Of course a properly made MF joint or crimp will never fail... the problem is knowing for certain that the joint was properly made before being made in-accessible.......
If (per the recent 'RCBO tripping' thread), the first thing I was taught during my higher education was "Common things afre common" (given that students have a horrible tendency of thinking first of the very uncommon!), the second thing I was probably taught was "Never say 'Never' " (or 'always', or 'none', or 'all'!). However, with that proviso, I essentially agree with you.
The third thing may have been "Trust no-one other than yourself"! My personal concern with 'MF' JBs involving spring contacts is essentially a (probably irrational) 'psychological' one. With a screwed connection, I am 'reassured' (perhaps because of arrogance) by the fact that I have been personally responsible for tightening the screws satisfactorily. With an 'MF' JB, one is essentially totally reliant on the thing having been manufactured (and inspected/tested) satisfactorily. Given that no manufacturing process is perfect/infallable and that I doubt that these JBs are subjected to 100% 'I&T' of the contact mechanism, I am left with some uneasiness.
I've often voiced my concerns about crimped joints. Again, I do not doubt that if "properly made" a crimped joint is very unlikely to ever fail, but here we have an issue of skill/experience. Even if I say so myself, I'm usually quite good at learning practical skills. However, as I've said before, over the years I have had countless attempts 'on the bench' to develop/perfect crimping skills, using tools ranging from the fairly cheap to the very expensive, and, despite have attempted to make hundreds, if not thousands, of 'test crimps' over those years, I've never been able to convince myself that the results were sufficiently reliable for me to be happy to use them 'in anger', certainly in inaccessible locations. I do not doubt that there are some people who are so good/experienced that they can more-or-less guarantee a 'perfect crimp' every time - but, given my personal failure to get to that point, I can't help but wonder how 'good' some of the people making crimped joints (particularly if only occasionally) actually are.
If I had to say what sort of joint I would be most comfortable about being 'inaccessible', it would probably be soldered/brazed (with strain relief) - but, again, there is the issue of it being 'done properly'. ... and why is it so uncommon these days (cheaper?!) to see screwed connections with two screws? A few decades ago it was quite common (remember the beautifully-engineered Wylex 'Standard' CUs?), and the redundancy surely must have decreased the risk of joint failure consdierably.
Kind Regards, John
