16 Oct Specifying Underground Ducts For Power And Telecom Runs
A duct route that looked straightforward on the services drawing has a way of turning complicated at utility submission. The depth gets queried, the formation does not match what the authority expects, or the product specified turns out to be certified for a different application from the one it is going into. None of these is hard to avoid, and all of them are far easier to settle before the trench is open.
Underground ducting sits awkwardly for specifiers because no single authority owns it. Depth of cover in a public street is LTA territory, the incoming supply arrangement answers to the electricity licensee, telecommunications provision falls to IMDA, and the product itself answers to whichever standard the manufacturer produced it to. Knowing which document governs which decision is most of the work. A run of uPVC electrical conduit inside a building and a duct buried under a carriageway attract entirely separate requirements, even where the two look alike on a materials schedule.
How Deep Do Underground Ducts Have To Be?
For works in a public street, depth of cover is set by LTA. Its Code of Practice for Works on Public Streets specifies minimum depths measured from road surface to the top of the utility pipe or cable, and the figures differ by road type. Services are not permitted in an expressway carriageway at all. Elsewhere the code sets a minimum of one metre under major arterial, primary and local access road carriageways, and 1.2 metres at sidetables, with a shallower alternative of 0.3 metres available for fibre optic cables only and subject to safeguarding conditions.
Those are street minimums, not a universal rule. Depths inside a development, under a building or across private land are set by the relevant authority or the project’s own engineer, and a duct crossing beneath a drain or sewer picks up clearance requirements from PUB on top of everything else.
Establish the governing depth for each segment of the route before fixing levels. A duct laid too shallow is among the more expensive things to put right, because correcting it means reopening the trench and relaying the formation.
What The Incoming Supply Arrangement Requires
The lead-in from the street to the intake position follows the electricity licensee’s requirements instead of the general street works code. SP Group’s guidance on applying for an electricity connection sets out what the customer provides: uPVC lead-in pipes running from the intake point to a defined distance beyond the boundary line or the roadside drain abutting the public road, laid within a stated maximum depth from finished road level. It also requires the installed pipes to pass mandrel tests before inspection, and draw pits to be positioned strategically, including at each turn of the service cable.
The mandrel test is the requirement that catches contractors out, because it tests the finished route and not the individual components. A duct deformed by point loading during backfill, or a joint pulled slightly out of line, will fail the mandrel even though every length arrived on site in good order. Bedding and backfill discipline is part of the specification, then, and the same care that goes into burying uPVC pipes underground for drainage applies just as directly to a duct formation. Telecommunications provision runs on a separate track again: IMDA’s Code of Practice for Info-communication Facilities in Buildings governs what a development has to provide for telecoms infrastructure, and developers should work from the current published edition, since IMDA periodically reviews the code.
Getting The Formation Right
Duct formation affects both installation and every future cable pull. Ducts laid in a regular grid, held apart by spacers and bedded in fine material, keep their alignment through backfill and leave predictable positions for anyone opening the route years later. Ducts tipped loosely into a trench shift during compaction, and a formation that has wandered makes the eventual pull harder than it needs to be. Spare ways are cheap at the point of laying and very expensive afterwards, which is why most routes are specified with more ducts than the immediate cable schedule calls for.
Which Standard Does The Product Answer To?
This is the question that most often goes unasked. The BS EN 61386 series covers conduit systems for cable management and is divided into parts by application: Part 21 sets the particular requirements for rigid conduit systems, applied together with the general requirements in Part 1. Buried applications are addressed by a different part of the same series, so a product produced to Part 21 has been assessed against the requirements for rigid conduit, which is not the same as being assessed for direct burial.
Vicplas manufactures its electrical conduits and accessories to BS EN 61386-21. Where a project involves buried ducting, confirm the intended application directly against the manufacturer’s product documentation before ordering, and confirm it with the receiving authority as well. Asking that question at tender stage takes a few minutes. Asking it after delivery rarely does.
What To Confirm Before Ordering
Most avoidable problems on a duct route trace back to a handful of details that were assumed instead of checked, and each is a short conversation at tender stage but a variation order once the trench is open:
- The governing authority and minimum cover depth for every segment of the route
- Whether the product is documented for the buried application, not only for rigid conduit use
- The lead-in requirements of the electricity licensee, including the mandrel test
- Draw pit positions, particularly at every change of direction along the cable route
- The number of spare ways the route should carry beyond the current cable schedule
Conclusion
Underground ducting rewards a specifier who reads the right document for each decision. Establish the governing depth for every segment, follow the licensee’s requirements for the lead-in, lay a formation that survives backfill, and confirm that the product on the schedule has been assessed for the application it is going into.
Our team can walk you through the conduit and accessory range and point you to the documentation a submission will need. Reach out to us and we will help you work out what fits.