EFT Immunity is the ability of an electronic/electrical device to withstand the interference of electrical fast transients (EFTs). EFTs are generally high-frequency pulses caused by sparking or arcing. The sparks occur during circuit connection or disconnection from a power supply, switchgear operation, or when lines touch each other. The EFT pulse can be filtered easily but it becomes rather difficult when the burst of lower frequency (5 or 100 kHz) occurs. If the low-frequency burst is repeated after every 300 ms, another component of 3.33 Hz is also introduced. This hampers the operation of the devices. 

Embedded controllers and devices use high-speed serial communication clocks that have timing specifications. Hence, EFTs if occurred during their operation can interfere with the clock signal which may ultimately degrade its outputs. Hence, if a device has a quality protection system to withstand this transient noise, then a higher level of reliability can be achieved.

What is EFT Immunity Testing?

EFT testing is conducted by subjecting Equipment Under Test to a series of fast rise time and duration pulses (5ns by 50ns respectively) to ensure compliance and meet product reliability requirements. The bursts or EFTs are generated by a burst generator that is connected to the EUT coupled onto power lines using a coupling-decoupling network (CDN). Along with power lines, EFTs testing is also conducted on communication, data, and signal lines. 

What is the Standard used for EFT Immunity Testing?

The IEC 61000-4-4 is an EMC standard by the International Electrotechnical Commission for the immunity requirements of electrical and electronic equipment to repetitive electrical fast transients. This standard specifies the limits, requirements, and procedures to evaluate the performance of the Equipment Under Test (EUT) when exposed to repetitive fast transients (bursts) on supply, signal, and control ports. These transient bursts are the disturbances created by switching power currents, the opening of circuit contacts leading to sparks, and the switching of loads in a circuit. 

The standard also classifies the test levels for EFT/B testing according to the requirements of the electromagnetic environment. These levels are: 

• Level 1: Well-protected Environment e.g., a computer room.
• Level 2: Protected Environment e.g., a control room of an industrial plant.
• Level 3: Typical industrial Environment e.g., a relay room of open-air, high voltage power plant or substation of manufacturing plant.
• Level 4: Severe Industrial Environment e.g., substation switchyards.

The range of test levels for the electrical fast transient test, applicable to power supply, protective earth (PE), signal, and control ports of the equipment as per IEC 61000-4-4 are given as follows:

What is the Equipment used for EFT Immunity Testing?

Following equipment is required for EFT immunity testing according to IEC 61000-4-4:

• EUT
• Burst Generator
• Coupling-Decoupling Network
• Ground-Reference Plane

What is the Test Method for EFT Immunity Testing?

Fig: EFT Test Set-up

EFT testing is conducted on AC and DC power supply lines, signals, earth, and control ports. EFTs or bursts of high-frequency pulses are injected into the system, with delays in between by using coupling-decoupling networks. The capacitive clamps are used for injection into the signal and control ports. These EFT test pulses are single 5 kHz unidirectional pulses of duration of 15 milliseconds each, repeated at three bursts per second for about one minute. 

The EUT is usually kept stand-alone and to be mounted on an insulating support. The Ground-Reference plane is extended outwards from the EUT placed on it by at least 100 mm in all directions. The test generator is also placed on the ground plane. According to IEC 61000-4-4, the EUT must be isolated from other conductive structures by a distance of 500 mm.

EFTs are applied at certain injection points in various methods of EFT testing depending upon the types of cables. For example, EFT pulses are applied to both the line and neutral lines for testing on an AC power source.