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In this study preceding tests are designed to determine the cause of the fire on #72 cable of Seohae Bridge, and the results are as follow. (1) It is confirmed molten marks located on the point 56 m apart from a installation part of #72 cable-main tower are composed of the zinc by EDS test. (2) It is confirmed the HDPE cover burnt by external gas torch is dissimilar to the burnt HDPE cover in the site. (3) It is discovered the Dimple which is the characteristic of ductile fracture from the break end of the #72 strand. (4) It is confirmed the Dielectric Breakdown of the HDPE by artificial lightning of 100 kV ~ 200 kV from a impulse voltage generator. (5) It is confirmed the P.C strand cable’s HDPE and wax are discontinuously ignited by short-circuit current approval test. Based on these results, The cause of the fire is estimated at lightning currents. And reenactment experiment that approves overcurrent to a HDPE P.C strand cable which used in the bridge is designed to verify the conclusion of the tests. The result is that the cable can be discontinuously ignited by currents of 2.0 kA for 0.15 seconds or by currents of 1.5 kA for 0.20 seconds. These current values are less than the limit that LINET system of KMA is able to dectect. the fire on #72 cable of Seohae Bridge can be caused by the small currents lightning with long strokes of IEC 62305-1.