Weigh in. 
I'm not sure it's that powerful as compared to many other Ranges. However, let's look at a Range on average. They are Personal, Touch, Eye, Voice, Sight, and Arcane Connection. So one Range category, on average, encompasses 1/6 of all spells designed. That's pretty close to a full Technique. And then there are a couple Vim spells acting on Arcane Connections. Also, "Personal Transformation" is a Minor, and it's much smaller than a Range category. So I would definitely go with Major. However, I would worry about opening a floodgate. While Arcane Connection might be OK, what about the others? Could you make good a case for others at the table that one is OK while another is not?
Chris
I'm with callen on this (as far as I can tell, sorry, feverish).
I've considered it before, and decided not to allow it.
Too broad, even for a major I think.
And I certainly don't want that precedent - because then I'd have to argue why Magical Focus (Voice) is not an option.
Alternatively, you mean casting spells on targets to which you have an Arcane Connection, not the range?
I have considered that as well (that's what lead to the above actually), and I'm still not a huge fan.
If this was my saga, I would rule that this is not a good definition for a Focus. "Arcane Connection" is simply flavour-less. Something more easily associated with a type of effect would be more appropriate for a Focus.
Using an arcane connection might make for a good definition for Special Circumstances, though.
That's what I settles on at the time indeed.
That's another idea I had as well. But I was interested to see how people would judge a corresponding Magical Focus.
IMO, it fails as a focus, because it is far too broad.
I have an AC to my enemy in my hand, I target him with a Pilum of Fire, and because of this "focus" I can enhance Pilum of fire's CT by adding the lower Art to the total? And then I do the penetration calculation, because I'm holding the AC, too? It's kinda broken.
Most of you seem to have understood that this was about casting spells at AC range, or using ACs for penetration... But what about ACs as the topic of the spells? As in preserving, detecting or destroying ACs? That would seem more clearly delineated within Vim.
I agree, far too powerful. It's not only that you get that huge bonus to casting, but you get it to learning the spells in the first place, and that makes Arcane Connections far too trivial. It will work as Special Circumstances, though (after all, it's narrower than Puissant Penetration and has only a slightly higher effect).
You could however have a Minor Magical Focus in Opening the Intangible Tunnel and related spells. I'm not sure exactly what to call it, but it's clearly smaller than one TeFo combination, so it's legal. That would allow a magus to open a Tunnel through some serious Magic Resistance, then follow with a Touch Range spell that would still have the Arcane Connection bonus to Penetration and might also go through the same Magic Resistance. The downsides being that it requires two separate TeFo, so more spread out Arts, and that the target can retaliate if they notice the Tunnel.
As for ACs as the target of the spells, that would probably make it a Minor Magical Focus.
The spell Pilum of Fire as written has no relation to Arcane Connections at all, and thus wouldn't apply. If used as a range focus, it would apply if that spell were designed to be cast with a Range of Arcane Connection.
That was also the idea. But preserving ACs doesn't use a casting/lab total, and I'm not sure what you mean with "detecting" them, so from your list that basically leaves destroying them. Another application I might see though is casting spells that create temporary Arcane Connections.
If you take into account that darn near all objects are arcane connections to something then it becomes much too strong.
I can also see it being much to weak, if you only get your bonus casting your spells on arcane connections that are being used as arcane connections then I don't see it as much good. It would clearly apply on creo and perdo vim spells to preserve or decay arcane connections, but I don't see any other obvious candidates that the focus would apply to.
I can see interpretations that are too strong and too weak. Intuition tells me that there should be some way to chart some middle paths that would be appropriate for either a major or minor focus. I haven't come up with anything yet, and I suspect that my intuition may well be wrong.
I get the need to make a thread about the actual abundance (or not) of Arcane Connections - because in my sagas, I don't feel they're too abundant. 
Sidetracking my own thread here, but: Are there actually any Creo Vim spells that preserve Arcane Connections? I thought there were, but in our recent ArM4 saga we realized there weren't any guidelines or spells for it. I checked ArM5 afterwards and couldn't find anything there either.
Alright... I usually only do this on technical boards... but you started this discussion with a topic about a focus, and a request to weigh in.
You need to provide much more information then about what you want to do.
Yes, there are. Go to the Atlas site and take a look at the .pdf with supplemental guidelines. Look at the CrVi5, CrVi10, and CrVi15 guidelines. Looks like I found those in MoH, on page 113.
Chris
I like it as major.
I don't like the way it seems increasingly trivial to obtain an arcane connection.
I'm sorry, but you're to blame for that misunderstanding - you didn't bother explaining what you meant by the title, just
So you're basically looking for a focus that deals with CrVi and PeVi (extend or weaken ACs), possibly coupled with a spell to create a temporary AC (ie. Plucking the Hair from the Billygoat's Chin from RoP: tI)?
First part: fine with me.
Second part: could you go into some detail on this perhaps?
I would also include Opening the Intangible Tunnel effects. That bunch together would seem to make a quite suitable Minor Magical Focus.
Chris
I thought about that, but the tunnel isn't intrisically tied to ACs - the same guideline can be used for eg. R: Sight, per Hermetic Projects.
I'm not concerned with its Range. I thought it essentially opened a conduit as/with an arcane connection. To me that seemed like a manipulation/creation of an arcane connection (essentially like the temporary AC thing in RoP:tI), and so seemed to me to qualify with the other magics that directly affect arcane connections. That's why I mentioned it. But I see what you're saying.
Chris
I think that I would be fine with a Major Focus that applied to spells that manipulate Arcane Connections (mostly CrVi and PeVi effects) and I think that I would also allow it to apply to spells made from guidelines that specifically use Arcane Connections as part of the guideline (e.g., InCo Level 3: Locate a person to whom you have an arcane connection). But it wouldn't apply to every spell cast at Arcane Connection Range.
I would also be fine with a (different) Minor Focus applying to the manipulation/identification of Intangible Tunnels (but not to the casting of spells down the tunnel).