This paper describes the effects on SUS tube of power optical fiber composite cable on underground transmission lines. The effects on grounding, air gap between SUS tube and metal sheath, contact resistance between outer semi-conducting layer and metal sheath and grounding of SUS tube application or not are variously analysed using EMTP in normal operating condition as well as single line to ground fault. From these results, in this paper, the scheme for protecting the electrically abnormal phenomena will be established on power-optical fiber composite cable of underground transmission lines. This paper can contribute to specification of grounding reference of SUS tube of optical fiber composite power cable system.