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.

MS01 said:

From a containment perspective, what is the minimum separation distance between LV power (230V-400V) and unscreened UTP cable in the UK? 

For copper data cabling indoors, the minimum separation for safety is 50 mm, but in some circumstances, 150 mm is required (see Clause 5.4.4.2 of BS 6701). The requirement  applies even if one or both cables are in metallic containment, and/or separated by metallic screen, and apply regardless of whether BS 7671 requirements for zero separation of different voltage bands.

There are also separation requirements for EMC, in BS EN 50174-2 for indoor cables (required by Clause 5.4.4.1 of BS 6701). This aligns with the guidance in Section 444 of BS 7671 (see A444.4).

Outdoors, an outdoor grade cable is to be used, and again an assessment of EMC, surge protection (if required) as well as insulation for different voltage bands. The requirements are in BS EN 50174-3. You might need to consider Table A444.2 of BS 7671 outdoors, depending on what power cables you are close to.

For both indoor and outdoor cases, the worst-case minimum should be used when safety and EMC are taken into account - for example, if safety says 50 mm and EMC says zero segregation, then use 50 mm - or of EMC says 150 mm and safety 50 mm, use 150 mm.

Some guidance on the above, and the wording of the relevant Clauses in BS 6701, can be found in:

• Annex I, Section I2 and I3 (pages 181-183) of the IET Code of Practice for Electrical Energy Storage System, 3rd Edition; and
• Section 5.13.2 (pages 54-57) of the IET Code of Practice for Electric Vehicle Charging Equipment Installation, 5th Edition.

Data cabling that is susceptible to interference. 

MS01 said:

Data cabling that is susceptible to interference. 

Potentially, all copper data cabling is susceptible to interference, more so unscreened (as UTP). Having said that twisted pair (where the pairs are used as a pair, as in RS 422, RS485 and Ethernet) is less susceptible then untwisted, unscreened, data cable, for example as used for older serial links/RS232 etc. RS423 is a bit of a mix of both, being differential receive and common-mode transmit, so needs care to get the maximum lengths of run.

Provided you meet the safety requirements, follow the guidance in A444.4 for segregation in BS 7671, either the BS EN 50174-2/BS EN 50174-3 approach, or Table A444.2 if the conditions for Table A444.1 are not met.

You can also consider earthed metallic containment (conduit or trunking).

If you let us know the specific application of the UTP, and what are the power cables are doing, and their voltages and currents, we may be able to be more specific.