Distances To Be Aware Of When Installing Surge Protectors 2—long-line Oscillation Effects
When the surge protector is used to protect equipment or installed on the input distribution board, if it is too far away from the protected equipment, due to the existence of the cable distributed capacitance and its own inductance, an LC resonant circuit is formed. When the protection voltage Up of the surge protector is transmitted to this time, the LC resonant circuit is excited by it and oscillates.
The voltage protection level Up value of the surge protection device will increase with the extension of the transmission distance, and may generate an oscillating voltage twice or even higher than Up on the terminal equipment. Although the equipment is protected by SPD, this oscillating voltage will still damage the equipment.
This long-line oscillation phenomenon is described in IEC61643-12 "surge protective device for Low Voltage Distribution Systems - Part 12: Selection and Use Guidelines": The impact of oscillation on the protection distance (called separation distance in some countries). When using SPD surge protectors to protect specific equipment or when the SPD located at the main distribution box cannot provide sufficient protection for some equipment, the SPD should be installed as close as possible to the equipment that needs protection.
If the distance between the surge protector and the protected device is too large, the oscillation can usually cause the voltage on the device terminal to rise to 2 times Up, and in some cases it may even exceed this voltage level. Although the SPD is installed, this voltage may damage the protected device.
GB50343 Section 6.1.2 The impact of oscillation on the protection distance: When the SPD is used to protect specific equipment or when the SPD is installed on the main distribution board and cannot provide adequate protection for certain equipment, the SPD should be as close to the protected equipment as possible. 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, and in some cases, it even exceeds this level. Therefore, despite the installation of the SPD, the oscillation phenomenon can still cause the protected device to fail.
When we install a surge protector, it is usually not enough to use an SPD close to the protected device. For EMC reasons, the SPD is best installed at the entrance of the device, which can better transfer current to avoid electromagnetic interference caused by surge current. If the device is not installed within the protection distance of the SPD at the entrance, another SPD needs to be installed close to the device to avoid the risk of damage caused by long-line oscillation.
