go 95 rule 57.4

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 the 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 .

(1) Messengers and Insulated Cables: The basic clearances specified in Rule 37 , Table 1 for conductors, as modified by provisions of Rule 54.4–A , are applicable to messengers and insulated cables (See Rule 57.8 ).

(2) Grounded and Bonded Messengers and Insulated Cables: The basic clearances specified in Rule 37 , Table 1, Column D , are applicable to grounded cables having metallic sheaths, and to effectively grounded messengers (see Rule 57.8 ).

(1) Which Transport Freight Cars: The clearances specified in Table 1, Case 1, Columns A, B, and D are based upon the maximum height of standard freight cars, 15 feet 1 inch from the top of 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 , 26 feet and 27 feet respectively, are based upon a maximum trolley pole throw of 26 feet, the usual maximum height of a free trolley pole above the top of rails or surface of streets used by trolley cars or coaches.

EXCEPTION: Messengers, and cables, which are bonded and grounded as specified in Rule 57.8 , may have clearances above the rails or surface of streets 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 with a minimum thickness of 7/8 inch (or other suitable material having the same minimum electrical insulating qualities and mechanical strength) for distances 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 by messengers are not required to be any specified distance from their supporting messengers.

E. Between Messengers

The clearance 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:

(1) Attached to Surfaces of Poles and Crossarms: Such grounded messengers and cables may be attached to the surfaces of poles or crossarms at less than the clearance specified in Table 1, Column D, Cases 8 and 9 . When attached to pole or crossarm at less than 15 inches from centerline of pole such grounded messenger or cable shall not be 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 Figure 57–1 A ).

(2) Supported on Same Crossarm above 750 Volts: 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 Figure 57–1 B ).

(3) Supported on Same Crossarm 0 - 750 Volts: Such grounded messenger and cable when supported on the same crossarm with unprotected conductors of 0 - 750 volts, shall be placed at not less than pin spacing ( Table 2, Case 15, Column D ) beyond the outermost unprotected conductor on the same side of pole (see Figure 57–1 C ).

(4) Between Cable and Unprotected Conductors: The clearances between a cable and unprotected conductors shall not be held to apply between a grounded cable (and its messenger) and unprotected conductors of the same circuit on poles where unprotected conductors enter (or leave) a cable. On such poles no grounded section of messenger or cable shall be less than 15 inches from surface of pole.

(5) Two or More Cables Attached to the Surface of Pole: Where two or more cables are attached to the surface of the same wood pole they shall be placed on the same side of pole when their vertical separation is less than 8 feet but in no case shall the vertical 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 should be arranged so as not to hamper or endanger workers and firefighters while performing their duties.

H. From Nonclimbable Street Lighting or Traffic Signal Poles or Standards

Messengers and metal–sheathed cables which are bonded and grounded as specified in Rule 57.8 , passing (unattached) nonclimbable street lighting and traffic signal poles or standards, including mastarms, brackets, and lighting fixtures may have a clearance less than the 36 inches specified in Table 1, Case 10, Column D , but shall have a clearance of not less than 15 inches, except when the conductors are mechanically protected from abrasion by materials specified in Rule 22.8 . Such mechanical protections shall extend not less than 15 inches in each direction along the cable from centerline of pole, standard, attaching mastarm or fixture, whether passing above, below or alongside. The conductors shall be installed in such a manner so as not to interfere with light distribution from lighting fixtures and shall not hamper workmen changing lamps or maintaining equipment.