- Not to connect pin1 to the shell of an RCA?
or- Something else?
Something else. There's something called the 'pin1 problem' dealing with balanced lines. Quite often the ground tab of the XLR connector gets tied to pin 1 (they are right beside each other on the connector, contributing to the confusion around this).
You can get into noise problems with this. We avoid that by simply leaving the ground tab alone and only using pin 1.
Another area where this is problematic is trying to get single-ended gear to work with balanced equipment, while also supporting AES48. I'm not going into this in depth, but to give you an idea of the mayhem that can go on let's take the example of a magnetic phono. Its a balanced source, but 99 44/100ths% of the the time is treated as single-ended. But you wind up with the 'ground wire' that other single-ended sources don't seem to need.
(Yet I still run into tonearm manufacturers that are unaware that magnetic phono cartridges are a balanced source!)
You can ground the ground wire to the ground of the RCA and quite often it will work fine. But every now and then it doesn't.
Its that fiddly bit where it doesn't always work that's the problem.