Below is a list of Zig Zag Transformers manufacturers from Foster City, California. A zig zag transformer is a special-purpose type of 3 phase power transformer that is intended for use in providing earthing for ungrounded electrical systems as well as filtering and controlling harmonic currents. Since ungrounded systems need protection against earth faults, zig zag transformers work especially well because they have a zero sequence current that is common to all three phases, which provides a path that earth faults can follow to exit.