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Developed and maintained
by the NFCC

Sealing end compounds


Where a high-voltage underground cable joins an overhead line, the transition from one to the other usually takes place at a 'sealing end compound'. The overhead line finishes on a terminal tower. Downleads bring the conductors down to join the ends of the underground cables where they come out of the ground.

There may also be some monitoring equipment, but usually no transformers or switchgear.

Lower-voltage lines could transfer from overhead to underground on the tower or pole itself, without needing a separate compound. However, the space required at 275kV and 400kV to maintain all the clearances usually precludes this.

Figure 3: A terminal tower - photograph courtesy of Peter Martin

This equipment is easy to recognise due to construction and location. In addition, utility companies provide ample signage warning of high voltages and relevant dangers.

Hazards (for further information refer to National Operational Guidance: Utilities and fuel)

  • High risk of electrocution for anyone entering the perimeter of the compound
  • Collapsed steel framework caused by the intense heat of a fire
  • Exploding glass and porcelain insulators, throwing fragments with considerable force

References and further reading