Download Contact Voltage - IEEE Standards working groups

6.1 Contact Voltage
Contact voltage is by definition a problem that appears when there is an electrical system
condition that should not normally exist. Contact voltage is only present when there is a
system fault (i.e. short or open) that cannot be eliminated by protective devices.
Under fault conditions the voltage that may be present at nearby publicly and privately
accessible locations can be as large as system voltage. Contact voltage is far more
hazardous than stray voltage (defined in section XXX) because of the greater voltages
that may be encountered. Under specific exposure conditions this voltage can be large
enough to drive lethal amounts of current through a person or animal that makes contact
with the energized surface.
Whether or not the voltage present is large enough to cause harm is dependent not only
on the measured contact voltage, but on all elements of the exposure circuit, especially
the fault impedance. If the fault impedance is high it will limit the amount of current that
flows when a person or animal makes contact and closes the exposure circuit. If the fault
impedance is low, the fault current (i.e. the exposure current) is only limited by the
source and contact impedance, and by the body path impedance of the person or animal
making contact. Fortunately, the vast majority of contact voltage exposures are not
harmful. They are generally perceived as a mild shock or tingling sensation. While
electrocutions are rare, there are often many surfaces energized with significant voltages
within some systems.
Contact voltage is a problem that appears in public areas as well private areas. The
abnormal condition can be on either the utility or the customer side of the revenue meter.
The revenue meter is often considered the point where responsibility for equipment
maintenance passes from utility to customer.
The hazardous nature of contact voltage exposure and a number of recent injuries and
deaths among humans and animals has raised safety concerns throughout the public and
electric utility industry. Some regulatory agencies have responded to these incidents with
increasing interest and additional testing and reporting requirements for the utilities they
regulate.
Contact voltage hazards must be removed. Consideration should be given first to
identification of the possible underlying hazards, addressing the areas of highest risk, and
development of efficient risk mitigation methods. When considering contact voltage, the
severity of the underlying hazard may not be well defined as a function of the measured
potential at the publicly accessible surface. In field investigations of cases where only a
few volts were found on the surface, further investigation has shown situations where
damaged insulation and high impedance faults were the root cause. The variability of the
fault condition should be considered when evaluating the possibility of injury.
A person, or animal, that comes in contact with an energized surface and a return path,
will have an electric current flow through them, if the current is high enough to cause
various nerve and muscle reactions, this is commonly referred to as receiving a shock. A
shock that is severe enough to be lethal is referred to as an electrocution. An energized
surface represents one leg of the shock triangle. Shown in the figure below, the shock
triangle, much like the fire triangle, depicts the requisite conditions for a shock to occur.
Hu
fa
ur
dS
ma
n
ize
/A
ni m
erg
al
En
ce
Shock
Ground
The probability that a shock will occur is dependent upon a combination of probabilities.
The legs of the triangle are comprised of an energized surface, a ground, and a human or
animal in contact with both the surface and ground. The ground can be the earth itself, or
some extension of the earth such as a concrete sidewalk. There is variability in the
resistance of the human/animal contact with the surfaces, the level of voltage on the
energized surface, and the resistance of the ground. The variability can be expressed as
probabilities of causing a shock at a given location. Some consideration of each leg is
appropriate when discussing contact voltage risk reduction.
Operators, designers and regulators of electric delivery systems must take great care to
ensure the appropriate control and safety of devices and structures comprising their
facilities. This is the only leg of the triangle that is under the control of the utilities. The
probability of hazardous contact voltage shocks can be reduced by testing accessible
devices and structures and correcting those that are energized.
The presence of a human or animal, in contact with an energized structure and a ground
is another probabilistic element. Increasing or concentrating testing and correction
efforts to locations where large numbers of people travel on foot will yield the greatest
reduction in the probability of hazardous contact voltage shocks.
The remaining leg of the shock triangle is the ground. Ground does not necessarily mean
the earth. It is any surface that is at, or near to, the voltage level of the earth in that area.
The variability of earth resistance, and the internal resistance of surfaces in contact with
the earth/ground, affects the probability of a hazardous contact voltage at a specific
location. While it is true that lack of a contactable ground near an energized structure is
an important element in assessing the danger of that structure, it is important to consider
all possibilities. The “reach” of a ground or energized structure can easily be extended
when standing water, a conductive pet leash or other variable is introduced.
A conceptual understanding of the elements in the shock triangle and the probabilities
they occur in unison, form a good basis for assessing risk and developing a meaningful
and appropriate risk reduction strategy.
6.1.1 Contact Voltage Discovery
Contact Voltage investigations are typically triggered by one of three discovery events;
Incident, Inspection, & Detection. Examples of these discovery events include; a report
from a member of the public, a worksite report from an employee, a routine inspection
and test program. In all cases, a discovery event is followed by some type of testing,
investigation, and ultimately a repair activity. In many instances, this investigation can
be performed by skilled and trained troubleshooters and repair personnel. In other cases,
it will require comprehensive testing performed by a utility engineer. The outcome for
this type of investigation will be the elimination of the contact voltage by repairing the
fault that was attributed to this undesirable condition.
Incident
A shock report, from a member of the public that has come in contact with an energized
structure, is a key form of discovery. In addition to the technical aspects of investigation,
some detective work may be required to understand the conditions in which the shock
occurred. If the reported structure shows no measurable voltage in the investigation,
conditions may have changed, or details may be missing in the exact manner in which
contact was made during the shock.
Inspection
Utilities perform inspections of their electric delivery assets. These inspections are
programs where the inspection of a specific item is repeated in a cycle of once every
three to five years, and, work area inspections every time construction or maintenance is
performed. An inspection usually consists of visual evaluation of the condition of the
equipment, with work area inspections including electrical measurements to confirm
contact voltage conditions are not present.
Detection
A recent utility industry trend in contact voltage testing is focused on system wide shock
prevention in the public right-of-way. Two methods have been employed, manual
surveys and mobile detection.
A manual survey tests utility owned structures with a handheld contact voltage detector
to sense the presence of voltage. This method requires a technician to walk around and
make physical contact with all structures on an asset list. Identifying voltage on the
surface of structures is the first step and is followed by additional testing to determine the
source and cause of the found voltage.
Mobile detection scans an area with a mobile e-field detector capable of determining the
presence of voltage on any structure by changes in the electric field strength. This is a
remote technique where physical contact to the structure under test is not required. When
the goals are focused on hazardous shock elimination, it is important to consider not just
the obvious utility assets, but all structures that may be inadvertently energized. In
underground distribution areas, many surface level structures are susceptible to being
electrically energized by problems occurring within the buried infrastructure.
6.2 Contact Voltage Sources
Contact Voltage is ultimately the result of some type of failure in materials or practices
that result in an electrical fault (i.e. short or open). A group of specific failures and how
they produce contact voltage are discussed in the sections below.
6.2.1 Contact Voltage Root Causes
Insulation Degradation:
Insulation may disintegrate or deteriorate over time due to exposure to different
environmental conditions. When cables are buried in the ground a small breach in the
cable insulation will increasingly get worse.
Porcelain insulators used on overhead lines can crack, from mechanical and/or thermal
stress, and polymer insulators can become covered with contaminants. Both of these
factors can allow electrical tracking across the insulated path, creating high-impedance
faults and the potential for a contact voltage condition.
Neutral Corrosion/Burn Out:
High impedance faults can develop as a result of neutral corrosion or burn out in direct
burial cable. Deterioration of the neutral conductor can cause voltage on street lights and
utility customer’s equipment. Loss of a neutral conductor in a service can result in
energized earth, including sidewalks and driveways, as the return current flows through
this path.
Workmanship:
Reversed polarity and improperly insulated connections have been found to be causes of
contact voltage conditions. Improperly insulated and exposed connections can lead to
direct contact between energized conductors, as well as wire corrosion and high
resistance connections. These cause contact voltages to appear on structure surfaces due
to short-circuits/faults and opens between the wires and structures. .
Construction Damage:
Buried infrastructure can be damaged by a variety of activities. Road, sidewalk and
building construction typically involve contractor and construction crews to working near
the electrical infrastructure. Contractors not hired by, and outside the control of, the
utilities have been known to create potentially hazardous contact voltage conditions when
trying to fix problems they create without the knowledge required to understand all the
ramifications of their actions. Many of these conditions result from broken or damaged
cables that are covered up without proper repair. Other conditions occur due to making
electrical connections on unfamiliar facilities.
6.2.2 Energized Structures
Some common examples of what has been found in contact voltage investigations are as
follows. Manhole and hand hole covers have been electrified by exposed energized
conductors in contact with the cover or cover support, exposed conductors inside a
manhole full of water, and exposed copper mounted on a rack in the manhole. Street
lights have been electrified by open neutrals, loose connections in the lamp base, reversed
energized and grounded conductor connections, defective street light neutral connection
in the manhole that feeds the light, and internal city wiring. Fences have been electrified
by pierced insulation on a service conductor. Sidewalks have been electrified by
damaged conductors, and service conductors that have been disconnected from a
structure that was removed, but the conductors were not disconnected from their source.
Pool ladders have not been bonded to the pool ground and sometimes energized due to
wiring errors at the property of even a neighboring property.
6.3 Contact Voltage Investigation
Contact voltage investigation begins with the discovery process described in section
6.1.1. Following the discovery of an energized structure, an investigation is needed to
locate the condition causing the fault. An array of tools and methods are available to aid
in the assessment of contact voltage conditions.
6.3.1 Test and Measurement Equipment
A number of test instruments are available for use in the discovery and measurement of
contact voltage conditions and energized structures. The following section describes the
tools and their applications. The descriptions are roughly organized in the order in which
they are employed in the field work associated with contact voltage measurements.
6.3.1.1 Hand Held Detector
The hand held detector is a contact device used to identify structures that are energized
within the landscape. This device utilizes the capacitive coupling between a user’s body
and the handle to provide a ground reference. Once the detector tip is placed in contact
with a structure it will indicate if there is a measurable voltage difference present. This
device is used in manual surveys.
6.3.1.2 Mobile E-field Detector
A mobile e-field detector is a non-contact device, mounted to a mobile platform,
generally a vehicle, used to detect changes in electric field strength, which can be
indicative of electrified structures.
6.3.1.3 Hand Held E-field Detector
The hand held e-field detector is a non contact device used to indicate the presence,
strength and direction of an electric field. This device is used in field investigations
following mobile detection.
6.3.1.4 Voltmeter
A voltmeter is a measurement device used to verify the voltage levels on an energized
structure. A true RMS, high-impedance, AC voltmeter will provide the best accuracy in
voltage measurements.
6.3.1.5 Ground Lead
Field investigations require the use of a remote earth ground to make accurate
measurements. The ground lead should be long enough to reach from the ground point to
the structure being measured. Extended ground leads and extension spools are useful in a
field investigation.
6.3.1.6 Switchable Shunt Resistor
A switchable shunt resistor is a (typically 500-3000Ω) resistor connected, by a switch,
across the terminals of a voltmeter. Switching the resistor into the circuit creates a lower
impedance path for current than the voltmeter. If the current source is strong enough to
drive current through the resistor, it will create a measurable voltage drop across the
shunt resistor. A shunt resistor is useful in differentiating phantom voltage conditions
from contact voltage conditions. The voltage measured across the resistor is used to
calculate the current. The investigator can use the shunt voltage along with a number of
other measurements to draw conclusions about the hazard level of the electrified
structure. Care must be exercised when using a shunt resistor as the test equipment
creates a conductive path between the energized structure and the ground point selected
for the measurement. Safety hazards can be created by providing this path for current.
6.3.1.7 Ground Rod
A ground rod is a long piece of metal that is driven into the earth to create a low
impedance ground connection. During field investigations ground connections are not
always available. Ground rods can be used to make temporary low impedance ground
connections for making field measurements.
6.3.2 Safety Equipment
Anyone performing testing/investigations for contact voltage must utilize appropriate
personal protection equipment. Investigators need to be aware that they are potentially
exposed to hazardous contact voltages. Proper safety clothing/equipment includes, but is
not limited to:
Hard Hat
Eye Protection
Safety Vest
Insulated Boots Rated for low voltage electrical work
Proper clothing, non flammable, cotton clothing
Gloves rated for the voltages being worked on.
6.3.3 Calibration of Equipment
Test equipment should be maintained and calibrated regularly. A calibration schedule
should be developed so that meters get checked and certified at least every year. Also,
before going into the field on an investigation, all detection and measurement equipment
should be tested by the investigator doing the work.
6.3.4 Locating, Measuring and Mitigating Contact Voltage
Investigation of contact voltage conditions should follow a logical sequence to an
accurate conclusion. A thorough understanding of typical fault circuits, possible
measurement circuits, tools and techniques, is needed to achieve the desired outcome.
The following sections describe the methods used in a chronological sequence for the
discovery, measurement and verification of contact voltage conditions.
All aspects of an investigation should be conducted in accordance with electrical codes
and safety regulations.
6.3.4.1 Discovery Methods
The following sections describe the processes used for each discovery method.
6.3.4.1.1 Incident
Reports of individuals and animals receiving shocks can occur anyplace at anytime
without warning. A plan should be in place to deal with such events. Each individual or
organization should maintain a documentation procedure designed to gather important
information about the shock incident. Record keeping and effective communications
between departments are important factors to ensure that proper testing and required
repairs are made. Information gathering may include names, addresses, phone numbers,
equipment descriptions, weather conditions and any other pertinent facts that can assist
the utility investigator.
6.3.4.1.2 Inspection
Inspection of utility owned infrastructure is required by the National Electrical Safety
code, and is most often conducted every three to five years. Inspections are performed by
technicians that can visually identify safety and reliability issues on system components.
Exposed wires, open covers, and structural damage on utility owned structures are
examples of conditions that can be found by inspection. Measurements are sometimes
performed to ensure equipment is not energized and conditions are safe. A record
keeping effort should be employed to document the inspection process. Equipment serial
numbers, locations, GPS coordinates and findings are typically recorded as verification of
the inspection process.
6.3.4.1.3 Detection
Detection processes typically yield the greatest number of findings. This is primarily due
to the fact that detection processes are able to find energized surfaces that are not
included on asset lists. Detection is used primarily in the public right-of-way where
there is a higher potential for the public to come in contact with an electrified structure.
Streetlight supports appear to be more susceptible to contact voltage conditions than most
other infrastructure. Obviously, the power is on at the light only at night. Therefore, to
maximize effectiveness of the detection program, it should be performed during the night
hours, or provisions should be made to turn the streetlights on during the test.
The geometry of the distribution system is an important factor in selecting a discovery
method for contact voltage conditions. In the case of overhead distribution, much of the
system is out of reach to the public. The greatest proportion of the equipment is
inaccessible. The system elements that are accessible are system endpoints (assets), such
as utility poles, conduits, and ground conductors. Street light fixtures, conductors, and
transformers, are out of reach. This in itself has two major implications. First, many
elements are out of reach and therefore do not impose a shock hazard. Any accessible
energized structures will likely be either the system endpoints that reach ground level, or
structures nearby those endpoints. Second, it is appropriate to test those endpoints and
upon successful test, it is reasonable to draw conclusions about the safety of the system.
In underground delivery systems, nearly the entire infrastructure is located below the
surface. Faults in the system can use the earth as a current path and can energize publicly
accessible surfaces. These surfaces can be either utility or non-utility owned. In this case,
testing system endpoints, such as manhole covers, service box covers and lighting
structures may not provide a complete indication of the system contact voltage situation.
Under these conditions, asset list based testing will, by definition, miss energized
structures and surfaces not captured on an asset list.
6.3.4.1.3.1 Manual Survey
The manual survey process identifies utility owned structures that have voltage present
by the investigator making contact with a hand held detector. Manual surveys of utility
facilities are usually conducted by technicians on foot. The manual survey relies heavily
on making good contact between a structure and a ground plane that is at a low potential.
Identification of voltage on a structure leads to a more comprehensive test using a
voltmeter and shunt resistor. The follow-up measurements will determine if there is need
for mitigation.
In overhead distribution areas, the opportunity for inadvertent contact between the
electric delivery system and publicly accessible surfaces is significantly lower than in
underground distribution areas. For this reason, manual surveys working from an asset
list are generally successful in identifying potential contact voltage occurrences in areas
with overhead distribution.
Areas served by underground electric delivery systems have an increased probability that
an energized surface might be missed in a manual investigation. This is primarily due to
the asset list dependence of manual surveys. An energized surface could be a non-utility
facility in electrical contact with some faulted item in the delivery system, but not
required to be tested by the technician.
6.3.4.1.3.2 Mobile Detection
Mobile detection is a scan of a landscape with a mobile e-field detector. Through the use
of a mobile e-field detector, changes in electric field strength can be monitored from a
distance while maneuvering through a street or pathway. Trained technicians monitor the
e-field magnitude for sudden increases indicative of an energized structure. Once an
increase is detected, technicians perform an investigation of the area with a handheld efield detector to isolate the exact structure(s) that have become energized. Measurements
with voltmeters are then made on the energized structure(s) and surfaces to determine if
mitigation is necessary.
Fault currents on underground delivery systems will use all parallel paths available.
These pathways can be influenced by environmental variables. Electric field detectors
capable of detecting change in field strength work well in underground areas, and have
proven to be very effective in underground distribution areas. The use of a mobile e-field
detector increases the efficiency of the detection process allowing the investigation to
cover more area in less time. In areas where overhead distribution is limited to secondary
voltages, mobile detection via sensing of e-field is also effective. However, in areas
directly underneath primary overhead distribution or transmission systems, e-field
detection of contact voltage is limited by effects of the strong e-field emanating from the
primary high voltage conductors.
6.3.4.2 Measurements
When an energized surface is discovered, an investigation will require field
measurements to determine if a fault condition exists. The field measurement should be
done by a trained technician to ensure accurate and safe methods are employed.
A number of voltage measurements and possibly some current measurements may be
useful when attempting to locate the fault and in understanding the possible fault
conditions. An understanding of the measurement circuit is important when interpreting
results.
The measurements are intended to answer the following questions:





What is the voltage level on the energized surface?
Is the voltage supplied through low or high impedance?
How much current can be sourced?
Is the voltage related to a fault, neutral resistance, or other condition?
Is the voltage likely to change?
6.3.4.2.1 Measurement Circuit
In its simplest form, the measurement circuit consists of an energized surface, a
measuring device and a reference ground.
Rsource
~
Voltmeter
Rshunt
AC
Source
Reference Ground
6.3.4.2.2 Voltage Measurements
Purpose: Determine if contact voltage conditions exist.
Method: Measure voltage between the energized structure and the system neutral, and
between the structure and remote earth. Measure the voltage between the system neutral
and remote earth.
Confirm remote ground is not energized by placing the hand held e-field detector near the
object representing earth (e.g. fire hydrant).
The open circuit voltage, Voc can be measured between the energized structure and a
remote earth ground point. A remote earth ground point is a ground that is clear of
voltage or the influence of return currents in the earth. Selecting a good ground is
important to the accuracy of the measurement. Some examples of good grounds found in
the public right of way are fire hydrants, drainage grates, water pipes, street signs, water
valves, or any long piece of metal driven into the ground.
The ground should be at some distance from an electrified structure to ensure that it is not
at an elevated potential. A handheld e-field detector or voltage detector can be used to
ensure there is no voltage on the object being used as a remote earth connection.
Once the remote ground point has been checked for voltage, good contact should be
made between the voltmeter probes and the two measurement points. At times it might
be necessary to clean or scrape the points of contact (e.g. painted surface.) The AC
voltage should first be measured without switching the shunt resistor into the circuit. The
value displayed on the voltmeter is Voc. This measurement should be made with at least
two different remote earth ground points to verify there is consistency between the
measurements. Consistency validates the remote ground point is not energized.
6.3.4.2.3 Current Measurements
Safety Note: A closed circuit measurement connects the voltage source to a
grounded object through a shunt resistor. The grounded object should be carefully
selected to avoid creating new hazards. Gas shut offs are an example of an
improper ground. In the rare case a leak is present, any spark could cause ignition.
Once voltage is found on a structure, additional measurements are necessary to
understand the source of the voltage and the impedance present between the distribution
system and the energized structure. At first glance, this may seem like a simple process.
Unfortunately, there are a number of situations where results can be misinterpreted and
incorrect conclusions can be draw.
The method often used to evaluate source impedance of a contact voltage condition is a
shunt voltage measurement. A shunt voltage measurement is a current measurement. A
shunt resistor is attached to the input terminals of a voltmeter (which converts it into a
current meter.) The shunted voltmeter is connected between the energized structure and a
remote ground point, carefully chosen by the test technician or engineer. The shunt
provides a burden to the source of voltage on the energized structure.
The goal of this measurement is to confirm that a voltage on an energized structure is the
result of a fault, and is sourced through an impedance sufficiently low to drive enough
current to potentially cause a shock. It is important to note shunt voltage measurements
are used in contact voltage investigations to evaluate the source impedance of a fault.
This is quite different than the purpose of shunt voltage measurements used in stray
voltage investigations where the measurement goal is to gauge the possibility of
physiological effects.
The current measurement circuit is depicted in figure xxx below.
Rcontact
Rsource
Rcontact
Voltmeter
Rground
~
Rshunt
AC
Source
Reference Ground
As shown in the figure above, there are several resistances in series within the
measurement circuit. A number of considerations are important if we are to make
unambiguous determination of the source resistance, Rsource. Field measurements
present special challenges as access to a low impedance ground can be limited. If steps
are taken to ensure contact and ground resistances are low, it is possible to determine
Rsource with reasonable accuracy. The following sequence is useful in obtaining
repeatable measurement results.
Select a remote measurement ground
Measure Voc, the open circuit voltage
Measure Vcc, the closed circuit voltage, by engaging shunt resistor
If Vcc is less than 90% of Voc, select another ground, check connections, repeat
The measurement is concluded when:
A voltage indicative of fault conditions is measured
Or
Repeatable low and acceptable values of Vcc are obtained from measurements using
multiple grounds
Flow Chart
The following concepts are important in understanding the possible outcomes from this
measurement. The goal of the measurement is to evaluate Rsource, but it can not be
measured directly in this measurement circuit. Rsource can only be determined with
confidence when the ground and contact resistances are low. This is why an iterative
process is used and decisions are based on the change in voltage when the shunt resistor
is engaged. A switchable shunt resistor, typically applied using a push button, is
desirable as it can be employed without disturbing the contacts to the circuit at the test
probes.
It should be noted that the value of the shunt resistor can impact measurement accuracy
when using this measurement technique. Low values for shunt resistance, such as 500
ohms, make it critical that low contact and ground resistances are obtained. If the sum of
ground and contact resistances equals 50 ohms, a 10% measurement error will be result
due to the series connection of the 500 ohm shunt and the contact and ground resistances.
A shunt value of 3000 ohms, when used with a total ground and contact resistance of 50
ohms, results in a measurement error of 1.6%.
“Phantom” voltages can appear on surfaces due to the influence of electric fields and
magnetic fields emanating from a power source in proximity to the surface. While these
phantom voltages can produce a measurable voltage on a high impedance voltmeter, they
do not have the capability of causing an appreciable current to flow through an
impedance and therefore are not hazardous to the general public or animals. Phantom
voltages will collapse under the burden of a shunt resistor. If sufficient effort has been
given to ensure contact and ground resistances in the measurement circuit are low,
accurate determination of phantom voltage conditions can be made.
Fences, railings, store fronts, long runs of metal and loose wires are examples of
structures where phantom voltages could appear. When these structures are identified by
one of the discovery methods, they should be tested for open and closed circuit voltages.
Manhole covers, service box covers, storm grates, streetlamps, and sidewalks are not
usually susceptible to phantom voltages, but, should still be measured without and with a
shunt to obtain information about the source of the voltage present.
The next section should further develop the
understanding of the fault circuit with examples. These
diagrams and text are representative place holders.
Furthermore, for this investigation it is necessary to make measurements between ground
points to indicate if there is an elevated ground voltage, or a bonding issue. If there is a
bad neutral, the return current will travel through the ground rods. Also it is important to
install a ground rod outside the immediate investigation area to ensure that any elevation
in voltage does not affect the reference ground point. The ground rod should be installed
at a distance at least 4x the length of the service panel ground rod, and away from any
utility pole or box ground rods. Voc measurements should be made between the
reference ground point and the different grounds throughout the secondary side of the
electrical system. These measurements will reveal if there is an elevated neutral-to-earth
(NEV) voltage present in the system.
(More clarity for the investigation of fault conditions on primary and secondary side of
the distribution.)
If an elevated contact voltage is present it must be determined if it is caused by fault
conditions on the secondary system (homeowner’s side) or primary system (utility side).
Best practice would be to eliminate high and low impedance low current faults on the
secondary side first. One way to accomplish this is to first turn off each secondary load
individually while monitoring the elevated contact voltage. If the elevated voltage goes
away when a specific load is turned off, the de-energized load is likely shorted and
should be repaired or replaced. If the elevated voltage does not disappear after all loads
are turned off, check the electric meter to see if it has stopped moving. If it continues to
move with all loads de-energized then there is likely a high or low impedance fault in the
wiring on the customer’s side of the meter. An investigation of the circuits and current
measurements, with a clamp on ammeter, on the suspected circuits will point to the faulty
wire. This wire should then undergo an insulation test to verify it is the correct wire and
aid in identifying the type of fault. Additionally, if the fault has not been located the
system can be checked for high impedance or open neutral connections. This is
accomplished by applying a known phase to neutral load at the service panel on the
secondary side and measuring the secondary neutral and ground lead current at the
serving transformer. If the majority of return current is finding its way back to the
transformer by paths other than the secondary neutral, further investigation may be
necessary
Considering now that the elevated NEV is still present and no high impedance faults were
found on the secondary system. The problem could be stemming from the primary
system (Utility side). Investigation of loads on the primary side will help to eliminate the
elevated voltage. Neutral isolators, load balancing and investigation of fault conditions
on other customer’s secondary might provide resolutions.
6.3.4.2 Safety Standards
This section will include details of how to interpret the voltage readings, and trigger
levels for mitigation activities. Low voltage / high voltage conditions often dictate
increasing urgency in repair efforts.
The New York State Electric Safety Standards currently state the following:
When interpreting results of an investigation, voltage that measures 1 volt or greater with
a 500 ohm shunt should be barricaded and guarded until properly mitigated. Any
temporary repair should be periodically monitored until permanent repair is made. The
higher the contact voltage the more hazardous the situation, more stringent safety
measures should be taken to ensure the public remains a safe distance from the structure.
Voltages that measure less than 1 volt with a shunt are considered safe levels of voltage
and do not require mitigation. The measured voltage of the structure should be
documented for reference.
6.4 Mitigation
After the energized structure has been identified and field measurements have been
completed, an assessment can be made to determine the most appropriate mitigation
method. A properly trained electrician or a repair crew should be notified of the
structure, the location, and the nature of the fault so that repairs can be made accordingly.
Repair of the fault condition is usually all that is required. It may require making new
connections, repairing and resealing bonds, splicing a new section of cable, or replacing
an entire cable. A record of all repairs, and a post repair voltage measurement is
necessary in all cases.
(Specific mitigation methods used by utilities and the specific fault application)
6.5 Case Studies
Case Study #1
1. Details of Customer Report
A rural residential customer reported getting shocked in his basement shower.
This had been going on for about two weeks when the utility received the
complaint.
The customer is served by a single phase, overhead, 25 KVA transformer. The
premise is near the end of a rural single phase tap, approximately 7 miles from
the substation and 5.5 miles from the nearest 3-phase. There is 50 KVA of
connected load downstream of this premise.
2. Previous Actions & Findings by Service Technician
The utility service man responded and measured 6.4 – 7.0 volts between the
faucet handles and metal floor drain cap in the shower.
Initial efforts by the service technician included:
 Disconnected the customer meter, to kill service to the house. This had
no affect on the voltage.
 Disconnect the service neutral, a well as the hot leads. This had no affect.
(It was determined later that disconnection of the service neutral had no
effect because there was a conductive bypass created by a shared water
service. This complicated the investigation. )
3. Engineering Diagnostic Actions & Findings
An Engineer joined technician to continue the investigation:
 Measured 3.5 volts in shower.
 Measured ground at 18 ohms, with 194 milli-amps (equates to 3.492 volts, by
Ohms Law).
 Disconnected hot legs at service box, no affect.
 Added bond from house ground to water pipes, no affect.
 Measured voltage across neutral connections in house, all OK.
 Customer reported problem started about two weeks ago. Around that
same time a neighbor had some sort of electric failure and had to get the
utility to fix it.
 We then disconnected power to the neighbor, by pulling the transformer
fuse, which dropped the shower voltage from 3.5 to 0.5 volts.
 Upon further questioning, the customer told us that he shared a well
water system with the neighbor across the street.
Next step was to investigate a possible ground fault at the neighbor’s premise:
 Neighbor is a farm operation (no house) served from a meter pole in the
yard, with 4 branch circuits running to each of four buildings, three
underground and one overhead.
 The farmer indicated that the 120 volt, 2-wire, overhead branch circuit to
a garage building had been torn down a couple of weeks prior, when hit
by a combine harvester. We checked the currents on these wires and
found 1-2 amps on the hot leg, but 5-6 amps on the neutral. We opened
the main breaker in the garage, which dropped the hot leg current to zero,
but left 3-4 amps on the neutral.
 Suspecting a ground fault elsewhere, returning thru this neutral, we shut
off the main breakers in the other three buildings, one at a time, with no
affect on the measured currents.
 We then opened the main switch at the meter pole, and the neutral
current went to zero.
 With the building breakers all open, we closed the main switch and
measured the currents on all of the branch circuits. The only circuit
carrying load was the overhead branch to the garage, on its hot leg.
(curious)
 The farmer then mentioned that he had reconnected the overhead wires
himself, and was not sure if he had done it correctly; all of the wires were
black. Maybe he had reversed them?
 We then killed the power at the main switch and checked continuity on
the 2 conductors going back up the meter pole to feed this overhead
branch circuit. Sure enough, he had cross-connected them. The overhead
‘neutral’ wire was energized. We corrected the connections, which fixed
the neighbor’s shower voltage problem across the street.
NOTE: Even though the overhead ‘neutral’ wire was securely bonded to a ground
rod at the garage’s breaker panel and energized at 120 volts, no fuses blew. There
was no noticeable voltage on the ground rod, when we touched it. The lights and
door opener in the garage all worked fine. The current to the ground rod measured
only ½ amp, even though we measured 3-4 amps on the overhead neutral. The only
measurements clue was that the ground impedance meter would not give a good
measurement on that ground rod.
4. Problem
This was a contact voltage problem that initially appeared to be a stray
voltage problem.
It had all of the symptoms of a Stray Voltage problem, from the point of
view of the complaining customer. Shower voltage levels were perceptible
and there were no electrical wiring problems on his premises. Since the
voltage source was off premise, the voltage at first appeared to be related
to normal operation of the distribution system.
A wiring error at the neighbor’s farm, however, had energized conductors
that should not have been energized. It was only through the luck of soil
conditions and insulation that there was no dangerous voltage at either
the fault location or the residence across the road.
5. Remedial Actions Taken
The wiring error at the neighbor’s farm was corrected.
6. Results
The shower voltage was reduced to levels that could not be perceived.
7. Any Next Steps (Optional)
None anticipated.