Permanent faults in medium-voltage cable joints significantly impact the reliability of distribution networks.Radial breakdowns caused by water ingress often lead to several self-extinguishing arc discharges—referred to as incipient faults—before developing into permanent faults.Effective monitoring of incipient faults can help reduce outage costs associated with permanent faults.However

the specific fault scenarios of incipient faults remain insufficiently understood.To address this gap

this study designed a simulation experiment replicating incipient fault conditions in medium-voltage cable joints under humid environments

based on actual operating scenarios.The experiment compared the insulation strength required to trigger incipient faults and examined both non-electrical fault characteristics

such as insulation damage and arc flame intensity

and electrical characteristics

such as fault current and impedance.Experimental observations show that

in cable joints

gaps without accumulated water retain sufficient insulation strength to prevent breakdown.However

the infiltration of accumulated water shortens the effective insulation path

thereby lowering the breakdown threshold.The peak current of an incipient fault can range from hundreds to thousands of amperes

with a duration of approximately 1/8 to 1/4 of a power–frequency cycle.During incipient faults

arc burning on the pore wall leaves conductive traces

which progressively reduce the insulation strength of the surrounding environment.As these traces accumulate over multiple events

the likelihood of breakdown increases

ultimately resulting in a permanent fault.Permanent faults are characterized by intense

sustained arc discharges that persist over a macroscopic time scale and exhibit flatshoulder waveforms within individual cycles

with discharge intensity increasing progressively over time.