Generally, most jurisdictions require GFCIs only in damp or wet spaces. Garages, bathrooms, un-finished basements, etc are usually classified as damp or wet spaces. Shops may not necessarily be classed as a wet or damp space. If there is a means such that the motor housing or enclosure can get wet - say condensation for instance then you want a GFCI. GFCIs are sometimes waived where the branch service is dedicated to a particular appliance. The interpretations for what constitutes "dedicated" can vary from jurisdiction to jurisdiction. In some cases the AHJ will accept a receptacle in immediate proximity to the equipment and labeled as such, in some it may be a screwy receptacle style and your appliance being the only item with a matching plug, and in some it means a hardwired pigtail. You can ask the AHJ and see what they will accept. Residential GFCI devices are Class A devices, meaning that they protect against electrocution as well as damage to equipment. Class A devices range from 4 mA to 254 mA of leakage, while industrial (Class B) GFCI devices are set to trip above 254 mA and do not provide protection from electrocution -- they only protect equipment. The AHJ should not normally permit Class B devices in a residential application where GFCI devices are invoked. As Tim and Mark points out, an RFI filter at the VFD can supply sufficient leakage to trip a GFCI. In fact, many lighting dimmer devices can also trip GFCI devices. An isolation transformer between the GFCI and the appliance (in your case the VFD) will thwart GFCI tripping as well as practically eliminate the shock risk - but they are pricey. AFCI devices only guard against arc faults and do not do what GFCI devices do. AFCIs will trip if connected to anything that sparks - say a motor using brushes for instance. AFCIs are usually required where a receptacle is in close proximity to combustible material - like drapes in a living room, etc. GFCIs work and have saved lives while I am not so persuaded for AFCIs. If you have a damp situation with connected equipment, you want a Class A GFCI device even if the AHJ says you do not need it.