Original General Order 95

Section V

Detailed Construction Requirements for Supply Lines

(Class H, L and T Circuits)

58.3    Transformers

A.    Position on Pole

Where more than one transformer is installed on a pole, all transformers shall be placed on the same side of pole.

Transformers shall not be supported on pole top extensions.

B.    Case and Lead wire Clearances

(1)    Above Ground

a)    Lead and bus wires: The clearances above ground specified in Table 1 are applicable to unprotected lead and bus wires of transformer installations except as modified by the following provisions.

Clearances above ground unprotected lead and bus wires of transformer installations may be less than 25 feet as specified in Table 1, Column E case 3 and 4, but shall not be less than 22 ½ feet except where a clearance of not less than 18 feet above ground is permitted by the provisions of Rule 54.4-A2b in which case the clearance above ground of such lead and bus wires shall be not less than 18 feet,

Clearances above ground of unprotected lead and bus wires of transformer installations may be less than 30 feet as specified in Table 1, Column F, Cases 3 and 4, but shall be not less than 27 feet.

Clearances above ground of unprotected lead and bus wires of transformer installations on structures of two or more poles may be less than 22 ½ feet or 27 feet as specified above, or less than 25 feet as specified in Table 1, Column F, Case 5 but shall not be less than 20 feet above ground, provided such lead or bus wires are guarded by transformer platform flooring which extends not less than 1 foot horizontally outside the vertical planes of all such lead and bus wires on the structure.

b)    Cases: Cases of transformers supported on poles or structures shall not be less than 17 feet above the ground except that in areas which are not in any way accessible to vehicles, the clearance of cases above ground shall be effectively grounded.

(2)    From Buildings: Transformers on poles shall be so located that normally Unenergized parts clear the surfaces of buildings by not less than 3 feet horizontally or by not less than 8 feet vertically.  Lead and bus wires carried as unprotected conductors shall have the clearances from buildings as specified in Table 1, Cases 6 and 7.

In situations where foregoing clearances of cases and lead bus wires from walls of buildings (not windows, fire escapes, etc.) are impracticable to obtain, such as the location of transformers on poles in alleys, these clearances will not be held applicable provided wood barriers authorized by this Commission are used.

(3)    Cases from conductor levels above and below: Transformers shall be so installed that normally Unenergized metal parts clear unprotected conductors, except the transformer connecting leads by distances specified in the following provisions:

a)    From 0-750 volt conductors below: The vertical clearance of transformer cases and hangers from the level of 0-750 volt conductors below (whether such conductors are supported on crossarms or racks) shall not be less than 10 inches except for certain conductors as provided in Rule 58.3-B4.

b)    From 0-750 volt conductors above: The vertical clearance of Unenergized metal parts of transformers from 0-750 volt conductors supported on crossarms above shall be not less than 3 inches or, in lieu of such vertical clearance, the Unenergized parts of transformers shall not be less than 6 inches horizontally from such conductors.  The vertical clearance of unenergized metal parts of transformers from 0-750 volt conductors supported on racks above shall be not less than 4 feet.

c)    From 750-7500 volt conductors below: The vertical clearance of Unenergized metal parts of transformers from the level of 750-7500 volt conductors below shall be not less than 12 inches.

d)    From 750-7500 volt conductors above: The clearance between Unenergized metal parts of transformers and 750-7500 volt conductors above or alongside shall be not less than 12 inches vertically or 12 inches horizontally, except that conductors of the circuit to which the transformer is connected may be less than the 12-inch vertical clearance from such Unenergized parts but shall be not less than 6 inches vertically from the transformer case and not less than 3 inches radially from the hanger provided no line conductor which is less than 12 inches horizontally from the case or hanger is less than 3 inches (Table 1 Case 9) above the level of the top surface of the crossarm.

(4)    Transformer Cases From Certain Conductors Less Than 10 Inches Below The Cases:

a)    Transformer Leads on Heel Arms: Heel arms shall not used to support lead wires or taps except where necessary to clear the lower voltage transformer leads from the transformer case or other conductors.

b)    Line conductors less than 10 inches below cases: Where a transformer case is unusually long, a crossarm supporting line conductors of 0-750 volts may be used as a heel arm or such conductors on an arm may be less than 10 inches below the transformer case or the hangers) provided all of the following conditions are met:

No more than a single transformer with lower voltage of 0-750 volts is supported on the pole at the same level;

The vertical clearances between conductors on the hanger arm and such line arm below shall not be less than as specified in Table 2, Cases 9 to 13;

It is not practicable to obtain the clearance of at least 10 inches specified in Rule 58.3-B3;

Such 0-750 volt conductors clear the transformer case by not less than 15 inches horizontally;

Service drops are not run from the crossarm supporting 0-750 volts conductors at that location; and

The vertical clearance of 0-750 volt conductors below the lowest point of the transformer primary leads is not less than

18 inches for primary leads of 750-7500 volts,

24 inches for primary leads of 7500-20,000 volts,

36 inches for primary leads of 20,000-35,000 volts.

(5)    From Hardware: Transformer cases, hangers, and other metal parts in contact therewith shall clear through bolts, arm braces and other hardware by not less than 1 ½ inches; except that transformer cases and hangers shall clear crossarm braces and crossarm through bolts by not less than 1-inch air-gap distances and 1 ½ –inch creepage distance.

(6)    From Guys: Transformer cases and hangers shall be not less than 4 inches from all portions of guys which are 6 inches or more from the surface of poles or crossams at the guy attachment.  Transformer cases and hangers shall be not less than 1 ½ inches from all portions of guys which are within 6 inches of the surface of poles or crossarms at the guy attachments.

(7)    Treatment of lead wires: Vertical and lateral leads between line conductors and transformers shall comply with Rule 54.6 and 54.4-C6; and with the clearances specified in Table 1, Cases 8 and 9; and Table 2, Cases 15, 16 and 17.  Where such leads enter cutouts or switches Rule 58.5-C shall also apply.  Such lead wires may be installed in the climbing space.

All lead wires shall clear braces, bolts and other line hardware a distance of not less than 1 ½ inches.

C.    Grounding

(1)    Grounding of Windings: Transformer windings not exceeding 250 volts (except those exclusively for energizing street lighting circuits and those used exclusively for energizing signal and track circuit) shall be effectively grounded as follows:

a)    Single phase systems: In two wire (nominal 120-volt system one wire shall be grounded; in two wire (nominal 240-volt) systems where the mid-point or some intermediate point of the windings is not available, one wire shall be grounded; in two-wire (nominal 240 volt) systems where the mid-point or some intermediate point of the winding is available, that point shall be grounded; in three-wire (nominal 120/240-volt) systems, the mid-point of the winding shall be grounded.  (See App. G, Fig 57.)

b)    Two phase systems: In three-wire (nominal 240 volt) systems, the point common to both windings shall be grounded; in four-wire (nominal 120/240-volt) systems, the mid-point of the winding on one phase shall be grounded; in four wire (nominal 240-volt), and five wire (nominal 120/240-votls) systems, the mid-points of both windings shall be connected and grounded.  (See App. G, Fig. 58.)

c)    Three phase Systems: In three-wire delta (nominal 120 or 240 volt) systems the mid-point of one transformer winding shall be grounded, or if the midpoint is not available on any of the transformer windings, a point common to any two windings (one phase wire) shall be grounded; in three-wire star (nominal 120, 208, or 240 volt) systems, the point common to all windings shall be grounded or, if the common point is not available on a three-phase star-connected transformer of such a system, one of the phase wires shall be grounded; in four-wire star (nominal 120/208 volt) and three-wire T or Scott (nominal 240 volt) systems, the common point shall be grounded.  (See App. G, Fig 59.)

(2)    Location of Transformer Winding Grounds: Transformer ground connections shall be provided at one of the following locations:

At the transformer pole,

At a pole adjacent to the transformer pole, or

At the load end of each service supplied from the transformer, separate from the usual house ground, except that where three or more services are supplied from one transformer or bank of transformers, ground connections at the two services nearest the transformer pole and one ground connection at services at approximately 500-foot intervals will suffice.

Transformer ground connections other than those occurring on common primary and secondary grounded neutral systems shall have a conductivity not less than that of No. 6 AWG copper wire.

Where a common primary and secondary grounded neutral system is used, ground connections shall conform to the requirements of Rule 59.4-A.

(3)    Transformer Case Grounding or Bonding: Cases of transformers and metal parts in contact therewith shall not be grounded where supported on wood poles or wood structures.

The hanging or placing of transformers on metal poles or structures is not recommended, particularly with respect to transformers connected to circuits of less than 14,000 volts.  Transformers shall not be supported on metal poles or metal supports in contact with the ground unless the cases are securely bonded to the metal poles or parts of structures in contact with the ground and such poles or structures are effectively grounded.  No transformer case shall be in contact with a metal crossarm or a metal beam attached to a wood pole or a wood structure, excepting a metal heel arm or rest which does not extend beyond the sides of any transformer case.

The bonding of cases of transformers whose high voltage windings are connected to circuits of less than 20,000 volts is not recommended but where such cases are bonded the case bonding system shall not be electrically connected to any unassociated hardware or to other bonds.

Except from the provisions of this Rule 58.3-C3 applying to the grounding of transformer cases supported on wood poles or structures are the following:

Any transformer whose high-voltage winding is connected to a circuit of more than 14,000 volts, which may have its case grounded provided all such transformer installations on the system are so grounded, warning signs calling attention to the case grounding condition are posted on the structure so as to be readily legible from the climbing space or spaces, and no such grounded transformer case is less than 8 feet vertically or 4 feet horizontally from the unprotected conductors of any other supply-line circuit than those to which the transformer windings are connected;

Any transformer whose high-voltage is connected to a circuit of 750-14,000 volts, which may have its case grounded provided no unprotected conductors (including lead wires) of 750-14,000 volts shall be less than 8 feet vertically or 4 feet horizontally from the nearest part of such grounded case; and

Any transformer the case of which is less than 8 feet above the ground.

Transformer cases which are grounded in accordance with any provision of this rule shall be effectively grounded (see Rule 33.3).

D.    Cutouts or Other Disconnecting Devices

Transformer cutouts, fuses, disconnects or switches shall be located so that they are readily accessible from climbing and working spaces.  Such devices or their connecting leads shall not extend into the climbing space but may extend wholly or in part into the working space.

The vertical clearances of transformer cutouts, fuses, etc., above the levels of conductors of other circuits shall not less than the clearances required between conductors as specified in Table 2, Cases 8 to 13.