Memo on the traceability, calibration, and recalibration requirements for Fluke coils

When dc current is applied to the current coil, it produces a static magnetic field with a force that is related to the amount of current applied and the number of turns in the coil. Clamp meters that are specified for dc have Hall-Effect sensors built into the jaws of the clamp which are capable of measuring the field that is produced by the turns of the coil.

Recalling that when an ac RMS current value is applied, it produces the mmf equivalent to the amount that is applied by dc current. The mmf produced by ac current is a dynamic field due to the amplitude variation of an ac signal. AC current clamps are designed to sense the mmf by electromagnetic induction.

In order to confirm the properties of Fluke coils, an ISO/IEC 17025 accredited calibration has been developed for each coil that Fluke sells. This calibration procedure ensures that there are no manufacturing defects, and that the mmf generated by each coil is equivalent to an ideal conductor. The calibration is performed at a low frequency ac value (50 to 60 Hz) because this is the typical use case for Clamp meters. Due to the ampere-turn definition not being dependent on frequency, there is no discontinuity at DC and there is no need to calibrate the coils at DC.
 

These calibration processes have been evaluated by the accreditation bodies that regularly assess Fluke. As a result, the calibrations associated with Fluke coils have a calibration certificate that is endorsed by the accreditation body logo and is evidence of metrological traceability.

The reason that instruments are calibrated are because the physical properties of the instrument may drift over time (i.e. electronic components drift, physical standards like weights or gauge blocks wear). In the case of Fluke Coils, there are no components that degrade over time. The fixture configuration for coils also maintains their mechanical stability. All Fluke Coils that are purchased as of 2017 are delivered with an accredited certificate of calibration. This calibration certificate meets requirements for measurement traceability. Due to fact that Fluke coils do not exhibit degradation over time, it is the recommendation of Fluke that there is no need for Fluke coils to be recalibrated. However, it is possible for Fluke coils to exhibit catastrophic failure due to mishandling. This is best addressed through the application of intermediate checks. These intermediate checks can be performed by the end user and consist of a visual inspection and possibly a confidence check with a Clamp meter (e.g. confirm with the clamp meter that one amp applied to the coil is multiplied by 50 turns instead of 37 turns, which would indicate a coil failure due to something such as short caused by a loss of conformal coating). These intermediate checks can be performed at a periodic interval that is appropriate for the level of risk of damage to the coil in its application by the end user, such as annually or semi-annually. Intermediate checks maintain confidence in the calibration status of Fluke coils.