PG 141-144 Underground Electrical Distribution Systems

Flashcards reviewing key concepts, definitions, and components of underground electrical distribution systems, including cables, conduits, enclosures, transformers, and risers.

What are the main advantages and disadvantages of underground distribution systems compared to overhead systems?

Underground systems eliminate poles and wires from sight and are immune to high winds, ice, and trees, but they are costly to install and maintain.

What are the two key classifications for the installation of underground distribution line equipment?

Above ground (padmounted) equipment and below grade (subsurface) equipment.

Why does the public prefer subsurface equipment over padmounted equipment?

The public prefers subsurface equipment because of its improved appearance due to the elimination of visible equipment.

What operational advantage does padmounted equipment offer over subsurface equipment?

Padmounted equipment is visible and much easier to maintain and operate.

What material was used for insulation in the first underground cables?

Paper-insulated lead cables.

What material is modern underground cable insulation made of, and why is it preferred?

Modern cables are made with polyethylene-type insulation, which makes them much easier to splice and terminate.

How do conductor constructions differ between larger and smaller underground cable sizes?

Larger cables use stranded aluminum or copper, while smaller sizes use either stranded or solid conductors.

What material is most commonly used for underground conduit installations?

PVC conduit.

Where is steel conduit used in underground electrical installations?

Steel conduit is used for certain main feeders in locations like substations or areas where extra protection is warranted.

What is 'cable in conduit' and how is it manufactured?

It is a product made by extruding a continuous one-piece conduit over the cable, which increases installation efficiency.

How is the term 'enclosure' defined in underground electrical distribution?

It is a broad term used to classify concrete, fiberglass, or composite-type housings used to splice cables, terminate cables, or house equipment such as switches and transformers.

What are manholes, and where are they typically installed?

Manholes are larger concrete structures typically installed in larger cities to house heavy cables and equipment.

What is the function of a pedestal in an underground system?

Pedestals are located above ground and are used to splice low-voltage secondary and service conductors.

How does three-phase transformer service delivery differ between overhead and underground construction?

In overhead construction, three single-phase transformers are commonly connected in a bank, whereas in underground construction, single-phase units are combined into one three-phase unit.

What types of pads are padmounted transformers installed on?

Pads made of either concrete or some form of polyurethane.

Where are subsurface-type transformers installed?

In an enclosure or a manhole.

What is a 'riser' pole in an electrical circuit?

A pole used to identify a transition from an overhead circuit to an underground circuit, or from an underground circuit to an overhead circuit.

What protective components are commonly installed on riser poles?

Switches to open the circuit, fuses to protect the line, and lightning arresters to protect the underground system from lightning surges.