Under British regulations I'm not aware of anything which prohibits LV power sharing a duct with optical fibre; it isn't unusual to see this type of installation.
We don't see a lot of conduit in this country, and most of what we have is small diameter (20mm / 25mm) and wouldn't be used for a fibre route.
@ Scotty.... Why 20mm / 25mm dia. conduits aren't used for the FO circuits as what you have mentioned ? I believe these should be OK for smaller cables like F.O 4 core etc.
21mm is the trade size for 3/4" conduit in the NEC. The minimum bending radius permitted for 3/4" conduit in Chapter 9, Table 2 of the NEC is 4 1/2" or 114mm, so with some fiber cable you might need to use a bender with a larger bend radius. The biggest issue would be conduit fittings which would have a smaller bending radius than what would be permitted for the fiber cable.
770.110(B)(2)indirectly permits non-conductive fiber cables to be installed in the same raceway with power conductors.
Since it is Fiber Optic, there is no risk of electromagnetic interference. Also, please note that in a 220 kV Over Head Line (OHL), it is common to take the fibers through the OPGW (Optic Fiber + Ground Wire) conductor. These fibers are along with the phase conductors for many miles.
Please don't forget that these fibers are used for critical applications such as substation to substation communication, SCADA, Feeder Differential protection etc. This is a common practice, even in KSA (Kingdom of Saudi Arabia??). By the same token, I do not see any reason, why the same conduit cannot be used, unless there is some abnormal temperature rise issues are the limitation.
To be honest we rarely see conduit used in the UK except for indoor small / medium power installations in factories and commercial premises. The majority of external work and heavy industrial applications use single wire armoured cables. Most fibre I see in the UK is either wire armoured, or kevlar armoured in applications where it's desirable to avoid metallic interconnection of the two locations, because that follows typical British practice. If the fibre in question was non-armoured and it was possible to stay within bending radius constraints then I guess it could be routed through conduit.
krisys - it is true that fibre itself is immune to electro-magnetic interference, but that's not always true of the overall FO cable where metal wire, braid and tape armours are all possible construction variants. Metallic armour can introduce touch and transfer potentials if only one end is earthed, or provide a path for circulating currents if both ends of the armour are earthed.
