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Electrical Fast Transient/Burst (EFT/B) Susceptibility
of an LV Circuit Breaker
The coupled voltage at component level has been simulated using CST MWS for two PCB releases of the same Low Voltage
Circuit Breaker (LV-CB) electronic control scheme. EFT/B was applied to an auxiliary inputs/outputs cable, as in Figure 1. The
Low Voltage Circuit breaker (LV-CB) open/close control is performed by means of a digital signal exchange between an ASIC
and a micro-controller, each located on two separate boards (horizontal and vertical boards) and linked together with a flexible
flat multi-conductor cable. The first PCB release allowed only one GND conductor of the flexible cable between the two boards,
while four GND conductors were allowed for the second one.
The assumption was made that a spurious open/close operation occured when noise was added to the digital signals
(SER_load signals) and that major coupling between EFT/B and SER_load signals came from common mode coupling
between the two separate board GND planes. In such a case, any common mode voltage between the two board GND planes
(Umc) is entirely added to the SER_load signals ( Fig.2).
Figure 1: Simulation focuses on the case (3), where EFT/B is applied to auxiliary inputs/outputs.
Electrical Fast Transient/Burst (EFT/B) Susceptibility of an LV Circuit Breaker
© 2016 CST AG - http://www.cst.com
Page 1 of 3
Figure 2:
Equivalent electronic circuit with common mode voltage.
Coupled voltage between the two boards is the result of a fully 3D field propagation effect including a complete direct and
reverse wave combination between the source and all the connected loads on the PCB. The 3D model, where the electric path
between EFT/B to SER_load included EFT/B generator, coupling capacitor, cables, and PCB GND plane, has been entered in
CST MICROWAVE STUDIO® (Figure 3). One pulse of the EFT/B time signal was applied at the excitation port (Port 1) and the
common mode time signal voltage (Umc) between the two GND planes was recorded.
Figure 3: Equivalent 3D model of the Low Voltage circuit breaker PCB GND plane.
The simulated Umc voltage for PCB (a) is plotted in Figure 4. It can be seen that it significantly exceeds the allowed noise
margin even for the lowest EFT/B level (2 kV). The SER_load signals are disturbed and a spurious open/close operation
occured during EFT/B injection. This result matches well with the experiment.
Electrical Fast Transient/Burst (EFT/B) Susceptibility of an LV Circuit Breaker
© 2016 CST AG - http://www.cst.com
Page 2 of 3
Figure 4: Simulated PCB (a) Umc voltage.
The simulated results of Umc voltage for PCB (b) are plotted in Figure 5. It can be seen that the voltage only touches the
allowed noise margin for the highest EFT/B level (3 kV). This result correlates well with experimental results.
Figure 5: Simulated PCB (b) Umc voltage.
The time to simulate the Umc voltage in the CST MWS per PCB release was less than 2 hours on a Pentium II 233 MHz, 256
MB RAM.
Electrical Fast Transient/Burst (EFT/B) Susceptibility of an LV Circuit Breaker
© 2016 CST AG - http://www.cst.com
Page 3 of 3