When doing EFT test, there have L1, L2, L3, N and PE interface. PE and earth are two different concepts. Electrical fast pulse interference is common mode, in the experimental setup figure of the standard, we can see the signal cable which is from the test generator can connect with corresponding power lines (L1, L2, L3 , N and PE) through the selectable coupling capacitor, and the signal cable shield connects with the chassis of coupling/decoupling network, and the chassis is connected to the reference ground terminal.
This indicates that burst interference is actually applied between the power line and the reference earth, so the interference which is added on the power line is common mode interference, and for the experimental method of using coupled folder, the electrical fast pulse will be added into the tested cable through the distributed capacitance which is between coupling plate and the tested cable. But the pulse which tested cable received is still relatively to the reference ground plane.
Thus, the interference applied on the tested cable by coupling clamp is still common mode. After confirming the nature of the interference, we can take the appropriate measures to make the equipment passed the test. So we can see, the X capacitor (differential mode capacitor) which used in power filters cannot inhibit EFT interference.
If the device included metal shell, the Y capacitors (common mode capacitance) will work. We can bypass the high frequency EFT to the shell, and then back to the signal source via the distributed capacitance which between the device shell and the reference ground, so as not to enter the circuit.
The mechanism of electrical fast pulse interference causes equipment failed, according to foreign scholars’ research, the single pulse energy is small, it will not cause malfunction of the equipment. However, the interference signal of burst charges apparatus line junction capacitance, when the above energy accumulated to a certain extent, it is possible to cause malfunction of the power line (as well as the system).
Therefore, there will be a time process for the line error, and there will be some chance (cannot guarantee the time interval of the error, especially when the tested voltage reaches the critical point). And it is difficult to judge whether the device is more likely to fail for separately applying a single pulse or applying pulse group together. And it is hard to say whether the device is more sensitive for positive pulses or negative pulses.
Practice shows that one device is usually particularly sensitive to a polarity of a power cable under a certain kind of tested voltage. Experiments show that the signal line is much more sensitive than the power line for the electrical fast pulse interference.
Effective measures for equipment passing the electrical fast pulse test. First we analyze the methods of interference injection: EFT interference signal is coupling to the main power lines by coupling the 33nF capacitance of decoupling network (and the signal or control cables applies interference through capacitive coupling clamp, the equivalent capacitance is 100pF). For 33nF capacitance, it’s cutoff frequency is 100K, which means
the interference signal which is more than 100KHZ can be passed; but for 100pF capacitance, the cut-off frequency is 30M, only allows interference whose frequency is more than 30MHz passed. The interference waveform of the electrical fast pulse is 5ns / 50ns, repetition frequency is 5K, pulse duration is 15ms, burst repetition period is 300ms. According to the Fourier transform, its spectrum is from 5K to 100M discrete spectral lines, and the distance between each spectral line is the pulse repetition frequency.
After the above, we can know that the coupling capacitance which applies the interference plays a role of a high-pass filter, because the impedance of the capacitance decreases as the frequency increases, the low-frequency in the interference will not be coupled to the EUT, only the interference signal with higher frequencies will enter the EUT. When we add the common mode inductance into the EUT circuit (with particular attention to that, the common mode inductance must be added to the main power line and its return line or it will reach saturation and thus cannot reach the purpose of attenuating interference), it will attenuate some of the high frequency interference, because the impedance of the capacitance increases as frequency increases. Therefore, the interference signal which actually applied to EUT only left the intermediate frequency.