ground one end to a main earth but not the other.. this prevents circulating currents and interference signals.. however with devices such as mobile phones th

ground one end to a main earth but not the other.. this prevents circulating currents and interference signals.. however with devices such as mobile phones the phone frame should be connected to the ground of the usb so it is a floating earthed frame around the phone internals but does not link itself to earth. The real problem comes about when connecting two or more devices that have a connection to the main earth... A detailed understanding of the circuitry and operation is need to make a decision.