NG TS 3.04.11 PDF

GENERAL DESIGN

The hoop spacer shall be designed to be attached to the upper sub-conductor of an offset pair of conductors. The spacer is intended to prevent conductor - to - conductor contact during bundle collapse under post-fault loads. Spacers shall be fitted at equally-spaced (approximately 60 m) intervals along the conductor. The hoop spacer shall have an outside diameter of 610±10 mm and be made from a double loop of aluminium alloy tube with an outside diameter of 30-35 mm and sealed at both ends. The spacer shall be attached to the conductor at two points within the loop using elastomer-lined clamps. To reduce audible noise on contact with the opposite sub-conductor and also to provide some damping to aeolian vibration the tube shall be filled with approximately 1 kg of kiln dried, silver sand. A pre-formed repair sleeve or armour rod of similar weight to the hoop spacer shall be fitted to the lower sub-conductor to avoid damage during contact with the hoop spacer.

The design of the clamp shall use latching or bolted mechanisms with captive clamp fasteners. Clamping systems shall ensure that the conductor is undamaged by installation of the spacer, either immediately, or in service. Where special tools for installation are required they shall be submitted for The National Grid Company plc (NGC) approval together with the spacers.

The spacer shall not cause opening or damage to the strands when first installed and shall not cause fretting, fatigue or corrosion of the strands in service. The spacers themselves shall be designed to perform satisfactorily for a service life of at least 20 years and shall take full account of environmental factors, including conductor temperature variations between -20bC and +90bC, ultra-violet radiation, ozone and atmospheric pollutants.