Hardware fault tolerance sometimes requires that broken parts be taken out and replaced with new parts while the system is still operational (in computing known as "hot swapping"). Such a system implemented with a single backup is known as single point tolerant and represents the vast majority of fault-tolerant systems. In such systems the mean time between failures should be long enough for the operators to have time to fix the broken devices (mean time to repair) before the backup also fails. It helps if the time between failures is as long as possible, but this is not specifically required in a fault-tolerant system.