Formula	Select the formula you wish to use. When you select a formula, it is displayed below the list. The formula’s calculation inputs appear in the Calculation inputs area. The formulas and their inputs are: • Single independent protected function • MTED = Mean time between failure for the protected function • MTIVE = Mean time between failure for the protective device • MMF = Allowed mean time between multiple failures
	• Multiple failure modes on a single protective device • MTED = Mean time between failure for the protected function • m1 = Mean time between failure for protective device 1 • m2 = Mean time between failure for protective device 2 • m3 = Mean time between failure for protective device 3 • m4 = Mean time between failure for protective device 4 • m5 = Mean time between failure for protective device 5 • MMF = Allowed mean time between multiple failures
	• Multiple redundant protective devices Protected system with multiple independent, fully redundant protective devices • MTIVE = Mean time between failure for the protective device • MTED = Mean time between failure for the protected function • MMF = Allowed mean time between multiple failures • n = the number of redundant protective devices
	• K of n voting system Protected system with a set of parallel protective devices • MTIVE = Mean time between failure for the protective device • MTED = Mean time between failure for the protected function • MMF = Allowed mean time between multiple failures • n = the number of parallel protective devices • k = is the number of protective devices required to activate the system • r = the number of units which have to be in a failed state in order to cause the whole system to fail: r = n - k + 1
	• Failure finding task causes a hidden failure The failure-finding task could cause the very failure which it is suppose to check and so leave the protective device in a (hidden) failed state from the moment the test is completed • MOTHER = Mean time between failure caused by phenomena other than the test • MTED = Mean time between failure for the protected function • MMF = Allowed mean time between multiple failures • p = if P is the probability that the failure finding test will leave the device in a failed state, p (as a decimal) is the unavailability caused by the testing process
	• Optimized failure-finding interval for a single protective device • CM = cost of the multiple failure (cost per event) • CFF = cost of doing the failure finding task (per task) • MTIVE = MTBF of the protective device • MTED = MTBF of the protected function
	• Optimized failure finding interval for fully redundant protective devices Or, optimized failure-finding interval for a fully redundant non-voting protective device and negligible probability that the failure-finding task will cause the device to fail • CM = cost of the multiple failure (cost per event) • CFF = cost of doing the failure finding task (per task) • MTIVE = MTBF of the protective device • MTED = MTBF of the protected function • n = the number of redundant protective devices

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Failure finding interval (FFI)

Calculated interval that it is considered safe to wait before performing failure-finding maintenance. The calculation is based on the values you input for the selected formula.

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Calculated optimal frequency

Frequency at which failure-finding maintenance needs to be performed. The frequency closest to, but not longer than, the failure-finding interval is displayed.

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