AC Interference
Columbia Gas of Ohio/Kentucky
Tim Jenkins
Corrosion Front Line Leader
Objectives





Develop a basic understanding of the
principles and components of AC
Develop an understanding of the
different types and effects of AC
Influence
Develop methods of mitigation
Understand safety protocols
Cover AC calculations
Basic Electric - AC
SINE WAVE
Half Cycle
0
Peak of positive side
of cycle.
AC - alternating current
will reverse in polarity 120
times per second. A full
cycle is considered one
hertz. Typical AC has 60
hz per second.
0
0
Half Cycle
Single
Phase
Peak of negative side
of cycle
Basic Electric - AC
Peak of positive side
of cycle.
AC – Three Phase, each
conductor has the same
amount of current and are
120 degrees out of phase
SINE WAVE
Half Cycle
0
0
0
Half Cycle
Three
Phase
Peak of negative side
of cycle
Fault Currents


If any of the AC waveforms are to get of
frequency with each other greater than or
less than 120°, then a possible fault current
can occur.
Fault currents are large magnitude of current
that can occur in brief amount of time
(normally in milliseconds, typically .1 second)

Normally electrical towers or structure has
grounding and protection devices for this situation
that limits the fault current to a brief amount of
time
Fault Currents


It’s not possible to know when, how or
where fault currents will occur, in which
makes it difficult to predict the effects of the
fault and the mitigation required to protect
both the pipeline and personnel
Need to calculate locations of more
acceptable for fault currents to occur,
such as –



Electrical storms, ice storms, & high winds
Distance from the Power lines
Information provided from the electric company
Fault Currents

Even though the fault current is brief, it
still presents a danger to personnel and
the pipeline



Coating damage can occur
Pipeline failure due to melting or cracking
of the pipeline wall
Discuss more in Conductive coupling
Three Phase –
Three conductors
Shielded wires
Counterpoise Lines – Used
for the grounding system,
normally buried and above
connected to each tower
to provide grounding
Method of measuring AC
voltage on Structures

Connect to ground with one lead and measure the AC
volts onto the structure with the other lead.

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Use an accurate volt meter, set meter on AC volts
Use rubber gloves during measurement and/or
Use a rubber mat for added insulation
High dielectric boots are available as well
Common method, use a copper-copper sulfate half
cell with the meter set at AC volts

Must have good soil contact with half cell
Effects of AC Influence

Two key factors to consider with AC
Influence


Safety
Corrosion
Effects of AC Influence

Two key factors to consider with AC
Influence


Safety
Corrosion
Safety

Electrical Shocks

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Step voltages
Touch voltages
Arcing
Ignition of volatile liquids
Safety

Maximum allowable AC voltage = 15
Vac

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
Based on a typical individual is at 1000
ohms body resistance
And the individual can tolerate up to 15 mA
Ohms law = 15 volts
Anything above 15 Vac, could cause
muscular contractions

Prevents the person from letting go
Safety

Electrical Shock, such as fault currents
 Can occur by physical contact or standing
in the vicinity of an energize structure in
contact with earth
 Short time frames of electrical shocks are a
concern when currents are above 50mA or
greater



Can cause ventricular fibrillation
Certainly occurs at body currents of
greater than 100 mA
Death will occur unless De-fibrillation is
given
Safety

Electrical Shock


Fault currents - passes from the structure to
ground creating a voltage gradient
Step Voltage –


Is the potential difference between two points on
earth’s surface separation by a distance of 1 pace
(approx. 1 meter) in the direction of max.
potential gradient
Touch Voltage –

Potential difference between the grounded
metallic structure and the point of earth’s surface
separated by a distance equal to the normal
maximum horizontal reach (approx. 1 meter)
Safety
If (Fault Current)
10 kV
Ouch!!!
Potential
Touch
voltage =
2kV
9 kV
8 kV
7 kV
Safety
If (Fault Current)
Ouch!!!
10 kV
Potential
Step
voltage =
1kV
9 kV
8 kV
7 kV
Safety –
(Maximum Current Calculation)


Maximum current IB a human body can
tolerate depends on shock duration ts
(seconds) and body weight
calculated as follows:
IB = 0.157/  ts ( for a 70 kg body)
 IB = 0.116/  ts ( for a 50 kg body)

Safety –
Step and Touch Voltage Calculation



Maximum voltage that human body can
tolerate by touch or step –
Step formula  VStep = (1000 + 6) 0.157/  ts ( for a 70 kg body)
 VStep = (1000 + 6) 0.116/  ts ( for a 50 kg body)
Touch formula  VTouch = (1000 + 1.5) 0.157/  ts ( for a 70 kg body)
 VTouch = (1000 + 1.5) 0.116/  ts ( for a 50 kg body)
Safety –
Step and Touch Voltage Calculation


Pipe line running parallel to a power line
may exhibit 500 volts for a duration of
½ second during line to ground fault
What is the tolerable touch voltage for a
50 kg individual with a soil resistivity of
50 ohms m touching the structure
during the fault?
Safety –
Step and Touch Voltage Calculation
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VTouch = (1000 + 1.5) 0.116/  ts - ( for a 50 kg body)
VTouch = (1000 + 1.5 • 50) 0.116/  (.5) = 176 VAC
Since the possible fault voltage is 500 V then we need to
raise the soil resistance
Try 3000 Ω-m of crush stone added to the site
Now the calculation equals to 902 VAC
Which exceeds the maximum pipe to earth voltage of
500 VAC, the pipe is now safe
Voltage gradient mats could provide a higher earth
voltage to decrease the potential difference between the
hand or feet touching the pipe
Safety
If (Fault Current)
Cool !!!
10 kV
Potential
Step
voltage =
0 kV
Voltage
gradient
Mat =
10 kV
10 kV
10 kV
7 kV
Gradient control
mats – Placed at all
test station
locations in the AC
Corridor
Zinc Grounding Mat
Dimensions =
4’x4’
Wire
connected
to the
zinc
ribbon
Zinc
ribbon
6” Low
resistance
material –
Coke breeze
or benonite
Note : You can use the
native soil, providing soil
has good moisture content
Cut hole for
Test station
12”
crushed
gravel
6” Low
resistance
material –
Coke breeze
or benonite
Zinc Grounding Mat
Copper
rods
installed to
get low
resistance
with
grounding
mat
Wire
connected
to the
zinc
ribbon
Connected
to pipeline
in Test
station box
Safety
(Calculation for Arcing)


One of the greatest concern in dealing
with fault currents between a power
line structure and the pipeline is
whether or not there is enough energy
available to create an electric arc
through the soil.
Could result in pipeline damage
Safety
(Calculation for Arcing)


Greatest prevention of Arcing with fault
currents is to maintain safe distance between
power lines and the pipeline
One must obtain information from the electric
company or producer such as


fault currents maximum measurements
Need to find soil resistivity in area

Perform sufficient amount of testing samples in
order to accurate obtain average
Safety (Calculation for Arcing)


One Safe distance calculation by Sunde
- for prevention of arcing
Distance r (m) over which an arc could
occur, based on soil resistivity  in (Ωm) and fault magnitude If (kA).
Safety (Calculation for Arcing)

R(m) = 0.08 If •  ( = < 100-m)
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R(m) = 0.047 If •  ( = > 1000-m)
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Use this formula with lower resistivity
Use this formula with extremely high
resistivity
R(m) = Distance measured in meters
If = Magnitude of fault current
 = Soil resistivity measured in meters
Safety (Calculation for Arcing)
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For an example,
Soil  = 6700 ohms-cm = 67 ohms -m
Fault Current If =17.9 kA
Use formula
R(m) = 0.08 If •  ( = < 100-m)
R(m) = 87.6 meters
Lightening
Fault
Currents
90
Meters
Lightening
Fault
Currents
90
Meters
Safety (Calculation for Arcing)

If safe distance can not be obtain,

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Screening electrodes between the pipeline
and towers maybe used to intercept the
fault currents
Such as zinc ribbon, or banks of sacrificial
anodes
Lightening
Fault
Currents
Pipeline
Lightening
Fault
Currents
Pipeline
Zinc
Ribbon
Zinc
Effects of AC Influence

Two key factors to consider with AC
Influence


Safety
Corrosion
AC Corrosion on Pipelines


AC influence can cause corrosion to take place on
coated steel pipe line
Study performed in Germany, recently in the
1990’s, had determined that corrosion occurs at
specific AC current density 


(>100 A/m²) = Corrosion will result
(20 A/m² - 100 A/m²) = Corrosion is unpredictable
(< 20 A/m²) = Corrosion will not result
AC Corrosion on Pipelines


There has been documented cases of pipe to
soil potentials being above -1.170VCSE with
pH samples at 11, indicating pipe being
cathodically protected, but corrosion was
found due to AC current density in the range
of 800 A/m²
Pipe must be mitigated by dropping the AC
voltage with the use of grounding devices
such as zinc ribbon, copper wire, etc..
AC Calculation for Current
Density

Calculation to determine AC current
density 
Iac = 8◦Vac/ ••d



Iac = Current density
 = soil resistivity in meters
d = holiday area in cm’s
AC Calculation for Current
Density

Calculation to determine AC current density  Iac = 8Vac/ ••d


Resistance and area of holiday will be the
key factors in determining the AC current
density
For an example – 1cm² holiday found with 5
Vac in a soil resistivity of 10 Ohms m (1000
ohms CM)


= 127 amp/m² ((( Corrosive)))
But below the 15 Vac
Documented cases of AC
Corrosion Found Pipe to soil
potential readings
were above –1.0v
CSE DC
Pipe met DOT criteria for CP –
above .850- V CSE
Documented cases of AC
Corrosion Found -
AC Stray Current –
Interference Methods

Electromagnetic Coupling –


Electrostatic Coupling –


Inductive
Capacitive
Conductive Coupling 
Direct path
AC Stray Current –
Interference Methods

Electromagnetic Coupling –


Electrostatic Coupling –


Inductive
Capacitive
Conductive Coupling

Direct path
Electromagnetic Coupling
– Inductive

Works in the same capacity of a
inductive pipeline locator –


Induces an audio signal onto the buried
pipeline
Or in the same capacity of a
transformer

Primary coils inducing current by a
electromagnetic field to the secondary
windings
Electromagnetic Coupling
- Inductive

Primary characteristics include:


Medium to High Voltages
High induced current levels
Electromagnetic Coupling
- Inductive

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The level of interference decreases with
increasing separation of conductors
The strength of the magnetic flux is in direct
proportional to the current magnitude and
inversely proportional to the distance of the
conductor
Induction effects experienced during power
line faults can be a hazard to personnel
Normally peaks at the point of entry of AC
corridor and at the point of exit
Electromagnetic Coupling
- Inductive (Remediation)

Installation of a low resistance grounding
systems to reduce current and voltage levels

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Grounding mats for test stations (safety)
Zinc ribbon
Copper wire with the use of PCR or ISP
Achieve at least a 25 ohm impedance system


Ideally one ohm system
Normally deeper is better
AC Stray Current –
Interference Methods

Electromagnetic Coupling –


Electrostatic Coupling –


Inductive
Capacitive
Conductive Coupling

Direct path
Electrostatic Coupling –
Capacitive


Any two conductors separated by a dielectric
material (insulator) is considered a capacitor
Electrical Field Gradient between the
transmission line and conductor takes place,
builds up a electrical charge on the structure


Such as a capacitor function
Primary characteristics include :

Very High Voltage peaks on power lines
Electrostatic Coupling –
Capacitive

Conductors acceptable to capacitive
coupling

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Pipelines suspended above ground on skids
Any above ground equipment isolated such
as vehicles or backhoes with rubber tires
Electrostatic Coupling does not penetrate
the earth
Long parallel exposure of buried metallic
structures to power lines
Electrostatic Coupling –
Capacitive
Electromagnetic
charge
(
VAC
VAC
Voltage
Gradient
–
electromagnetic
field
The voltage builds
up until it has path
to ground to
discharge
Ground
Electrostatic Coupling –
Capacitive
Electromagnetic
charge
(
VAC
VAC
Voltage
Gradient
–
electromagnetic
field
The voltage builds
up until it has path
to ground to
discharge
Ground
Circuit is
open, the
voltage
charge will
build to
high
voltage
static
capacity,
until a
ground
source is
provided.
Electrostatic Coupling –
Capacitive
Direct Path
to ground
–
Electromagnetic
charge
Voltage
Gradient
–
electromagnetic
field
VAC
(
You……
By
touching
the
structure
and ground
at the
same time.
VAC
The voltage
discharges
Ground
Pipe
suspended
500 VAC
Electrostatic Coupling –
Capacitive (Remediation)

Temporary repair –

Ground vehicles and equipment

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Use temporary grounding rods (copper rods) normally
in 3 meters in length
Use #2 Cable
Use ½ in diameter rods in normal soils
Refuel away of influence area to prevent
accidental ignition, bond to refueling tanks
Due to high resistance soil, you need to place
multiple rods, space about 6 feet apart
Metal chains dragging from the vehicle’s bumper
in High AC voltage corridors are commonly used
Pipe is being
grounded by
making
contact to the
soil
Electrostatic Interference
– Capacitive (Remediation)

Permanent repair –

Above ground pipelines or valves

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
Install Zinc ribbon
Install Zinc Grounding or voltage gradient Mats
Grounding Rods

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3 Meters in length
Design cable size based on potential fault currents
Set depth until achieved a minimum of 25 ohms
impedance
Lower the impedance as low as possible
One ohm is desirable
Electrostatic Interference
– Capacitive (Remediation)

In most cases, the Electrostatic charge
can not generate enough body current
to create a shock hazard, more of a
nuisance shock similar to static
electricity.
AC Stray Current –
Interference Methods

Electromagnetic Coupling –


Electrostatic Coupling –


Inductive
Capacitive
Conductive Coupling

Direct path
Conductive Coupling



Occurs when line to ground shorts or faults
take place
On High Voltage power lines faults normally
occur during lighting strikes
Fault currents can occur by accidental contact
with other structures


Such as construction equipment or cranes
Fault currents is conducted to the pipeline
through its coating

Higher the coating dielectric strength, the less
amount of the transfer current on the pipeline
Conductive Coupling

Occurs in milliseconds


Voltage and current is higher than steady
state but happens very briefly
.1 or a tenth of a second is the normal
time frame that voltage is present due to
the fault protection system
Conductive Coupling

Failure to the pipeline


Coating damage
Cracking and melting of the pipe wall
Materials Used for AC
Mitigation

Two reasons for mitigation of AC
influence



Prevent corrosion on the pipeline
Prevent of hazardous shock from contacting
the pipeline
Materials commonly used





Zinc grounding and/or voltage gradient mats
Zinc ribbon or heavy gauge copper wiring
Blind face test stations
Galvanic anodes
PRC or Inductive capacitive coupling
Materials Used for AC
Mitigation

Materials –

Zinc Ribbon – to mitigate the AC currents
from the pipeline to the soil


Zinc is used in some low resistivity areas as a
galvanic anode to protect structures
The AC currents will take the path of less
resistance to the ground


Zinc provides this path
Depending of soil resistance, distance to the
tower, the location of structure to the towers and
the amount of magnitude influence of the towers
must be calculated in the design of the amount of
Zinc ribbon needed and the location
Materials Used for AC
Mitigation

Programs available to profile the
pipeline for AC mitigation

PRCI
Materials Used for AC
Mitigation

Materials –

Zinc Ribbon –

Installation –



Placed below the pipeline
 Depending on soil resistance
 Place in the lowest resistance area
 Minimum Two feet away from the pipeline
Make connection to the pipeline in a junction box or
test station
Commonly used, a minimum of a no. 4 gauge wire
connected to the pipeline and zinc ribbon
 May need to increase size of cable due to
greater magnitude of fault currents
Materials Used for AC
Mitigation

Materials –

Zinc Ribbon –

Installation –



Placed between the pipeline and tower
 To mitigate fault currents and prevent
coating damage
Splice zinc ribbon by striping the zinc off the
wire and crimp the connections together
Make crimp repair with epoxy resin kits, heat
shrink sleeve, or electrical rubber tape
Chart for Zinc Ribbon
Standard – ½ inch comes in
wooden spools
Ribbon is bonded to the
main in the test station to
be able to test AC mitigation
such as AC current density &
grounding system
resistance
Zinc ribbon is placed
below the pipeline and
at least two feet away
Installed at test
station facility –
Coupon for AC
measurements
Grounding mat or
voltage gradient
mat for test point
reader safety
Zinc ribbon
connection
Structure
connections
What's wrong with the
next slide?
Zinc ribbon
Slide “B”
Tower
Slide “A”
Pipe line
Zinc
ribbon
Zinc Ribbon is on the wrong
side of the pipeline
Tower
Zinc ribbon is above the
pipeline
Pipe line
Zinc
ribbon
Polarization cells or Insulated
surge protections are great
for grounding the pipeline or
structure with out shorting
out the DC cathodic
protection currents. It will
block the DC and allow the AC
currents flow to ground.
Dead Front Test Stations
To prevent
electrical shock in
making contact
with wire
connection to
mainline