Electrical Protection

Transcription

1 Electrical Protection Excessive current in any electrical circuit is hazardous and not desired, and these maybe caused by the following; 1. Overloads, and 2. Short-circuits. Overload Currents: These are caused by excessive loads connected in which may result in an excessive current more than few times its normal current. Causes of Overload Current: Note: 1. Large amount of electrical loads or equipment connected to a single circuit. 2. High rated electrical loads or equipment connected to a low rated capacity circuit. 3. Overloading motors due to mechanical stress. When an excessive current is increased considerably due to overload, cables will begin to heat up, If this overload current is sustained, the cable insulation deteriorates causing fire or electric fault in the system (such as short-circuits). Short-circuit currents: These are caused by direct contact between live conductors (or live and neutral) that may result in an excessive current more than hundred times or even thousand times its normal current. Causes of Short-circuit Current: 1. Direct contact between two or more current-carrying conductors having opposite polarities. 2. Contact between live wires due to poor insulation of cables or equipment. 3. Incorrect connections during initial installation or after modification current. 1

2 ELECTRICAL PROTECTION All electrical circuits and appliances must be protected against excessive currents (overloads and shortcircuit currents) that may lead to hazards such as: fire, thermal, mechanical and electrical damages, namely: 1. Fuse, and 2. Circuit Breaker. Fuse: Is a device that opens a circuit by means of a strip of wire, which is designed to melt when an excessive current flows through it. The current rating for the fuse should not exceed the current rating of the smallest conductor in the circuit it protects. Basic Parts of a Fuse: 1. Fuse element: Is a strip of copper, silver or tin-lead alloy, which melts when heated due to excessive flows of current. 2. Fuse links: Is part of a fuse which comprises a fuse element, a cartridge or other container, and capable of being attached to fuse contacts. 3. Fuse contact: Is part of a fuse where the fuse link is attached or fitted. Types of Fuse: 1. Rewireable (Semi-enclosed) Fuse. 2. Cartridge Fuse. 3. High-Breaking Capacity (H.B.C.) Fuse, or 4. High Rupturing Capacity (H.R.C.) Fuse. 2

3 REWIREABLE OR SEMI-ENCLOSED FUSE. BUILDING SERVICES ENGINEERING DEPARTMENT, MKJB This type of fuse consists of a tinned copper or a tin-lead alloy fuse element (rewireable) which is directly screwed to the contact terminals of a semi-enclosed porcelain fuse carrier to be plugged into the socket fuse base. FUSE LINK Fuse element (tinned doper or lean-tin alloy) Porcelain fuse carrier Contact screw Tinned copper contact Standard rating of rewireable (semi-enclosed) fuse: 5A, 15A, 20A, 30A, 45A, 60A, 100A. Advantages of rewireable fuse over the other: 1. Cheapest and most economical type of fuse. 2. Blown fuse element is easy to replace. Disadvantages of rewireable fuse over the others: 1. Wrong types of fuse element is dangerous to the house hold. 2. Slower to blow compare to other types of fuse. 3. Sparking or arcing is not protected. 3

4 CARTRIDGE FUSE: It is a totally enclosed type of fuse, whose fuse element is sealed by a tube which is filled with fireproof material. It has metal cap on both ends where the fuse element is welded or soldered. Cartridge fuse (for low rated appliances. Standard rating of cartridge fuse: 5A, 15A, 20A, 30A, 45A, 60A, 80A, 100A. Advantages of cartridge fuse over the others: 1. It is simple and quick to replace. 2. The fuse element is protected against sparking or arcing. 3. It operates faster than the rewireable fuse during fault conditions. Disadvantages of cartridge over the others: 1. It is more expensive than the rewireable fuse. 2. It is disposable. 4

5 HIGH BREAKING CAPACITY (H.B.C.) OR HIGH RAPTURING CAPACITY (H.R.C.) FUSE: It consists of a pure silver fuse element totally enclosed in a ceramic tube which is filled with fireproof materials such as silica and it is capable or interrupting a circuit under a heavy fault current at the shortest period of time. Standard rating of H.R.C. and H.B.C. fuse: 5A, 10A, 15A, 20A, 25A, 32A, 40A, 50A, 63A, 80A, 100A, 125A, 160A, 200A, 250A, 315A, 400A, 500A, 630A, 800A. Advantages of H.R.C. and H.B.C. fuse: 1. It operates or interrupts heavy current fault quickly. 2. It is safe to use and less arcing or sparking effect. 3. It is accurate and reliable. 4. Blown fuse element is visible through its indicator. Disadvantages of H.R.C. and H.B.C. fuse over the others: 1. It is more expensive than the others. 2. It is disposable. Fuse Terminology: Fuse current rating :- It is the maximum amount of current the fuse can carry without melting its fuse element. Eg. 5A, 10A, 15A, 20A,.100A. Fusing current :- It is the maximum amount of current, which is required to melt fuse element. Fusing Factor = Fusing current Fuse current rating 5

6 CIRCUIT BREAKER. It is a mechanical switching device designed for making and breaking a circuit, both under normal and abnormal conditions, such as short-circuits and overloads. It can be switch ON or OFF manually. It will switch OFF automatically during abnormal conditions such as short-circuit or overload. Two method of circuit breaker operation: 1. Magnetic tripping :- This is due to magnetic field sets up by a coil carrying heavy current which attracts the iron part and breaks the circuit. 2. Thermal tripping :- This relies on extra heat produces by high current that warms the bi-metal strips bond and breaks the circuit. Advantages of circuit breaker over the fuse: 1. When tripped it can be reset. 2. Faulty circuit is easily identified. 3. It has an accurate tripping current. 4. It will be tripped for a switched overload. Disadvantages of circuit breaker over the fuse: 1. It is expensive 2. It has mechanical moving parts. 3. It needs regular testing to ensure satisfactory operations. Note: Thermal operation of C.B. is rather slow and nor suitable for interrupting heavy short-circuit currents quality. However, it is widely used as overload protection. 6

7 Types of Circuit Breaker for domestic electrical installations: BUILDING SERVICES ENGINEERING DEPARTMENT, MKJB 1. Moulded case circuit breaker (M.C.C.B.):- It is designed to province circuit protection for medium-voltage distribution systems. It is also known as an air-break circuit breaker, designed with moulded housing for insulating material, capable of making, carrying and breaking current under normal and abnormal circuit conditions and used as main circuit protection. M.C.C.B. standard current rating: 15A, 30A, 60A, 100A, 150A, 200A, 250A, 300A, 400A, 500A, 600A, 800A, 1000A and 1800A. 2. Miniature Circuit-breaker (M.C.B.):- It is used as final sub-circuit protection in domestic and commercial installation which is similar in construction with M.C.C.B. except that it is designed for lower current rating and short-circuit capacity. M.C.B. standard current ratings: 6A, 10A, 20A, 32A, 40A, 50A. Voltage rating : Up to 440V. Short-circuit capacity: Up to 9KA. Back up protection or submain Main protection M.C.B. Final circuit M.C.C.B. M.C.B. Final circuit M.C.B. 7

8 PROTECTION AGAINST EARTH LEAKAGE CURRENTS. Earth Leakage current:- It is a current which flows to earth, or to extraneous conduction parts, in a circuit which is electrically sound. Wherever there is a live supply, there is always a risk that metalwork not intended to carry a current (e.g. metal casing of a cooker) may come into contact with a live conductor. If this happens, the current in the live conductor will leak in the metalwork that may result to: 1. The risk of shock to a person touching the metalwork. 2. The risk of fire due to a heavy currents flowing undetected. Methods of Earth-Leakage Protection: The methods recommended to prevent the danger of earth-leakage currents are as follows: 1. Enclosing live parts in insulation to provide basic protection against electric shock. This is called basic insulation. 2. The use of double insulation, now increasingly used for portable appliance such as electric shavers, hair dryers, etc. 3. Earthing exposed metalwork. 4. Isolation and separation of metalwork from live parts, such that there is no way that it can come into contact with live conductors. 8

9 Earthing of Exposed Metalwork: This purpose earthing is to prevent all metalwork associated with an installation from becoming live and reaching a potential that would cause the risk of electric shock to human being and to reduce the risk of fire. Let us consider what will happen under earth-fault condition as illustrated in figure below, if a non-earthed metalwork or casing not intended to carry a current become live when the phase conductor carrying a current touches it, the persons touching that metalwork may receive a shock while he is in contact with the general mass of the earth. Figure 1 shows a non-earthed metalwork. The person may received an electric shock. However, if the metalwork was effectively connected to earth by means of a suitable conductor as shown in figure 2, under earth-fault condition, the conductor would prevent the earth-leakage current from flowing into the person s body. The current would just flow directly into earth. The person may not receive an electric shock. An efficient earthing arrangement is an essential part of every electrical installation. The reason for having properly-earthed metalwork in an installation is to provide a low-resistance discharge path for earth leakage currents which would otherwise prove fatal to any person touching the metalwork associated with a fault circuit. 9