What is Surge Immunity?

The ability of electronic/electrical devices to withstand the sudden current and/or voltage surge from power lines is surge immunity. Indirect lightning strikes, routine power switching events, insulation faults, switching of reactive loads, etc. cause a sudden surge in the AC power lines. These sudden surges cause electrical, thermal, and/or energy overstress. The surges occurring at the AC lines usually are rated in some kV. Even a surge of 1 kV can easily burn a PCB of a device that is only rated for 100 V. Hence, it can be said that devices meeting surge standards are more reliable to withstand the surges.

Fig: Surge Immunity Test

What is the EMC Standard used for Surge Immunity Testing?

The IEC 61000-4-5 is an EMC standard by the International Electrotechnical Commission on surge immunity. It specifies the tests, methods, and recommended limits for equipment to unidirectional surges caused by overvoltages from switching and lightning events. The IEC 61000-4-5 focuses on immunity tests and procedures for power transmission and telecommunication lines.

IEC 61000-4-5 specifies several levels of tests in association with an environment. The standard has classified the operating environments for which the equipment has been designed. They are as follows:

• Class 1: Partly protected electrical environment.
• Class 2: Electrical environment where the cables are well-separated, even at short runs.
• Class 3: Electrical environment where cables run in parallel.
• Class 4: Electrical environment where the interconnections are run as outdoor cables along with power cables, and the cables are used for both electronic and electric circuits.
• lass X: Special conditions specified in the product specification.

The following table summarizes the surge voltages for different classes:

What is the Test Equipment Required for Surge immunity Testing?

Following test equipment is required for Surge Immunity Testing:

• Surge Generator
• 3 Phase Coupling/Decoupling Networks
• Telecom/Data Line Couplers
• Connecting cables of the length of or less than 2 m

What is the Test Method for Surge Immunity Testing?

As per IEC 41000-4-5, surge immunity testing is done in two methods: the open-circuit test and the closed-circuit test. These two tests are conducted at the same time as a 1.2/50 µs voltage impulse into an open-circuit, and as an 8/20 µs impulse into a closed-circuit test. A similar method is applied for surge testing in telephone lines. IEC 61000-4-5 requires AC or DC supply ports of the EUT to be tested for surge immunity. However, EN 61000-4-5 directs that all the ports of the EUT should be tested for immunity. Application of at least five positives and five negative surges is advisable. A gap of one minute should be given between two consecutive surges to allow for the recovery of any protection devices.