51 | B a s i c E l e c t r i c a l I n s t a l l a t i o n ... 100 6.53 ... 150 8.70 ... 200 4.35 - Spot Light 100 6.53 ... 150 13.05 Seasonal Appliances Wattage Cost (RM/Months) - Energy saving lamp (equivalent to 40 watts lamp) 9 0.39 (equivalent to 60 watts lamp) 11 0.48 (equivalent to 75 watts lamp) 15 0.65 (equivalent to 100 watts lamp) 20 0.87 Air Conditioner (average usage 4 hrs/day - 75% on time) Window type - 9000 Btu/hr 1200 31.32 - 12000 Btu/hr 1500 39.15 - 16000 Btu/hr 2400 62.64 Split Unit - (Single split) - 9000 Btu/hr 1000 26.10 - 12000 Btu/hr 1300 33.93 - 18000 Btu/hr 1950 50.90 Multi Split - 2 x 9000 Btu/hr 1950 50.90 - 9000 + 12000 Btu/hr 2100 54.81 - 2 x 12000 Btu/hr 2500 65.25 Fan (average usage 6 hrs/day) Table type - 12" 35 1.83 - 14" 55 2.87 - 16" 70 3.65 Ceiling fan 72 3.76 Stand type 75 3.92 Box type 100 5.22
52 | B a s i c E l e c t r i c a l I n s t a l l a t i o n Chapter 6: Electrical Wiring Protection
53 | B a s i c E l e c t r i c a l I n s t a l l a t i o n UNDERSTAND OVER CURRENT PROTECTION This provides protection from dangers caused by electrical currents, such as over current, earth leakage current, short circuit, lightning, etc. to the wiring system, electrical equipment or consumer. The most fundamental requirement in any electrical system is proper over current protection of conductors and equipment. According to the NEC Article 100, over current is the condition where the current in amperes is greater than the rated current of the equipment or conductors, resulting from an overload, short circuit, or ground fault. The two dangers to be prevented are fire and shock to people. In turn these dangers can arise from three kinds of fault, namely a short circuit, an overload and a fault to earth. An over current protection device protects the circuit by opening the device when the current reaches a value that will cause an excessive or dangerous temperature rise in conductors. Most over current protection devices respond to both, short-circuit or ground-fault current values as well as overload conditions. CURRENT PROTECTION In general, protection from the dangers of current can be divided into two aspects, namely: i. Overcurrent Protection (Over Load or Short Circuit) ii. Earth Leakage Current Protection 1. Overcurrent Protection (Over Load or Short Circuit) An over current is a current greater than the rated current of a circuit. It may occur in two ways. i. As an overload current. ii. As a short circuit or fault current.
54 | B a s i c E l e c t r i c a l I n s t a l l a t i o n Overloads Overloads are over currents, occurring in healthy circuits. They may be caused, for example, by faulty appliances or by connecting too many appliances to a circuit. Short circuits A short circuit is the current that will flow when a ‘bridge’ occurs between live conductors (phase-to-neutral for single-phase and phase to-phase for threephase). Prospective short-circuit current is the same, but the term is usually used to signify the value of short circuit at fuse or circuit-breaker positions. Prospective short-circuit current is of great importance. However, before discussing it or any other over current further, it might be wise to address the subject of fuses, circuit breakers and their characteristics. These conditions need to be protected against in order to avoid damage to circuit conductors and equipment. In practice, fuses and circuit breakers will fulfil both of these needs. Properly rated circuit breakers or fuses suitable for over load or short circuit protection must be used. The circuit breakers or fuses must be installed on the live conductors only. For three phase circuits, all the circuit breakers or fuses must be combined in one set of circuits. Selection of overcurrent devices must be based on the short circuit fault current levels of the circuit breaker or main switch (kA). Figure 6.1: Example of overload
55 | B a s i c E l e c t r i c a l I n s t a l l a t i o n 2. Earth Leakage Current Protection Properly rated Residual Current Devices (RCD) must be used for protection from earth leakage currents (to prevent electric shocks). No commentary on protective devices would be complete without reference to residual current devices. The IEE Regulations specify that, provided the required disconnection times can be met for circuits and all bonding is in place, fuses and/or CBs will provide the necessary protection against electric shock. RCDs are only required; when disconnection times cannot be met when they are needed as supplementary protection against direct contact for all socket-outlet circuits in TT systems (they are also preferred for other circuits, but over current devices are acceptable) for socket-outlet circuits from which it may be reasonably expected that portable appliances may be supplied for use outside the main equipotential zone For socket-outlet circuits and supplementary protection against direct contact, the RCD rating must be 30 mA or less. For other applications, the rating will depend on the earth fault loop impedance. Figure 6.2: Residual Current Devices (RCD)
56 | B a s i c E l e c t r i c a l I n s t a l l a t i o n FUSES AND CIRCUIT BREAKERS Fuses and circuit breakers are two different ways of protecting against suddenly large overloads of electrical flow. Large power overloads are dangerous, potentially destroying electrical equipment or causing a fire. Both fuses and circuit breakers will automatically block against an incoming surge of electrical power past a certain safety limit. But while they both accomplish the same task, each uses different technology in the way that it stops the flow of electricity. Fuses are typically small objects that plug into a fuse box or other central location. They are an early technology, dating back to the 19th century. Inside the fuse is a small piece of metal, across which the electricity must pass. During normal flow of electricity, the fuse permits the power to pass unobstructed. But during an unsafe overload, the small piece of metal melts, can stopping the flow of electricity. When a fuse is tripped, it should be thrown away and replaced with a new fuse. As there are many varieties of fuses available that handle different capacities of electricity, care should be taken when choosing replacement fuses. Circuit breakers are a more recent invention and improve on fuse technology. Circuit breakers are switches that are tripped when the electrical flow passes a safe limit. The excess of electricity typically triggers an electromagnet, which trips the circuit breaker when an unsafe limit is reached. Once tripped, the switches simply turn off. That stops the flow of electricity, which will remain off until the switch is reset. To reset the flow of electricity after the problem is resolved, the switch can simply be turned back on. Circuit breakers are often located in a cabinet of individual switches, typically inside of an apartment or other central place. While often used in homes, circuit breakers can be used for much larger industrial applications as well. As we know, a fuse is the weak link in a circuit, which will break when too much current flows, thus protecting the circuit conductors from damage. It must be remembered that the priority of a fuse or MCB is to protect the circuit conductors, not the appliance or the user. Calculation of cable size, therefore, automatically involves the correct selection of protective devices. There are many different types and sizes of fuse, all designed to perform a certain function.
57 | B a s i c E l e c t r i c a l I n s t a l l a t i o n 1. Fuses A fuse is simply a device which carries a metal element, usually tinned copper, which will melt and break the circuit when excessive current flows. The three types of fuse are: i. The rewirable or semi-enclosed fuse ii. The cartridge fuse and fuse link iii. The high-breaking-capacity (HBC) fuse. Figure 6.3: Rewirable fuse The Rewirable fuse A rewirable fuse consists of a fuse, holder, a fuse element and a fuse carrier (the holder and carrier being made of porcelain or Bakelite). The circuits for which this type of fuse is designed have a colour code, which is marked on the fuse holder and is as follows: 45 A – green 30 A – red 20 A – yellow 15 A – blue 5 A – white
58 | B a s i c E l e c t r i c a l I n s t a l l a t i o n Figure 6.4: Rewireble fuse holder Although this type of fuse is very popular in domestic installations, as it is cheap and easy to repair, it has serious disadvantages. The fact that it is repairable enables the wrong size of fuse wire (element) to be used. The elements become weak after long usage and may break under normal conditions. Normal starting-current surges (e.g. when motors are switched on) are ‘seen’ by the fuse as an overload and may, therefore, break the circuit. The fuse holder and carrier can become damaged as a result of arcing in the event of a heavy overload or short circuit. Cartridge fuse Figure 6.5: Glass Cartridge Fuse A cartridge fuse consists of a porcelain tube with metal and caps to which the element is attached. The tube is filled with silica.
59 | B a s i c E l e c t r i c a l I n s t a l l a t i o n These fuses are found generally in modem plug tops used with 13 A socket outlets, in some distribution boards and at mains intake positions. They have the advantage over the rewireble fuse of not deteriorating, of accuracy in breaking at rated values and of not arcing when interrupting faults. They are, however, expensive to replace. High-breaking-capacity (HBC) fuses The HBC fuse is a sophisticated variation of the cartridge fuse and is normally found protecting motor circuits and industrial installations. It consists of a porcelain body filled with silica with a silver element and lug type end caps. Another feature is the indicating element, which shows when the fuse has blown. It is very fast-acting and can discriminate between a starting surge and an overload. Figure 6.6: Glass Cartridge Fuse
60 | B a s i c E l e c t r i c a l I n s t a l l a t i o n 2. Circuit Breakers (CBs) These protective devices have two elements, one thermal and one electro-magnetic. The first, a bi-metal strip, operates for overloads and the second, a sensitive solenoid, detects short circuits. These devices have the advantage over the fuse in that they may be reset after they have operated (provided the fault current has caused no damage).The design includes the following components: 1. Actuator lever - used to manually trip and reset the circuit breaker. 2. Actuator mechanism - forces the contacts together or apart. 3. Contacts - Allow current when touching and break the current when moved apart. 4. Terminals 5. Bimetallic strip. 6. Calibration screw - allows the manufacturer to precisely adjust the trip current of the device after assembly. 7. Solenoid 8. Arc divider/extinguisher Figure 6.7: Circuit Breaker
61 | B a s i c E l e c t r i c a l I n s t a l l a t i o n CLASS OF PROTECTION It will be evident that each of the protective devices just discussed provides a different level of protection, i.e. rewirable fuses are slower to operate and less accurate than CBs. In order to classify these devices, it is important to have some means of knowing their circuit breaking and fusing performance. This is achieved for fuses by the use of a fusing factor. Fusing factor = Fusing current Current rating Where the fusing current is the minimum current causing the fuse to operate and the current rating is the maximum current which the fuse is designed to carry without operating. For example, A 5 A fuse which operates only when 9 A flows will have a fusing factor of Fusing factor = Fusing current @ 9/5 = 1.8. Current rating Rewirable fuses have a fusing factor of about 1.8. Cartridge fuses have a fusing factor of between 1.25 and 1.75. HBC fuses have a fusing factor of up to 1.25 (maximum). Circuit breakers are designed to operate at no more than 1.45 times their rating. Breaking capacity of fuses and circuit breakers When a short circuit occurs, the current may, for a fraction of a second, reach hundreds or even thousands of amperes. The protective device must be able to break or make such a current without damage to its surroundings by arcing, overheating or the scattering of hot particles. Position of protective devices When there is a reduction in the current-carrying capacity of a conductor, a protective device is required. There are, however, some Protection 71 exceptions to this requirement. These are listed clearly in the IEE Regulations. As an example, protection is not needed at a ceiling rose, where the cable size changes from the
62 | B a s i c E l e c t r i c a l I n s t a l l a t i o n 1.0 mm2 to, say, and the 0.5 mm2 for the lamp holder flex. This is permitted as it is not expected that lamps will cause overloads. ADVANTAGES AND DISADVANTAGES FOR THE FUSES AND CIRCUIT BREAKER Fuses and circuit breakers have unique advantages and disadvantages. One advantage of fuses is that they are cheap and can be purchased from any hardware store, but they have the drawback of needing to be replaced once they stop an overload. That can be challenging in a darkened room. Alternatively, circuit breakers can simply be reset with a flip of a switch after an overload. However, the technology can be more expensive than a fuse box. Electricians are best qualified to determine whether fuses or circuit breakers are better for a particular electrical installation. Circuit breakers and fuses are important safety devices in any electrical grid. Though they perform the same function, there are advantages and disadvantages to each. Fuses A fuse contains a length of filament which melts when exposed to an abnormally high current, breaking the circuit. Fuses are one-use only, however, and must be replaced. Circuit Breakers A circuit breaker is a mechanical switch which "trips" when current faults are detected, and can be reset by cycling the switch off and on.
63 | B a s i c E l e c t r i c a l I n s t a l l a t i o n Cost As a circuit breaker is a complex mechanical device, outfitting an electrical panel with breakers is more expensive than using fuses. Failure When a fuse fails, it can simply be unscrewed and replaced with another of the same type, as long as you have spares on hand. If a circuit breaker fails, you'll have to remove the breaker and wire in a new one. Human Error When replacing a fuse, you need to take care to ensure the replacement is rated for the same amount of current. Using a fuse rated for too much current is a fire risk, and using one rated for too little will cause the fuse to blow prematurely.
64 | B a s i c E l e c t r i c a l I n s t a l l a t i o n UNDERSTAND EARTHING Improper grounding hazards When an electrical system is not grounded properly, a hazard exists. The most common OSHA electrical violation is improper grounding of equipment and circuitry. The metal parts of an electrical wiring system that we touch (switch plates, ceiling light fixtures, conduit, etc.) should be grounded and at 0 volts. If the system is not grounded properly, these parts may become energized. Metal parts of motors, appliances, or electronics that are plugged into improperly grounded circuits may be energized. When a circuit is not grounded properly, a hazard exists because unwanted voltage cannot be safely eliminated. If there is no safe path to ground for fault currents, exposed metal parts in damaged appliances can become energized. Extension cords may not provide a continuous path to ground because of a broken ground wire or plug. Earth Electrodes In normal earthing, the earth and the neutral are quite separate. The load current flowing through the neutral must cause a potential difference between the two ends of the neutral. Since the end at the supply transformer is earthed, the end at the consumer’s service terminal must inevitably be at some potential above earth. It cannot, therefore, be used as an earth point. Nevertheless, an effective earth has to be found for the earth continuity conductors of the permanent installation in a building. In urban areas the sheath of the Electricity Board’s service cable is normally used for this purpose, but there is no obligation on the Board to provide an earth, and in rural areas where the supply may be by overhead cable, it may not be possible for them to do so. In such cases, the consumers must provide their own earth electrodes and the design of these become part of the design of the building installation. An earth electrode is a metal rod, which makes effective contact with the general mass of earth. A common type consists of a small diameter copper rod which can be easily driven to a depth of 6 m or more into ground reasonably free of stones or rock. The soil remains practically undisturbed and in very closes contact with the electrode surface. Since resistivity is lower in the deeper strata of earth and not very affected by seasonal conditions deep driving gives a good earth.
65 | B a s i c E l e c t r i c a l I n s t a l l a t i o n Rods of this type are practically in corrodible. Also it is easy to get access to the connection at the top of the electrode. A typical arrangement is illustrated in Figure 6.8. Where the ground is shallow but has low resistivity near the surface, a plate electrode, either of copper or of cast iron, can be used. When the soil resistivity is high, a cast iron plate can be used with a coke surround. This method is illustrated in Figure 6.9. Figure 6.8: Copper rod electrode Figure 6.9: Cast iron plate electrode Standard cast iron plates are made for use as earth electrodes. They are complete with terminals for the earth continuity conductor. These terminals consist of two copper sockets each secured by a drift-pin, the two being joined by
66 | B a s i c E l e c t r i c a l I n s t a l l a t i o n a tinned copper strand to which the earth conductor is bound and soldered. The completed connection is sealed and covered in bitumen before the electrode is buried. It is, therefore, not as accessible as the connection of the rod type electrode. Long copper strip can also be used as an earth electrode, and the method of doing this is shown in Figure 6.10. It will be seen from this that the strip is a useful type of electrode for shallow soil overlying rock. Strip may be arranged in single lengths, parallel lengths or in radial groups. Figure 6.10: Copper strip electrode Standard strip is commercially available for use as earth electrodes. When current flows from the electrode into the soil, it has to overcome the resistance of the soil immediately adjacent to the electrode. The path of the current is shown in Figure 6.11. The effect is equivalent to a resistance between the electrode and the general mass of earth, and this resistance is the resistance of the electrode. Figure 6.11: Current from electrode into earth
67 | B a s i c E l e c t r i c a l I n s t a l l a t i o n Furthermore, the surface of the ground near the electrode becomes live when current flows from the electrode to earth and Figure 6.12 shows a typical surface distribution near a rod electrode. It can be seen that an animal standing near such an electrode could have a substantial voltage applied between its fore and hind legs, and in fact fatal accidents to livestock from this cause have been known. Figure 6.12: Voltage at surface of ground due to rod electrode The earth electrode should, therefore, be positioned well out of harm’s way. It should perhaps be noted that the deeper the electrode is below the ground, the smaller will be the voltage gradient at the surface. The effectiveness of earth protection depends on the low resistance of the electrode when current flows through the electrode into the soil. This resistance cannot be accurately predicted in advance and must be checked by testing. After installation, the electrode should be periodically examined and tested to ensure that its initial low resistance is being maintained. The scheme for testing an electrode is shown in Figure 6.13. The electrode under test is indicated by X; two auxiliary electrodes, Y and Z are driven in for the test. Y must be placed sufficiently far from X for the resistance areas not to overlap, and Z is placed approximately half way between X and Y. The test electrode X is disconnected from its normal continuity conductor and connected to
68 | B a s i c E l e c t r i c a l I n s t a l l a t i o n the test instrument as shown in Figure 6.13. A low voltage alternating current is passed between X and Y. Figure 6.13: Earth electrode resistance test The current is measured and so is the potential between X and Z. The resistance of the earth electrode is given by the dividend of voltage and current. Check readings are taken with the electrode Z nearer to and further from electrode X, and the results are accepted only if all three readings are substantially the same. If they are not, the test must be repeated with a greater distance between X and Y.
69 | B a s i c E l e c t r i c a l I n s t a l l a t i o n PROTECTIVE MULTIPLE EARTHING This is an alternative method of earthing in which the neutral of the incoming supply also forms the earth return path. In other words, instead of the neutral and earth of the incoming supply being separate, they are combined. The name protective multiple earthing is usually abbreviated to P.M.E. The installation within the building is carried out in exactly the same way as for any other system, and separate earth continuity conductors are used. The main earth point at the intake is not, however, connected to a separate earth but to the neutral of the incoming service cable. The Electricity Board affording the supply earths the neutral conductor at a number of points on the distribution network and is responsible for seeing that maximum resistance to earth from any part of the neutral conductor does not exceed a prescribed value. Because with this system the neutral is relied on as the earth, there must be no fuses, cut outs, circuit breakers or switches anywhere in the neutral. In the United Kingdom an Area Electricity Board may not adopt P.M.E. without the permission of the Secretary of State for the Environment, and stringent requirements are made to ensure that the neutral conductor is adequate to carry earth fault currents, that it is truly kept at earth potential and that it is protected against breaks in continuity. The permission of British Telecom is also required. This is because the currents into and through the ground at the points of multiple earthing could cause interference to adjacent telephone and telegraph cables in the ground. After early hesitations, P.M.E. is becoming increasingly widespread in the United Kingdom. Experience has shown that it is in practice as safe as previously used methods, and it has important advantages. In rural areas it makes it unnecessary for consumers to have their own earth electrodes and therefore removes the risks of earth electrodes in the care of unqualified persons. In urban areas it makes the Electricity Board’s distribution network cheaper.
70 | B a s i c E l e c t r i c a l I n s t a l l a t i o n Reference 1. Porges, F., The design of electrical services for buildings.—3rd ed. 2. Advantages & Disadvantages of Circuit Breakers & Fuses | eHow.com http://www.ehow.com/facts_5914160_advantages-disadvantages-circuitbreakers-fuses.html 3. Darrell Locke, Guide to the Wiring Regulations, 17 th ed.,John Wiley & Sons, Ltd. 4. Ray McReynolds, Home Wiring, Step By Step Guide Book Co., 4 th ed. 5. W. E. Steward and T. A. Stubbs, Modern Wiring Practice Design and Installation, Revised edition 6. Electrical Safety – Safety and Health for Electrical Trades; Student Manual Niosh Revised Edition. 7. http://electricalinstallationblog.blogspot.com/2009/12/socket-outlet-switchinstallation.html 8. Laws Of Malaysia - Act 447 of Electricity Supply Act 1990; Suruhanjaya Tenaga 9. Edisi 2008, www.st.gov.my –Jabatan Keselamatan Elektrik, Suruhanjaya Tenaga 10. Ahmad Fadli, Electrical Wiring, First Edition 2014, Politeknik Kuching Sarawak 11. Yahya Emat, Teknologi Pemasangan Elektrik, Edisi Pertama 2008, IBS BUKU Sdn Bhd.
71 | B a s i c E l e c t r i c a l I n s t a l l a t i o n Appendix: Practical Work
ISBN 978-967-0797-69-4