31    Classification For Grades Of Construction.

For the purpose of determining grades of construction (see Section V), circuits are classified and designated as follows:

(a)    Class H.  This includes constant potential alternating current supply circuits having 5000 volts or more between any two conductors, or 2900 volts or more between any conductor and ground; constant current series circuits exceeding 7.5 amperes; constant potential direct current circuits exceeding 750 volts between any conductor and ground; except, however, railway trolley circuits and feeders metallically connected thereto.

(b)    Class L.  This includes constant potential, alternating current supply circuits of lower voltage than Class “H”; constant current series lighting circuits, alternating and direct current not exceeding 7.5 amperes; constant potential direct current supply circuits of lower voltage than Class “H”; except however, railway trolley circuits and feeders metallically connected thereto.

(c)    Class S.  This includes signal circuits as defined in 17(d).

(d)    Class T.  This includes electric railway or trolley circuits and feeders metallically connected thereto, as defined in 17(c).  In determining the grade of construction required, Class “T” conductors will be considered as ordinary supply conductors of equal voltage and will fall into Class “H” or “L” accordingly.  Where Class “T” lines are involved in crossing or in conflicts they shall take the grade of construction required of supply lines.