Trenching for Underground Mains: Is There a Depth Requirement?

September 6, 2019

Power distributing networks form intricate, line-heavy diagrams when they're set down on paper. Operating as step-down systems, they march out from their generating stations, head towards transmission substations, then take to the air on heavy cables. It's only after they arrive at an area substation, that's when a linesman hands off the job to a trenching service who needs to safely sink the mains cables to a pre-approved depth.

How Deep is Deep Enough?

Well, that depends. The power carried in a mains cable could carry multiple wires or "phases." Technically speaking, the voltage carried in-line is low, at least when compared to grid power, although they can still transmit dangerously high currents to high-energy machinery. Perhaps the trenched cables are being routed towards an apartment block. Alternatively, they're headed into a factory complex, one that contains several floor-length assembly lines. The point being: every project is different. As a project conformity baseline, we need to drop those cables deep so that they and the world above remain unaffected by the power contained inside that wire armouring and rubber sheathing. In Melbourne, Australia, that depth is specified at 500mm, at least that's what the minimum depth should be for a consumer mains electricity supply.

Weighing All Depth-Altering Contributing Factors

It's not easy to define a single depth minimum. The 500 mm baseline is standard, but then there are conditional aspects to add. For every project, those conditions will be different. That's why electricians and trenching teams are ruled by compliance books and electrical regulations manuals. In those books, compliance tables list different depths. They exist to offer alternative options, which again depend on the unique circumstances encountered by a trenching project. Obstructions above ground, numbers of conductors, the spacing between the conductors, and the kV (kilovolt) rating demanded by a structure, represent a tiny but important cross-section of a broad range of impact-heavy depth controlling determinants.

That 500 mm limit stands firm. From here, though, a trench can sink as far as 1000 mm. Sure, a seasoned contractor can judge the conditions, both above and below ground, and come to a fairly accurate depth estimate, but there's no need for all that number crunching, not when the electrical regulations tables are right there, ready to be consulted. Generally speaking, a mains electricity cable for consumers will obey that 500 mm rule. Dropping down, perhaps when a power-hungry three-phase power supply needs extra business-actuating energy, the cables might come with a heavier steel-wired sheath or be sent through a conduit. Concrete blocks and cable spacing considerations drop the depth lower, again maybe as low as 1000 mm, where the fully bedded cables are mechanically protected and the traffic overhead cannot cause subsidence damage.

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