The grounding practices and regulations for power systems and the associated equipment have evolved over time based on experiences and knowledge that have been gained. The preponderance of power companies outside of the state of California follow a practice of permanently grounding transformer cases. California power companies typically do not permanently ground transformers. The question is why is California the only state with this practice and why? This paper is intended to answer this question Northwest Lineman College www.lineman.edu

1. California Practice
of Not Grounding
Overhead Distribution
Transformers

2. Background
The grounding practices and regulations for power systems and the associated equipment have evolved over
time based on experiences and knowledge that have been gained. The preponderance of power companies
outside of the state of California follow a practice of permanently grounding transformer cases. California power
companies typically do not permanently ground transformers. The question is why is California the only state
with this practice and why? This paper is intended to answer this question
Regulations
Regulations regarding power systems started to appear in the early 1900’s as a result of an abnormally high rate
of serious accidents to both lineworkers and the public. The first national efforts were undertaken in 1913 by the
National Bureau of Standards. This effort spawned the National Electrical Safety Code which is today’s national
standard. Individual states had the authority to issue their own regulations. While many states developed regulations for inside wiring few developed any for power lines. California was an exception and they recognized
the need for lineworker and public safety early on and established regulations for all power companies to follow.
Many power companies established their own standards based upon their experiences that were at least as stringent as the state requirements.
NESC
The NESC became the standard that many states adopted and typically becomes the standard for any legal
litigation in many states. The NESC has a general requirement for grounding equipment including transformer
cases but allows an exception for installations over 8 ft. above the ground.
NESC Rule 215 Grounding of circuits, supporting structures, and equipment
C. Non current-carrying parts
1. General
Metal or metal-reinforced supporting structures, including lamp posts; metal conduits and raceways; cable sheaths; messengers; metal frames, cases, and hangers of equipment; and metal switch handles and operating rods shall be effectively
grounded.
EXCEPTION 1: This rule does not apply to frames, cases, and hangers of equipment and switch
handles and operating rods that are 8 ft. or more above readily accessible surfaces or are otherwise
isolated or guarded and where the practice of not grounding such items has been a uniform practice
over a well-defined area.
California and its long standing practice of not grounding transformer cases is in compliance with the NESC. In
California G.O. 95 is applicable standard.
California Public Utilities Commission
California’s power line regulations can be traced back to 1911 and the Railroad Commission of the State of
California. At this time the Railroad Commission had jurisdiction over power companies. In 1911, the state passed
an act which created chapter 499 which regulated the erection and maintenance of overhead
lines. In 1929 the Railroad Commission issued its General Order No. 64-A covering the erection
and maintenance of overhead lines. G.O 64 did not include any requirements for grounding or
not grounding transformer cases.

3. In 1942 the Railroad Commission updated G.O. 64-A with many changes. Company representatives from power
companies including PG&E, SCE, SDG&E and LADWP were instrumental in the changes that were made. At
this time General Order 64-A was revised as General Order 95. G.O 95 included the following rule:
GO 95 Rule 58.3 - C(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.
Exceptions
Any transformer whose high voltage winding is connected to a circuit of more than 14,000 volts, which may
have its case grounded provided that 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 and no such grounded transformer case is less than 8 ft. vertically or 4 ft. 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 winding is connected to a circuit of 750 -14,000 volts, which may have its
case grounded provided no unprotected conductors of 750-14,000 volts shall be less than 8 ft. vertically and or
4 ft. horizontally from the nearest part of such grounded case.
Any transformer which is less than 8 ft. above ground.
Keep in mind that in the 1940’s the majority of the distribution systems in California were 4 wire 2400/ 4160 volt
systems with a primary neutral conductor. Linemen were trained to work on these systems off the pole with rubber gloves. The preponderance of representatives from the power companies who worked on the revised G.O.
95 concluded that having the transformer case and hanger brackets grounding presented a significant hazard to
lineworkers as a second point of contact when working on the primary circuits with rubber gloves.
This conclusion resulted in the practice of not grounding transformer cases which is still followed to a large degree
in California.
The current GO 95 is somewhat silent to any requirement to ground or not to ground transformer cases.
GO 95 Rule 58.2 - A(3):
Transformer Case Grounding or Bonding: (See Rule 54.4–G for Grounded Equipment Clearances)
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. Where transformer cases are bonded, the case bonding system
shall not be electrically connected to any unassociated hardware or to other bonds.
Note: Revised January 13, 2005 by Decision No. 0501030.
GO 95 Rule 54.4 – G:
From Grounded Metal Boxes, Hardware and Equipment
All grounded metal boxes, hardware and grounded metal cases for equipment, on non–metallic poles or non–
metallic structures shall be a minimum of 36 inches above and 30 inches below the next level of unprotected
conductors. Such equipment shall have a minimum clearance of 48 inches above or 72 inches below communication line conductors, cables and messengers of different ownership.
Conclusion
There is no question that California’s regulations have resulted in safe and reliable line construction and maintenance performance over the years. This is particularly true for linemen working
off the pole. During the era of working off the pole linemen who had worked in other states were
always complementary of California’s line construction.

4. At the time the no transformer case grounding requirement was adopted one can readily understand the logic
behind eliminating a large mass of grounded metal as a second point of contact in the work area. Nationally, experience with maintaining and operating overhead lines has confirmed that best practice is to ground the transformer
case. This allows the transformer protection to isolate a transformer where the primary winding has shorted to
the case and minimize the chance of high voltage getting into the secondary winding.
It is possible over time California power companies will gradually shift over to grounding overhead transformer
cases. In the meantime lineworkers working on systems with ungrounded transformer cases should understand
that potentially the cases could be energized and take proper precautions.