The NFPA 70E Standard details multiple levels of Hazard Risk Categories (HRC) [see table 1.1], also called Levels. The greater the risk: the higher the HRC. Arc Rating is the factor that indicates which HRC is necessary, and can be measured in either ATPV or EBT.
The optimal pH range for brook trout appears to be 6.5-8.0,with a tolerance range of 4.0-9.5.Brook trout occur in waters with a wide range of alkalinity and specificconductance, although high alkalinity and high specific conductance usuallyincrease brook trout production (Cooper and Scherer 1967).
For example, the brook, brown and rainbows that inhabit Big Spring Creek in Cumberland County, Pennsylvania (a limestone spring creek) for the most part live their entire lives in water temperature from 46 to 50 f. The trout's metabolism in these waters is geared to these temperatures and the trout grow all year round. Take the same temperature and move a few miles away to a freestone stream and those trout are barely active in such cold water! So how does a fly angler ascertain which temperatures are best.