ABB REL670 Installation And Commissioning Manual page 187

Repair instruction Chapter 13
Fault tracing and repair
If the REx670 IED has the increased measuring accuracy feature (Optional), a file with unique
calibration data for that particular transformer module is stored in the Main processor module.
Therefore it is not possible to freely change only one of these modules with maintained accuracy.
To maintain the accuracy feature the calibration file stored in the NUM must match the TRM
and ADM used together with the NUM. As a consequence of this a faulty NUM can be replaced
without replacing the TRM and ADM only if the original serial number is supplied when order-
ing a new NUM. Because these modules are factory calibrated, all three modules will have to be
replaced if the TRM fails and the increased accuracy feature is to continue functioning.
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