A string of overheating PS Vitas has prompted an investigation in Japan, but Sony maintains that the cause isn’t a defect, instead chalking it up to user error. As reported by Wired, the total number of cases has now reached 31, 23 of which happened in Japan. The problem appears to stem from the charging port; Sony says users are trying to charge their Vitas while either the port or the cable’s terminal are wet or dirty.

The agency performing the investigation is the National Institute of Technology and Evaluation (NITE), an independent body charged with evaluating electronics safety. In response to the increased scrutiny, Sony Computer Entertainment Japan issued a press release yesterday, saying:

"Rarely, when the PS Vita or multi-use terminal on its USB cable have foreign matter or liquids on them and are connected for charging, etc., a partial short that leads to the terminal burning out can occur. Symptoms of this can include the console becoming unable to charge or start up. However, this phenomenon is not a safety problem."

While the investigation is ongoing, the Yomiuri Shimbun daily says there are no plans for a recall. A representative from Sony is quoted as saying, "it is not a defect. In the case of a burned-out part, we will exchange it for a fee."