Purpose It is a well-established practice in calibration laboratories to use current coils for clamp meter calibrations. Many laboratories either build their own coil, or simply make several loops of a test lead for the purpose of increasing the current value sensed by the clamp meter. Fluke has designed and manufactured several current coils for the purpose of clamp meter calibration. Customers have requested calibration for Fluke coils and some accreditation bodies have established policies regarding traceability of Fluke coils. This letter is intended to inform the reader about the theory of operation of a current coil, and its subsequent traceability and calibration requirements. What is a current coil? When a current is passed through a single strand of wire, it produces a magnetic field.i The force that produces the magnetic field is called magnetomotive force (mmf). The unit of mmf is defined as the ampere-turn (At). One ampere-turn is the amount of force that is generated by a direct current of 1 ampere flowing in a single loop turn in a vacuum. The total mmf that is produced is defined by the product of the number of turns and the current.ii If a single strand of wire is looped into 50 turns (N), the current in the wire would be multiplied by 50 to obtain the mmf.
Clamp meters are designed to interact with the mmf that is produced in a wire and to produce an accurate representation of the current flowing through the wire.iii Most calibration standards used in the calibration of clamp meters operate in the range of zero to 120 amps, but the top range of typical clamp meters are between 1,000 and 2,500 amps. In order to properly calibrate clamp meters at or near the top off their range, current coils have been developed which take advantage of the physics of ampere-turns by using multiple turns, so that the current sensed by the clamp meter is the product of the current being sourced by the calibrator multiplied by the number of turns in the coil. Fluke produces several different current coils for the purpose of calibrating clamp meters. Metrological design characteristics to consider for current coils Optimally, current coils should be designed to minimize the possibility of stray magnetic fields.iv The Fluke 5500A/Coils utilize a large coil design to minimize the impact of stray magnetic fields interacting within the device under test. When Fluke designs smaller coils such as the 9100-200, the impact of stray magnetic fields are minimized by the use of magnetic shielding materials. Another important consideration is the minimization of impedance due to the coil when AC current is applied. Impedance affects the current source (calibrator) by increasing the compliance voltage sensed by the calibrator, which will eventually cause the calibrator to cease operating. The two components of impedance in this situation are DC resistance and inductive reactance. DC resistance can be minimized by using heavy gauge wire. The use of heavy gauge wire also minimizes the heating due to high current flow, which also drives up resistance. The impedance of an inductor is dependent on its inductance value and the frequency. This is why Fluke Coils are specified for use over a limited frequency range. Operation at dc 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. Operation at ac 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. Confirmation by calibration 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. Recalibration requirements 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.
i Halliday, Resnick, Walker “Fundamentals of Physics” 4th Edition. John Wiley and Sons Inc. 1993 Chapter 30
ii Floyd “Electronics Fundamentals – Circuits, Devices and Applications” 3rd Edition. Prentice Hall 1995
iii Fluke “Application Note: Clamp Meter ABC’s” Fluke Corporation 12/2015
iv Roberts, Paul “The design, confirmation and use of a compact Current Coils set for clampmeter calibrations” Fluke Corp. Available at flukecal.com in the articles and education section