Getting your Portable Appliance Tester calibrated yearly is an important process for anyone that regularly undertakes testing and tagging. Just as your car requires a service periodically, your PAT tester requires maintenance to ensure it is working as it should. If a PAT tester isn't calibrated properly, results will often become skewed and the tester will pass items that should be failing and vice-versa.
For example, when performing tests on Class II appliances - as the values are so small to begin with, a small discrepancy in the results could mean the difference between an item passing or failing incorrectly.
The AS/NZS 3760 standard states that regular intervals are required to ensure your PAT tester is working accurately. In saying this, the general consensus is that PAT testers will basically require a calibration every 12 months, regardless of what type of tester you use.
It usually takes about five business days to have a tester calibrated, so it's certainly worth taking the time to ensure your results are correct.