In order for the power to work, there must be a completely closed circuit, so that the current flows from the power source, passes through the load, and then returns to the power source. This continuous line (including power supply, load and wires) is called a circuit. Usually the circuit should have a control device, such as a switch. Figure 1 shows a closed circuit with power supply, load, connecting wires and switches. The box marked “load” can be an electric motor, an electric light or other electrical appliances. Please note that the current flows from the negative pole of the battery and returns to the positive pole of the battery after passing through the load.
In the circuit, if the load is bypassed by a wire, the load is initially short-circuited (see Figure 2). If the load is short-circuited, the current in the circuit may be very large. Because the resistance of the wire is not enough to limit the current, the current will rise sharply to a dangerous level. When a short circuit occurs, a melt or a circuit breaker can be used to break the circuit.

①Ground
Proper grounding is an important safety requirement for the design and construction of electrical systems. In the electrical system, the ground is the common reference point in the circuit, and the earth ground can usually be used as the ground. In the circuit, the ground is at zero volt potential relative to other points, and the ground provides a low-resistance path to the ground. The grounding conductor is usually connected to the metal frame (shell) of the electrical equipment, so when an ungrounded conductor touches the frame, the current will flow into the ground through this path (see Figure 3a).

②Electrical safety device
Safety devices such as melt and circuit breakers (see Figure 4) can cut off the current path in the event of a short-circuit fault or circuit overload. Ground fault circuit breaker (GFCI) (see Figure 4) (Note: Residual current circuit breaker or leakage protector) can protect people from being exposed when the metal shell or tool of the appliance touches an exposed live wire electric shock. When a small current (usually in the milliamp level, such as 0.003~0.005A) passes, the leakage protection switch can be turned off. The leakage protection switch compares the current on the neutral wire (ground return wire) with the current on the live wire (ungrounded), and if the current is not equal and exceeds the set value, the circuit will be disconnected instantaneously. Although some circuit breakers have built-in leakage protection functions, leakage protection switches cannot replace melts and circuit breakers. The function of the circuit breaker and the melt is to protect the circuit when the current exceeds the specified value due to a short circuit or overload fault of the equipment. When the current on the power line (live line) and the neutral line are equal, the residual current circuit breaker cannot detect the large current caused by the short circuit or overload of the equipment, and the melt and the circuit breaker are connected in series with the protected circuit elements in series. of.

There are two types of grounding wires commonly used in residential and other electrical systems. The neutral wire (usually covered with white insulation) is grounded, and the equipment grounding wire (usually bare wire or covered with green insulation) is also grounded. These two kinds of wires have different uses. The neutral wire is also called the return wire, and it usually has current flowing through it, while the equipment grounding wire only has current when the equipment is faulty and the housing is electrified. The equipment grounding wire is connected to the metal shell or frame of the equipment, and the frame is at ground potential at this time. If the circuit is short-circuited with the metal shell or frame, a current path will be formed. This can happen when a live conductor touches the metal part of the frame. It is usually connected to the third contact (usually in the middle position) of a 120V plug or socket. If the fault current exceeds the rating of the melt or circuit breaker, the melt or circuit breaker will open the circuit. In some cases, the current will “leak” to the ground of the equipment. Although its size is not enough to trigger the circuit breaker or the melt, it can cause personal injury. In this case, a properly installed leakage protection switch will disconnect the circuit.