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 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.
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.
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.
