The Circuit Designer's Companion: The safety earth and wiring/cables
Peter Wilson
www.edn/design/components-and-packaging/4431867/The-Circui t-Designer-s-Companion--The-safety-earth-and-wiring-cables
电路设计师指导手册(4)：安全地以及电线/电缆
作者：Peter Wilson
[Part 1 begins a look at grounding: when to consider it, how chassis materials affect it, and the problem of ground loops. Part 2 discusses power supply returns and I/O signal grounding. Part 3covers inter-board interface signals, star grounding, and shielding.]
[第一部分开始讨论接地问题：何时考虑接地，机箱材料如何影响接地，以及接地环路问题。第二部分讨论电源回路和I/O信号接地。第三部分覆盖了板间接口信号、星形接地和屏蔽。]
1.1.12 The safety earth
A brief word is in order about the need to ensure a mains earth connection, since it is obvious from the preceding discussion that this requirement is frequently at odds with anti-interference grounding practice. Most countries now have electrical standards which require that equipment powered from dangerous voltages should have a means of protecting the user from the consequences of component failure. The main hazard is deemed to be inadvertent connection of the live mains voltage to parts of the equipment with which the user could come into contact directly, such as a metal case or a ground terminal.
1.1.12 安全地
确保市电地连接的安全需求非常迫切，因为从前面的讨论可以明显看出，这个需求与抗干扰接地措施经常发生冲突。大多数国家现在都有相关的电气标准，这些标准要求使用危险电压供电的设备在元器件发生故障时应该能够保护用户的安全。主要危害被认为是市电的火线电压被意外连接到用户可能直接接触的设备中的某些部分，如金属外壳或接地端子。
Imagine that the fault is such that it makes a short circuit between live and case, as shown in Figure 1.20. These are normally isolated and if no earth connection is made the equipment will continue to function normally
but the user will be threatened with a lethal shock hazard without knowing it. If the safety earth conductor is connected then the protective mains
fuse will blow when the fault occurs, preventing the hazard and alerting the user to the fault.
想象一下这样的故障，即在火线和外壳之间发生了短路，如图1.20所示。火线和外壳正常情况下是隔离的，如果没有地连接，设备将继续正常工作，但用户将在不知情的条件下受到致命电击的威胁。如果连接了安全的地导体，那么起保护作用的市电熔丝将在故障发生时熔断，从而防止危害发生，还能提醒用户故障情况。
FIGURE 1.20 The need for a safety earth
图1.20：需要一个安全地。
For this reason a safety earth conductor is mandatory for all equipment that is designed to use this type of protection, and does not rely on extra levels of insulation. The conductor must have an adequate cross-section to carry any prospective fault current, and all accessible conductive parts must be electrically bonded to it. The general requirements for earth continuity are:
基于这个理由，对所有设计使用这类保护措施的设备而言安全的地导体都是必须的，不能依赖于高度的绝缘等级。这种导体必须具有足够大的横截面以承载任何预期的故障电流，而且所有可接触到的导电部件都必须在电气上绑定到导体。对地连续性的一般性要求是：
∙The earth path should remain intact until the circuit protection has operated.
∙The impedance should not significantly or unnecessarily restrict the fault current.
∙地路径应该保持完好无损，直到保护电路启动工作。
∙阻抗不应显著或不必要地限制故障电流。
As an example, EN 60065 requires a resistance of less than 0.5 Ω at 10
A for a minute. Design for safety is covered in greater detail in Section
9.1.
举例来说，EN 60065要求在10A、1分钟的条件下保持导体电阻小于0.5Ω。安全性设计在第9.1节有更详细的讨论。
1.2 WIRING AND CABLES
This section will look briefly at the major types of wire and cable that can be found within typical electronic equipment. There are so many varieties that it comes as something of a surprise to find that most applications can be satisfied from a small part of the range. First, a couple of definitions: wires are single-circuit conductors, insulated or not; cables are groups of individual conductors, separately insulated and mechanically contained within an overall sheath.
1.2 电线和电缆
本节将简要介绍在典型电子设备中经常见到的电线和电缆的主要类型。由于种类太多，因此发现大多数
应用在局部范围内就能得到满足多少有点令人惊讶。首先，明确一些定义：电线是一些单独的电路导体，可以是绝缘的，也可以是非绝缘的。电缆是成组的单个导体，各导体单独绝缘，并且从结构上看包含在总的护套中。
1.2.1 Wire types
The simplest form of wire is tinned copper wire, available in various gauges depending on required current-carrying capacity. Component leads are almost invariably tinned copper, but the wire on its own is not used to a great extent in the electronics industry. Its main application was for links on printed circuit boards, but the increasing use of
double-sided and multilayer plated-through-hole boards makes them redundant.
1.2.1 电线类型
最简单的一种电线是镀锡铜线，根据要求的电流承载能力的不同有很多种规格。元件引脚几乎总是不变的镀锡铜材，但电线本身在电子行业中并没有广泛使用。其主要应用是印刷电路板上的连接，但越来越多双面板和多层电镀通孔板的使用使得它们经常显得多余。
Tinned copper wire can also be used in rewirable fuselinks. Insulated copper wire is used principally in
wound components such as inductors and transformers. The insulating coating is a polyurethane compound which has
self-fluxing properties when heated, which makes for ease of soldered connection, especially to thin wires.
镀锡铜线还可以用在可重新接线的熔丝连接中。绝缘式铜线主要用于绕线元件中，比如电感和变压器。其绝缘涂覆层是一种聚氨酯化合物，在加热时具有自熔特性，因此非常方便焊接，尤其是很薄的电线。
Table 1.2 compares dimensions, current capacity and other properties for various sizes of copper wire. In the UK the wires are specified under BS EN 13602 for tinned copper and BS EN 60182 (IEC 60182-1) for enamel insulated, and are sold in metric sizes. Two grades of insulation are available, Grade 1 being thinner; Grade 2 has roughly twice the breakdown voltage capability.
表1.2对各种规格铜线的尺寸、电流能力和其它属性进行了比较。在英国，镀锡铜线要符合BS EN 13602标准，搪瓷绝缘型铜线要符合BS EN 60182 (IEC 60182-1)标准。这些电线出售时都用公制标注，共有两种绝缘等级，等级1的绝缘层比较薄，等级2的击穿电压能力基本上是等级1的两倍。
Wire inductance
We mentioned earlier that any length of wire has inductance as well as resistance. The approximate formula for the inductance (L) of a straight length of round section wire at high frequencies is
电线电感
reactive metal我们以前曾提过，任何长度的电线都有电感和电阻。截面为圆形的一段直线在高频时的电感值近似计算公式是：
L = K × l× (2.3 log10 (4l/d) - 1) µH
where: l and d are length and diameter respectively, l>> d and K is 0.0051 for dimensions in inches or
0.002 for dimensions in cm.
其中：l和d分别是长度和直径，而且l>> d，尺寸单位为英寸时K等于0.0051，为厘米时K等于0.002。
This equation is used to derive the inductance of a 1 m length (note that this is not quite the same as inductance per meter) in Table 1.2 and you can see that inductance is only marginally affected by wire diameter. Low values of inductance are not easily obtained by adding cross-section and the reactive component of impedance dominates above a few kiloHertz whatever the size of the conductor.
这个公式可以用来算出表1.2中1米长度电线的电感(注意，这与每米电感值并不完全一样)，从表中可以看出，电感只受电线直径的少许影响。增加横截面获得较低的电感值并不容易，不管导体尺寸是多少，当频率大于几kHz时阻抗中的电抗成份就占主导地位。
A useful rule of thumb is that the inductance of a one inch length of ordinary equipment wire is around 20 nH and that of a one centimeter length is around 7 nH. This factor becomes important in high-speed digital and RF circuits where performance is limited by physical separation, and also in circuits where the rate-of-change of current (di/dt) is high.
一个有用的经验法则是，普通设备电线一英寸长度的电感约为20nH，或者说一
厘米长度的电感约为7nH。在性能受物理分割限制的高速数字和射频电路中以及电流变化率(di/dt)很高的电路中，这个因素变得非常重要。
Equipment wire
Equipment wire is classified mainly according to its insulation. This determines the voltage rating and the environmental properties of the wire, particularly its operating temperature range and its resistance to chemical and solvent attack. The standard type of wire, and the most widely available, is PVC insulated to BS 4808 which has a maximum temperature rating of 85°C. As well as current ratings at 25°C you will find specifications at 70°C; these allow for a 15°C temperature rise, to the maximum rated temperature, at the specified current. Temperature ratings of 70°C for large conductor switchgear applications and 105°C to American and Canadian UL and CSA standards are also available in PVC. PTFE is used for wider temperature ranges, up to 200°C, but is harder to work with.
设备电线
设备电线主要根据其绝缘性能进行分类。绝缘性能决定了电线的额定电压和环境属性，特别是其工作温度范围以及对化学物品和溶剂侵袭的抵抗能力。应用范围最广的标准类型电线是PVC，其绝缘性能符合BS 4808，最大额定温度为85℃。和25℃时的额定电流一样，你可以到70℃时的电流值。在特定电流
时允许高

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