Giải đề IELTS Premier – Test 1 – Reading passage 3 – Surge Protection
A. BÀI ĐỌC
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With more devices connecting to the world’s electrical networks, protecting electrical systems and devices from power surges-also known as distribution overcurrent-has become more important than ever. Without adequate overcurrent protection, interruptions to electrical service can have catastrophic effects on individuals, cities and entire nations.
In a normal electrical system, customers are supplied with a steady electrical current-a predetermined voltage necessary to safely operate all electrical equipment connected to that system. This steady electrical supply is subject to minimal variations-variations that are imperceptible to the consumer and do not normally harm electrical devices. An overload current is any surge that exceeds the variances of this normal operating current. The higher the overcurrent, the more potential it has to damage electrical devices. One of the most important principles of overcurrent protection, therefore, is that the higher the magnitude of the overload current, the faster the overcurrent must be disrupted.
How do overcurrents occur? Most overcurrents are temporary and harmless, caused when motors start up or transformers are energised. Such things as defective motors, overloaded equipment or too many loads on one circuit, however, can cause harmful, sustained overcurrents, which must be shut off quickly to avoid damaging the entire distribution system. An inadequately protected system can cause damage ranging from electrical shocks to people coming in contact with electrical equipment, to fires caused by the thermal ignition of electrical materials on the overloaded circuit.
Electrical storms and lightning are among the biggest causes of major distribution overcurrent worldwide. In the United States alone, 67 people are killed every year by these types of storms (including those killed by falling trees and power lines-not only surges). The intense current of a lightning discharge creates a fleeting, but very strong, magnetic feld. A single lightning strike can produce up to a billion volts of electricity. If lightning strikes a house, it can easily destroy all the electrical equipment inside and damage the distribution system to which that house is connected.
To protect people and devices adequately, overcurrent protection needs to be sensitive, selective, fast and reliable. IN the interest of conservation, most power systems generate different loads at different times of day; overcurrent protection must therefore be sensitive enough to operate under conditions of both minimum and maximum power generation. It also needs to be selective so that it can differentiate between conditions that require immediate action and those where limited action is required; in other words, it should shut down the minimum number of devices to avoid disrupting the rest of the electrical system. Overcurrent protection also needs to be fast; it should be able to disconnect undamaged equipment quickly from the area of overcurrent and thus prevent the spread of the fault. Of course, the most basic requirement of protective equipment is that it is reliable, performing correctly wherever and whenever it is needed.
When an overcurrent occurs at a major electricity supply point such as a power station, the resulting surge, if it is not checked, can damage the entire distribution system. Like a flooding river-which breaks its banks and floods smaller rivers, which in turn flood streets and houses-the extra voltage courses through the network of wires and devices that comprise the distribution system until it discharges its excessive energy into the earth. This is why each piece of equipment within the electricity manufacturing and distribution system must be protected by a grounding or earthing mechanism-the grounding mechanism allows the excess electricity to be discharged into the earth directly, instead of passing it further down the distribution system.
Within the distribution system, surge protection is provided by overcurrent relays. Relays are simply switches that open and close under the control of another electrical circuit; an overcurrent relay is a specific type of relay that operates only when the voltage on a power line exceeds a predetermined level. If the source of an overcurrent is nearby, the overcurrent relay shuts oﬀinstantaneously. One danger, however, is that when one electrical circuit shuts down, the electricity may be rerouted through adjacent circuits, causing them to become overloaded. At its most extreme, this can lead to the blackout of an entire electrical network. To protect against this, overcurrent relays have a time-delay response; when the source of an overcurrent is far away, the overcurrent relays delay slightly before shutting down-thereby allowing some of the current through to the next circuit so that no single circuit becomes overloaded. An additional benefit of this system is that when power surges do occur, engineers are able to use these time delay sequences to calculate the source of the fault.
Fuses and circuit breakers are the normal overcurrent protection devices found in private homes. Both devices operate similarly: they allow the passage of normal currents but quickly trip, or interrupt, when too much current flows through. Fuses and circuit breakers are normally located in the home’s electrical switch box, which takes the main power coming into the house and distributes it to various parts of the home. Beyond this level of home protection, it is also advisable to purchase additional tripping devices for sensitive electrical devices such as computers and televisions. While many electrical devices are equipped with internal surge protection, the value of these devices usually warrants the additional protection gained from purchasing an additional protective device.
The modern world could not exist without reliable electricity generation and distribution. While overcurrents cannot be entirely avoided, it is possible to mitigate their effects by providing adequate protection at every level of the electrical system, from the main power generation stations to the individual home devices we all rely upon in our daily lives.
B. BÀI TẬP
Choose the correct Jetter: A, B, C or D.
Write your answers in boxes 28-33 on your answer sheet
28. In a normal electrical system,
29. The writer suggests that most overcurrents
30. What does the writer state is the most basic requirement of overcurrent protection equipment?
31. What is an essential safety requirement for every device in an electrical system?
32. In which of the following circumstances might the shutdown of an overcurrent relay be delayed?
33. The writer suggests that most household electrical devices
Do the following statements agree with the information given in Reading Passage 3?
In boxes 34-40 on your answer sheet, write
TRUE if the statement agrees with the information
FALSE if te statement contrdicts the information
NOT GIVEN if there is no information on this
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