Often on forums and within suppliers literature reference is made to three points “Precision Resistors“, “Calibration Certificates” and “Traceability“. Often these references are made to support and warrant over-inflated prices of other Checkboxes in the Market.

Quote 1 – “Only a calibration check on traceable equipment will determine that your test gear was inside its stated specification.”

This is true for calibration of the instrument, but the fundamental stipulation of Checkboxes is “to verify the on-going accuracy”, a purely relative measurement.
You do not need to employ traceability standard procedures.

A lot of reputable suppliers quote their use of “Precision Resistors”.

Using Precision Resistors of high tolerance is simply not required for resistance Checkboxes.

The measurements are purely relative not absolute.
All commercial resistors will provide consistancy throughout their lives. The important factor which effects repeatability is the assembly process and connections of the CheckBox; not the quality or tolerance of the resistor. It is important to understand for on-going checking it is not the absolute resistance measurement; it is the relative measurements from an established instrument baseline.
This Baseline is established on the first measurement; future measurements should remain the same and not drift from this reference baseline.

A reputable supplier is supplying resistor Checkboxes with calibration certificates.

Resistance Checkboxes do not require certification, calibration, traceability or precision.
These check measurements are relative and not the absolute.

The CalCard is not designed for RCD testing or Zs testing. An accepted procedure for carrying out these tests are described in another document downloadable here.