异己观点，不能成为谩骂对方的理由。
坛子上随时可窥探到为人的高度和深度
冷静一下吧，封贴一晚，看亚冠去。。。
异己观点，不能成为谩骂对方的理由。
坛子上随时可窥探到为人的高度和深度
大鼻山 发表于 2012-5-15 20:03 http://www.jzdq.net.cn/club/images/common/back.gif
混账透顶的言论！
和人身攻击何干？
注意：“您可以回头看看，有没有发生过人家好好讨论问题时，我恶声恶气发言的？
实在是看不下去。我尺短是不会顾及自己形象的。谁怕谁？”
下面这些话，又是谁的留言？
“有些菜鸟也太扯了，一根筋，当论坛都当科学圣殿了；
别哪天看到网上言论不合口味，一时想不开从金茂大厦跳下去，广大版友可承担不了这连带责任”
“你这种人不配给别人道歉；
我已在会议室建议撤销你这种人的贵宾身份；
若换在5年前，我会把这种人骂进黄浦江淹死或上东方明珠羞愧摔死”
唉工程设计行业中 电气是最悲剧的
没有哪一个专业有电气的规范编的这样乱，编制规范的人水平这样差
人，是需要逐步逐步提升自身修为的，包括你我他所有人。没在提升，就是一种退步。
如果历史可以重来，肯定项羽当皇上而不是刘邦
尺短本就是一个既没有高度也没有深度的人。
但非常令人奇怪，偏偏有些人，自认为高人。
非诚勿扰20120513 2号男嘉宾高光威
http://www.jzdq.net.cn/club/viewthread.php?tid=23658&extra=
你明天或后天再看你今天的冲动，估计你会后悔。
可惜啊可惜……
5. Earthing and Earth Leakage
Earthing of electrical systems is required for a number of reasons, principally to ensure the safety of people near the system and to prevent damage to the system itself in the event of a fault. The function of the protective conductor, or earth, is to provide a low resistance path for fault current so that the circuit protective devices operate rapidly to disconnect the supply. The resistance of the earth path must be low enough so that the potential rise on the earth terminal and any metalwork connected to it is not hazardous; a value of 50 volts is often quoted, but some locations and circumstances require a lower limit.
With the massive increase in the use of electronic equipment in commercial and industrial premises, especially personal computers and their related communications equipment, the situation has changed. Safety under fault conditions is now just one of the functions of the protective conductor; it must also carry the leakage currents from electronic equipment and must provide a noise-free ground reference required by the equipment. There are now three aspects to consider in the design of the earthing system:
• Safety
• Leakage currents
• Noise
5.1. Earthing for safety
The primary purposes of the earthing system of a building are to ensure that a safe environment is maintained for the occupants and to protect equipment from damage in the event of a fault. This is achieved by:
• providing a safe low resistance path to earth for fault currents so that protection equipment can operate swiftly
• establishing an equipotential platform on which equipment can be safely operated
• bonding metalwork to earth
BS 7671:1992, ‘Requirements for Electrical Installations’ gives detailed regulations covering earthing within buildings, and BS 7430:1991, ‘Code of Practice for Earthing’ gives detailed guidance on the design and specification of the earthing system. CDA Publication 119, ‘Earthing Practice’ gives an overview of the subject as a whole.
5.1.1. Typical earthing systems
Earthing systems are referred to by a letter code indicating their characteristics.
The first letter indicates
T one or more points of the supply are directly earthed (e.g. the neutral at the supply transformer)
I either the supply is not earthed at all, or it is earthed through a deliberately inserted impedance to limit the fault current. This system is not permitted for public supplies in the UK.
The second letter indicates
T all exposed metalwork and conductive metalwork is connected directly to earth
N all exposed metalwork is connected directly to an earthed supply conductor provided by the electricity supply company.
The third and fourth letters indicate
S neutral and earth conductor systems are quite separate
C neutral and earth conductors are combined.
Combinations of types are common; in the TN-C-S system the earth and neutral conductors are combined in the supply but are separated at the point of common connection and remain separate throughout the installation. On the supply side, there may be multiple connections between the combined neutral/earth and the mass of the earth - referred to as protective multiple earth (PME) - or a single connection - referred to as protective neutral bond (PNB). The majority of systems in the UK are TN-C-S systems with PME.
5.1.2. Ground connections
In earlier practice, it was considered sufficient merely to provide a suitably low impedance connection to the mass of the earth. This was often achieved by providing a single earth rod or a buried horizontal conductor of sufficient length to give the required resistance value. This is no longer adequate, and consideration must now be given to the potential field that arises around the earth connection when a large fault current flows into it. A brief outline of the principles follows;
a fuller discussion can be found in CDA publication 119, ‘Earthing Practice’.
If a single rod is used, electrons will flow in all directions into the mass of the earth (assuming that the resistivity of the earth surrounding the electrode is homogeneous). As the current spreads out, the area through which the current flows increases as the square of the distance from the rod, and the voltage gradient reduces similarly.
A person standing near the earth rod is standing on the voltage gradient, and will be at the voltage appropriate to his position - the closer to the rod, the higher the voltage. If our person stands as far as possible from the rod while he can just reach the earth rod (or any metal work connected to it), then the voltage difference between the two will be a maximum. This is called the touch voltage. If he were to stand with one foot next to the rod, and the other foot a pace away, radially, then there would be a voltage difference between his two feet, and this is called the step voltage. There will be a voltage between his feet wherever he is standing, but it will be highest closest to the rod.
The transfer voltage is the voltage difference between the earth rod (and any metalwork
connected to it) and an insulated cable connected to a remote earth. It applies equally to the complimentary condition of an insulated cable from the earth rod taken to the vicinity of a remote earth.
Modern earthing practice aims to reduce these voltages by careful design of the earthing system.
The transfer voltage simply depends on the size of the fault current and the resistance of the earth connection - it is in fact the traditional consideration of earlier practice - and the lower the earth resistance, the better. The factors that affect resistance are the physical dimensions and form of the earth electrode and the soil condition. For example, the resistance of the earth contact will reduce as the length of the rod (and to a lesser extent as the radius of the rod) increase, as the side length of a square plate increases, or as the length of a horizontal conductor increases. Several formulae are available to calculate the effective resistance from the resistivity of the soil and examples can be found in BS 7430 and in CDA Publication 119. Obviously, there are limits to the real benefits that can be gained merely by increasing size, and careful calculations need to be
made.
The touch voltage depends not only on the earth resistance but also on the physical positioning of the earth point and the equipment. By installing a buried perimeter conductor around the installation, connected to the earthing system, an equipotential area is established covering the whole of the area within the conductor loop. Now any person who can reach out and touch the enclosure metalwork must be standing within the perimeter conductor and is therefore at the top of the voltage gradient where the touch voltage is low. The larger physical size of the earth electrode system has two additional advantages; the current density in the mass of the earth is reduced, making the voltage gradient much less steep, lowering the step voltage, and the earth resistance is also lower, reducing the maximum fault voltage.
National regulations regarding the use of structural and utility metalwork for earthing vary greatly from country to country. The use of gas and fuel pipes as earth electrodes is generally prohibited, while some codes (but not the UK) allow the use of water pipes with permission of the owner of the pipe. The use of structural steelwork is generally permitted, but there may be
大家都需要为自己的一言一行负责。
别自以为是，就好。
下面这些话，又是谁的留言？
尺短寸长 发表于 2012-5-15 20:28 http://www.jzdq.net.cn/club/images/common/back.gif
你为什么不引用我最近几年的留言呢？
2 Standards and Legal Framework
2.1 Philosophy Underlying the Standards
As a general rule, the standards provide the design limits to be met and (together with
supporting codes of practice) explain how the earthing system can be designed to meet these.
They generally include formulae to enable the necessary calculations to be carried out or
detailed guidance on practical aspects - for example, how to connect items of equipment or where to position the electrodes. In this chapter the potentials on which the design limits are based will be described, based on supply industry practice. Readers should note that there are differences in the design limits appertaining to the supply industry and consumer electrical installations. For example, the shock voltage limits are lower within electrical installations than in supply industry substations. It is important to refer to the appropriate standard to check the design limits which apply to each situation.
Previously, it was established practice to design the earthing system to achieve a certain
impedance value and the main electrodes were usually positioned near the equipment where fault current was expected to pass (for example transformers). This has changed during the last ten years, as the approach in the standards has moved towards that of north American practice. The most significant change is that now the earthing system must be designed to ensure that the potentials in its vicinity during a fault are below the appropriate limits. When an earth fault occurs and current flows to ground via the earth electrode, the potential on the electrode and any equipment connected to it, will rise above true earth potential. The potential reached under severe fault conditions, can be several thousand Volts. As the earth fault current flows into the soil surrounding the electrode, the potential within the soil and on its surface will rise. Moving away from the electrode system towards a remote point, the potential will progressively reduce until eventually it becomes that of true earth. This situation is shown in Figure 2-1, where the potential rise on the surface of the soil surrounding a single vertical earth rod, has been illustrated in three dimensions. This attempts to explain the potentials involved, in a semi-structural way.
Reference to Figure 2-1 shows that the rate of reduction of soil surface potential, or the
potential gradient, is greatest near the rod and reduces as one moves away towards a remote point. Imagine that a person is walking away from the rod in a straight line towards a remote (reference) earth, i.e. down the potential “slope”, taking equally spaced steps. The potential difference between the feet would be higher near the rod (for example at position A1, where it would be the potential difference between points A1 and A2) and would fall rapidly with each successive step (for example it is lower at position B1, i.e. B1-B2) before leveling out some distance away. This effect is recognised in the standards and is the basis of the term “step potential”, which is the potential difference between two points on the surface of the soil which are one metre apart. The situation described for a single rod is similar to that for all electrode systems and the step potential is highest in the area immediately beyond the buried electrodes, in uniform soil conditions. Step potential is a directional quantity and calculations are required to find the highest value in a full 360 degree radius.
We have recognised that the potential on the surface of the soil differs according to the
position in relation to the electrode system. This has implications for the second type of
potential difference, the “touch” potential. Whilst fault current is flowing through the
impedance of the earthing system, all of the exposed metal connected to this will experience a rise of voltage. For small systems, this is assumed to be the same value on all metalwork and is referred to as the GPR (Grid Potential Rise). In the example shown in Figure 2-1, the GPR is approximately 420V. The potential at a point on the surface of the soil will be lower than this, by an amount dependent on the buried depth of the electrode and the horizontal distance away. If a person is in contact with exposed metalwork and is standing on the soil, then their hands will be at same potential as the GPR, whilst their feet will be at a lower potential. This potential difference will be lowest if the feet are directly above the buried rodand will increase as they move further away. For example, Figure 2-1 shows that the touchvoltage is significantly higher at position B1 than at position A1. The touch potential is normally the potential which dictates the design of the earth electrode system within an outdoor substation and it will be greatest in areas furthest away from buried electrodes where it is still possible to touch exposed metalwork. In chapter 7, examples of earth electrode arrangements are discussed and the new arrangements attempt to reduce touch voltages. It is also important to ensure that a potential difference cannot be experienced between hands
which are in simultaneous contact with different pieces of exposed metalwork and this is
catered for by inter-equipment bonding as discussed in chapter 4.
Figure 2-1 Touch, Step and Transfer potentials around an earth rod electrode
Finally, if an insulated cable which is connected to a remote (reference) earth, is brought near the rod, the potential difference between the cable and the rod is called the “transfer potential”. The same transfer potential would be present if an insulated cable were taken from the rod to a remote point, where metalwork connected to a remote (reference) earth electrode system was present. The highest value of transfer potential is thus the GPR and this is the value normally used for calculations. At present, transfer potential limits are set by communication directives. They are 430 V and 650 V in the UK, depending on the type of installation, above which additional precautions are required.
Whether a person experiencing any of these potentials is at risk depends on a range of
factors, including the GPR. The standards attempt to take these factors into account and
establish limits, below which the design is considered acceptable. The ultimate risk of these potentials is that they will be sufficient to cause an electric shock which causes ventricular fibrillation of the heart. In arriving at the present limits, it was necessary to predict the proportion of current which would flow in the region of the heart and then establish limits based on its magnitude and duration. In UK standards, curves C1 and C2 of IEC 479-1, 1989
(International Electrotechnical Committee, Effects of Current Passing Through the Human Body) are used. These curves illustrate, for two probability levels, the current required for different time durations to cause ventricular fibrillation in a human.
你为什么不引用我最近几年的留言呢？
大鼻山 发表于 2012-5-15 20:55 http://www.jzdq.net.cn/club/images/common/back.gif
我的留言，是在怎样的语境下和针对谁的故意曲解、恶毒评论呢？
自己是清楚的吧？
老诚老诚，尺短第二次求您了——
神啊——哦不，“老诚老师”，您就宽恕我们吧……阿门！
