Probably a few considerations. Firstly (safety first) you probably have to take a step back and decide how shock protection is provided for the PTC circuit. Depending on the circumstances it might be SELV/PELV or FELV. If the latter you'd be obliged to treat it like mains - i.e. mains rated insulation on the cables, but segregation from LV circuits is less of an issue. Treating it as SELV/PELV and using more lightly insulated cables, you need to ensure that it cannot attain mains voltages - either due to a fault at the supply end, or due to lack of separation between the sensor and live bit at the machine end, as well as adequate separation/insulation along cable runs.

Next there are functional requirements - might EMI from the mains cables interfere with the PTC signal for instance? If so screened cables or wider separation might be called for.

(and before anyone tells me off, if this is part of a "machine" rather than building wiring, it may well come under a different standard to the usual wiring regs, and while the same general principles apply the details might vary somewhat.)

   - Andy.