What is direct burial cable?
Indoor network cable soaks up moisture, and its PVC jacket breaks down outdoors. Direct burial cable is designed to go into a trench and be covered with soil, with no conduit around it. Typical uses are Ethernet runs between buildings, to a detached garage or outbuilding, to outdoor cameras and wireless access points, and coaxial runs to satellite dishes and antennas.
How it is built
- Tough outer jacket. Usually polyethylene (PE), which resists moisture, abrasion, and soil chemicals better than PVC. Many direct burial cables are also UV-resistant for sections that run above ground.
- Water blocking. Either a gel compound fills the spaces between the pairs (often called gel-filled or flooded cable), or dry water-swellable tape or yarn stops water from traveling along the cable. Gel must be wiped off with a cleaner before the conductors can be terminated.
- Shielding. Many direct burial Ethernet cables have a foil or braid shield, and some add a second jacket or metal armor against rodents and digging damage. CompTIA lists direct burial under shielded twisted pair in objective 3.2.
- Same category ratings. A direct burial Cat 6 cable has the same Cat 6 performance and the same 100-meter limit for an Ethernet channel as indoor Cat 6.
Installation
- Before digging, have underground utilities located. In the United States, calling 811 starts that process.
- Follow local code and the cable maker's instructions for burial depth.
- Many installers still lay direct burial cable in conduit for extra protection and so it can be replaced later. Underground conduit counts as a wet location, so cable inside it still needs a wet-location rating; ordinary indoor cable is not acceptable.
- Bond the shield to ground as the manufacturer and code direct, and install surge protection where the cable enters each building.
- Transition to indoor-rated cable at a termination point inside the building, unless the outdoor cable is also rated for the indoor space it passes through.
Copper vs. fiber between buildings
Copper cable between two buildings can carry lightning-induced surges and suffers when the two buildings' electrical grounds sit at different voltages. That is why shield bonding and surge protection matter. Fiber optic cable carries light through glass, which does not conduct electricity, so it avoids those problems and also handles runs longer than 100 meters. Fiber also comes in direct burial versions. For a short run to a garage, direct burial copper is common and inexpensive; for longer runs or campus links, fiber is usually the better choice.
Troubleshooting buried runs
- Link fails after heavy rain: water has entered the cable or a connection. Check for indoor cable used outdoors or an unsealed splice or enclosure.
- Intermittent errors or a dead link after nearby digging or landscaping: physical damage. A cable tester shows the open or short, and a time-domain reflectometer can estimate the distance to it.
- Equipment damaged after a storm: a surge came in over the copper run. Add surge protection or switch the link to fiber.
Direct burial cable on the CompTIA A+ exam
Core 1 objective 3.2 lists direct burial under copper network cables, beneath shielded twisted pair, and lists plenum-rated and optical fiber in the same objective. Know that direct burial cable can go straight into the ground without conduit because it has a waterproof jacket and water-blocking fill, that it is often shielded, and that it is about the environment, not fire safety. If a scenario asks for an underground Ethernet run to a detached building, choose direct burial cable. If the run passes through an air-handling ceiling, the answer is plenum-rated instead.