L4

L4 is a tentative and unclear designation for certain 132 kV towers. The towers in question were already CEB L132-compliant and the term “L132” is also used for these types in a confusing manner.

The designations L4 and L4(H) were used for PL16 and Eve 0.4 towers respectively within ESI Standard 43-3 (Issue 1, November 1972) “Design of Lattice Steel Transmission Towers for Overhead Lines at 132 kV and Higher Voltages”. No explanation for the term “L4” is given. “H” presumably denotes “heavy duty” or similar owing to Eve 0.4 taking Zebra conductor instead of Lynx conductor. Eve 0.175 is not shown, suggesting that it was abandoned in favour of PL16, perhaps simply for consistency.

The only apparent corroboration of this terminology is [Castlereagh–Eden], which states (section 2.1.1, page 4):

In 1966, following the construction of Castlereagh substation, the section of circuit between Finaghy and Rosebank was turned into the new substation. This was achieved using L4 towers strung with ACSR Lynx conductor (rating 82/95/103 MVA).

Examination of the line shows the towers to be PL16. Either “L4” in this document is erroneous, or it refers here to “imperial L4”, specifically PL16.

ENA Technical Specification 43-7, covering L4(m), notes:

The L4 specification was originally issued in 1973 by the CEGB to cover 132 kV double-circuit transmission lines supported on steel towers with 1 x 175 mm² Aluminium Conductor Steel Reinforced (ACSR) phase conductors and 1 x 70 mm² ACSR earthwire against the Electricity (Overhead Lines) Regulations 1970. Subsequently the L4 specification was incorporated into ENA Technical Specification 43-7 Issue 1:1978

ESI 43-3 (1972) pre-dates the CEGB L4 specification (1973), but nothing yet is known to date as far back as 1966.

Contradicting all of this are the L4(m) drawings, the earliest of which date back to 1973. These drawings refer to specification L4(M)-1972 (also written simply L4M 1972).