Megger testers of any vintage have NOT used the DC blocking method (D lok) of No-trip testing.
Originally the DC blocking method was patented by Kyoritsu the original equipment manufacturers of Robin test gear and Robin trademarked D lok in the UK, all the other test gear manufacturers were forced to seek other ways of performing No-trip testing, the method worked well with passive RCDs of the time, but with the introduction of more sensitive RCDs it became less effective or reliable.
Most of the other manufactures adopted variations on the sub 16mA test current method or the sub half mains cycle method. In fact after Fluke acquired Robin they were forced to adopt the later method on the early Robin badged 165x series MFTs, because a) they would have to licence DC blocking from Kyoritsu and b) Fluke had also acquired LEM who had already solved the problem successfully with a sub half mains cycle method and Fluke solely owned the rights to that method so they did not need to “re-invent the wheel”.
Initially Megger used a variation of the now common single pulse low current (15mA) EFLI test method which injects a current below the residual trip current of the RCD. Later this was superseded by their patented low current multi-pulse method that is much less sensitive to electrical noise. The sophisticated method of noise cancellation that the Megger uses can resolve the impedance from the low level "No-trip" current pulses, even in an electrically noisy environment. It is because of this multiple current pulse and noise cancellation technique that the Megger gets unfair criticism from users of other MFTs (Fluke particularly) for it's apparently slow "No-trip" test, however, Megger users get the last laugh because they very rarely have their test inhibited by a noise error, a common problem with Fluke in particular.
[FONT=&]Even though Megger's clever No trip method is better than its competitors it is still not fool proof, in the real world notable differences between Hi current loop and No-trip loop testing can often be seen with ALL modern testers.[/FONT]
So you should not blame your Megger MFT. Using the No-Trip test on any other tester would likely as not produce similar results. In laboratory conditions both No-Trip and Hi current loop test measurements are likely to produce similar results, certainly within the calibrated accuracy of the MFT, but add in the oxidized joints and contacts of a typical real world installation and result is likely to be completely different.
My money would be on the Hi current loop test giving the more accurate result because the high current applied for multiple mains cycles is more likely to be able to break down the oxide or tarnish film.
[FONT=&]If your using a Megger MFT17xx (or 18xx) and it has a firmware version earlier than 2.01 installed it would be well worth getting it calibrated and the latest firmware update (usually inclusive in the calibration charge), as this has addressed virtually all the previous variability, instability and RCD test issues. Two wire loop testing is still less than successful, but each firmware update seems to produce a slight improvement[/FONT]
