It is nowadays extremely important. Unfortunately, the straight MFT won't test for this. Shark fin is actually a waveform of a pulse which is responsible for nuisance tripping. It is not standardized, however. German vendors will use the capital letter K, with the same meaning. I have a secret tip how to test this: make a circuit on the table being protected with the RCD under test, and apply a Martindale EZ-150 socket tester (any other won't fit!). They apply an 500 usec pulse train. If the RCD won't trip, then it is fine, otherwise avoid it or limit its use. Of course, any other tests must pass, too. There are a lot of RCDs on the market, which may not have the shark fin mark, but withstand those ugly short pulses. It is worth to select RCDs for this feature otherwise the customer may become unsatisfied.
