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Industrial process interruption is one of the major power quality issues. Load-shedding schemes based on sub-frequency monitoring are normally employed to disconnect loads so that the balance between the system generation and loading is restored. This procedure should be very reliable and well adjusted. In this paper, a case study is presented, wherein an industrial process of a petrochemical plant was tripped out by frequency relays that were not suppose to operate as the system disturbances were not associated to the imbalance between load and generation, but to a voltage dip in the supply transmission system. This case study is based on actual recorded data, showing evidences that voltage dips may affect the frequency measurements, leading to load interruptions that should be avoided. Recommendations to prevent this sort of false trip are discussed.