No single cable characteristic should be
emphasized to the serious detriment of others. A balance of cable
characteristics, as well as good installation, design, and construction
practices, is necessary to provide a reliable cable system.
Service conditions
a) Cables should be suitable for all
environmental conditions that occur in the areas where they are installed.
b) Cable operating temperatures in
substations are normally based on 40 °C ambient air or 20 °C ambient earth.
Special considerations should be given to
cable installed in areas where ambient temperatures differ from these values.
c) Cables may be direct buried, installed
in duct banks, conduits, and trenches below grade, or in cable trays, conduits,
and wireways above ground. Cable should be suitable for operation in wet and
dry locations.
High-voltage power cables are designed to
supply power to substation utilization devices, other substations, or customer
systems rated higher than 1000 V.
NOTE — Oil-filled and gas-insulated cables
are excluded from this definition.
Low-voltage power cables are designed to
supply power to utilization devices of the substation auxiliary systems rated
1000 V or less.
Control cables are applied at relatively
low current levels or used for intermittent operation to change the operating
status of a utilization device of the substation auxiliary system.
NOTE — leads from current and voltage
transformers are considered control cables since in most cases they are used in
relay protection circuits. However, when current transformer leads are in a
primary voltage area exceeding 600 volts they should be protected as required
by the NESC, Rule 150.
As used in this document, instrumentation
cables consist of cables for Supervisory Controls and Data Acquisition (SCADA)
systems or event recorders, and thermocouple and resistance temperature
detector cables.
Instrumentation cables are used for
transmitting variable current or voltage signals (analog) or transmitting coded
information (digital).
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