What is the difference between PFD and PFH in safety calculations?

PFD (Probability of Failure on Demand) is used for low-demand mode SIS (demand rate less than once per year) and represents the probability that the system will fail to respond to a demand. PFDavg is the average PFD over the proof test interval. PFH (Probability of Failure per Hour) is used for continuous/high-demand mode systems (demand rate greater than once per year or system in continuous operation) and represents the dangerous failure rate per hour. SIL levels have different target values for PFDavg vs PFH. Most process industry SIS operate in low-demand mode.