What is EFT Testing?
EFT or Burst testing is a conducted immunity test where an EUT is subjected to a series of fast rise time and duration pulses (5ns by 50ns respectively) an effort to ensure compliance and meet product reliability requirements. This EMI event is typically coupled onto power lines through the use of a CDN, but testing can be required on communication, data, and signal lines as well.
Electrical Fast Transients (EFT), also going by fast transients or bursts (EFT/B) are a series of quick high frequency pulses mainly caused by arching (sparking). A common cause of power line transients occurs when an AC/DC connection is made or broken, equipment powered down, or circuit breakers switched.(1)
The image, a switch closing and arching across, provides a easy to understand example of how this type of electromagnetic interference (EMI) can occur. It is most commonly tested in accordance with EN/IEC 61000-4-4 (Buy/Preview Standard) and often used to duplicate field failures. This standard falls under the basic EMC publications under HF conducted disturbances.
The arc(s) associated with switching, is often several arcs and have the potential to very based upon the distance and breakdown voltage of air gap.(3)
This article is a basic overview of this occurrence and is by no means a complete explanation. Its purpose is to provide the ground work of the concepts and associated testing. For additional information, please view the resources section and associated standards.
Where It Occurs
This type of interference has the potential to occur in a large amount of different electrical systems and devices. The interconnection of relays to power mains offers many situations for potential arching and electrical transients. Commonly they are tested to and occur in:
- AC Power Mains
- Automotive DC Electrical Systems
- Ethernet & Data Lines
- Electrical Systems & Subsystems(2)
Each electromagnetic (EM) environment has an impact on how this commonly occurring transient occurs and is defined. In automotive, given the lower voltage automotive DC systems, they're often considered "microbursts". As electronics system associated with the occurrence changes, so does how they're defined and tested.
This common conducted immunity standard is used for testing electrical and electronics system ability to function given the interference (often called stress) or a burst event. Immunity is commonly thought of as what interference or noise equipment is able to withstand given the electromagnetic environment which it functions. These often involve several tests, the most common being surge, EFT, and electrostatic discharge (ESD) events.
The IEC version has been harmonized with EN 61000-4-4 and call under the Electromagnetic Compatibility (EMC) Directive(3).The most recent version of this standard is the 2012 version and the International Electrotechnical Commission (IEC) offers a Reline version for comparing changes.
The table below, in Texas Instruments IEC 61000-4-x Tests for TI’s Protection Devices(4) provides associated test levels with corresponding line types. The associated test levels, 1-4 are defined by the standard and should be referenced accordingly.
The waveforms associated with these types of transients are generally categorized by pulse time/duration (tp) and rise time (tr). For EFT a 5 nanoseconds rise time by 50 nanoseconds duration 5/50. These relatively quick pulses in nanosecond range and are specified as repeated pulses/bursts for 15 milliseconds duration(4).
The below image from the Teseq NSG 3040 manual provides the shape and associated timing. Keep in mind, the voltage (kV) and associated current (A) will very based upon associated test level, upwards of 4kV for level 4.
This standard offers both testing at both 5kHz and 100kHz frequency being applied every 300 milliseconds. The diagram to the right taken from the EM Test NX5 user manual illustrates the different associated frequencies. You can notice both the 5kHz and 100kHz shown to IEC 61000-4-4 requirements. Testing by the standard also specifies a 50Ohm source impedance, built into the test generator. Both frequency options are often provided, however typically the 100Khz option tends to be the most commonly used.
As mentioned above, the pulse time/duration, rise time, test level as well as many other factors are determined by the associated standard. Pulses are often categorized by the rise time/duration, Burst 5/50ns, Surge 1.2/50μs, etc.
The associate voltage (open circuit) rise times based around common immunity tests are provided below.
EFT Pulse Generators
Burst pulse generators often provide additional testing capabilities for combination wave surges. Test systems offer unique features, monitoring ports, software capabilities, interface systems, remote control, and coupling decoupling networks (CDN)s.
Many test systems have built-in coupling decoupling networks, often mains capabilities as high as 300 volts 16 amps designed for single phase applications. For high voltages, currents, and 3-phase applications external CDNs are required.
High Voltage/Current 3-Phase
The power requirements of the equipment under test determine the need for external CDN’s. If the equipment requires more than 300v and/or draws more than 16a, an external CDN may be required.
These external networks designed for EFT are often manual, meaning it doesn't require communication with the generator (line synch communication), allow for use with a variety of pulse generators. For combination wave surge testing to IEC 61000-4-5 line synchronization communication is required.
The associated diagram, from the CDN 3083-B100 manual demonstrates how the coupler provides 3-phase testing capabilities. It is crucial when setting up a manual CDN that a solid connection is made and correct gauge wire is used.
Coupling onto data or communications lines for burst and electrical fast transients is accomplished through the use of a capacitive coupling clamp. These pieces of test equipment are generally built in accordance with IEC 61000-4-4 and are usually not generator specific.
Most coupling clamps can be used with a variety of generators given the proper connection to the generator is provided.
Given the nature of data lines, there is no sine wave, with that being said, no communication with generator is required. It is possible to use a Teseq clamp with a Haefely EFT System, so long as the right connector is provided. An example of a EFT setup with a CCL (Teseq CDN 3425) can be seen below.
EFT Test Setup
Given the quick risetimes and voltages associated with burst testing, grounding is especially important. The majority of generators and systems offer grounding cables ensuring that proper connection and paths are maintained.
When referencing setup requirements, spacing can play an important consideration given the potential for arching. All safety precautions shown in the associated standard and with the generator should be followed to avoid potential health issues.
Table Top Setup Requirements
- Ground Reference Plane
- Pulse Generator
- CDN (CCL if applicable)
- Insulating Support
- Connection/Grounding Cabling
Ground Reference Plane - Usually made of aluminum, copper, brass or other metal. Often times same ground plane can be used for IEC 61000-4-2 if EUT sizing requirements are met. Copper and aluminum work excellent for this requirement, are malleable, and commonly referenced in a variety conducted immunity setups.
Insulating Support - Most commonly polystyrene (Styrofoam), polypropylene or insulating foam-based product. When selecting for compliance to a particular standard, referencing relative permittivity εr. A table of the permittivity/dialectic constant of common materials can be found at this Website.
Grounding Strap/Cable - These are especially important given safety with this type of testing. They are often included with the generator, however for specific sizing requirements can be purchased here as well.
What are electrical fast transients (EFT)?
Electrical fast transients and conducted immunity interference most commonly associated with connection, disconnection, and switching of electrical and electronic devices. Given the arching that often occurs they are uses test in groups of pulses called bursts.
What else does EFT go by?
Electrical fast transients can be abbreviated EFT, which are also called burst, or BEFT. They are commonly referred to as fast transients, given the quick rise time and duration in comparison with other common associated transient pulses.
Why do we need to test for EFT?
Testing for this is crucial from both a compliance and product reliability standpoint given the destructive impact these events can have on electronic equipment. The underlying causes of this type of interference are also quite common, see definition above, exposing a vast number of systems and equipment.
How do I choose the right test platform?
Choosing the right test platform usually depends upon your testing needs and preferences. Often times when a failure to comply to IEC 61000-4-4 has occurred, it's best practice to match the same generator with the lab if possible.