专业英语13章 电气工程 英语翻译_专业英语和电气工程
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12.3 Grounding of Electrical Systems
In general, most electrical systems must be grounded.The purpose
Fig.12.4 Secondary high-voltage radial distribution system
of grounding is to limit the magnitude of voltage caused by lighting, momentary surges, and accidental contact with higher voltages.System grounds must be seaweed to provide a path of minimum impedance in order to ensure the operation of over-current devices when a ground fault occurs.Current should not flow through the grounding conductor during normal operation.Direct-current systems generally have the grounding conductor connected to the system at the supply station, and not at the individual service.Alternating-current systems, on the ether hand, must be grounded on die supply side of the main disconnect al each individual service.For specific information an the location and methyl of funding, refer to NEC Article 250.(a)Secondary high-voltage distribution system;high-voltage radical.low-voltage loop
(b)Consumer distribution system with high-voltage and low-voltage loops Fig.12.5
12.4 Grounding of Electrical Equipment
Metal conduit and cases which enclose electrical conductors must be grounded.If the ungrounded(hot)conductor comes in contact with a metal enclosure which is not grounded, a voltage will be present between the enclosure and the ground.This presents a potential hazard.Persona comic in contact with the enclosure and ground will complete a circuit.All non-current-carrying metal parts of electrical installations should be tightly bonded together and connected to a grounding electrode.Good electrical continuity should be ensured through all metal enclosures.The current caused by accidental grounds will be conducted through the enclosures, the grounding conductor, and the grounding electrode to the earth.If the current is false enough, it mill cause the over-current device to open.12.5 Ground Fault Protection
A ground-fault protector(GFP)is a device which senses ground faults and opens the circuit when the currant to ground reaches a predetermined value.A ground-fault circuit interrupter(GFCI)is a device which opens the circuit when very small currents flow to ground.There is no way to determine in advance the impedance of an accidental ground.Most circuits are protected by 15 A(ampere)or larger over-current devices.If the impedance of a ground fault is low enough, such devices will open the circuit.What about currents of le than 15 A? It has been proven that currents as small as 50 mA through the heart, lungs, or brain can be fatal.Electrical equipment exposed to moisture or vibration may develop high-impedance grounds.Arcing between a conductor and the frame of equipment may cause a fire, yet the current may be le than 1 ampere.Leakage current caused by dirt and /or moisture may take place between the conductor and the frame.Portable tools are frequently not properly grounded.and the only path to ground is through the body of the operator.The ground-fault circuit interrupter was developed to provide protection against ground-fault currents of le
than 15 A.the GFCI is designed to operate on two-wire circuits in which one of the two wires is grounded.The
standard circuit voltages are 120 V and 277 V.The time it takes operate depends upon the value of the ground-fault current.Small currents of 20 mA or le may flow for up to 5 s before the circuit is opened.A current of 20 mA will cause the GECI to operate in le than 0.04 s.This time/current element provides a sufficient margin of safety without nuisance tripping.The GFCI operates on the principle that an equal amount of current is flowing though the two wires.When a ground fault occurs, same of the currant flowing through the ungrounded(hot)wire does not flow through the grounded wire;it completes the circuit though the accidental ground.The GFCI senses the difference in the value of current between the two wires and opens the circuit.GFCIs may be incorporated into circuit breakers installed in the line, or incorporated into a receptacle outlet or equipment.Ground-fault protectors are generally designed for use with commercial and/or industrial installations.They provide protection against ground-fault currents from 2 A(special types go as low as 50 mA)up to 2 000 A.GFPs are generally installed on the main, submain, and/or feeder conductors.GFCls are installed in the branch circuits.GFPs are generally used for three-wire, single-phase and for three-phase installations, while GECls are used for two-wire, single-phase circuits.A ground-fault protector installed on supply conductors must enclose all the circuit conductors, including the neutral, if present.When operating under normal conditions, all the current to end from the load flows through the circuit conductors.The algebraic sum of the flux produced by these currents is zero.When a phase-to-ground fault occurs, the fault currents returns through the grounding conductor.Under this condition an alternating flux is produced within the sensing device.When the flux current reaches a predetermined value, the magnetic flux causes a relay to actuate a circuit breaker.Sometimes the GFP is installed on the grounding conductor of the system.Under this condition, the unit senses the amount of phase-to-ground current flowing in the grounding conductor.When the current exceeds the setting of the GFP, it will cause the circuit breaker to open.The ground-fault protector is actually a specially designed current transformer connected to a solid-state relay.12.Three-Phase Systems
The various three-phase systems in normal use will lie described.Under ideal conditions, these systems operate in perfect balance, and if a neutral conductor is present it carries zero current.In actual practice, perfectly balanced systems are seldom encountered.The electrical worker, therefore, must be to calculate values of current and voltage in unbalanced systems.Single-phase loads are frequently supplied from three-phase system.The single-phase load requirements vary considerably, making it virtually impoible to maintain a perfect balance.In a balanced three-phase system, the currents in the three lines are equal.The currents in the three phases are also equal.In other words, ILX=ILY=ILZ and Ip = Ip = Ip.if, however, ILX≠ILF≠ILZ, then IPX≠IPY≠IPZ and the system is unbalanced(see Fig.12.6).To calculate the line currents in an unbalanced three-phase system, the method in the following example may be used.Example 1
Three pure resistance, single-phase loads are connected in a delta configuration acro a three-phase supply, as illustrated in Fig.12.6.Load X requires 30 A, load Y requires 50 A, and load Z requires 80 A.Calculate the current through each line wire.Example 1 applies to loads of 100 percent power factor connected in delta.With loads of different power factors, the phase angle will vary from 120°.For a wye connection, the line current is equal to the phase current.Some connections may be a combination of singe-phase and three-phase loads.Under these conditions, the phase angle between three-phase load and the single-phase load must be considered.12.7 Harmonic Effect of Fluorescent Lighting Fixtures
Most distribution systems in tile United States and Canada operate on a frequency of 60 Hz.certain types of electrical equipment produce secondary frequencies are multiples of the supply frequency.These secondary frequencies are called harmonics.For example, the second harmonic of 60 Hz is 120 Hz, the third harmonic is 180 Hz, and so on
The alienating flux developed by transformers, used in the ballasts of fluorescent lighting fixtures, produces a voltage which has a frequency of 180 hertz.This results in an additional current flowing in the supply conductors.The value of the current in the phase conductors is usual about 25 percent of the supply current.This third harmonic current adds to the supply current, causing a greater heating effect in the conductors.This increased heating effect is rather small, poibly in the vicinity of 380% greater than if the third harmonic current did not exist.CAUTION: When installing supply, feeder, and branch circuit conductors for heavy fluorescent loads, the size of the neutral conductor should be at least equal to that of the phase conductors.
