My Experience With "Noise": Part IIIb

EFT: Understanding Test Methodologies To Avoid Failures

In the previous post, the source of EFT noise is presented in brief. The idea behind this article is to provide a highlight of this topic and hence I'm not going into depth. But we need to know a bit more on the EFT test wave characteristics and the test methodology that is described in IEC 61000-4-4. That might help in understanding what kind of noise might get into the system during testing if proper precautionary measures are not implemented in the design. The waveform below shows the EFT test waveform as suggested in the standard.

EFT Waveform Per Standard IEC 61000-4-4

The "Repetition Period" above depends on the frequency selected. As recommended in the latest standard, the pulse repetition period could be 5KHz (as it was prescribed in earlier versions) or 100KHz (suggested as more similar to practical, but not mandatory). Hence the repetition period could be 200us (for 5KHz) or 10us (for 100KHz). The "Burst Duration" above is the time duration for 75 pulses, i.e. the burst duration is 15ms when the pulse repetition frequency is selected as 5KHz. Please note that, irrespective of the pulse repetition frequency selected as either 5KHz or 100KHz, the energy imparted by the EFT pulses remain same for both of these cases.

The peak voltage level of the EFT noise pulses could be selected as it is recommended in the product specific standards for the specific channel type (power/IO/Communication) for the specific equipment under test (EUT). For example the peak test voltage levels could be +/-2KV for AC power lines and +/-1KV for other lines (DC power/signal/communication etc). The EFT noise needs to be applied to the EUT for 1 second for each polarity of the peak voltage level tested. 

  

The link below could serve as a quick reference to the waveforms, spectral characteristics, equations and test methodology etc.

http://www.teseq.com/com/en/service_support/technical_information/4_Transient_immunity_testing.pdf 

Next we will discuss more on the design measures to avoid failures due to EFT noise. 

My Experience With "Noise": Part IIIa

Electrical Fast Transient (EFT):

Anybody having experience with EMC testing should be well conversant with this term "EFT". For others, EFT is also a transient noise, that occurs due to the switching of power relays or the interruption of inductive loads on the power mains. The standard IEC 61000-4-4 or EN61000-4-4 describes the test procedures for subjecting EFT noise to the equipment under test, trying to simulate the real life EFT noise that might be present in an industrial environment. While designing an electronic device or system, the designer must take precautions against the design's vulnerability to EFT. As I have experienced, things gets much harder to fix a circuit failing in EFT test if the appropriate measures had not been incorporated in the design proactively.

Understanding the source of EFT:

When an electrical circuit is switched off, the current flowing through the switch contacts is interrupted more or less instantaneously. Hence at the moment of switching there is a high (ideally infinite) di/dt. Due to stray inductance associated with the wiring, types of inductive loads such as motors or solenoids, the voltage developed across an inductance L by a changing current i is:

V = - L . di/dt

This high instantaneous voltage causes the increasing air gap across the contacts to break down and a current flows again, which collapses the voltage spike, so that the briefly formed arc extinguishes. But this re-interrupts the current, hence another voltage spike appears, creating a further arc. This process repeats itself until the air gap is large enough to sustain the applied voltage without breakdown. At this point the circuit can be said to be properly switched off. The visible effect of this is a brief spark between the breaking contacts of the switch, but actually this consists of a series of microsparks whose repetition rate and amplitude depend on the circuit and switch parameters.

This burst of noise is the EFT noise which can appear at the power mains inlet of the device. Since the pulses are very fast (order of nanoseconds) they couple effectively through mutual capacitance and inductance to other wiring in close proximity to the source wiring such as control, communication or sensor lines. Voltage spikes typically of hundreds or thousands of volts, may appear on any such coupled circuits. 

When we should worry about:

Though it is not impossible for the transients to be coupled into a victim in close proximity inductively, but generally EFT noise enters the product via the cable connections, getting coupled capacitively. Any electronic device with power cable(s), IO cable(s) or communication cable(s) is the candidates susceptible to failure due EFT noise if the proper design methodologies are not followed to prevent failure due to EFT. On signal ports the EFT spikes are almost invariably in common mode. Common mode coupling on mains includes the protective earth wire.

In the next part (IIIb) we will discuss more on the measures to avoid the design getting susceptible to failures due to EFT noise.