A unit is said to be a reserve (Back-up) able to make up for the failure of another to function, normally coupled “in parallel” to perform the same function. The reserve is used to be defined as “hot” in the case in which the back-up unit has a zero or almost zero intervention or switching time (in practice it works in parallel to the other); the reserve is instead defined as “cold” when a significant time interval and a specific procedure are required (possibly with the intervention of an external operator) to activate the back-up unit and bring it to work at full capacity, to replace the failed one or to be taken out of service
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