Hiya Michael
I don't have any personal experience of mounting a mast that close to a building transformer, but a few ideas popped in to my head with regards possible problems, so I thought I would 'throw in my pennyworth'.
The first obvious point is that because of the potential danger of what could happen if the mast became somehow connected to the transformer is something you will have to give serious consideration to. If the mast were to be securely fastened to the building then the risk of the mast falling over on to the transformer should be minimal, but it's worth keeping in mind.
Also, depending on whether or not there is public access to the mast you have to also consider the possibility that someone (children perhaps, or drunks) might decide to climb the mast, just as a prank, or maybe even to steal your antennas! So some form of anti-climbing 'shroud' around the mast base might be in order (better safe than sorry), just in case someone were to climb the mast and fall onto the building transformer
As regards interference it would be a good idea to ground the mast if possible, since there is a possibility that you could have capacitive (or even inductive) coupling occurring between the mast and the transformer, and in so doing give rise to wide-band noise problems while receiving on HF etc.
The biggest problem these days is all of the HF noise carried on the mains supply from the many various switch mode supplies that are in use in increasing numbers in electrical goods. Such 'noise' tends to travel for considerable distances along supply lines, and can render whole chunks of the HF bands useless. There is also a growing problem with BPL (Broadband Over Power-line) devices, they really do chew up the HF spectrum for considerable distances around the point of origin.
So worst case scenario is that with a few of those devices in use around the neighbourhood, the noise could end up 'focused' around the general area of the building transformer and cause problems with your club receiver/s.
I'm not sure if you do intend to use HF antennas such as wire dipoles, but again I'm going to state the obvious and mention that you need to be sure that any wire or feeders can't fall down and make contact with the transformer, or indeed fall on some unsuspecting member of the public.
Just thinking about the possible interference problems themselves, I've found that BPL devices and fluorescent light fittings etc. cause the most headaches on HF/6m, whereas computer monitors and digital TV decoders are the main culprits on 6m-VHF/UHF.
All of the above is a 'worst case' response to your question, but I thought I would set the ball rolling by throwing in a few ideas (hope I haven't put you off the project too much!)
Best 73, Mark.
