Multi-phase electrical machines are widely adopted in high-capacity propulsion systems, such as those in electric ships, due to their advantages in torque performance and fault tolerance. However, reliable operation requires effective online monitoring of stator winding insulation to anticipate latent short-circuit faults. While partial discharge monitoring is commonly used in grid-connected generators, it is susceptible to interference from inverter switching transients in drive systems. Alternatively, leakage current measurement methods leverage inverter-generated harmonics as excitation to assess insulation impedance without interference. Existing methods, however, are primarily designed for three-phase machines and are not directly applicable to open-end multi-phase configurations. This paper proposes a novel online insulation condition monitoring method tailored for open-end multi-phase machines. The approach uses a common-mode current sensor encircling all phase conductors in opposite directions to cancel load current components and isolate leakage current related to insulation health. Experimental results from a fifteen-phase induction motor demonstrate the method’s high accuracy in detecting changes in insulation capacitance and conductance under various operating conditions, confirming its effectiveness for online prognosis and fault prevention.