Does anyone know if the USB charger is permanently energised (when fitted as part of 13A sockets) or whether it's turned on via a micro switch when the USB plug is inserted (I'm guessing the former)?

I know the current draw of a USB charger is tiny when open circuit but, being 'old school', I always remove the USB lead plug when not charging our phones but this may be the cause of several USB leads becoming faulty. Maybe I should leave the USB leads connected permanently and relax?