if you buy one without calibration, you're taking the chance that it's not accurate, or even faulty. best to be sure. the kewtech is OK, but i prefer megger as no.1 and fluke as no.2, the megger is top for me as you don't need to press a test button for continuity or loop tests.also has auto RCD test. all you need to do is set to auto, connect to circuit, press "test" and stan by RCD and reset 4 times. then back to the megger and read off the stored times. saves a lot of time.