When selecting a contactor, it’s important to understand its reliability and expected service life. Two commonly used metrics are MTTF (Mean Time To Failure) and B10 Life.
What is MTTF?
MTTF (Mean Time To Failure) refers to the statistical average time a product operates before failure. It is used primarily for non-repairable components and indicates overall reliability in typical usage conditions.
What is B10 Life?
B10 Life represents the point at which 10% of the product population is expected to fail, meaning 90% of the units are still operating without failure at this point.
AC-3 Duty vs AC-4 Duty: Why It Matters
Contactors are tested under different operating conditions based on the IEC utilization categories, mainly AC-3 and AC-4:
| Duty Type |
Description |
Typical Use |
| AC-3 Duty |
Switching motor loads during running condition (motor starting & stopping without plugging or jogging) |
Standard motor control |
| AC-4 Duty |
Frequent switching, plugging, or inching operations (higher electrical stress) |
Reversing, inching, jogging |
Because AC-4 duty involves more severe switching conditions, MTTF and B10 values for AC-4 are usually lower than for AC-3. This reflects the additional wear from frequent arcing and higher inrush currents during switching.
Summary
When reviewing the contactor life ratings:
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Use MTTF to understand the average life expectancy.
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Use B10 Life for maintenance planning and reliability analysis.
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Pay attention to AC-3 vs AC-4 duty ratings, as they directly affect product lifespan