In your specific context, I suspect this simply means that the trip threshold is fixed (i.e. what you and I in the street might consider a 'normal' circuit-breaker — one that has a set current rating); as distinct from more exotic industrial types that can be adjusted for all sorts of parameteres, including trip current, trip delay, etc., etc.

This would seem to be consistent with the fact that this document (have you looked at it?) is quoting current ratings for fuses (inherently 'fixed' devices!) and circuit-breakers; clearly it isn't so easy to categorize adjustable circuit-breaker current ratings alongside fixed fuse ratings!

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Note added at 4 hrs (2013-09-24 07:38:38 GMT)
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In Asker's specific context, the [US] NEC 240.6 (a) table lists the various standard fuse and circuit-breaker ratings; none of the other refs. I have found seems to specifically mention 'fixed-trip', which makes me more convinced than ever that the writer of this source text was simply using the term in an everyday sense (perhaps even unaware that at least one European manufacturer uses it with a more specific sense).

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Note added at 4 hrs (2013-09-24 07:39:51 GMT)
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Note that the document cited has nothing whatever to do with LTBs.

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Note added at 4 hrs (2013-09-24 07:51:03 GMT)
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Note, too, that the official usage of 'fixed-trip' isn't confined to LTBs either, though!

However, I remaing convinced that the writer has unfortunately used this term confusingly in an everyday way here.

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Note added at 13 hrs (2013-09-24 16:13:58 GMT)
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Sorry, I was so intent on getting the actual meaning correct, I made a slip in my typing and inadvertently followed the EN convention.

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Note added at 13 hrs (2013-09-24 16:15:56 GMT)
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Just for the sake of clarity, it should of course be 'non réglable' — in this specific context of circuit-breakers, I find 'réglable' and 'réglage' used all the time in FR docs; I do a lot of FR > EN translation in this specific field, and that is the term that is used exclusively in the case of my customer, at least — a major player in the industry.