Original General Order 95

Section V

Detailed Construction Requirements for Supply Lines

(Class H, L and T Circuits)

57.4    Clearances

Basic minimum clearances for messengers and insulated cables are specified in Rules 37 and 38, Tables 1 and 2 respectively and shall be measured to nearest surfaces of the cable and messenger assembly, including cable rings and messenger supports.  The basic clearances specified for conductors, as modified by provisions of Rule 54.4 are applicable to insulated cables.  These clearances for messengers and cables are also subject to the following modifications.  The clearances required for supply conductors of 0-750 volts shall be applied to suitably insulated cables of any voltage having a metallic sheath which is bonded and grounded as specified in Rule 57.8.

A.    Above ground (see Rule 54.4-A)

B.    Above Railways and Trolley Lines

(1)    Which Transport Freight Cars: The clearances specified in Table 1, Case 1, Columns A and B and D are based upon the maximum height of standard freight cars, 15 feet 1 inch from the top rail to top of running board.  These clearances shall in no case be reduced more than 5% because of temperature, wind or mechanical loading.

(2)    Operated by Overhead Trolley:  The clearances specified in Table 1, Case 2, Columns A and D are based upon a maximum trolley pole throw of 26 feet.

Messengers, and Cables, which are bonded and grounded as specified in Rule 57.8, may have clearances above the rails or running surfaces used by trolley cars or coaches less than the distance specified in Table 1, Case 2, Columns A and D for trolley-throw clearance, provided the messengers and cables are not less than 4 feet above the trolley contact conductor and are encased in wood boxing or moulding 7/8-inch or more in thickness for distance of at least 2 feet horizontally from the vertical plane through each trolley contact conductor.

C.    Between Conductors in Cables

No specified clearance is required between the individual conductors of suitably insulated cables, whether single or grouped and therefore the clearances specified in Table 2, Cases 15, 16, and 17 do not apply between such conductors.  Any such multiple-conductor group shall be treated as a single conductor in the application of other clearance requirements.

D.    Between Cables and Messengers

Cables supported be messengers are not required to be any specified distance from their supporting messengers.

E.    Between Messengers

The clearances specified in Table 2, Case 1, Column A does not apply and no clearance is required where messengers of the same electrical system branch or cross.

F.    From Poles, Crossarms and Other Conductors

Messengers, and metal sheathed cables, which are bonded and grounded as specified in Rule 57.8 shall have clearances not less than the minimum clearances required for conductors of 0-750 volts with all of the following provisions being applicable:

Such grounded messengers and cables may be attached to the surfaces of poles or wood crossarms at less than the clearances specified in Table 1, Column D, Cases 8 and 9.  When attached to pole or wood crossarm at less than 15 inches from center line of pole such grounded messenger or cable shall be not less than 4 feet vertically above or 6 feet vertically below any unprotected supply conductor except that where a guard arm (or arms) is placed above messenger and cable as specified in  Rule 57.7, the clearance of 6 feet may be reduced to not less than 4 feet below unprotected conductors of 0-750 volts (see App. G, Fig. 53).

Such grounded messenger and cable when supported on the same crossarm with unprotected conductors in excess of 750 volts shall be placed on the side of pole opposite the unprotected conductors (see App. G, Fig. 54)

Such grounded messenger and cable when supported on  the same arm with unprotected conductors of 0-750 volts, shall be placed at not less than pin spacings (Table 2, Case 15, Column D) beyond the outermost unprotected conductor on the same side of pole (see App. G, Fig. 54).

The clearances required in  this Rule 57.4-F between a cable and unprotected conductors shall not be held to apply between a grounded cable (and its messenger) and protected conductors of the same circuit on poles where unprotected enter (or leave) a cable.  On such poles no grounded section of messenger or cable shall be less than 15 inches from surface pole.

Where two or more cables are attached to the surface of the same wood pole in accordance with the provisions of this Rule 57.4-F, they shall be placed on the same side of pole when their vertical separation is less than 8 feet but in case shall the vertically separation be less than 1 foot.

G.    From Buildings and Other Structures

Messengers and metal sheathed cables, which are bonded and grounded as specified in Rule 57.8 are not required to clear buildings and other structures any specified horizontal distance and the clearance specified in Table 1, Case 7, Column D does not apply in such instances.  Such cables, and messengers, shall be installed so that they do not interfere with the free use of fire escapes, exits, etc., and shall be so arranged as to hamper and endanger workmen and firemen as little as possible in the performance of their duties.