DEFINITION

  FIT  =  Failure In Time
      (number of failures per 10⁹ operating hours)

  Variables :
    FIT   failures per billion hours

  Rationale : standard unit on component datasheets since the 1960s
  (origin MIL-HDBK-217, Bell Labs telecom). Adopted by IEC 62380,
  Telcordia SR-332, SN 29500. Allows transistors, relays and sensors
  to be compared on a single reliability scale.


CONVERSIONS

  MTBF_h  =  10⁹ / FIT
  MTBF_y  =  MTBF_h / 8760           (8760 h = 1 year of continuous operation)
  FIT     =  10⁹ / MTBF_h

  Variables :
    MTBF_h   Mean Time Between Failures (hours)
    MTBF_y   Mean Time Between Failures (years)

  Rationale : MTBF is the mathematical inverse of FIT, more intuitive
  for a human reader. "100 FIT" is abstract ; "MTBF 1140 years" is
  easy to picture.


TYPICAL VALUES

  FIT  =  10        MTBF ≈ 11 400 years   (very reliable component, mil-spec)
  FIT  =  100       MTBF ≈  1 140 years   (good industrial component)
  FIT  =  1000      MTBF ≈    114 years   (common component)
  FIT  =  10000     MTBF ≈     11 years   (low-reliability component)

  Rationale : a typical industrial pressure transmitter runs around
  200–500 FIT. A SIL 3 safety logic solver : 50–100 FIT. A consumer
  grade component (resistor, capacitor) : 1–10 FIT for the element
  alone.


INTERPRETATION — CAUTION

  MTBF is NOT a guaranteed lifetime. It is the mathematical expectation
  of time to failure, under the assumption of an **exponential law**
  (constant failure rate).

  Rationale : a component with MTBF = 100 years may fail in year 1.
  The survival probability up to t follows P(t) = e^(−t/MTBF). At
  t = MTBF, the cumulative failure probability is already 63 %.
  MTBF is useful for PFD / availability CALCULATIONS — not to promise
  a commercial lifetime.