A spark plug for an internal-combustion engine comprises an attachment fitting 2, an insulator 3, a center electrode 4 and an earth electrode 5. The restriction sensor system may be used as a retrofit system that is capable of being installed on existing clothes dryers. In an embodiment, the selected storage interface is an ATA storage interface to convert the exchanged data in accordance with an ATA protocol when selected responsive to the selection signal. The control circuit calculates the phase-to-phase voltages from the phase-to-ground voltages. Other embodiments and related methods are also disclosed herein.