On a single phase installation there is a small but real chance that an upstream open neutral will result in a dangerous voltage on the neutral, hence the need to isolate the neutral for safe working.

On a three phase system, exactly the same could happen, but isolating the neutral on three phase carries its own risks. If the 4 pole switch opens the neutral just BEFORE the phases, or closes the phase connections just before the neutral, then gross over voltages could destroy single phase loads. For this reason, not isolating the neutral has historically been acceptable on three phase.

These days special 4 pole switches are available, that by design will always open the neutral last and close it first. I expect that neutral isolation via such switches will become a requirement in future on three phase systems.

On a single phase installation there is a small but real chance that an upstream open neutral will result in a dangerous voltage on the neutral, hence the need to isolate the neutral for safe working.

On a three phase system, exactly the same could happen, but isolating the neutral on three phase carries its own risks. If the 4 pole switch opens the neutral just BEFORE the phases, or closes the phase connections just before the neutral, then gross over voltages could destroy single phase loads. For this reason, not isolating the neutral has historically been acceptable on three phase.

These days special 4 pole switches are available, that by design will always open the neutral last and close it first. I expect that neutral isolation via such switches will become a requirement in future on three phase systems.