Cable Duct Sealing

Catastrophic Consequences

The Risks Of Cable Duct Sealing Failure

Failure to correctly and effectively seal cable ducts can inflict catastrophic damage to LV HV electrical distribution networks, substations and power supply.


Duct seals are installed to reinforce the flood defences of LV MV HV substations protecting critical low, medium and high voltage cables and electrical equipment. Typically at:

         Medium Voltage (11kV/33kV)

         High Voltage (66kV-132kV)

         Transmission & Distribution Grid Networks

T&D distribute proven cable duct sealing products from leading manufacturers Filoform with widespread DNO and water utility approvals and industry acceptances, including WIMES.

        + Trusted alternatives to traditional “sticky and tricky” to install                       mastic seals or regulation violating expanding foam

        + Guaranteed protection without the risk of failure

FiloSeal+ HD Fire FiloSeal+
FiloSeal+ Water And Gas Cable Duct Seal Filoform

Filoform FiloSeal +HD is a duct sealing system that is able to seal large heavy armoured power (multicore XLPE SWA) and high voltage cables.


  • Strong support system using hexagonal tubes
  • Re-enterable seal with honeycomb structure
  • WIMES compliant (3.02 Clause,
  • Approved for use by UK DNO's and utilities
  • Gas and water tightness for cable ducts up to 250mm outside diameter
FiloForm Duct Seals

Filoform FiloSeal+HD FIRE is a new highly certified fire, gas and water duct sealing system for both ducts and cable transit frame systems.


Tested to BS EN1366-3 2009.


  • Strong support system using fire resistant hexagonal blocks
  • Made of specially formulated material called ‘Formite’
  • Suitable for sealing empty or any cable configuration or pipes contained in one duct
  • Suitable for sealing LV MV HV cables into substation ducts where fire protection is required


Blog: Fire Stopping Cable Ducts

FiloForm Duct Seals

FiloSeal+HD FIRE has now been tested by London Underground for the safe and effective sealing of cables and ducts on the underground rail network.


The duct seal system has been rigorously and successfully tested against smoke and toxicity emissions according to LUL S1085 Standard.


London Underground Tested Cable Duct Seals

A case study of a cable sealing and substation asset protection project in the USA.


The MV medium voltage cables are installed 20 meters underground in a substation that is providing electrical power distribution to a subway metro station.


This is one of many sites where they have ongoing moisture problems associated with floodwater and environmental conditions.


By using FiloSeal+HD the client improved the environment inside the substation, reducing moisture, which was their electrical and substation protection objective.

Underground Substation Cable Sealing

Severn Trent Water


The Whitacre / Warwickshire team identified a shortfall in the way cable ducts are sealed following installation of new and the maintenance of existing assets.


Several options to securely seal cable ducts have been investigated with support from Water R&D at the water utility.

Duct Sealing To Prevent Pollution Of Potable Drinking Water

Project Information


All 3 types of FiloSeal cable duct seal systems were required; original Filoseal+ with foam backing, newer Filoseal+HD for heavier cables and the newest seal type Filoseal+HD FIRE which adds BS EN1366 Part 3 with up to 4 hours of fire protection.


All four cable duct seals were installed according to instructions whilst special guidance and tips were given on site by the manufacturer

Sealing Ducts And Cables Against Water, Gas & Fire

Project Information


The Filoseal cable duct seal system was selected to seal 50mm and 110mm outside diameter cable ducts to provide up to 2 bar Pressure Resistance and prevent water ingress into the low voltage control rooms effecting sensitive electronic and electrical equipment.

Sealing Cable Ducts Containing Low Voltage Control Cables

Filoseal+HD duct sealing kits have been tested to provide watertight, gastight and/or fire resistant cable seals and were thus specified to fill in the voids in the concrete core drilled holes at the data centre.


The duct sealing system has been heavily tested against pressure, pulling forces, chemicals, gases and hydrocarbons for use with low, medium and high voltage cables.

Sealing Cable Ducts In A Data Centre
Cable Jointers LV MV HV 11kV 33kV

Thorne & Derrick supply 3M Electrical, Prysmian, Filoform and SPS low voltage cable joints suitable for connecting and jointing single and multi-core/pair power, control and instrumentation cables up to 600/1000V (3.3kV) in industrial and hazardous area locations.


Specialist applications include fire resistant, low smoke zero halogen, hydrocarbon resistant and utility cable joints utilising Cold Shrink or Scotchcast resins.

LV Cable Joints - Cold Shrink, Heat Shrink & Resin

Heat shrink, cold shrink, push-on and slip-over cable accessories enable the jointing, terminating and connection of 11kV-33kV and 66kV-132kV cables to oil, air or gas insulated switchgear, transformers, motors and overhead lines distributing electricity at medium/high voltages.


Stockists of separable connectors including Pfisterer CONNEX and Nexans Euromold for inner or outer cone cable termination.

HV Cable Joints, Terminations & Connectors

Arc flash clothing and protection equipment including coveralls, gloves, helmets, face shields, insulating gloves and general head-to-toe PPE is used to protect against flashover. 


Contact us to discuss and calculate your arc flash protective clothing requirements.


Arc Flash Helmets

Insulating Boots


BLOG : Can Arc Flash Clothing Save Utility Workers Lives?



Arc Flash Clothing, PPE & Flash Protection Kits

A complete range of cable cleats manufactured by Ellis Patents used to clamp and retain low, medium and high voltage cables in the event of a short circuit, including 11kV/33kV MV-HV power cables – single, trefoil, quadrofoil and multiple bundled cable cleat configurations.


LV MV HV EHV Cable Cleats


BLOG : IEC 61914 – Cable Cleats & Short Circuit Protection Calculations



Cable Cleats

The complete range of electrical connectors for jointing and terminating cables and connecting conductors into low, medium and high voltage switchgear, control panels and transformers using compression (crimping) or mechanical shearbolt (“snap-off”) type cable lugs and connectors.


BLOG : BS7609 | Crimping Cables With Cembre Cable Lugs & Tools

Cable Crimps, Lugs & Connectors

Cable cutting tools to suit LV-HV cables with copper or aluminium conductors, steel wire armoured (SWA) / braided cables with lead sheath and insulation types including XLPE, EPR and paper types – commonly used to cut armoured LV cables and medium/high voltage power cables 11kV-33kV, single and 3 core type.


Cable crimping tools using hydraulic or battery operated to crimp conductors 1.5sqmm-1000sqmm suit aluminium and copper conductors with sector shaped, stranded or solid cores.

Cable Cutting & Crimping Tools

Cable jointing tools for stripping and preparing low, medium and high voltage power cables including 11kV, 33kV, 66kV and 132kV-400kV polymeric cables with XLPE or EPR insulation and bonded/peelable semi-conductive cable screens.





T&D distribute Cable Jointing Tools from leading manufacturers including Alroc, Ripley, Boddingtons and Hivotec for all types of LV, MV or HV cable stripping, preparation and jointing requirements – insulated tools are available for live-working on low voltage cables, joints and electrical equipment up to 1,000v.

Cable Jointing Tools

Cable protection covers, tile tapes and underground warning tapes are installed over LV, MV and HV power cables in trench and ducted cable installations with high visibility colour providing a visual warning of danger when excavating cables.


11kV and 33kV cable protection tapes (Tapetile and Stokbord covers) are manufactured from heavy duty plastic for reliable protection.


Underground protective covers are used to provide a clear and effective visual warning to the presence of LV-HV underground cables, cable duct or high voltage cable joints.


BLOG : Tape Tile & Stokbord Cable Protection Covers For MV HV Power Cables

Cable Protection Covers

Thorne & Derrick distribute the most extensive range of Cable Pulling & Cable Laying Equipment to enable the installation of low, medium and high voltage power cables into underground trench or duct. 


Cable Rollers | Socks | Duct Rods | Jacks | Lubricant

Cable Pulling & Cable Laying

Cable Transits manufactured by Roxtec to provide effective sealing of single and multiple low and high voltage cables and ducts against constant water pressure, fire spread and gas migration.


Extensively specified in the Utility Power, Transmission & Distribution sectors for sealing cable ducts and protecting 11kV/33kV medium/high voltage substations against flood risk.

Cable Transits

Earthing & Lightning Protection products including bare and covered copper earth tapes to provide effective earthing protection to LV, MV & HV substations, buildings and electrical infrastructure supported by a complete range of earthing products such as earth rods, plates and lattice or solid type copper earth mats.


BLOG: Earthing System Design Services 

Earthing & Lighting Protection

T&D supply both standard and bespoke feeder pillars in a selection of materials, paint finishes and dimensions to meet all indoor and exterior power distribution, control and lighting requirements up to 1,000 volts with IP65 door seal – this includes LV substation and network pillars with DNO supply cut-outs for utility applications.


Feeder Pillars

Non-radial, rough and jagged semicon screens with protuding points at the cutback will cause termination or joint FAILURE.




See our range of Jointing Tools including fully bonded and peelable ("easy strip") semicon screen removers


Also : MV Heat Shrink Cable Terminations Can Fail Despite Passing VLF Tests

Cable Jonting Tools 11kV

In this BLOG we discuss to processes and Furse products used for the Earthing of electricity substations with site installation photographs from an EHV 275kV substation earthing project.


Thorne & Derrick promote professional businesses providing leading technical skills and services to the power distribution industry.


Take a look at our Guest Articles.


The Distribution Network Operator (DNO) Transition to Distribution System Operator (DSO)


UPS – The Key Role of Back-up Battery Testing & Maintenance For Reliable Power Supply by BCL Power


Cable Ploughing | Meet ATP The European Utility Ploughing Specialists

Cable Ploughing MV HV 11kV 33kV

The following sequence of photographs provide a behind the scenes glimpse of Cable Splicer Training at the Quanta Services Lazy Q Ranch, a state-of-the-art facility where the Art & Craft of Cable Splicing is alive and kicking.


Cable Splicing At The Lazy Q


PILC - Cable Longevity


Cold Shrink Splicer Training


“You’re Just A Cable Splicer!”


“You’re Still Just A Splicer!”


Special thanks to James Craven & John Perez - Quanta Services.

More Info

BASEC are collaborating on knowledge and content sharing with Thorne & Derrick.


With a shared passion and ambition to enhance LV MV HV electrical safety by the education of industry stakeholders to specify and buy accredited, tested and certified Quality Cables & Cable Accessories.

Cables LV MV HV

The following article has been republished with kind permission of Martha Davis (Senior Director of Content – T&D World and Utility Analytics Institute) from Transmission & Distribution World and their video sponsor Burns & McDonnell.


The 5 Videos videos are real-life and shot-on-site, showing transmission and distribution crews and splicers at work.

High Voltage Electrical Cable Jointers
Nexans Cables Nexans Cables
Nexans Cables Nexans Cables

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Thorne & Derrick are specialist distributors of LV, MV & HV Cable Installation, Jointing, Earthing, Substation & Electrical Eqpt – servicing UK and global businesses involved in cable installations, jointing, substation, overhead line and electrical construction at LV, 11kV, 33kV and EHV.

Product Categories: Duct Seals | Cable Cleats | Cable Glands | Electrical Safety | Arc Flash Clothing & Protection | Cable Jointing Tools | Cable Pulling & Laying | Earthing | Feeder Pillars | Cable Joints  LVJoints & Terminations MV HV | Cable Transits | Fuses MV HV | Cold Shrink