All professionals working in the electrical environment have to test and verify their installations as both safe and correct. This is achieved by measuring the insulation and continuity of the installation using calibrated and accurate instruments.
The accuracy and serviceability of the instrument when returned from periodic calibration will state it was within calibration parameters; but only at that time only. it certainly could not guarantee that the instrument or its leads and probes would still be fit for purpose at any time after that.
The CalCard provides the solution. Verifying the serviceability and ongoing accuracy of an instruments ability to make the same truthful resistance readings throughout the time between these formal calibrations period.
In the UK this monthly procedural test became an integral part of the compliance requirements for electricians with all industry regulatory, health and safety bodies.