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An advanced technique for detecting incipient inter-laminar insulation failure in stator cores of large electric machines is proposed in this paper. The proposed scheme is an offline method based on low flux stator core excitation that employs a new probe. An iron-core probe is placed in the wedge depression area of the stator and scanned axially to detect the variation in the measured flux due to a fault. Two fault indicators that provide clear indication on fault severity and location are proposed in this paper. A 120-MW ac machine stator was tested under a number of intentionally inserted fault conditions using portable field equipment recently developed. The experimental results show that the fault existence, location, and severity can be determined reliably using the proposed scheme. It is also shown that the fault detection sensitivity and reliability, inspection speed, and user convenience are significantly enhanced.