Transcript COH Interference 7-10

Agenda
•
•
•
•
•
In class training over PowerPoint
Open discussion and review
Lunch
Hands on and field testing
Follow up back in class room setting
Definition of Stray Current
Interference
The National Association of Corrosion Engineers
(NACE) recommended practice on cathodic
protection underground structures provides
several insights to the definition and evaluation
of interference.
Stray current is defined as “current through
paths other than the intended circuit” or “the
deterioration of a material, usually a metal that
results from a reaction with its environment”.
Interference Objectives
• Recognize
• Test
• Mitigate
Recognize
• Pipe to soil potentials indication of
possible interference situation
• Areas of high voltage gradients or
polarization
• Structural effect of stray current pickup
and discharge
Test
• Basic survey measurements techniques
• What are the minimum requirements?
• Selection of testing equipment and
materials
• IR and Polarization measurements
Mitigate
• Why mitigate?
• Difficulties in interpreting data?
• Selection and method of mitigation
Fundamental Review
• Corrosion diminishes the integrity of the
pipeline, increases the probability of pipeline
failures and loss of system reliability.
• Failure to properly analyze and arrest
corrosion may result in loss of life and
property, jeopardize pipeline integrity and
corporate image, not to mention the economic
forfeiture of pipeline system revenues.
Fundamental Review
Continued
This training session encompasses interference
stray current, effect on pipeline polarization,
recognition and role of voltage gradients,
importance of the consideration of voltage
drops, and understanding when stray current
interference may result in corrosion.
Basic Faradays’ Law
Rates of Metal Loss
Fe (Iron) / STEEL
Al (Aluminum)
Cu (Copper)
Pb (Lead)
Mg (Magnesium)
Zn (Zinc)
20LBs /Amp/ Year
6.5
45.7
74.5
8.8
23.6
Given: 1 amp of current discharging from a
pipeline for 1 year.
Metal loss: Approximately 20 lbs.
EQUIVALENT METAL LOSS
Equivalent Length
Pipe Diameter/W.T.
Pipe Weight/Foot
of Pipe Loss
4” = 4.500” O.D. x 0.188 W.T
8.66 lbs/ft.
2.31 ft.
6” = 6.625” O.D. x 0.280 W.T.
18.97 lbs/ft.
1.05 ft.
10” = 10.750” O.D. x 0.188 W.T.
21.21 lbs/ft.
0.94 ft.
16” = 16.000” O.D. x 0.250 W.T.
42.05 lbs/ft.
5.70 in.
20” = 20.000” O.D. x 0.250 W.T.
52.73 lbs/ft.
4.55 in.
Stray Current cases can result in high Amp
discharges, in which results to rapid corrosion or
metal loss
Remember the basic law of corrosion - For
corrosion to occur all the components of a basic
corrosion cell must be present.
Conventional Current Flow
Metallic connection
Conventional current
Flow - is the flow of
current from positive to
negative in a electrical
circuit.
Electron Flow
Electrolyte
Current Flow - needs to
return to it’s original
power source.
Cathode
Positive
Anode
Negative
Electromotive Force
is the potential
difference between
the two structure.
Conventional Current Flow –
Rectified System
Anode bed
Rectifier
Unit
Current Flow
(+)
Positive (+)
Negative (-)
Current Flow
Structure - Pipeline
(-)
Recognize Stray Current
• Foreign structures nearby
– other pipelines
– buried tanks or petroleum facilities
• Increase leakage in area
• Readings increase more negative or positive
(Annual monitoring, Bi-monthly inspections)
• Corrosion focus on a pinpoint location onto
structure
• Coating disbondment near foreign line
Types of Interference
There are two types of interference
1. Dynamic
2. Static
Dynamic Interference
• Dynamic interference is recognized by
pipe-to-soil measurements that fluctuate as
a result of the source of stray current.
These currents are continually varying in
amplitude and/or continually, changing
their electrolytic paths.
• Light rail systems.
• DC mining activities.
• DC welding on the pipeline.
Two areas of discharge
Dynamic Interference Example
Two Interference bonds
connected
Bond between pipeline and DC substation
Reverse Switch or Diodes Used
• The rail system potential builds up
positive
• In prevention of shorting out company
system with the return interference bond,
in which bring the pipe line structure
positive with the rail system, diodes are
used
Static Interference
• Static interference is a steady, continuous
stray current source, such as, an impressed
cathodic protection rectifier.
• This session deals primarily with the
definition, recognition, testing, and
mitigation of static interference stray
current.
Conventional Current Flow
Current returns
through the soil
+
Current
Discharge
–
Corrosion
Current
Discharge
–
Corrosion
Static interference caused by a cathodic protection system
Conventional Current Flow
Current
Discharge –
Corrosion
Foreign Line Crossing and Stray Current Pickup and Discharge
Conventional Current Flow
Current
Discharge –
Corrosion
Testing non crossing foreign pipeline for stray current interference
Current
Discharge –
Corrosion
Voltage Gradients
• Remember the rectifier transformers,
– The voltage gradient is build around the
primary coils, energize a iron ore, which the
secondary coils picks up the voltage potential
and current flows through the circuit
– Current still returns back
– Stray current works in the same principal
– When ever pipe passes through one of these
voltage gradients from a foreign line, it picks
up current
Ion Flow
• Remember basic corrosion, electrons flow
the opposite direction as current
• At the cathode area (pick up), electrons
will flow and build up to a negative
potential
• At the anode area (discharge location),
loss of electrons builds up to a positive
potential
Conventional Current Flow
• Conventional current will flow, from
positive to negative,
– From the foreign ground bed system to the
companies pipeline pickup area
– Through the pipeline to the anode
– From the anode (discharge area) to the
foreign structure through the electrolyte
Interference Consideration
Factors Impacting Corrosion Severity
• Separation and routing of facilities
• The location of the interfering current source
• Magnitude and density of the current
• Coating quality
• Absence of external coating on the structures involved
• Presence and location of mechanical joints of high electrical
resistance
Interference Detection
• When stray current interference is
detected
– Time is of the essence
– Leakage can occur with in days or weeks
Testing
Effect of Interference Stray Current
Pickup and Discharge
Laws of Electricity and Interference
Current
•
Current will always take the path of
least resistance.
•
Stray current will always return to the
source.
Effect of Interference Stray Current
Pickup and Discharge
Current Discharge May Only Reduce
Polarization.
• Where the stray current discharges, a detrimental
affect will occur.
• Pipe-to soil readings will be less negative.
• May result in possible corrosion
• Whether actual structural damage of corrosion
occurs depends upon the existing level of
cathodic polarization.
• If there is adequate polarization, current
discharge may not cause metal loss.
• Potential shifts less negative are not necessarily
indicative of interference corrosion
Electronic or Ionic
When the stray current discharges, one of
two reactions will occur.
1. If adequate polarization exists, the
current discharge will result in an
electronic exchange electrochemically.
–
–
No corrosion occurs.
Reduces the cathodic polarization.
Electronic or Ionic
2. When there is a lack of cathodic
polarization present, the discharge of
current will result in corrosion damage.
Foreign Stray Current Affect
on Polarization
• Stray current pickup increases
polarization, this is represented by the
higher,more negative, pipe-to-soil
readings
• Stray current discharge decreases
polarization, this is represented by a
more positive or less negative pipe-to-soil
readings
IR drop information needed to use 100
mV shift Criteria or -.850 V
polarization criteria
Pick Up and Discharge Areas
• Need to identify areas of Pick up and Discharge
– Pick up area,
• More negative
– Discharge area,
• More Positive
• Determine locations by CIS
– Interrupting foreign structure
– Data logger is the best tool to use
Connect Interrupter in
series with the structure or
ground cable. In this case,
we used the structure cable.
Interrupter
MCM used to
find peak and
valleys of reads.
Sincorder 2 for
the CIS
Discharge Area
• Indicated in CIS as the most positive potential
reading
• The area considered anodic
• The area that will corrode
– Faraday's law = 1amp = 20lb’s per yr
• Most likely found at the point of crossing or the
maximum exposure to the foreign line
• The location for the bond to be established
Graph of Stray Current Pickup and Discharge on Bare Pipeline
Graph of Stray Current Pickup and Discharge on Coated Pipeline
Pipe-to-Soil Readings Through Foreign Influence and Resultant
Depression in Potentials
Rules of Thumb - Interference
Testing
1. Current will always take the path of
least resistance.
2. Current must always return to its
source.
3. Get the “big picture” of all metallic
structures and possible stray current
sources, and.
Interference Testing Rules of
Thumb
4. Follow the data if practical by finding
the corresponding stray current
discharge point when a stray current
pickup is found.
5. Simplest test is to measure the metallic
voltage shifts.
The greatest
voltage shift
Beware of Interference Testing
Difficulties
• Limited access to the pipelines due to
blacktop or concrete requires drilling to
obtain measurements
• Polarization testing is complex and time
consuming
• Polarization testing may require
substantial number of current
interrupters that are synchronizable
Beware of Interference Testing
Difficulties
If you have more then one rectifier, you
need to have synchronizable current
interrupters.
Time programmable
Master – Slave
GPS
Testing Criterion
• It is necessary prior to conducting any
field-testing to gain agreement on what
criterion will be utilized to test, evaluate,
interpret, and mitigate any stray current
problems that may be identified.
• Prior to conducting field tests all parties
should agree to the standard remediation
requirements.
Testing Criterion
• Columbia’s acceptable criteria
– 50 mV voltage shift, with foreign system
interrupted, more positive
– .850- V CSE Criteria, with foreign CP
system operating
• Both criteria's must be met
Interference Testing Outline
Summary
Data Needed to be Obtained:
– A survey of the pipelines with the existing current
from groundbeds in the area (On Potential Reading
of the foreign structure)
– A survey of each pipelines as if no foreign pipelines
were present with only the companies current (Off
Potential Reading of foreign structure)
– A survey of each pipeline depolarized to obtain a
static potential
Interference Testing Outline
Summary
How to Accomplish This?
• Conduct an ON/OFF survey of Columbia’s pipeline
with the existing bonds broken and all known foreign
influencing rectifiers interrupted (Columbia’s CP
system operating)
• Perform an ON/OFF survey of Columbia’s system with
only Columbia’s rectifiers interrupted. The foreign
companies are to be turned off at least 12 hours prior to
each survey
• Obtain potentials after all rectifiers have been turned
off for at least 48 hours
Interference Testing Outline
Summary
Why an ON/OFF Survey?
• An ON survey alone does not give insight into the
actual condition of pipe regarding its actual cathodic
protection
• The actual cathodic protection is demonstrated by
measuring the chemical activity at the pipeline surface
that regards corrosion
• On potentials have included in the measurement
– IR through the soil
– IR in the pipe
– Chemical activity representing polarization
– Native potential of the steel
Interference Testing Outline
Summary
Why an ON/OFF Survey?
Continued
• Instant OFF potentials only include the static potential
of the steel and the chemical polarization. By
simultaneously shutting off the current, the IR through
the soil and steel of the pipe is eliminated
• The actual chemical polarization of the pipeline is
determined after static potentials are obtained
Instant OFF – Static = chemical Polarization
Interference Testing Outline
Summary
Determine the Acceptable Amount of
Interference:
• Must have at least 100 mV of polarization in the “as
existing” condition. This results in a protected pipeline
with no metal loss
• Loss of polarization between the individual company
does not mean metal loss as long as at least 100 mV of
chemical polarization exists. The companies affected
must agree upon the acceptable level of polarization
loss or gain due to interference
Interference Testing Outline
Summary
Determine the Acceptable Amount of
Interference:
• If the potentials of the pipeline is above the .850- V CSE
criteria with the foreign line CP operating, this
indicates adequate polarization on Columbia’s pipeline
to prevent corrosion
• However, due to possible miss-interrupted readings due
to soil conditions with seasonal effects, -50 mV shift is
used as minimum accepted criteria with the foreign
system interrupted
Interference Testing Outline
Best Practice
• Interrupt the foreign structure
• Perform CIS over Columbia’s structure
• Set interrupter for 500 milliseconds “Off” and
1 second “ON”
• Log survey on data logger
• Identify Low points & High points on the “ON”
cycle
• Identify & measure Voltage shift to the most
positive direction (maximum exposure area)
• Mark locations
Mitigation
Mitigation of Stray Current
1. “Design and installation of electrical bonds of
proper resistance between the affected
structures.
2. Cathodic protection current can be applied to
the affected structure at those locations where
the interfering current is being discharged. The
source of cathodic protection may be galvanic or
impressed current anodes.
3. Adjustment of the current output from the
interfering cathodic protection rectifiers may
resolve interference problems.
Mitigation of Stray Current
4. Relocation of the groundbeds of cathodic
protection rectifiers can reduce or eliminate
the pickup of interference currents on nearby
structures.
5. Rerouting of proposed pipelines may avoid
sources of interference current.
6. Properly located isolating fittings in the
affected structures may reduce interference
problems.
7. Application of external coating to current
pickup area(s) may reduce or resolve
interference problems.”
Mitigation of Stray Current
• Applying coating to the pick up area, will
provide a high resistant barrier for
Columbia’s pipeline to pick up current from
the foreign ground bed system
Resolution of Interference
Problems
•
Indications that interference or stray current
problems have been resolved can be
demonstrated by:
Interrupt system (Foreign structure)
– 500 milliseconds “ON” and 1 second “Off”
•
•
•
Perform CIS with data logger
Indication of no voltage shift or less than 50mV
Indication of no potential readings below .850- V
CSE
Bond at foreign pipeline crossing
Setting a Resistant Bond – Best
Practice
• Attach two no. 8 and no. 12 wires onto both
structures (Columbia and foreign structure)
• Wire sizes may change due to design of higher
expected ampere output
• Mark the foreign structure wires for easy
identification (normally with white or red tape)
• Connect an high impedance volt meter to the
companies no. 12 wire and place the CSE over
the maximum exposure area
Setting a Resistant Bond – Best
Practice
• Connect an amp meter in series with Columbia
and the foreign structure to achieve the
maximum current drain reading
– Set meter at it’s highest setting to prevent blowing
fuses
• Connect a temporary bond rated for the
ampere measured
• Normal practice – set up a one ohm slide
resister, with the setting half way (= .5 ohms)
• Take potential reading at maximum exposure
area before and after temporary connected
Setting a Resistant Bond – Best
Practice
• If potential shift over structure goes from
a depressed state to an impressed state,
resistance is too low
• If potential shift over structure is still in a
depressed state, resistance is too high
• Keep adjusting slide resistance to desire
criteria is met by checking maximum
exposure area
BOND BOX
ON / OFF
SWITCH
# 8 GAGE
WIRE TO
COMPANY
LINE
2 # 12 GAGE
WIRE TO
COMPANY
LINE
ADJUSTABLE
RESISTOR
8 GAGE
WIRE TO
FOREIGN LINE
2 # 12 GAGE
WIRE TO
FOREIGN LINE
Galvanic anodes used to drain current
Installation of Anodes
• Method is not preferred
– Due to large amount of current discharge
normally consumes anode in rapid time
frame, requiring regular replacement
– Must connect the anode bed into the test
station box for amp drain measurements
– Decrease in amp drain measurements, may
indicate depletion of anodes
– Galvanic anodes used (Magnesium)
Hydrogen Embrittlement
• Pick area needs to be lowered below –2.00 V CSE
due to possible coating disbondment of hydrogen
build up and possible hydrogen embrittlement, in
which results in pipe failure
• Normal resolution to problem, after bond is set,
high potentials exist, add more coating to increase
resistance
– Bond may need readjusting after completion of task
Shield Installation
• Not preferred,
• Due to cost of excavation with material and
labor
• As like the anodes, will deplete over time
and need to be replaced
Example 1
• With our rectifier “on” the pipe-to-soil potential
for our line is -0.990
• Foreign pipeline has a pipe-to-soil potential of
-0.960
• Rectifier switched “off”
• Our potential becomes more positive (-0.850)
• Foreign pipeline becomes more negative (-0.980)
OUR GROUNDBED
FOREIGN
LINE
+
OUR RECTIFIER
_
STATION
Foreign Line
ON
-0.960 V
OFF -0.980 V
V + 0.020 V
OUR LINE
Our Line
ON
-0.990 V
OFF -0.850 V
V -0.140 V
Conclusion
• Based on the recorded test data, our line is
considered to be protected
• The potential on the foreign line decreased
(became more positive) when are rectifier was
switched on
• There is a possibility a holiday exists near the
point of crossing
• The reduction is not sufficient to indicate loss of
protection, no corrective measures required
Example 2
• With our rectifier “on” the pipe-to-soil potential
for our line is –1.150
• Foreign pipeline has a pipe-to-soil potential of
-0.580
• Rectifier switched “off”
• Our potential becomes more positive (-1.040)
• Foreign pipeline becomes more negative (-0.880)
OUR GROUNDBED
FOREIGN
LINE
+
OUR RECTIFIER
_
STATION
Foreign Line
ON - 0.580
OFF - 0.880
 V + 0.300
OUR LINE
Our Line
ON - 1.150
OFF - 1.040
V - 0.110
Conclusion
• Based on the recorded test data, our line is
considered to be protected
• The potential on the foreign line decreased
(became more positive) when are rectifier
was switched on
• Need to set a resistance bond to bring the
foreign pipeline on potential back to the off
potential
DOT
• P/P • DOT Part 192.465 (c)
Critical Bonds
6 times each calendar year, not to exceed 2 ½
months
Non-Critical Bonds
once each calendar year, not to exceed 15
months
DOT
• DOT Part 192.473 (a).
Each operator whose pipeline system is
subjected to stray currents shall have in
effect a continuing program to minimize
the detrimental effects of such currents.
Columbia’s P/P
• Critical bond is where the pipeline is
conducting current through the bond
and:
The bond current is 0.5 Ampere or more
Failure of the bond may result in a potential
change of 100 mV or more below (less
negative) the static potential of the pipeline
Bi-monthly
• All Critical bonds must be evaluated bimonthly or every two months not to
exceed 15 days
• Pipe to soil reading on company structure
Annual Monitoring
• Monitoring
Bonds utilizing a diode or reverse current
switch:
A pipe-to-soil potential reading
Bond current measurement
Test to ensure the blocking device is
operative
Annual Monitoring
• Monitoring
All other bonds:
Pipe-to-soil potential of all structures
with the bond connected
Pipe-to-soil potential of all structures
with the bond disconnected
Measurement of the bond current
Typically five readings obtained
Slide Resister
Application
Resister Wire Application
The amount of Resistance is
made by the Length of the
Wire.
Disconnect Bond Wire
for Amp Drain Reading
1.36
Amp
Current
Drain
Amp Drain
Reading
Make
Connection
in series
with the
Circuit
Slide
Resister
Lighting
Arrestor
Direct Bond
Connection, No
Resistance
Connection Made on
Resister from Foreign
Structure and
Columbia.
Disconnection to get Amp
Drain must be made in
series with the circuit.
Shunt Resister
Measuring
Amp Drain
by Measuring
Voltage Drop
Across the
Shunt. Ratio
is normally
found on
Shunt.
.001V = 1A
7 milivolts = 7
Amps
Make
connection
across Shunt.
Polarity does
not matter.
Ratio for Shunt
Shunts do not have to be
disconnected in BI-monthly
Quick Review -
Basic Corrosion Cell
1.
2.
3.
4.
Anode (more Neg.)
Cathode (more Pos.)
Metallic connection
(pipeline surface, wire,
any metal structure)
A common electrolyte
(water, soil, etc.)
Basic Corrosion Cell
To provide the driving voltage for current to flow in the
corrosion cell, there must be a potential difference
between the anode and the cathode.
Take away any one of the four elements in the basic
corrosion cell and it will stop the corrosion.
Role of the Environment
In the Corrosion Process, the Environment Plays a
Major Role.
• If soil resistivity is high, current flow is restricted,
• Non-uniform environments restrict currents flow at
the transition points,
• If moisture is present, the corrosion reaction may
accelerate,
• As temperature rises the corrosion rate accelerate,
• Other mechanisms, such as, differential aeration and
soil pH will impact how and where the corrosion
rates accelerate.
Role of the Environment-pH
• An understanding of pH is important in corrosion
and CP work
• For many metals, the rate of corrosion increases
appreciably below a pH of about 4
• Between 4 and 8 corrosion rate is fairly
independent of pH
• Above 8, the environment becomes passive and
the corrosion rate decrease
• Cathodic polarization increases the pH at the pipe
surface
Role of the Environment-pH
Neutral pH = 7
Acid pH < 7
Alkaline pH > 7
0
Acid
7
Neutral
14
Alkaline
The pH scale is logarithmic, for each unit of pH the environment
become ten times more acid or alkaline
Corrosion Prevention
Coatings.
• Coating is the first line of
defense in corrosion
prevention.
• Isolating the steel from the
environment disrupts the
basic corrosion cell.
• Coating damage on the
pipeline is called coating
holidays.
Figure 2 – Coal Tar Enamel Coating with Area of Coating
Removed
Corrosion Prevention
Cathodic Protection.
• Cathodic protection (CP) is a supplement in the
prevention of corrosion.
• CP is the application of a DC current.
• The desired effect is to shift the anodic (corrosion)
areas on the pipeline to cathodic (protected).
Polarization
• Cathodic protection current collects at the
coating holidays.
• This DC current forms cathodic “polarization”.
• This creates a protective layer at the coating
holiday, when sufficient DC current is
available.
• Polarization can be measured by conducting an
instant off shift test.
Pipe-to-soil Measurement
Measurement of the CP effectiveness is accomplished
by obtaining voltages, which are commonly called
“pipe-to-Soil readings”. Readings measure the three
components:
1. Chemical activity of the pipe or Polarization
1. Soil or electrolyte voltage (IR) drop
2. Metal voltage (IR) drop
Pipe-to-soil Measurement
• Each component adds voltage to the pipe-to-soil
measurement.
• Polarization adds protection to the pipeline.
• Polarization plus native potential most accurately represent
the level of CP protection achieved from the CP current.
• Soil and metal voltage drop, add error to the pipe-to-soil
readings.
Possible Pipe-to-soil Measurement
Errors
• Increases to the pipe-to-soil measurement are a result of current (I)
passing through the earth resistance (R).
• Large soil/metallic voltage drop may add significant error to the
pipe-to-soil reading.
• Small soil/metallic voltage drop will have a minimal impact.
• P/S reading with the current applied =
[static potential + polarization] + [soil IR + metal IR].
Possible Pipe-to-soil Measurement
Errors
Current Interruption to Remove Voltage Drops
• Interrupting the CP sources and measuring the instant off
reading removes the IR voltage drops.
• Failure to test for IR voltage drop error can give misleading
data and result in misleading conclusions.
• Polarized or Instant Off Potential =
[Static Potential + Polarization] + [Soil and Metal I x R = 0]
Recognize Interference
Cathodic Protection Voltage Gradients
•
There are two locations that generate
voltage gradients:
1. Anodic
1. Cathodic
Typical Anodic Gradient Field
Developed By An Isolated Anode
Cathodic Gradient Surrounding
Pipeline
Recognize Interference
• Metallic structure picks up stray current
from voltage gradients.
-Path of least resistance
• Voltage gradient effect on pipe-to-soil
measurements.
-Failure to recognize will result in
pipe-to-soil data interpretation error
Foreign Voltage Gradients
• When there are anodic voltage gradients present
from foreign cathodic protection systems, data
interpretation is more difficult
• Not only are the soil/metal voltage drops present
from the company’s CP system, there also from
the foreign system
• The presence of these voltage gradients dose not
prove that stray current has been picked up or that
interference corrosion will be present
Foreign Voltage Gradients
•
•
•
•
•
•
Current pick up is dependent upon:
Pipeline cathodic polarization
Coating resistance and quality
Foreign current driving force
The earth resistance
Magnitude of concentration
Current flow and path of least resistance
Cathodic Protection Criteria
• - 850 mV current applied criterion includes
IR drop for pipe-to-soil readings
• 100 mV shift criterion removes the IR drop
from the pipe-to-soil readings
Setting A Resistance Bond
Best Practice
5. Measure the temporary bond current (IT)
between the pipeline and its source of
interference and observe its direction.
At the same time, measure the change of
the pipe-to-soil potential (ET) caused by
the temporary bond current.
Setting A Resistance Bond
Best Practice
6. Measure the resistance (RT) of the
temporary bond.
7. Determine the pipe-to-soil voltage change
(E1) required to return the pipeline to the
original or desired potential from step 2.
8. Use the values (IT), (ET), and (E1) to
calculate the current required.
Setting A Resistance Bond
Best Practice
IMAX = IT / ET x E1
IMAX is the current required to correct the
stray corrosion at the point of maximum
exposure.
Setting A Resistance Bond
Best Practice
RMAX = IT x RT / IMAX
RMAX = maximum resistance of the final
drain wire
IT = temporary drain current value in amperes
RT = temporary drain resistance in ohms
IMAX = bond current in amperes required to
correct the problem
Setting A Resistance Bond
Best Practice
• Select a cable size of the required length
that will give a resistance of a little less that
the calculated RMAX.
• The final conditions at the maximum
exposure point must be checked after
drainage bond is installed to determine if
the return potentials are satisfactory.