Industrial control systems typically include both low-voltage and high-voltage control components. To maintain control signal communication while isolating electrical connections—that is, implementing low-voltage and high-voltage isolation—is crucial for ensuring system stability and the safety of equipment and operators. One purpose of electrical isolation is to separate interference sources and susceptible components from the circuitry, thereby isolating field interference. I. Signal Isolation One purpose of signal isolation is to cut off introduced interference channels, ensuring that the control device maintains only signal communication with the field, without direct electrical contact. Common isolation methods between industrial control devices and field signals include opto-isolation, pulse transformer isolation, relay isolation, and wiring isolation. 1. Opto-isolation Opto-isolation is achieved using optocouplers. A light source is configured at the input, and a receiver at the output, thus ensuring complete electrical isolation between the input and output. Because the input impedance of an optocoupler (100Ω～1kΩ) is relatively small compared to the impedance of a typical interference source (10⁵～10⁶Ω), the interference voltage at the input terminal of the optocoupler is small, and the current it can provide is not large, making it difficult for the semiconductor diode to emit light. Furthermore, the optocoupler has a large isolation resistance (approximately 10¹²Ω) and a small isolation capacitance (approximately a few pF), thus preventing electromagnetic interference generated by circuit coupling, making it difficult for various interferences from the controlled equipment to feed back to the input system. The optocoupler isolates the input signal from the internal circuitry, or the internal output signal from the external circuitry, as shown in Figure 1. After the switch input circuit is connected to the optocoupler, the isolation effect of the optocoupler blocks various interference pulses mixed in with the input switch signal from one side of the input circuit. Because the optocoupler does not directly couple the electrical signals on the input and output sides, but uses light as a medium for coupling, it has high electrical isolation and anti-interference capabilities. Currently, most optocouplers have isolation voltages above 2.5kV, with some reaching 8kV. These range from high-voltage, high-current, high-power optocouplers to high-speed, high-frequency optocouplers (frequency up to 10MHz). Commonly used devices include the 4N25, with an isolation voltage of 5.3kV; and the 6N137, with an isolation voltage of 3kV and a frequency above 10MHz. 2. Pulse Transformer Isolation Pulse transformers have fewer turns, and the primary and secondary windings are wound on opposite sides of a ferrite core. This process results in extremely low distributed capacitance, only a few pF, making them suitable as isolation elements for pulse signals. When transmitting input and output pulse signals, pulse transformers do not transmit DC components. PLCs, which use digital signal input/output control devices, do not require the transmission of DC components, thus leading to their widespread application in industrial control systems. Figure 2 shows an application example of a pulse transformer. The external signal of the circuit is filtered by an RC filter circuit and a bidirectional Zener diode to suppress normal-mode noise interference before being input to the primary side of the pulse transformer. To prevent excessively high symmetrical signals from damaging circuit components, the output voltage of the pulse transformer's secondary side is limited by a Zener diode before entering the measurement and control system. Generally, the signal transmission frequency of a pulse transformer is between 1kHz and 1MHz, while newer high-frequency pulse transformers can reach 10MHz. 3. Relay Isolation The coil and contacts of a relay are not electrically connected. Therefore, the relay coil can be used to receive signals, and the contacts can be used to send and output control signals, thus avoiding direct contact between high-voltage and low-voltage signals and achieving anti-interference isolation. Figure 3 is a schematic diagram of an example of relay output isolation. In this circuit, the low-voltage DC and high-voltage AC are isolated by the relay, preventing interference from the high-voltage AC side from entering the low-voltage DC side. 4. Wiring Isolation Separating weak signal circuits from circuits prone to noise pollution requires that signal lines be run separately from high-voltage control lines and power supply lines, maintaining a certain distance between them. When wiring, AC lines, DC regulated power supply lines, digital signal lines, analog signal lines, and inductive load drive lines should be distinguished and separated. The larger the wiring spacing and the shorter the wiring, the smaller the noise impact. However, the actual internal and external space of the equipment is limited, and the wiring spacing cannot be too large; it is sufficient to maintain the minimum spacing distance. The attached table lists the minimum spacing that should be maintained between signal lines and power lines. If environmental conditions limit the distance between signal lines and high-voltage lines and power lines, various measures to suppress electromagnetic noise, such as connecting capacitors to signal lines, must be adopted. II. Power Supply System Isolation Using a 1:1 isolation transformer is a traditional anti-interference measure, which is very effective against power grid spike pulse interference. Figure 4 is a typical schematic diagram of an isolation transformer. Its anti-interference principle is that the primary side presents a high impedance to high-frequency interference, while the metal shielding layer located between the primary and secondary windings blocks the distributed capacitance generated by the primary and secondary sides. Therefore, the primary winding only has distributed capacitance to the shielding layer, and high-frequency interference is bypassed to ground through this distributed capacitance. The effectiveness of the 1:1 isolation transformer often depends on the shielding layer's manufacturing process. It is best to use 0.2mm thick pure copper plates, with one shielding layer on each of the primary and secondary sides. Typically, the primary side shielding layer is connected to the secondary side shielding layer via a capacitor, and then connected to the secondary side ground. Alternatively, the primary side shielding layer can be connected to the primary side ground wire, and the secondary side shielding layer to the secondary side ground wire. A larger cross-sectional area for the grounding lead is also preferable. A 1:1 isolation transformer also effectively isolates common-mode interference from grounding loops. 1. Isolation of AC Power Supply Systems Because AC power grids contain a large amount of harmonics, lightning surges, high-frequency interference, and other noise, measures should be taken to suppress interference from AC power sources for control devices and electronic equipment powered by AC power. Using a power isolation transformer can effectively suppress noise interference entering the AC power supply. However, ordinary transformers cannot completely suppress interference. This is because, although the primary and secondary windings are insulated, preventing noise voltage and current from the primary side from being directly transmitted to the secondary side, thus providing isolation, the presence of distributed capacitance (between windings and core, between windings, between layers of turns, and between leads) allows noise from the AC power grid to couple to the secondary side. To suppress noise, a shielding layer must be added between the windings. This effectively suppresses noise, eliminates interference, and improves the electromagnetic compatibility of the equipment. Figures 5a and 5b show the distributed capacitance of isolation transformers with and without shielding layers. In Figure 5a, the isolation transformer is not shielded. C12 is the distributed capacitance between the primary and secondary sides. Under the action of the common-mode voltage U1C, the common-mode noise voltage coupled by the secondary winding is U2C. C2E is the capacitance to ground of the secondary side. Therefore, from the figure, the common-mode noise voltage U2C of the secondary side is: U2C = U1C12 / (C12 + C2E). In Figure 5b, the isolation transformer is shielded. C10 and C20 represent the distributed capacitance of the primary and secondary sides to the shield, respectively. ZE is the impedance to ground of the shield, and C2E is the capacitance to ground of the secondary side. Therefore, from the figure, the common-mode noise voltage U2C of the secondary side is: U2C = [U1CZE / (ZE + 1/jωC10)] [C2E / (C20 + C2E)]. Since C2 is the impedance to ground of the shield, in the low-frequency range, ZE << (1/jωC10), so U2C → 0. It is evident that after implementing shielding measures, the common-mode noise voltage through the isolation transformer is significantly reduced. Figure 6 shows a comprehensive scheme for AC power supply anti-interference. To isolate the measurement and control system from the power grid, eliminate coupling caused by common resistance, reduce the impact of load fluctuations, and for safety, a 1:1 isolation transformer is often added before the power transformer and the low-pass filter. Currently, foreign countries have successfully developed dedicated noise suppression isolation transformers (NCTs), which are multi-layer shielded transformers with shielding layers on both the windings and the entire transformer. The structure, core material, shape, and coil position of this type of transformer are quite special. It can cut off high-frequency noise leakage flux and winding linkages, making differential-mode noise less likely to be induced on the secondary side. Therefore, this type of transformer can cut off both common-mode noise voltage and differential-mode noise voltage, making it a relatively ideal isolation transformer. 2. Isolation of DC Power Supply Systems When isolation is required between the internal subsystems of control devices and electronic and electrical equipment, their respective DC power supplies should also be isolated from each other. The isolation methods are as follows: the first is to use an isolation transformer on the AC side, as shown in Figure 7a; the second is to use a DC voltage isolator (i.e., a DC/DC converter), as shown in Figure 7b. After adopting electrical isolation measures, most circuits can achieve good noise suppression, enabling the equipment to meet electromagnetic compatibility requirements.