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multiple connections between the utility ground and customer
ground can lead to stray voltage
problems; customer piping, swimming pools or other equipment may
develop objectionable
voltages. These problems may be difficult to resolve since they
often originate from places otherthan the customer's premises.
Distribution network configurations
Substation nearYellowknife, in the Northwest Territories of
Canada
Distribution networks are typically of two types, radial or
interconnected (see spot network). Aradial network leaves the
station and passes through the network area with no normal
connection
to any other supply. This is typical of long rural lines with
isolated load areas. An interconnected
network is generally found in more urban areas and will have
multiple connections to otherpoints of supply. These points of
connection are normally open but allow various configurations
by the operating utility by closing and opening switches.
Operation of these switches may be by
remote control from a control center or by a lineman. The
benefit of the interconnected model is
that in the event of a faultor required maintenance a small area
of network can be isolated andthe remainder kept on supply.
Within these networks there may be a mix of overhead line
construction utilizing traditional
utility poles and wires and, increasingly, underground
construction with cables and indoor or
cabinet substations. However, underground distribution is
significantly more expensive thanoverhead construction. In part to
reduce this cost, underground power lines are sometimes co-
located with other utility lines in what are called common
utility ducts. Distribution feeders
emanating from a substation are generally controlled by acircuit
breakerwhich will open when afault is detected. Automatic circuit
reclosers may be installed to further segregate the feeder thus
minimizing the impact of faults.
Long feeders experience voltage droprequiring capacitors or
voltage regulators to be installed.
Characteristics of the supply given to customers are generally
mandated bycontractbetween thesupplier and customer. Variables of
the supply include:
AC orDC- Virtually all public electricity supplies are AC today.
Users of large amounts
of DC power such as some electric railways, telephone exchanges
and industrial
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processes such as aluminium smelting usually either operate
their own or have adjacent
dedicated generating equipment, or use rectifiers to derive DC
from the public AC supply
Nominal voltage, and tolerance (for example, +/- 5 per cent)
Frequency, commonly 50 or 60 Hz, 16.6 Hz and 25 Hz for some
railways and, in a few
older industrial and mining locations, 25 Hz.[2]
Phase configuration (single-phase,polyphaseincluding two-phase
and three-phase) Maximum demand (usually measured as the largest
amount of power delivered within a
15 or 30 minute period during a billing period)
Load factor, expressed as a ratio of average load to peak load
over a period of time. Loadfactor indicates the degree of effective
utilization of equipment (and capital investment)
of distribution line or system.
Power factorof connected load
Earthing systems - TT, TN-S, TN-C-S or TN-C Prospective short
circuit current
Maximum level and frequency of occurrence oftransients
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