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A Certified Component Does Not Make a Certified Instrument
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Designers often assume that a bill of materials made entirely from certified components produces a conforming instrument. The assumption is wrong: component certification covers the component in the conditions of its own standard, and the instrument is assessed in its own right, in its own enclosure, at its own temperatures, with its own clearances.
Where the transfer fails
A relay approved for a certain creepage inside its own body does not provide that creepage on the PCB beneath it. A transformer approved as a component may have been approved at a different ambient or with a different thermal fuse. An optocoupler approved for reinforced insulation provides that insulation only across the specified path, not around the package.
What component approval does buy
It reduces the amount of testing required at the subassembly level and it provides evidence for the internal construction of the component, which is otherwise difficult to inspect. Many standards explicitly allow reduced clearances or omit certain tests where a suitable component certificate exists, but only within the component.
The right use
Use component certificates as inputs to the design, then verify the assembly: clearances on the board, temperature at the component in the actual enclosure, and the insulation path around, not through, the component. A file that contains certificates and no assembly-level measurements is not a safety file.
