Single-Core Cables Laid in Trefoil or Flat Formations – Advantages & Disadvantages

Single-core power cables can be run in a number of formations, the most common include flat or trefoil formations. Each cable formation has its benefits and drawbacks, we’re going to look at the differences between each way for laying these cables.

Trefoil Phase Formation

One of the main reasons trefoil formations are used is that it places the phases the same distance apart, so the magnetic field and circulating currents are equivalent for each cable phase. Typically, trefoil phase formation is more commonly used for Low and Medium Voltage applications up to 132kV due to ease of installation and the reduction in space the formation has across the containment system used. However, installing single-core cables in trefoil formation can mean that the touching cables will exhibit worse heat-dissipation when compared to flat formation, therefore lowering the current-carrying capacity. Considerations which should be taken into account by specifying electrical engineers when designing such systems.

Flat Phase Formation

Rarely used at voltages below 275kV due to the mounting centres having to allow for sufficient heat-dissipation. Running in flat formation, the central phase of the three-phase set is adversely affected by the magnetic fields around the neighbouring phases, leading to a higher running temperature on the middle phase and subsequent voltage imbalance. Phase transposition can be used to counter-act these affects, but the necessary mounting space required for such installations usually precludes their use in most industrial/commercial installations and flat formation tends to only be used by major DNOs in their distribution networks.

Trefoil formations are chosen for applications where space is at a premium and flat formations also tend to be less cost effective due to the increase potential cable route space they take up. Our experience of such installations has generally been with cables above 132kV, with cables under 132kV being laid in a trefoil formation.

Triplex Formation

An adaptation of the trefoil formation can be found with the increasingly popular Triplex formation. Triplex cables are three conventional single-core cables supplied pre-wound in trefoil formation by the cable manufacturer/vendor, the three cores having been slowly twisted together during the manufacturing process and supplied on one drum, offering significant benefits in installation time.

Due to the constant phase rotation of the cores, using conventional trefoil cleats is not possible as the position of the mounting base will not be constant. For this reason our Triplex Cable Former provides a cleating solution, allowing traditional trefoil cleats to be used.

Cable cleats suitable for each cable formation:

Flat Trefoil Triplex
2 Bolt Aluminium Alpha 2 Bolt Nylon LSF*
2 Bolt Nylon LSF Vulcan Vulcan+ Trefoil*
Emperor Single Way Emperor Emperor Trefoil*
Vulcan+ Single Way
*With use of triplex cleat liner

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