replacement unit (hot swap) (RU)

A replacement unit ( RU) is an electronic unit that can be used to replace defective components in the event of a fault.

If replacement units can be replaced during operation, this is referred to as hot swap, if the program has to be shut down for the replacement, this is referred to as warm swap, and if the system has to be switched off, this is referred to as cold swap. Replacement units that can be replaced by the user without special knowledge are called Customer Replaceable Units( CRU). Typical replacement units are boards, disk subsystems, power supplies and cables.

If the replacement units are already permanently installed and are automatically switched on in the event of a failure, this is known as hot spare.