📚 Mains Electricity Explained | 市电详解
Mains electricity is the alternating current (ac) supply delivered to homes, schools and businesses through the National Grid. In IGCSE and O Level Physics you need to understand its main features, the safety mechanisms built into plugs and appliances, and the standard calculations for power, energy and cost. This article covers the difference between ac and dc, UK mains specifications, three-core cable and plug wiring, fuses, earth wires, circuit breakers, double insulation, the power and energy equations, and how electricity bills are worked out.
市电是电网输送到家庭、学校和企业的交流电。在 IGCSE 和 O Level 物理中，你需要理解它的主要特性、插头与电器中的安全机制，以及功率、能量和电费的标准计算。本文将介绍交流与直流的区别、英国市电规格、三芯电缆与插头接线、保险丝、地线、断路器、双重绝缘、功率与能量公式，以及电费的计算方法。
1. What Is Mains Electricity? | 什么是市电
Mains electricity is electrical energy generated in power stations and distributed to consumers through the National Grid. It is an alternating current supply, which means the direction of the current reverses many times every second instead of flowing steadily one way.
市电是在发电站产生、并通过国家电网输送到用户的电能。它是交流电供应，也就是说电流方向每秒改变多次，而不是稳定地朝一个方向流动。
The supply reaches a building through two working conductors. The live wire carries the alternating voltage, and the neutral wire completes the circuit and stays close to zero volts. A third conductor, the earth wire, carries no current in normal use and exists purely for safety.
市电通过两根工作导体进入建筑物。火线携带交变电压，零线构成完整回路并保持在接近零伏。第三根导体是地线，正常使用时没有电流，只用于安全保护。
The potential difference of the mains is measured between the live wire and the neutral wire, and also between the live wire and earth. That is why touching the live wire is dangerous: a large potential difference is set up across your body and a dangerous current flows through it.
市电的电压（电势差）在火线与零线之间测量，也在火线与地线之间测量。这正是接触火线危险的原因：你的身体两端会产生很大的电势差，从而有危险电流通过。
- Live wire (brown): carries the alternating voltage. | 火线（棕色）：携带交变电压。
- Neutral wire (blue): completes the circuit, near 0 V. | 零线（蓝色）：构成回路，接近 0 V。
- Earth wire (yellow and green): safety only, no current in normal use. | 地线（黄绿相间）：仅作安全保护，正常时无电流。
2. AC vs DC Current | 交流电与直流电流
Direct current (dc) flows in one direction only and has a constant value, as supplied by a battery or a solar cell. If you plot current against time for dc, you get a straight horizontal line because the current never changes sign.
直流电（dc）只沿一个方向流动，大小恒定，例如电池或太阳能电池提供的电流。如果把电流对时间作图，直流电对应一条水平直线，因为电流的正负号从不改变。
Alternating current (ac) reverses direction periodically and its graph is a sine wave. The number of complete cycles per second is the frequency, measured in hertz (Hz). Two values describe an ac supply: the peak value, which is the maximum in either direction, and the root mean square (rms) value, which is the value quoted for the mains.
交流电（ac）会周期性地改变方向，其图像是正弦曲线。每秒完成的完整周期数称为频率，单位是赫兹（Hz）。描述交流电供应有两个值：峰值，即两个方向上的最大值；以及均方根值（rms），也就是市电标称的值。
Vpeak = √2 × Vrms ≈ 1.41 × Vrms
For a 230 V rms mains supply the peak voltage is about 325 V, so the voltage swings from +325 V to −325 V and back again 50 times every second.
对于 230 V（rms）的市电，峰值电压约为 325 V，所以电压在 +325 V 与 −325 V 之间往复变化，每秒完成 50 次循环。
Mains supply is alternating because transformers can only change the voltage of ac, not dc. This allows the grid to step the voltage up for efficient long-distance transmission and then step it down to a safe 230 V for homes.
市电采用交流，是因为变压器只能改变交流电的电压，不能改变直流电的电压。这使电网可以把电压升高以实现高效远距离输电，再降到安全的 230 V 供家庭使用。
3. UK Mains Specifications | 英国市电规格
In the UK, the mains supply has a potential difference of about 230 V and a frequency of 50 Hz. The 230 V figure is an rms value, not a peak value, and the current changes direction 100 times each second because each cycle contains two reversals.
在英国，市电的电压约为 230 V，频率为 50 Hz。230 V 是均方根值，不是峰值；由于每个周期包含两次方向改变，电流每秒改变方向 100 次。
The period of the supply is the time for one complete cycle and is found from the frequency:
市电的周期是完成一个完整循环所需的时间，可以由频率求出：
T = 1 ÷ f = 1 ÷ 50 = 0.02 s = 20 ms
Other countries use different standards; the USA, for example, uses about 120 V at 60 Hz. Always use the value given in the question rather than assuming 230 V, and remember that a.c. or d.c. may be stated as part of the answer mark.
其他国家的标准不同，例如美国使用约 120 V、60 Hz。答题时应使用题目给出的数值，不要一律假设为 230 V，并记住答案中可能需要标明是交流还是直流。
| Quantity | 物理量 | UK value | 英国数值 |
| Potential difference (rms) | 电压（均方根） | 230 V |
| Peak voltage | 峰值电压 | about 325 V | 约 325 V |
| Frequency | 频率 | 50 Hz |
| Period | 周期 | 0.02 s (20 ms) |
4. The Three-Core Cable | 三芯电缆
A mains cable contains three separate copper conductors, each wrapped in its own coloured plastic insulation, with a tough outer plastic sheath holding them together. Copper is used because it is a good conductor with low resistance, while plastic is used because it is a good insulator.
市电电缆包含三根独立的铜导体，每根都包在带颜色的塑料绝缘层中，外面再包一层坚韧的塑料外皮把它们固定在一起。使用铜是因为它导电性好、电阻小；使用塑料是因为它是良好的绝缘体。
The insulation prevents current from flowing between the wires or from the wires to anyone touching the cable. If the insulation becomes damaged or the cable is overloaded and overheats, the conductors may touch and cause a short circuit or a fire.
绝缘层防止电流在导线之间流动，也防止电流流向任何接触电缆的人。如果绝缘层损坏，或电缆过载发热，导体可能相互接触，造成短路或火灾。
The three wires in a modern UK cable are colour-coded: brown for live, blue for neutral, and yellow and green stripes for earth. Older cables may use red, black and bare copper, but the function of each wire is unchanged.
现代英国电缆中的三根导线有颜色标识：棕色为火线，蓝色为零线，黄绿相间为地线。旧电缆可能使用红色、黑色和裸铜线，但每根导线的功能不变。
5. Plug Wiring and the Fuse | 插头接线与保险丝
Inside a three-pin plug the wires must be connected to the correct pins. Looking at the back of the plug, the earth wire goes to the top pin, the live wire goes to the pin on the right, and the neutral wire goes to the pin on the left. Each wire is clamped under a screw terminal so that the metal touches the brass pin and no bare copper is left exposed.
在三脚插头内部，导线必须接到正确的插脚上。从插头背面看，地线接顶部插脚，火线接右侧插脚，零线接左侧插脚。每根导线都被螺钉端子压紧，使金属与黄铜插脚接触，不留裸露的铜线。
The cable grip, or cord clamp, holds the outer sheath firmly so that pulling the cable does not pull the wires out of their terminals. The outer sheath must extend into the plug, and the individual wires should be cut to the right length so that the earth wire is slightly longer than the others.
电缆固定夹（线卡）牢牢压住外皮，这样拉拽电缆时不会把导线从端子中拉出。外皮必须伸入插头内部，各根导线应剪到合适长度，地线应比另外两根稍长一些。
The fuse is always connected in the live wire, between the live pin and the live terminal. If the fuse blows, the appliance is disconnected from the high-voltage live side of the supply, so the appliance becomes safe to touch and to repair.
保险丝始终接在火线上，位于火线插脚与火线端子之间。如果保险丝熔断，电器就与供电的高压火线一侧断开，因此电器变得可以安全触摸和维修。
A switch must also be placed in the live wire for the same reason: switching off then isolates the appliance from the high voltage. Putting a fuse or a switch in the neutral wire is a common mistake and leaves the appliance live even when switched off.
出于同样的原因，开关也必须接在火线上：这样断开开关就能把电器与高压隔离。把保险丝或开关接在零线上是常见错误，会导致电器即使断开开关仍然带电。
6. Choosing the Correct Fuse Rating | 选择合适的保险丝额定值
A fuse is a thin metal wire that melts and breaks the circuit when the current through it exceeds its rating. Standard UK plug fuses are rated 3 A, 5 A and 13 A. The correct fuse must carry the normal working current but melt quickly if a fault causes a large current.
保险丝是一根细金属丝，当通过它的电流超过额定值时会熔断并切断电路。英国插头标准保险丝额定值为 3 A、5 A 和 13 A。正确的保险丝必须能承受正常工作电流，但在故障导致大电流时迅速熔断。
First calculate the normal current from the power of the appliance, using the mains voltage:
首先用市电电压和电器功率算出正常工作电流：
I = P ÷ V
le, a 1000 W microwave takes 1000 ÷ 230 ≈ 4.3 A, so a 5 A fuse is chosen, not a 13 A fuse.
然后选择大于该电流的最小标准额定值。例如，1000 W 的微波炉电流为 1000 ÷ 230 ≈ 4.3 A，因此应选 5 A 保险丝，而不是 13 A。
| Appliance | 电器 | Power | 功率 | Current | 电流 | Fuse | 保险丝 |
| Table lamp | 台灯 | 60 W | 0.26 A | 3 A |
| Television | 电视 | 150 W | 0.65 A | 3 A |
| Microwave |
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