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Essential instrumentation and controls include the following:
Other essential services that may be fed from the instrument and essential services power supply, include (but are not limited to) the following:
Non-essential services are those loads which are not required during general power outages, and whose loss may also be tolerated during infrequent maintenance and repair of their power supply.
Tolerable transfer and outage times are the longest supply interruption which will not have a significant effect on process or equipment operation.
Each unit or group of units considered critical and independent to plant operations should be specified by the Requisition Engineer. As a guide for large facilities, a unit or group of units are defined as critical and independent if they can operate without the other(s) and they represent or are needed for 20 percent or more of facility throughput. The unit(s) can be process units, utilities, or other non-process units. Facilities not meeting the 20 percent rule may also be defined as critical and independent for commercial reasons.