Single-core Cross-linked Polyethylene (XLPE) cables are commonly used to transmit 3-phase power in long distance transmission and distribution lines. Cross bonding of metal sheaths across phases limits induced currents. Assessing cable condition allows asset managers to prevent failure. On-line monitoring of Dielectric Loss (DL) indicates the general state of the cable insulation. In practice, DL measurement relies on measurement of a reference voltage and insulation leakage current. Circulating currents, which are generated in unbalanced lengths of interconnected cable sections, are superimposed on the leakage currents of each section. This makes measurement and differentiation of leakage currents in interconnected sections more complex. An on-line method of assessing relative DL in HV cross-bonded XLPE cables, with no requirement of reference voltage, has been developed. The method uses data from power frequency current sensors applied to collect sheath currents. The method, named Leakage Current Separation Method (LCSM), can distinguish the leakage currents under cross-bonded connection. The Relative Tendency of DL in Comparison of Three-phase Leakage Currents (RTDL-CTLC) has been put forward to determine the relative insulation conditions of interconnected sections. The actual data obtained from an 110kV cable tunnel, China, was analyzed to verify the consistency. Error analysis, including the characteristics of current sensors, the precision of data acquisition, synchronization error and the fluctuations of system frequency, was established to improve the on-line measurement.