For fixed surge protectors, routine installation should follow the following steps:
1) to determine the discharge current path.
2) Mark the wires causing extra voltage drop at the equipment terminal, as shown in Figure 2. 1 and 2.2.
Note: In Figure 2, Ures is the residual voltage, or more generally, the limit voltage of Class I and Class II SPDs.
3) In order to avoid unnecessary induction loops, PE wires of each device should be marked, as shown in Figures 2.3, 2.4 and 3.
Note: If single grounding is not feasible, two surge protectors are required (as shown in Figure 2.4).
4) Equipotential connection should be established between equipment and surge protector.
5) Coordinate the energy of multistage surge protectors.
In order to limit the inductive coupling between the installed protected part and the unprotected equipment part, some measurements are needed. Mutual inductance can be reduced by separating the inductance source from the sacrificial circuit, selecting the loop angle and limiting the closed loop area, as shown in Figure 2.
When the conductor of the current-carrying component is part of a closed loop, the loop and the induced voltage are reduced because the conductor is close to the circuit. See figure 3.
Generally speaking, it is best to separate the protected conductor from the unprotected conductor and the grounding wire. At the same time, in order to avoid transient orthogonal coupling between power cable and communication cable, necessary measurements are needed.
Oscillation effect related to guard distance
When SPD 1 is used to protect equipment or installed on the input switchboard, but it cannot provide sufficient protection for some equipment, the installation position of SPD2 should be as close as possible to the protected equipment. If the distance is too far, it may produce twice or even higher oscillation voltage on the terminal equipment. Although the equipment is protected by SPD, this oscillating voltage will still damage the equipment. Reasonable distance (also called protection distance) is related to SPD type, system type, steepness and waveform of surge source and connected load. In particular, voltage multiplication may occur when the device is equivalent to a high-resistance load or when there is isolation inside the device. To explain this phenomenon, Figure (4) gives an example of voltage multiplication in this case.
It is generally believed that oscillation will not occur when the distance is less than 10 meter. Figure 4 shows that voltage multiplication may occur even if the distance is 10 meter, but only when the load is pure capacitance. Sometimes the equipment has internal protection components (such as piezoresistors), even if the distance is far away, the oscillation will be significantly reduced. At this time, attention should be paid to the coordination between SPD and internal protection elements of equipment.
Note: Generally speaking, it is not enough to install an SPD only near the protected equipment. For the sake of electromagnetic compatibility (to avoid electromagnetic interference caused by surge voltage, it is best to shunt at the entrance) and protection equipment (to avoid flashover between wires), it is best to install SPD at the entrance of the equipment. If the equipment is not within the protection range of SPD installed at the entrance, it is necessary to install another SPD near the equipment, and its coordination should also be considered at this time.
Note: This phenomenon can be explained by oscillation and traveling wave related to surge frequency and conductor length.
Influence of connecting line length
In order to obtain the best overvoltage protection, the SPD connection wire should be as short as possible. If the conductor is too long, it will cause SPD voltage drop. In order to provide effective protection, it is necessary to lower the protection level of SPD installed here. The residual voltage delivered to the device is the sum of SPD and the induced voltage on the wire. These two voltages do not necessarily peak at the same time. For practical purposes, they can be added in general. Figure (5) shows how the induction of the connecting wire leads to the increase of the residual voltage of the surge protector.
It is generally assumed that the inductance of the conductor is1μ h/m. When the pulse steepness is 1kA/μ s, the induced voltage drop on the conductor is close to1kV/m. If the di/dt steepness is large, the induced voltage value will increase. If possible, when the influence of inductive reactance is considered to be significantly reduced due to the separation of the loop, it is best to choose scheme c) in Figure (6); When scheme c) cannot be adopted, scheme d) should be adopted and scheme a) should be avoided as far as possible.
Note: If the return line and the incoming line are magnetically coupled through compact wiring, the inductive reactance will be reduced.
When the surge voltage at the incoming line of the building is low, it is enough to install an SPD near the incoming line. However, in some special cases, such as the installation of very sensitive equipment (electronic equipment, computers) or the equipment that needs protection is too far away from the SPD installed at the entrance, and the electromagnetic field generated by lightning discharge and internal interference sources in the building, it is necessary to install additional SPD near or inside the protected equipment.
When the power supply system and signal network lines enter the protection zone, they should be close to each other and connected to the same metal object to realize equipotential connection, which is particularly important for buildings made of unshielded metals (such as wood and brick-concrete structures).
The withstand voltage level of most protected electronic sensitive equipment in the system should be considered. For SPD installed near the equipment, its UP value must be at least lower than 20% of the withstand voltage of the equipment. Assuming that the SPD installed at the incoming line is within the protection range, if the UPl of SPD at the incoming line is lower than UP2 after multiplying by the overvoltage factor, then only SPD at the incoming line can be used. (See Figure 7)
Note: users should pay attention to the immunity of the equipment, which can be tested by mixed wave generator according to IEC6l000-4-5 standard. In this case, the immunity of low impedance equipment is not only defined according to the withstand voltage UW, but also a part of surge current is shunted through the equipment, so a reasonable coordination needs to be designed.
When some high-energy switching surge (switching overvoltage) may occur inside the building, additional SPD should be installed at this time.
Functions and Additional Requirements of Surge Protector
The basic functions of 1 (speed)
For the low-voltage system under normal working conditions, the installed SPD should not have obvious influence on the working characteristics of the system and the equipment in the system device.
For low-voltage systems with abnormal working conditions such as surge, SPD should respond to surge in time, and the characteristics of SPD can limit transient overvoltage and shunt surge current, so as to reduce overvoltage to below the surge overvoltage rating of equipment in various positions specified in IEC60664- 1.
For the low-voltage system that has experienced abnormal state, that is, SPD that returns to normal state after surge, its high impedance characteristics should be restored, and measures should be taken to prevent or suppress the aftercurrent on the power line.
2. Additional requirements for using SPD
1) Protect against direct contact. SPD shall be installed in such a way that it is inaccessible or avoids direct contact (such as installing isolation equipment).
Safety of SPD fault events. When the surge voltage exceeds the designed maximum tolerance and discharge current capacity, SPD may fail or be damaged. The failure modes of surge protectors can be roughly divided into open circuit and short circuit.
When in open circuit mode, the protected equipment will no longer be protected. At this time, it is difficult to find that SPD is faulty, because it will hardly affect the system itself. In order to ensure that the failed SPD can be replaced before the next surge, SPD must be required to have the function of indicating failure.
In short-circuit mode, the system will be seriously affected by the fault of surge protector. Short-circuit current flows from distribution system to fault SPD. Because the failed SPD is usually not completely short-circuited and has a certain impedance, it will generate heat energy and cause combustion before opening the circuit. In this case, the protected system has no suitable equipment to separate it from the fault SPD. At this time, it is necessary to install appropriate isolation devices for SPD in short-circuit fault mode. (circuit breaker)
Selection steps of surge protector
The explanation is as follows:
A: UC, UT and Ic.
This paper expounds the value of Uc in different power supply systems. UT is the short-term overvoltage that SPD can withstand, which is theoretically a straight line. However, in practical application, some values (power frequency, DC overvoltage) may change with time, and will exceed the maximum continuous working voltage Uc within a certain time interval (generally between 0.05 second and 10 second), so the value of UT should be considered to be greater than UTOV. But in fact, it is impossible to require an SPD to have high short-term overvoltage resistance and low protection level at the same time, so we can only give up comparison or adopt multi-level protection.
When the continuous working voltage Uc is applied, the maximum continuous working current through SPD is Ic. In order to avoid unnecessary actions of overcurrent protection devices or other protection devices (such as RCD), the selection of Ic value is very useful. The choice of Ic can be determined by referring to the "five-point method".
B. Protection distance
Mainly refers to the installation position of SPD. SPD should generally be installed on the switchboard of the low-voltage power supply system at the entrance of the building, especially on the low-voltage side of the transformer (in particular, the installation of SPD in the public distribution system must be approved by the management department of the public distribution system, such as the power supply bureau). When the switchboard is far away from electrical equipment or electrical equipment needs multiple protection, SPD2 and SPD3 should be as close as possible to the protected equipment, and equipotential connection should be made at the junction of lightning protection area.
Generally speaking, there are six steps in the selection of SPD, as shown in Figure (8).
Matters needing attention in selecting and using surge protector
1. Surge protectors with different performances should be selected for different application ranges. When choosing the power surge protector, the form of power supply system and rated voltage should be considered. The SPD at the junction of LPZ0 and LPZ 1 area must be
It must be a product that has passed the 10/350us waveform impact test. When selecting signal surge protector, the compatibility between surge protector and electronic equipment should be considered.
2.SPD protection must be multilevel. For example, the lightning protection of the power supply part of electronic equipment should adopt at least two levels of protection: discharge SPD and voltage limiting SPD.
3. In order to achieve effective cooperation between surge protectors at all levels, when the distance between power lines or communication lines between two surge protectors does not meet the specified requirements, appropriate decoupling measures should be taken between the two surge protectors.
4. For computer rooms built in different environments such as cities, suburbs and mountainous areas, when designing and selecting SPD, we must consider the unstable factors of computer room power supply and choose SPD with appropriate working voltage.
5. For unattended occasions, the power SPD can choose a contact with remote communication; For unattended occasions, SPD with acousto-optic alarm can be selected. All power arresters are old.
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6. The signal SPD should meet the needs of signal transmission bandwidth, working level and network type, and the interface should be compatible with the protected equipment.
7. Because the signal SPD is connected in series in the line, the SPD with smaller insertion loss should be selected.
8. When selecting surge protector, the designated supplier shall provide relevant technical parameters of surge protector.
9. Correct installation can achieve the expected effect. SPD shall be installed in strict accordance with the installation requirements provided by the manufacturer.