How To Resolve The Harm Of Long-line Oscillation Voltage Of Surge Protector To Equipment
Some users clearly installed surge protectors, but their devices were still damaged by current breakdown. What is the reason?
1. Impact of long-line oscillation effect on devices
Standard document GB/T 18802.12-2014, Section 6.12: If the distance between the SPD and the protected device is too long, the oscillation voltage value generated at the device end is generally as high as twice Up. In some cases, it may even exceed this level. Despite the installation of SPD, the oscillation effect can still cause damage to the protected device. Therefore, the surge protector should be installed close to the protected device to avoid oscillation effect and damage to the device.
Example of oscillation phenomenon generating 2 times voltage
2. Reasons for long-line oscillation effect How is the long-line oscillation effect generated? It turns out that when the surge protector is installed too far away from the protected device, the LC resonant circuit is formed due to the presence of cable distributed capacitance and its own inductance. As shown in the figure below, when the protection voltage Up of the surge protector is transmitted to the cable, the LC resonant circuit is excited by it and oscillates. In this way, the oscillation voltage in the line will become larger and larger due to the oscillation effect. The longer the connecting wire is, the larger the oscillation voltage will be. Therefore, the surge protector should be installed close to the protected equipment to avoid the generation of oscillation voltage to damage the equipment.
3. What is the actual residual voltage at the front end of the equipment? The actual residual voltage Utotal at the front end of the equipment = Up/f + UL (UL is the induced voltage drop of the wire between the SPD installation position and the protected equipment). Although these residual voltages do not always reach the peak at the same time, they can be simply added for practical purposes. Up/f is called the effective voltage protection level, which is the sum of the residual voltages of the surge protector, backup protector and wire. As shown in the figure below, Up/f = residual voltage 1 (wire) + residual voltage 2 (surge protector Up) + residual voltage 3 (backup protector Up) + residual voltage 4 (wire) + residual voltage 5 (wire).
Therefore, the Civil Building Electrical Design Code (GB51348-2019) stipulates that the wire connected to the surge protector should be short and straight, and the total length of the lead should not exceed 0.5m. The surge protector can be installed in this way, which can not only reduce the actual residual pressure at the front end of the equipment, but also play a role in quickly discharging lightning current.
IV. Three ways to resolve the hazards In summary, the oscillating induced voltage generated by the long connecting wire will cause damage to the equipment. How to avoid it? We need to pay attention to the following 3 points when installing a surge protector: 1. When the length of the wire between the surge protector and the protected equipment is less than 10m, the oscillation effect can be ignored; however, when the length between the surge protector and the protected equipment is greater than 10m, a Class II or Class III surge protector should be added to the front end of the protected equipment as further protection to avoid damage to the equipment by the oscillating voltage. 2. The Up value of the selected surge protector should be lower than 0.8xUw (equipment withstand voltage value). The 20% margin is reserved to take into account the existence of induced voltage drop caused by the oscillation effect on the wire. Choosing a surge protector with a lower Up value can eliminate the harm of long-line oscillation effect to the equipment from the source.
3. Keep the connecting wires of the surge protector short and straight, and the total length of the connecting wires is less than 0.5 meters to reduce the residual pressure of the grounding wire.
