General Order 64-A
Section II
Definitions Of Terms Used In This Order
20.5 Circuit means a conductor or system of conductors located outside of buildings and through which an electric current is intended to flow.
A. Communication (Class C) Circuits mean circuits which are used for public or private communication service and which operate at not exceeding 400 volts to ground nor 750 volts between any two points of the circuit and the transmitted power of which does not exceed 150 watts. When operated at less than 150 volts no limit is placed on the capacity of the system.
Note Telephone, telegraph, messenger-call, clock, fire or police alarm circuits, and other systems conforming with the above are included.
(1) Major Class C Circuits mean
communication circuits which include the following:
More than four conductors (open, paired or in cable) used chiefly for local
exchange service.
Toll telephone or telegraph circuits used for transmission of messages of the
general public, and not including clock, messenger call and other special
communication circuits.
(2) Minor Class C Circuits mean communication circuits not included in the definition (see Rule 20.5-A1.) above.
(3) Communication Circuits Not for Public Use mean circuits used by a utility in the operation of its facilities. They may be classified under the above definition (see 20.5-A) and may be installed and operated as communication circuits for public use if so desired and if consistently so treated. Private circuits of this character which are carried on the same structure with class H supply circuits (see 20.5-C1) and which are not protected by high voltage arresters, drainage coils, insulating transformers or other suitable protective devices to prevent an induced potential in excess of 400 volts to ground normally, or which are not consistently treated throughout as communication circuits in accordance with Definition 20.5-A shall be classified as Class L circuits. (See 20.5-C2)
B. Railway Signal Circuits mean
those supply and communication circuits used primarily for supplying energy for
controlling the operation of railway block signals highway crossing signals,
interlocking apparatus and their appurtenances.
Circuits which operate at less than 400 volts to ground are considered as
communication (Class C) circuits and shall be so classified and treated
provided that, if the voltage exceeds 150 volts between conductors the power
transmitted shall not exceed 150 watts.
Where all circuits of a line are owned and operated by one utility, the
voltage between conductors carrying in excess of 150 watts may be increased to
not more than 250 volts and the signal circuits may be considered as
communication (Class C) circuits.
All railway signal circuits which do not meet the requirements above shall be
treated as supply circuits of corresponding voltage.
C. Supply Circuits mean those circuits which are used for transmitting a supply of electrical energy.
(1) Class H Circuits include the following:
Constant potential alternating current circuits of 5000 volts or more between any two conductors
Constant potential alternating current circuits of 2900 volts or more between any conductor and ground
Constant current series circuits exceeding 7.5 amperes. (For exception see Rule 20.5-C2.)
Constant potential direct current circuits exceeding 750 volts between any conductor and ground
(2) Class L Circuits include the following:
Constant potential alternating or direct current supply circuits of lower voltage than Class H
Constant current circuits not exceeding 20 amperes provided the open circuit voltage of the supply transformer does not exceed 5000 volts between conductors.
Constant current circuits not exceeding 7.5 amperes.
D. Electric Railway Circuit mean supply circuits as defined above:
(1) Class T Circuits mean electric railway or trolley circuits but sometimes require different construction details than other supply circuits, hence are specifically, mentioned in the rules. Class T circuits will be considered as those conductors which through the contact of a trolley or other collecting device, supply current to electric railway cars or locomotives and includes feeders or other conductors metallically connected to such contact conductors. This class of circuit (H or L) depends upon its voltage for determining grades of construction to be used in its installation