Cathodic Protection Study 3
THE 'IR DROP IN THE SOIL'
Cathodic protection specialists have now recognised that there is an error in the 'pipe to soil
potential' and have described this as 'the IR drop in the soil', as it is caused by the current (I)
passing from the cathodic protection anode to the pipeline metal, through the resistance (R) of
the soil. The term 'IR drop' comes from the formula E=IR and the value is commonly called
IMMEDIATE OFF POTENTIAL READINGS
It is believed that the corrosion status is retained for a short period before the effects of the
cathodic protection start to decay. It is now recommended that the voltage measurement is
made immediately after the cathodic protection current is switched off, thus removing the error
caused by the 'IR drop'.
A simple model of this feature can be examined on CPSTUDY3.EXL but a certain amount of
'license' has been taken in creating this model. It is assumed that the cathodic protection
current is flowing to an infinite remote earth and will therefore follow the inverse square laws
The model shows the groundbed (GB) in cell A4 (at the top left hand side of this exert) and
applies the inverse square law to the neighbouring cells progressing outward from A4. The
results are colour coded and show the amount of current passing through each cell.
In the model, the potential of the pipeline is used as a reference against which to measure the
potential of the electrode, plus the errors caused by the IR drop in the soil. In reality the
pipeline has very little electrical resistance and the voltage across a span of pipeline would
represent much less than 0.01% of the total reading.
In the model the potential of the soil, at the points where the electrode touches, is a result of
the current (I) passing through the resistance (R). When the output current from the anode,
shown as the value of cell A4, is reduced to zero the error is removed.
USING THE MODEL
The user can adjust the value of the resistance of the soil along the pipeline route to give the
readings shown in the sample, but of course, it is not suggested that the resistance of the soil is
the sole cause of the variation of the errors along a pipeline route.
The user can insert values into the 'pipe-to-soil potential' column, from their own records and
balance them to the resistances using the "goal seek" facility within EXCEL.
This model can be used to help students to understand potentials in the soil and potential
gradients which are manifest in the voltages measured in cathodic protection field work.
This model indicates that it much more likely that the so-called 'attenuation curve' can be
attributed to the geographical relationship to the groundbed than to the amount of protection
afforded to the pipeline.
This concept can be demonstrated, at cathodic protection groundbeds, by plotting the ground
voltage with respect to a fixed electrode in a remote position, or in relation to the pipeline
metal. The resulting voltages can be plotted as equipotential contours and reflect the degree
of variation suggested in model CPSTUDY3.EXL.
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