Site Preparation Overview

This chapter provides information one needs to help adequately prepare a site. If differences occur between this guide and local codes and regulations, then the local codes and regulations should be given preference. If a process is critical to plant operation and process interruptions must be minimized, secondary power sources, signal redundancy, and other forms of backup controls should be considered. If less stringent requirements are acceptable, one can make certain cost control decisions. Control system availability means that a system is available to perform its intended function. System availability can be defined mathematically as the percent of time that the system is available compared to the total time that the system is in use. Mean time to repair (MTTR) is an important consideration for system availability; the shorter the MTTR, the more the system is available. MTTR includes identifying a fault after its first indication, location of the fault, and the time required to correct the fault. For isolation from source electrical noise, it is recommended that the control system be supplied from an isolation transformer or power source that is separate from sources used for room lighting and other power loads.