Figure 1: Understanding conducted EMI and radiated EMI emission and immunity

Electromagnetic interference (EMI) refers to unwanted electromagnetic signals emitted from an external source that can interfere with the operation of, or sometimes damage, connected or nearby devices in the same environment. EMI can be caused by man-made sources, such as emissions during the normal operation of electrical and electronic devices, or by natural sources, such as lightning. EMI from a source can affect nearby equipment in two ways: conducted EMI (i.e., travels via wires or connected cables) and radiated EMI (i.e., propagates through the air). 

A surge or electrical transient refers to a short-duration, temporary sudden change in voltage or current waveform (voltage or current spikes), which can damage electrical and electronic devices/equipment connected to a power line or circuit. A transient spike or surge voltage can have a magnitude ranging from a few volts to several thousand volts and durations from several microseconds (µs) to milliseconds (ms), see figure 2. This is a conducted EMI issue. Common causes of surge or transient disturbances include capacitor bank switching, switching reactive loads (e.g., motors), device faults, load switching, and indirect lightning strikes.  

Figure 2: Understanding surge or electrical transient

Transient immunity refers to the ability of electrical and electronic equipment to withstand surges or electrical transients (i.e., voltage and current spikes) originating from external sources, such as nearby connected equipment (e.g., due to switching reactive loads) or natural phenomena like lightning. Electrical and electronic equipment should meet the surge immunity level requirements specified by relevant EMC immunity test standards to ensure proper function when exposed to surge disturbances that may occur in real-world electrical systems or circuits.

Note: Typically, transient protection or surge protective components (e.g., TVS diode) and circuits are used to ensure the immunity of electrical and electronic equipment and circuits against electrical transients.

EMC Standard for Transient Immunity testing:

EMC standards are the technical documents written and published by national and international organizations. There are two categories of EMC standards: emission and immunity standards. The emission standards are designed to ensure that the electrical and electronics equipment will not emit more than the conducted and radiated EMI emission limit specified in the relevant standards of the product. The immunity standards are designed to ensure that the electrical and electronics equipment can withstand external EMI such as conducted EMI, radiated EMI, ESD, surge, and EFT disturbances from various external sources (e.g., from nearby equipment or natural causes such as lightning). 

In short, the EMC standards ensure electromagnetic compatibility (EMC) of the equipment. Electromagnetic compatibility (EMC) refers to the ability of electrical and electronic devices to work satisfactorily without disturbing any other connected and nearby equipment in its real-world electromagnetic environment. A product can enter into a market of a country or region when it complies with requirements of relevant EMC standards. 

The IEC 61000-4-5 is the commonly used EMC standard for surge immunity testing that provides guidelines, test signal levels, and testing procedures for surge immunity testing. This testing evaluates a device's ability to withstand surge disturbances or transients; i.e., it verifies whether the device under test (DUT) has sufficient immunity against surge disturbances. This test ensures that the equipment can operate satisfactorily when exposed to surge disturbances that may occur in its real-world electrical system or circuits.

How is the surge immunity test or transient immunity test conducted?

Figure 3: Surge immunity testing setup

The Figure shows the surge immunity testing setup that can simulate real-world surge disturbances to the equipment under testing (EUT). The test setup consists of equipment such as a combination wave generator, coupling decoupling network (CDN), EUT, an oscilloscope, a computer, a power supply unit, and other required equipment to perform testing. This test is conducted inside a shielded chamber, such as an anechoic chamber or semi-anechoic chamber, according to the testing procedure described in the IEC/EN 61000-4-5 standard. 

Figure 4: Surges are applied to the AC sine wave at an angle of 0⁰, 90⁰, 180⁰, and 270⁰

During the transient immunity test, the EUT is in normal operating condition. According to immunity standard IEC/EN 61000-4-5 requirements, the combination wave generator generates the required surge test signals. The surge generator can produce a 1.2 µs/50 µs voltage waveform in open mode conditions and an 8/20 µs current waveform in a short circuit. 

Using a coupling decoupling network (CDN), the generated test surge signals are applied to the AC (or DC) power supply lines of the EUT. During testing, a minimum of five positive and five negative surges are applied to the AC sine wave at an angle of 0⁰, 90⁰, 180⁰, and 270⁰, with a repetition rate not exceeding 1 per minute. Some standards may also require that surge testing be applied to signal ports. The EUT's performance to the applied test pulses is evaluated. If the EUT withstands the applied surge signals and operates satisfactorily, the EUT passes the test and complies with the relevant immunity standard. If the EUT mal-operates or fails for applied test signals, then the EUT is considered as failed in the test or does not comply with the immunity test standard.  

The surge immunity testing ensures that the EUT can operate satisfactorily when exposed to surge disturbances that may occur in real-world electrical systems or circuits.

Table: Sows basic test signal levels as per IEC 61000-4-5 for surge immunity test

Figure 5: Generator open-circuit voltage waveform and short-circuit current waveform (from IEC 61000-4-5 document)

Methods to improve transient immunity of equipment:

• EMI Filter with Surge Protective Devices: Combining an EMI filter with surge protective devices (SPDs) provides a dual-layer protection strategy. The EMI filter suppresses high-frequency noise currents on power and signal lines, while the surge protective devices absorb and divert surge disturbances, preventing them from damaging the equipment. The EMI filters also help to mitigate radiated EMI disturbances caused by conducted EMI.  
• Unplugging Important Appliances and Electronics during a Storm: Physically disconnecting appliances and electronics from power sources during a storm helps to prevent damage caused by surge disturbances. 

These methods enhance the safety and longevity of sensitive equipment against surge or transient events, significantly contributing to improving the EMC performance of devices.