Clearly it is relatively common for Burndy cables on the NAC552 not to lock properly, or at least as easily as they do on other components. I wonder the same question as posed by Skip, above: does it need to be tightened until a click is heard? Does it matter? What difference would it make if it did lock? Or is it designed/intended to be like that?
That’s the issue with irregularities - they pose more questions than answers!
I've no experience with the 552, but, presumably, it would be the same as with other Burndies, in that (I'm guessing) it should click to signify that it is 'fully home', and securely connected/locked, and given that it carries power, this is presumably how it should be?
IME, if correctly lined up, Burndies aren't too difficult, but, as I say, I've no experience of the 552, and this thread suggests that it may be more 'awkward' than the norm (though not according to Richard)?
With Burndies in general, I suppose it also depends on how much/little room you have to work with round the back of the equipment, too.
Just another of the idiosyncrasies/mysteries of the Naim world, perhaps?