【正文】 第一篇：專業(yè)英語13章 電氣工程 英語翻譯 Grounding of Electrical SystemsIn general, most electrical systems must be purpose Secondary highvoltage radial distribution systemof grounding is to limit the magnitude of voltage caused by lighting, momentary surges, and accidental contact with higher grounds must be seaweed to provide a path of minimum impedance in order to ensure the operation of overcurrent devices when a ground fault should not flow through the grounding conductor during normal systems generally have the grounding conductor connected to the system at the supply station, and not at the individual systems, on the ether hand, must be grounded on die supply side of the main disconnect al each individual specific information an the location and methyl of funding, refer to NEC Article 250.(a)Secondary highvoltage distribution system。highvoltage loop(b)Consumer distribution system with highvoltage and lowvoltage loops Grounding of Electrical EquipmentMetal conduit and cases which enclose electrical conductors must be the ungrounded(hot)conductor es in contact with a metal enclosure which is not grounded, a voltage will be present between the enclosure and the presents a potential ic in contact with the enclosure and ground will plete a noncurrentcarrying metal parts of electrical installations should be tightly bonded together and connected to a grounding electrical continuity should be ensured through all metal current caused by accidental grounds will be conducted through the enclosures, the grounding conductor, and the grounding electrode to the the current is false enough, it mill cause the overcurrent device to Ground Fault ProtectionA groundfault protector(GFP)is a device which senses ground faults and opens the circuit when the currant to ground reaches a predetermined groundfault circuit interrupter(GFCI)is a device which opens the circuit when very small currents flow to is no way to determine in advance the impedance of an accidental circuits are protected by 15 A(ampere)or larger overcurrent the impedance of a ground fault is low enough, such devices will open the about currents of less than 15 A? It has been proven that currents as small as 50 mA through the heart, lungs, or brain can be equipment exposed to moisture or vibration may develop highimpedance between a conductor and the frame of equipment may cause a fire, yet the current may be less than 1 current caused by dirt and /or moisture may take place between the conductor and the tools are frequently not properly the only path to ground is through the body of the groundfault circuit interrupter was developed to provide protection against groundfault currents of lessthan 15 GFCI is designed to operate on twowire circuits in which one of the two wires is standard circuit voltages are 120 V and 277 time it takes operate depends upon the value of the groundfault currents of 20 mA or less may flow for up to 5 s before the circuit is current of 20 mA will cause the GECI to operate in less than time/current element provides a sufficient margin of safety without nuisance GFCI operates on the principle that an equal amount of current is flowing though the two a ground fault occurs, same of the currant flowing through the ungrounded(hot)wire does not flow through the grounded wire。it pletes the circuit though the accidental GFCI senses the difference in the value of current between the two wires and opens the may be incorporated into circuit breakers installed in the line, or incorporated into a receptacle outlet or protectors are generally designed for use with mercial and/or industrial provide protection against groundfault currents from 2 A(special types go as low as 50 mA)up to 2 000 are generally installed on the main, submain, and/or feeder are installed in the branch are generally used for threewire, singlephase and for threephase installations, while GECls are used for twowire, singlephase groundfault protector installed on supply conductors must enclose all the circuit conductors, including the neutral, if operating under normal conditions, all the current to end from the load flows through the circuit algebraic sum of the flux produced by these currents is a phasetoground fault occurs, the fault currents returns through the grounding this condition an alternating flux is produced within the sensing the flux current reaches a predetermined value, the magnetic flux causes a relay to actuate a circuit the GFP is installed on the grounding conductor of the this condition, the unit senses the amount of phasetoground current flowing in the grounding the current exceeds the setting of the GFP, it will cause the circuit breaker to groundfault protector is actually a specially designed current transformer connected to a solidstate SystemsThe various threephase systems in normal use will lie ideal conditions, these systems operate in perfect balance, and if a neutral conductor is present it carries zero actual practice, perfectly balanced systems are seldom electrical worker, therefore, must be to calculate values of current and voltage in unbalanced loads are frequently supplied from threephase singlephase load requirements vary considerably, making it virtually impossible to maintain a perfect a balanced threephase system, the currents in the three lines are currents in the three phases are also other words, ILX=ILY=ILZ and Ip = Ip = , how