From what little I know of metrology, I think even a PAT Test calibrator would need calibrating! Calibration labs need their kit calibrated (perhaps less frequently since it’s only used in controlled lab conditions) by a bigger calibration lab, or if they’re big enough by a Nation Standards Lab such as NPL.
Our PAT tester is showing signs of needing a new battery, so I was thinking a few weeks back of building my own simple go/no go PAT test tester with 5% resistors. Opening our PAT tester to change the battery would invalidate the calibration certificate, even though, with resonable care there’s no reason why it should actually put the calibration out. Such a go/no go tester would verify that it was still functional and with any error much less than could make a life or death difference. But I didn’t get around to it as I think the battery will just about hold out until the next biennial calibration date, and worst case, it’ll run off the charger. As with one or two other projects, I find that identifying a suitable box is harder than designing what to put in it! (It would need to be absolutely safe if some idiot pluged it into the mains instead of into a PAT tester!)