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The paper assesses safety of AC earthing systems in a DC traction-supply substation (TSS) by comparing the designed mesh and step voltages to the maximum tolerable voltages. Generally, IEEE 80 criteria are based on the maximum tolerable body current to prevent fibrillation of the human heart, and this current is then translated into tolerable touch and step voltages. As a result, safety assessment of AC earthing systems in a DC TSS is referred to a procedure by which the designed mesh and step voltages are computed and compared with the tolerable touch and step voltages. Computation results of AC earthing systems for the orange line at O19 TSS in Taipei rapid-transit systems are presented as an example.