 1.7 | Electrical Hazards and Protection 56 (b) Residual Current Protective Devices Fig. 1.7-10: Residual current circuit breaker Look at the consumer unit in your home and you can find a residual current circuit breaker (RCCB), or residual current protective device, like the one shown in Fig. 1.7-10. It is specially designed to protect us if there is an earth leakage current (in the range of 5 to 30 mA) in an electrical appliance or any part of the house. An RCCB can cut off the supply within 40 milliseconds to prevent an electric shock. Unit 1.7 | Electrical Hazards and Protection 57