As someone else has said, you may disagree with the regulations (and I
can understand why - the system is unlikely to be unsafe in practice)
but they do state that all cables run together should be insulated for
the highest voltage present:
"A band I circuit [e.g. telephone] shall not be contained in the same
wiring system as band II [e.g. mains] voltage circuits unless one of the
following methods is adopted:
(i) every cable is insulated for the highest voltage present
(ii) [discusses multicore cable]
(iii) [multicore again]
(iv) the cables are insulated for their system voltage and installed in
a separate compartment of a cable ducting or trunking system
(v) [trays and ladders]
(vi) a separate conduit, trunking or ducting system is employed.
For telecommunication circuits, data transfer circuits and similar,
consideration shall be given to electrical interference, both
electromagnetic and electrostatic. See BS EN 50081 and BS EN 50082."
Comments in  by me.
In the OSG this is tabulated in tables 7.3A and 7.3B, section 7.3.3 page
As you have found, there is also a very real risk of interference.
Standard mains twin and earth isn't a particularly noisy cable, as equal
and opposite currents flow in close proximity, but get *too* close and
you'll hear it. (Just thought - if it's a ring circuit it's worth
checking that both L and N are continuous right around the ring.
If one of them is broken this will lead to unbalanced currents in
the loop and will greatly increase the potential for
interference). Proper telephone cable should have a twist in each pair
which also helps reduce interference, but it isn't perfect.
If you *must* wire this closely, instead of standard telephone
cable, choose something like Cat 5 cable which is usually (though not
neccessarily always) insulated sufficiently (some brands are marked
1000V) and has a much tighter pair twist.
Martin Angove: http://www.tridwr.demon.co.uk /
Don't fight technology, live with it: http://www.livtech.co.uk /
Thank you those who replied. I have now checked the wiring & found no
visible fault. The cable looks correct: solid conductors; colour-
coded as on WPP website; pins 2 , 3 , 5 connected in correct colours
at both ends; 4 not connected. I cannot tell if the cable is twisted
without stripping some and there is no spare length. Massive resistance
between conductors ( > 10 megohm). Unplugged master socket, shorted one
end, there-and-back resistance about 3 ohm. Sometime I hope to lift a
floorboard & see how close it runs to the mains cable...
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