Title: “Little Falls Generating Station Grounding Problem”
Sponsor: Avista Utilities
URL of Sponsor: www.avistacorp.com
Team Name: Galena
Current grounding practices used in the electrical industry consist of two fundamentally different
practices. For purposes of this discussion, these are referred to here as NEC and NESC
methods. The NEC method is that method prescribed in the National Electrical Code (NEC) that
is applied to industrial, commercial, and residential buildings and facilities. The NESC method is
that practice prescribed in the National Electrical Safety Code that is applied to electrical utility
substation and generation facilities. Each of these methods, in order to adequately protect
personnel, need to be consistently applied to a facility.
At issue is what the effects of mixing these two grounding methods are. Each scheme
implemented individually at each site provides the electrical protection for personnel. When these
two schemes are mixed within the same facility, does this compromise the integrity of the
grounding system to render it ineffective? Does this create a false sense of security to personnel
who believe they are protected? Why or why not? What practices should be adopted, if any, to
remedy this type of application?
Project Description: The purpose of this project is to study the effects of mixing the different
grounding schemes and to provide a report on what issues, if any, exist in the facility. If the
study concludes that there are no issues associated with the grounding schemes, the design
team is to develop a model, specific to the Little Falls HED, that will calculate the touch and step
potential at the generator and station service locations. If time permits, a field measurement will
be taken to confirm the model.
If the study concludes that there are issues associated with the grounding scheme, the design
team is to develop detail designs to remedy the situation. These designs include connection
details and material lists. The design is to be accompanied by a computer model that
demonstrates the existing system problems and how the design teams solution improves the
Background: The NEC grounding method is summarized simply as a means where an easy
ground return path to a ground node is provided in order to minimize the electrical hazard and
prevent electrical shock. This easy path is via a ground wire that generally runs parallel with
electrical circuits. In the event of a fault, the electrical current will seek the easiest path to ground
that is through the ground wire. This, in effect, allows the person who may contact the fault to be
“bypassed” by the current.
The NESC works from a different premise. The NESC is prescribed because the electric current
due to a fault is extraordinarily high. Concerns are not only with ground current return paths but
also with step and touch potentials. With this method, a ground grid is used to distribute the fault
current creating an equipotential plane. The premise of this method is that all things on the plane
are at the same potential. This creates no potential difference between a person and anything on
the plane that would cause electrical current to flow.
The problem identified here is specifically related to the Little Falls and Long Lake hydroelectric
facilities. These are generating stations that were constructed circa 1905 to 1920. These
stations were constructed at a time where grounding methods were either not as standardized or
the standards were different.
At the time of construction, neither station had a NESC or NEC type of grounding system. Station
grounding was accomplished by means of utilizing bond connection to all metal pieces to provide
an electrical reference point. This included the generator and turbine frames, penstocks, re-
enforcing steel in the concrete, connections to the building superstructures and other bonded
connections. There is not a specific ground point or grid. Additionally, both of these stations
deploy a delta-delta station service connection. Thus, there is no specific electrical ground
reference for circuits connected to this system.
Over time, there have been a number of additions to the plants that affect the basic electrical
distribution system since original construction. This includes the installation of numerous
electrical sub-panels that supply an increasing number of auxiliary systems. Because many
electricians have been trained in application of the NEC code and the general guidelines followed
by our utility practices, these new panels were installed generally in accordance with the NEC
In addition, the electrical grid to which the stations were connected have been modified. The
original 60kV delta transmission system has been replaced with a 115 kV wye system. Whereas
at the time of commissioning the significant fault current source was probably the generating
station, the electrical system is now the dominant source if a “close in” fault was to occur.
Proposed Scope of Work: The design team would be expected to accomplish the following
1) Review material associated with different grounding practices and theories. This includes
IEEE, NEC, NESC, and other standards. The team will need to understand how
grounding schemes are applied to assure safety to personnel and the public.
2) Review the existing ground grid system at Little Falls HED by reviewing drawings, original
construction photos (as they are available), and field verification of the grounding scheme
that can be identified.
3) Identify specific areas where there is a possible compromise to the grounding scheme.
These should be identified on a drawing for record purposes. The items will need to be
numbered and a cataloged.
4) Through application of research, develop a computer model of the Little Falls grounding
scheme that determines step potentials, touch potentials, and any other grounding
effectiveness measurement. The model needs to be developed so that the effectiveness
of any recommended changes can be quantified.
5) Prepare a report that provides final recommendations and addresses questions such as:
What is the appropriate grounding method to be used at Little Falls and Long Lake? Do
the installation of sub-panels using NEC methods compromise the grounding protection
for personnel? Are they creating a ground current loops that may create more of a safety
hazard than if they did not run a ground?
Desired Outcomes: The design team would be expected to provide Avista with the following:
1) A written report on the statement of the problem, current methods used, and
assessment of alternatives for addressing the issue. This report is to derive a
recommendation with cost estimates for the hardware, labor, and other devices
needed to implement the recommended solution.
2) Provide a design package that documents the existing system and shows the
changes or modifications that will be necessary to the existing system in order to
implement their design.
3) Provide Avista with the computer model of the grounding, complete with appropriate
documentation of code, assumptions, and other criteria.
4) Provide a presentation to Avista engineering and management staff that addresses
Avista Contribution: (1) Avista would provide a project sponsor to support the design team(s)
that could provide access to existing documentation and arrange plant visits if necessary; (2)
Avista would provide copies of documentation requested by the design team as practical. (3)
Avista would provide limited funds to acquire project support items such as software, purchase of
codes or standards if required, etc.
Additional Discussion: The scope and outcomes described here are suggestions only. All
methods and procedures for managing the project would be in accordance with practices and
requirements of the student program. If there are objectives or methods that conflict with the
Senior Project Design requirements, the proposed project can be modified.
This is believed to be a complex project. It may be that portions of the project may be done as
part of a multi-year effort in order to make the project achievable for students.
Due to security issues, the project team will be subject to signing a confidentiality agreement. In
addition, security standards for critical infrastructure facilities continues to evolve and the design
team may have to submit personal information for a background check. Currently this is not a