A datasheet lists the power supply used across a fleet of parking meter payment terminals at 500,000 hours MTBF. A technician tells the client each unit will therefore run about 57 years before it fails. What does the section teach?

Select one answer.

• Only the units — the 500,000-hour figure is right but must be divided by the number of terminals in the fleet to get each unit's life, so the per-unit lifespan is that figure spread across the fleet rather than 57 years.
• Nothing — MTBF is the operating life of a single unit, so 500,000 hours is exactly how long each supply will run before it fails, and 57 years of continuous operation is the correct expectation to give the client. The number is the lifespan, so reading it as the time one unit lasts is precisely its meaning, and dividing the hours by a year gives the honest service life the client should plan around, since a mean-time-between-failures figure is the manufacturer's statement of how many hours the individual product is built to survive before reaching the end of its usable life.
• MTBF is a population statistic, not a single unit's lifespan: 500,000 hours means roughly one failure per 500,000 aggregate operating hours across a large population in useful life, not that any one supply runs 57 years — across a fleet the failures arrive far sooner than that per-unit reading suggests, and MTBF says nothing about when a unit wears out.
• The temperature — the 500,000-hour MTBF is a true per-unit lifespan as long as the supply is kept cool, so derating for temperature, rather than rethinking what MTBF means, gives the real service life.