1. Overview:

Surge refers to the current, voltage, or power transmitted along a line, characterized by a rapid rise followed by a slow decay. Surge (impact) immunity testing simulates severe interference caused by lightning. Surge current immunity testing for measurement and control devices in industrial processes simulates surge voltages and currents that equipment may encounter due to lightning strikes or switching operations under various environmental and installation conditions. Surge immunity testing provides a basis for evaluating the anti-interference capability of equipment power lines, I/O lines, and communication lines.

A. Surge immunity testing mainly includes surge voltage immunity testing and surge current immunity testing, primarily studying switching transients, lightning transients, and transient simulation.

B. System switching transients include: main power system switching disturbances, minor switching actions or load changes near instruments in distribution systems, resonant circuits associated with switching devices, and various system faults.

C. Lightning transients mainly include: lightning strikes on external circuits, where the injected large current flows through grounding resistors or external circuit impedances to generate voltage; induced voltages and currents on internal and external conductors from indirect lightning strikes; coupling of nearby direct ground discharge lightning current into the common grounding path of the equipment group's grounding system. When protective devices operate, voltage and current may change rapidly and may couple into internal circuits.

D. Transient simulation includes: signal generator characteristics should simulate the above phenomena as much as possible; if the interference source and the EUT port are in the same circuit, such as in a power network (direct coupling), then the signal generator should be able to simulate a low-impedance source at the EUT port; if the interference source and the EUT port are not in the same circuit (indirect coupling), then the signal generator should be able to simulate a high-impedance source.

2. Surge (Impact) Immunity Test Levels

Switching operations in the power grid and nearby lightning strikes can cause surge phenomena on AC power lines. Different equipment have varying sensitivities to surges, requiring corresponding test methods and different test levels, as shown in Table 1.

Table 1: Test Levels

Note: X* is an open level, which can be specified in the product requirements.

The test level should be selected according to the installation situation, which can be classified into the following categories.

a. Class 0: Well-protected electrical environment, often within a dedicated room.

All incoming cables have overvoltage protection. All electronic equipment units are interconnected by a well-designed grounding system, which is not affected by power equipment or lightning. Surge voltage does not exceed 25V.

b. Class 1: Partially protected electrical environment.

All cables entering the room have overvoltage protection. Equipment is well interconnected by a ground network, which is not affected by power equipment or lightning. Electronic equipment has a power supply that is completely isolated from other equipment. Switching operations indoors can generate interference. Surge voltage cannot exceed 500V.

c. Class 2: Electrical environment with well-isolated cables and short, well-isolated wiring.

Equipment groups are connected to the power equipment's grounding system via separate ground wires. This grounding system almost always encounters interference voltages generated by the equipment group itself or lightning. The power supply for electronic equipment is mainly isolated from other lines by specialized transformers. There are unprotected lines in this equipment group, but these lines are well isolated and limited in number. Surge voltage can exceed 1kV.

d. Class 3: Electrical environment where power cables and signal cables are laid in parallel.

Equipment groups are grounded through the power equipment's common grounding system. This grounding system almost always encounters interference voltages generated by the equipment group itself or lightning. Within the power facility, currents caused by grounding faults, switching operations, and lightning strikes can generate high-amplitude interference voltages in the grounding system. Protected electronic equipment and less sensitive electrical equipment are connected to the same power network. Interconnection cables may be partially outdoors but close to the grounding network. The equipment group contains unsuppressed inductive loads, but usually, no isolation is taken for different field cables. Surge voltage cannot exceed 2kV.

e. Class 4: Electrical environment where interconnection cables are laid along power cables as outdoor cables, and these cables are used as electronic and electrical lines.

Equipment groups are connected to the power equipment's grounding system, which is susceptible to interference voltages generated by the equipment group itself or lightning. Within the power facility, currents of several thousand amperes caused by grounding faults, switching operations, and lightning can generate high-amplitude interference voltages in the grounding system. Electronic and electrical equipment may use the same power network. Interconnection cables are routed like outdoor cables and may even be connected to high-voltage equipment. A special case in this environment is electronic equipment connected to communication networks in densely populated areas. In this case, there is no systematic grounding network outside the electronic equipment, and the grounding system consists only of pipes, cables, etc. Surge voltage cannot exceed 4 kV.

f. Class 5: Electrical environment where electronic equipment is connected to communication cables and overhead power lines in non-densely populated areas.

All these cables and lines have overvoltage protection. There is no extensive grounding system outside the electronic equipment. Interference voltages caused by grounding faults and lightning are very high.

g. Class X: Special environment specified in product technical requirements.

h. Table 2 provides guidance for selecting test levels.

Note: ① For distances from 10m to a maximum of 30m, with special structures and arrangements, no test is required for interconnection cables under 10m, and only Class 2 applies.

② Depends on the level of the local power system.

③ Usually tested with primary protection.

④ DB - Data Bus (data lines), SDB - Short Distance Bus, LDB - Long Distance Bus, NA - Not Applicable.