Check-boxes were never intended to replace an annual calibration check. They are intended to be used to confirm on-going accuracy between annual calibrations. Most check-boxes only have a very limited number of values that can be used to check against, compare this to the whole list of measurements that you receive from the calibration laboratory that will be checking your equipment on calibrators which costs tens of thousands of pounds each.
The check-box is a great idea to minimize the risk of any catastrophic failure and for spotting a trend towards out of calibration which may otherwise result in site revisits and retesting being required, and as such it is a great addition to the electrician's quality assurance system.
In my opinion the best check-box on the market currently is the Megger MTB7671 which actively measures the voltage and current output of the tester and confirms if it is within the limits prescribed by BS7671, GN3 Chapter 4 and BS EN61557, as well checking the readings. Testing the output voltage and current of the tester is so important because if it's low the circuit under test will not be stressed sufficiently and any readings obtained will be unreliable and maybe inaccurate.
The leading test equipment manufacturers including Megger, Fluke, Ethos, Di-log, Kewtech & Seaward all recommend annual calibration in a calibration laboratory to ensure ongoing accuracy verification of their equipment and to maintain their product guarantee. The only exceptions are for the tri-annual calibration of check-boxes, and approved voltage testers (AVT) that indicate voltage in broad bands that do not require calibrating. If the AVT incorporates a meter however, it should be annually calibrated to assure the accuracy of the meter.
