A while back I was helping to install a piece of 1200A switch-gear. When it came time to join the main-breaker bus to the gear-frame bus, the respective sets of bars wouldn't line up perfectly. As a result, the screw holes for each set of bars were out of alignment by about 3/8" at maximum.
No amount of shimming and jockying the main was making a difference, so I ended up taking a C-clamp and using it to squeeze the two sections of bars together until all the screw holes lined up and the bolts could be inserted.
In retrospect, I have a bad feeling that this probably made the torquing of the bolts almost useless: When I took the C-clamp off, and the bars pulled away from each other, it seems like the shear force on each bolt would mean that it would require a lot more torque to get the same compression VS if the bolts dropped straight in the holes and there was no shear force.
Is there the potential for a high-resistance connection there, or am I worrying over nothing? If it is a problem, how should I solve it in the future?
The gear was delivered in sections and assembled on sight, and it was assembled prior to the buses being joined.
As far as plumb, level, square, I honestly didn't check and have no idea if anyone did. Obviously something is out of alignment or else the buses would have been perfectly parallel and there would have been no connection difficulties. But like I said, we tried moving and shimming that main all over the place to get the buses to line up and it wasn't happening. Maybe the buses themselves were installed improperly, but they were factory installed.
Are you saying the only thing now holding the sections together is the bolted bus?
No, the sections are bolted independently of the bus. However, the buses do not line up properly at the joints. There were four bars per phase on the frame that had to mesh with four bars per phase on the main. When they were pushed together, the screw holes should have lined up all the way through. But they didn't, and had to be forced into alignment.
The problem should have been solved before the gear was put back together because it sure sounds like someone was in a big hurry, and /or does not know how to assemble large switchgear...and I don't mean you.
Alright, that answers me well enough, obviously I should have never had to force those bus bars into alignment. And you may not mean me, but I'm partly responsible here either way.
So, to go back to the heart of my original question, does having to force these bars compromise the tightness of the connection, or is there another issue here?
Out of line 3/8"? Did they line up vertically, and horizontally, but at different depths? Are you sure there was not some missing spacers? I remember one that had short sections of buss that went in between and you bolted through that. And it wasnt apparent because you olny needed these if using this section and that one - we had both...
Anyway the question of compromising the tightnes of the connection - I would have to say yes.... the connection is under tension. If they were held in contact at that tension when torqued, maybe not so bad... Hard to tell really, but you are at a reduced torque IMO.
Mark Heller "Well - I oughta....." -Jackie Gleason
YES, you have put the bus "in shear"since the bolts are only for connecting the bus together ..never for alignment !
you have placed stress on every point of that bus but since you are taking some of the blame for this, I'll go easy on you.
the Mfr. should have been notified of the problem, and they would have sent a tech out there if they could not fix the problem over the phone( along with faxing you an approval to make any field mods to their gear )
next time GET IT IN WRITING before you merely decide to just let it go !
I have had to crawl into gear when there have been problems and 1200A is big enough to do that , but I would suggest that the gear needs to be inspected because if the equipment fails.
.the ... will hit the fan, and your butt is in the breeze.
I'll bet that the wedge anchors at the base were never backed off while you struggled to align the bussing.
Could you expand on what the wedge anchors are or where they would've been located? That term isn't ringing a bell.
There is enough play in most assemblies such that you can bolt them together successfully even if they are a tad out of alignment.
This is a good bit to know for future reference.
I wasn't the only one on this gear, there were also a couple of old salts, so I sort of have to assume that at least one of them would have caught anything simple I would've missed. But the fact that there were a number of different guys working on this caused problems in its own right, so anything is possible.