11 kV power lines (UK)
Contents
General
These are the power lines typically used to feed villages and farmhouses (in some cases the final leg may be 400 V or 230 V). 11 kV power lines are single circuit on wood poles. Terminal poles with ceramic insulators always have one or two discs; those with three discs will be 33 kV. Unlike with 33 kV and above, junctions and circuit breakers are common. Terminal poles frequently feature a transformer, whereas higher voltage lines are transformed at substations.
The design appears to be broadly the same across the UK: a crossarm affixed to a wood pole. The centre pin insulator is typically offset to one side of the wood pole, alternating between the left and right sides of the pole along the power line. Section poles often have two crossarms back-to-back, one either side of the pole. Terminal poles may have one crossarm (on the far side of the pole, to brace the tension pull against the pole) or two as per a section pole.
Smaller deviation angles can be accommodated on basic pin insulators, while larger angles require section poles with tension insulators. All poles except intermediates are cable-stayed. The largest angles are formed of two terminations on the same pole, instead of a single extra-wide crossarm.
Transformers, fuses, circuit breakers and tee-off points feature frequently, and reclosers can sometimes be found. Some spurs are formed of two out of the three phases only.
11 kV lines can also be found on steel masts and steel tower lines. Steel tower lines running at 11 kV will have been de-rated from a minimum of 33 kV. The only known line using single-circuit Blaw Knox K4611 lattice towers is 66 kV de-rated to 11 kV.
BLX
As with other voltages, 11 kV lines typically use bare conductor, often HDBC (hard-drawn bare copper). There are also insulated-conductor lines, including ABC (aerial bundled cable: all three phases in a single cable) and BLX. The derivation of “BLX” is not entirely clear. “BL” stands for “Belagt Linesystem”, Norwegian for “coated line system”. The conductors are insulated with XLPE (the suggested derivation of the “X” in “BLX”), cross-linked polyethylene (also called PEX and XPE). BLX is a Scandinavian design that appears to be intended to withstand fallen trees without suffering shorting between phases or broken conductors. It is briefly covered on pages 32 and 33 of SPEN, severe storms. There is also some discussion of BLX on Facebook.
The insulation on the conductors allows for the conductors (which for SPEN is 50–150 mm² stranded aluminium alloy) to be much more closely spaced. SPEN note that this allows for “much reduced” wayleaves versus conventional 11 kV lines. The closer spacing and shorter crossarms mean that the impact from a fallen tree or wind-blown object would “apply much less torque to the pole top”.
SPEN also report better visibility to water birds, reducing bird strikes.
BLX is not without its disadvantages. Connecting a portable earth is more difficult as the conductors are insulated. SPEN note that, anecdotally, some BLX lines have had sections reconductored with bare wires, leading to clashing problems. Anecdotally, BLX is felt to harder to work with, and seemingly not the most popular system with line workers.
The coils around the wires are said to be vibration dampers.
The photos below are not proven to be BLX, but they match the description, although SPEN’s single photograph does not show the vibration dampers. One commenter reported that BLX is popular with SSEN’s England region (SSEN South), and two of the photos below are from SSEN South. Unfortunately, no photos of BLX under good light conditions yet.
Sindlesham, near Reading, Berkshire, SSEN South:
Note the (as suggested) vibration dampers on the standard-width 11 kV, suggesting that BLX conductor has been used on existing poles and crossarms.
Bricket Wood, Hertfordshire, UKPN:
The spacing is a little unclear (especially on the intermediate poles), but seems considerably narrower than regular 11 kV. Only two phases are present.