[stuarth]

In the music studio scenario, eg the BBC, you could have a band come into the studio with various pieces of equipment in assorted sizes and a box of mains leads and extension cables. You would have to check every piece of equipment, and both ends of every mains cable. If you miss one, including that extra cable someone nipped back to the van for, someone could die.

So you take the safe option and remove the earth at the transformer output. If there are any N-E swaps, the equipment will not power up, and it's all fail-safe.

Effectively, the isolation transformers in the studio have only one job, and never need an earth connection (no hum loops either!). For a wider range of applications, adding two 4mm sockets to link the output socket earth to mains earth if required is useful (wouldn't do in the BBC case, someone in the band could re-connect the earth when trying to get their equipment to power up - horses for courses).

Stuart