18 Way 20 Amp boxed PSU PTC Fuses Surge Protected. POWER SUPPLIES DIRECT range of multi way metal cased PSUs (STP-18WAYBOX & STP-9WAYBOX) use the latest technology PTC fuses, eliminating replacing fuses after overvoltage or short circuit occurs When traditional fuses are being used for overcurrent protection it can only protect once then it must be replaced. The PTC resettable fuse is a new-type of overcurrent protection element which has two functions providing overcurrent protection and automatic reset.