The Company, Inc. Model Monitoring System Control Unit was experiencing lightning-induced transient susceptibility failure modes. The control unit was disassembled and compared with schematics and drawings for good EMC-engineering practice. The layout of the internal wiring, printed circuit board traces, and EMI protection components was shown to have multiple deficiencies:
- The internal wiring from the circular connector to the PCB header is >10 cm.
- The input-power traces on the PCB meander several centimeters before reaching the EMI protection components.
- EMI protection components must be installed properly to be effective:
a. They should be physically located near the enclosure penetration.
b. The combined length of internal wiring and PCB trace before reaching the EMI protection components should be <3 cm.
c. The circular connector would be an ideal location for a 1-nF capacitor and a properly rated MOV.
- The long internal wiring and PCB traces defeat the EMI protection and induce externally created signals onto the sensitive electronics. This effect is known as crosstalk (see Figure 1 below).
Figure 1 – Current EMC Layout vs. Ideal EMC Layout
The lightning bolt represents the extent of penetration of external signals into the enclosure.