The CalCard works similar to the more commonly termed "Calibration Checkbox".

Basically it provides a range of fixed resistance's for confirming the operation of electricians insulation and continuity instruments.

This is a fundamental calibration checkbox requirement of "Part P" for domestic installers.

The CalCard EXT 1 calibration checkbox is currently been used by thousands of Part P registered tradesmen who have been certified and their records checked by inspectors against measurements using the CalCard Ext 1

These professionals are members of various institutes and recognised bodies. To date the CalCard Ext1 has been viewed and accepted by IEE, NICEIC, NAPIT, ECA, IET and ELECSA to name but a few.

Various aspects of electrical work require testing and inspecting using either a multifunction test meter or a set of individual function test meters.

These tests include;

1. Continuity: Using low resistance ohm meter. e.g. checking your wire connections are good & sound through the length of a circuit, continuity of CPC!
2. Insulation Resistance: Using a meter capable of generating a test voltage and measuring any high resistance break-down between the cables insulation. (e.g. 500v d.c. for 230v ac circuits). checks your cables haven’t been damaged during installation!
3. Earth loop impedance: need to measure the Ze and Zs values for a circuit. Verifying max Zs values are met to ensure circuit breakers will operate within set time in the event of a direct live-earth fault.
4. RCD tests: ensure RCDs operate within their specified times.
5. Earth electrode resistance testers: used to verify integrity of an earth rod resistance through earth on TT installations.
   

In order to know that your test readings are good you need to know that your test instrument is working correctly within its designed tolerances/capabilities. This is generally assumed to be good with a new instrument but with age and normal wear & tear, ongoing checks need to be made.

General recommendations for verifying test instrument integrity can be found in publications such as IEE guidance note 3 “Inspection & Testing”; HSE guidance note 38, “electrical equipment for use by electricians” and various Part-P scheme providers have their own guidance notes re Test instrument accuracy.

The common accepted methods of checking test instrument accuracy are: -

1. A formal calibration by a UKAS accredited calibration company, these will provide you with a certificate of calibration for that particular test instrument, identified by its serial number.
2. Intermediate testing between calibrations, with a known check-box, set of resistors, or against another known good calibrated test instrument!

The actual intervals between these checks & calibrations will be dependant upon the type of use the meter is put through.

But either way to ensure conformity and to demonstrate competence, formal records should be kept verifying the results of tests & checks.

So, that’s the reason for having a check-box, either purchased off the shelf or home made.