[eigenvector]

Are GP mentioned, the safety procedure to protect you from a line you're working on becoming inadvertently energized is temporary protective grounding. A grounding jumper is connected to each phase of the power line near the work location so that the worker touching the line works in a zone of equipotential. Because everything the worker is touching bonded together, if the line becomes energized the worker's hands and feet are elevated to the same voltage simultaneously, so there is no potential difference across the body and no current flows through the worker.

But since lives are at stake here, we prefer to have layers of protection rather than just one thing. Grounding jumpers can fail if not properly connected (a large mechanical force will be applied to the jumper when the phase-to-ground fault current flows through it).

Bottom line, respect the electrical code and connect generators through a transfer switch. These rules are in place for a reason.

[mycall]

Could an isolating transformer be an alternative to connecting generators through a transfer switch?

[swiley]

Transformers only block DC. Unless your house is wired for that (pretty rare in almost every country) You'll backfeed AC which will go through transformers.

In fact it's likely the transformer on the street between your house and the network that's responsible for the death of the linemen. IIRC that's a step down transformer (so step up when run backwards like this.)