Due to rectifying, harmonics are produced. To reduce the harmonic components of the input current, input chokes have to be used to meet the requirements of the domestic power distributors. Usually the 5th harmonic should not exceed 30% of the ground wave of the current. Input chokes are used to reduce the harmonics. Frequency inverters of type vkv... in connection with vka... or vks... relay-cabinets have a built in input-choke