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Loss-of-field relays operating on an impedance-measuring principle provide an important part of the protection of a synchronous generator by minimizing possible damage to the machine and low voltages detrimental to system operation. These relays require an external source of reactive power, such as a utility tie or other in-plant generators, to insure proper operation on loss of generator field. In a multigenerator isolated system, the reactive current provided by the remaining generators may not be sufficiently large to allow proper operation of the loss-of-field relays on the generator in question. Some guidelines are provided as to the relative size of synchronous generators needed to insure proper operation of the impedance type loss-of-field relays.