In preparing a work of this nature, it will be realized that the subject is so broad and varied in its scope as to preclude an author's intimate personal knowledge of each and every system described. Attempt has been made to cover the subject from the first principles of Protective Relays to the protection of high tension net-works, the object being to make the work of value not only to the operator and tester who has a fair knowledge of electricity and is seeking more information, but also to the designer of the system who may find many points not previously taken into consideration in his calculations. Consequently, many reliable sources of information were freely consulted and in several instances, passages were quoted verbatim from a booklet entitled Performance of Instrument Transformers and from Protective Relays which in turn was reprinted from an article by Mr. L. N. Crichton in The Electric Journal. Full credit is hereby given to the Westing house Electric Manufacturing Company for such excerpts; also to the American Institute of Electrical Engineers for several paragraphs quoted from its June, 1919, Proceedings.