Trefoil Cables – Sealing Ducts & Cleating Trefoil Cables Up To 33kV
Published 22 Nov 2017
by Chris Dodds T&D - estimated reading time 5 minutes
Before we demonstrate an effective duct seal solution for trefoil cables let us first define trefoil and cover the importance of selecting correct cable cleats to prevent short-circuit inflicted damage to power systems and electrical infrastructure.
Cable installations for 3 phase power installations, whether LV or MV (600/1000 Volts to 11kV/33kV) utilise single conductor cables typically in flat or trefoil cables formation – trefoil cable cleats are used to restrain 3 single core cables installed in the “shamrock” style formation, commonly referred to as trefoil.
Where a 3 phase cable system design is based on trefoil cables formation The 17th Edition Wiring Regulations (BS7671) can be consulted to provide current ratings and voltage drop values for the installation.
Trefoil cable formation refers to the installed method for low, medium and high voltage power cables where three single core (phases) distribute the electricity supply rather than a single 3 core cable cable.
Depending upon the short circuit calculation according to IEC 61914 the appropriately fault-rated cable cleats should be used to provide cable restraint depending on whether the LV, MV or HV power cable system generates short, medium or high short circuit levels.
According to the installation application and environmental conditions trefoil cleats are available in a selection of materials including nylon, stainless steel and aluminium.
Clearly cleating cables is essential, as is effective sealing of cable ducts to prevent flood damage to substations and buildings or the spread of potentially flammable gases.
Sealing Trefoil Cables In Substation Ducts
Single core 11kV-33kV cables in “triplex formation” according to British Standard BS7870 require effective sealing of cable duct entries against water, gas and fire on medium/high voltage substations as well as cleating to protect against short circuit situations. Triplex cables are characterised by the appearance of 3 core cables twisted around each other.
Trefoil cable arrangements entering substation ducts can be difficult to seal using traditional mastic or inflatable cable duct sealing systems, however the FiloSeal+HD duct sealing system effectively seals single, trefoil or multiple cables in duct. The single core cables are carefully separated and supported using the FiloSeal+HD hexagonal tubes – note the central positioning of the hexagonal tubes to establish the required HV cable separation and allow maximum flow of the duct sealant into the trefoil cable formation.
The hexagons separating the pictured 11kV cables must have the smaller hexagonal tubes nested in the large one to create additional strength.
Where the cable ducts are located in a designated hazardous area according to the ATEX Directive we can provide expert advice and product recommendations for sealing cables in potentially explosive atmospheres.
Thorne & Derrick International
Since 1985 Thorne & Derrick based in the UK have been specialist distributors of LV, MV & HV Cable Installation, Jointing, Substation, Arc Flash Clothing & Electrical Safety Equipment – customers include UK and worldwide contractors, specifiers, traders and end-users involved in cable installations, cable jointing, substation, overhead line and electrical construction at Low, Medium & High Voltages.
We are specialist stockists and distributors of 11kV-33kV Cable Joints, Terminations & Connectors from manufacturers including 3M, Pfisterer, Prysmian and Nexans Euromold in a selection of jointing technologies including heat shrink, cold shrink and push-on.