There are really two considerations insulation failure /damage- what sort if cable is the UTP (would the jacket of the lower rated cable hold off mains voltages ) if so then they could be as close as you like,otherwise it should be segragated by split duct or similar.

More accurately BS 7671 prohibits Band I and Band II cables sharing the same cable enclosure or multicore cable unless: every cable is insulated for the highest voltage present, or each conductor in a multicore cable is insulated for the highest voltage present, unless conductors of the two bands are separated by an earthed metal screen,
or they are installed in separate compartments of a trunking or ducting system, or they are installed on a tray with a partition providing separation,
or a separate conduit or ducting system is provided for each band.

but the other factor is EMC - how much voltage can we induce in the UTP from the mains side before things go wrong. This depends on how long is the region of close approach - a long parallel section is far worse than a cross at 90 degrees.

Different standards call up differing figures  but the closest used is usually 50mm but more is good ;-)

however it is more complex than that see here (note the non UK standard classes of signals)

www.electrical-installation.org/.../Wiring_recommendations

Mike.

There are really two considerations insulation failure /damage- what sort if cable is the UTP (would the jacket of the lower rated cable hold off mains voltages ) if so then they could be as close as you like,otherwise it should be segragated by split duct or similar.

More accurately BS 7671 prohibits Band I and Band II cables sharing the same cable enclosure or multicore cable unless: every cable is insulated for the highest voltage present, or each conductor in a multicore cable is insulated for the highest voltage present, unless conductors of the two bands are separated by an earthed metal screen,
or they are installed in separate compartments of a trunking or ducting system, or they are installed on a tray with a partition providing separation,
or a separate conduit or ducting system is provided for each band.

but the other factor is EMC - how much voltage can we induce in the UTP from the mains side before things go wrong. This depends on how long is the region of close approach - a long parallel section is far worse than a cross at 90 degrees.

Different standards call up differing figures  but the closest used is usually 50mm but more is good ;-)

however it is more complex than that see here (note the non UK standard classes of signals)

www.electrical-installation.org/.../Wiring_recommendations

Mike.