Underground Distribution Construction and Maintenance
Underground distribution covers cable, ducts, vaults and padmounted equipment. The exam tests cable construction and ratings, installation limits such as bending radius and pulling tension, termination and splicing practice, and how you prove and locate a cable fault.
Cable and ratings
- URD cable is typically aluminium or copper conductor with TRXLPE or EPR insulation and a concentric copper neutral, often with an outer jacket (jacketed concentric neutral) to resist corrosion and dig-ins.
- Ampacity tables assume standard soil thermal resistivity, burial depth, ambient temperature and grouping. Apply derating for duct bank, soil type, depth and parallel cables or the conductor will run hotter than its rating.
- Cables come in 15, 25 and 35 kV classes, with 100% or 133% insulation levels. The shield and neutral must be bonded and grounded per design.
Installation
- Bed cable in sand, install warning tape and leave slack at structures; keep every bend above the manufacturer's minimum radius, commonly 8 to 12 times the overall diameter.
- Pulling tension and sidewall pressure set the maximum pull length, lubricant choice and number of bends. Exceeding sidewall pressure damages insulation even when tension looks acceptable.
- Hand-expose within the tolerance zone around located plant; vacuum excavation is the standard method.
- Concentric neutral corrosion and third-party dig-ins are the dominant failure causes on older URD systems.
Termination and testing
- Stress cones control the electrical field at the end of the insulation shield; a termination usually fails at a poorly cut shield or a contaminated surface.
- Insulation resistance testing screens gross defects; VLF or tan-delta testing suits XLPE cable, while TDR and fault thumping locate the failure point.
Worked example
A cable with a 35 mm outside diameter is pulled through a vault. At a 10× rule the minimum bend radius is 350 mm, so both the duct sweep and the sheave must meet or exceed that. If the sheave is 250 mm, the cable is over-bent even with correct pulling tension, and sidewall pressure at the sheave rises. The reasoning: bend radius is set by cable diameter, and a tight sheave fails the cable independently of the pull.
Traps that cost marks
Using a DC hi-pot on XLPE insulation; backfilling without bedding or warning tape; trusting locate marks alone inside the tolerance zone; and taking ampacity from a table without derating.