IEC 61000-4-4 EFT Test

Test Equipment

Latest Version IEC 61000-4-4:2016

EFT Background

Electrical Fast Transients (EFT) are caused anytime a gaseous discharge occurs (a spark in air or other gas), the most common being the opening of a switch through which current is flowing. As the switch is opened, arcing occurs between the contacts; first at a low voltage and high frequency while contacts are close together, and later at a higher voltage and lower frequency as the contacts become separated. Coupling of the EFT into electronic products occurs when power cables handling high currents are run in close proximity to power, data, and/or I/O cables.

Electronic products are tested for EFT immunity to insure their continued reliable operation if subjected to realistic levels of fast transients. The European Union's EMC Directive mandates EFT testing for virtually all electrical and electronic products as a condition for obtaining the CE Mark before shipping to a member state of the European Union.

Applicable Standards

Generic Immunity, Product and Product Family Standards require that EFT tests be performed in accordance with Basic EMC Standards: IEC 801-4, IEC 61000-4-4 or EN 61000-4-4. Application Note, EMC Standards Overview, provides an overview of European Standards for electromagnetic compatibility, describes how the Standards relate to one another, and lists sources for procuring copyrighted documents.
1 IEC 801-4, IEC 61000-4-4, and EN 61000-4-4 are essentially the same for test voltages and levels.

Basic EMC Standard

The Basic EMC Standards for EFT define methods of generating consistently reproducible fast transients for test purposes. They specify generator and coupler/decoupler design and performance in an attempt to produce correlation results between test sites. The Basic EMC Standard specifies how to perform EFT testing, the Generic, Product and Product Family Standards specify the test levels and pass/fail performance criteria.

Test Levels

Test Set-Up

Coupling Methods

Capacitive Coupling via 33nF capacitors is the required coupling method to AC or DC power mains. These coupling capacitors are included as part of a Coupler/ Decoupler (C/D) in commercially available EFT simulators. The design of the decoupling portion of the C/D, which prevents the EFT burst from traveling back onto the power mains, is also specified in IEC 61000-4-4.

A Capacitive Coupling Clamp is used to couple EFT bursts onto data, I/O, and telecommunications lines. Construction of the clamp is shown in Figure 5 of IEC 61000-4-4; however, considerable differences exist in commercially available clamps. Some clamps use higher quality materials and some designs allow for the use of an optional safety interlocked cover.

Waveforms

Waveform Verification

IEC 61000-4-4 requires that the simulator output be verified periodically. A 50 ohm load and attenuator rated for high voltage is required and the measurement must be made using an oscilloscope with at least 400MHz bandwidth.

One problem noted with IEC 61000-4-4 is that simulator designs can be quite different and although each has a 50 ohm source impedance and provides the specified pulse into a 50 ohm termination, some simulators provide significantly more or less energy than others into loads that are not 50 ohms - the AC power input of most electronic products is something other than 50 ohms. To deal with the problems, changes to the standard were proposed to require waveforms be verified into a 1000 ohm load. Unfortunately, these changes were not accepted; however, this and other proposed improvements to IEC 61000-4-4 will be included in a forthcoming Annex.

Test Execution

According to IEC 61000-4-4, testing must be carried out according to the manufacturer�s test plan, which shall specify:

EUT Performance Criteria

For EFT tests, the Generic Immunity Standards and Household Appliances Product Family Standard require that products operate as intended after the test. No degradation or loss of function is allowed below a performance level specified by the manufacturer. During the test, degradation is allowed, but not a change in the actual operating status or data storage. Refer to the tables located in the Generic, Product and Product Family Standards for specific Performance Criteria. The product cannot become unsafe under any conditions.

EFT Simulators

EFT Simulators produced by Thermo KeyTek meet all the simulator requirements of |IEC 61000-4-4, including discharge pulse energy, output type, dynamic source impedance, and all waveform characteristics. Additionally, Hilo test simulators meet the requirements for coupling/decoupling networks for AC and DC mains as described in paragraph 6.2 of IEC 1000-4-4.