In the study, investigators bought 400 Apple chargers on the internet from different retailers around the world. Each charger was put through a basic test that would be required for the chargers if they were to be sold in brick-and-mortar shops.In the test, high voltages were applied to the chargers. The authors wrote, "If the current is above the threshold, the unit is determined to have insufficient isolation with potential for electric shock."Out of 400 chargers, 397 failed.This study comes just a few weeks after Apple conducted its own study on third-party power accessories. That study estimated that 90% of Apple chargers and cables sold through Amazon were unsafe. This led to a lawsuit from Apple , which was filed in October.The lesson here: Be careful where you get your chargers. They might be cheaper, but they may not be safer.