Earth Electrode Test Calculator

An earth electrode test establishes the measured resistance of an installed grounding electrode or grounding-electrode system to earth. The calculation organizes resistance readings taken at multiple probe positions and produces a clear resistance summary: Average resistance, Minimum resistance, Maximum resistance, and Reading spread.

The primary result is the average of the entered Measured resistance values. In a fall-of-potential or three-point test, that value supports field documentation and qualified evaluation of whether the readings are stable enough to represent the tested electrode condition. It does not size a grounding electrode conductor, establish feeder ampacity, calculate voltage drop, or determine whether a particular installation satisfies an applicable code, project specification, or utility requirement.

Fall-of-Potential Test Readings

Select Test method as Fall of potential / 3-point when recording readings obtained through that method. The selection documents the method used for the measurements; it does not create a test procedure, determine probe placement, or validate instrument setup.

Each test location is entered as a row under Test-point readings:

Field	Electrical meaning
Measured resistance (ohm)	The resistance value recorded by the earth-ground tester at that probe position
Probe distance (ft)	The associated distance or probe position recorded in feet
Test-point count	The number of resistance-reading rows included in the result

The probe distance provides traceability for the test record. The calculator does not determine whether the distances, electrode spacing, test lead routing, or probe locations were appropriate for the site conditions.

Resistance Summary Formula

For (n) entered readings \(R_1, R_2, \ldots, R_n\), the calculator produces:

\(\displaystyle \text{Average resistance} = \frac{R_1 + R_2 + \cdots + R_n}{n}\)

\(\displaystyle \text{Minimum resistance} = \min(R_1, R_2, \ldots, R_n\)

\(\displaystyle \text{Maximum resistance} = \max(R_1, R_2, \ldots, R_n\)

\(\displaystyle \text{Reading spread} = \text{Maximum resistance} - \text{Minimum resistance}\)

\(\displaystyle \text{Spread vs average} = \frac{\text{Reading spread}}{\text{Average resistance}} \times 100\%\)

A low spread indicates that the entered resistance values are relatively close together. A larger spread identifies variation that should be evaluated against the test method, instrument instructions, soil conditions, lead placement, nearby grounding paths, and the project’s acceptance criteria.

Calculation Example

Enter the following Test-point readings:

Row	Measured resistance (ohm)	Probe distance (ft)
Row 1	8.2	100
Row 2	8.6	150
Row 3	8.4	200

The average earth resistance is:

\(\displaystyle \frac{8.2 + 8.6 + 8.4}{3} = 8.4\ \text{ohm}\)

The resulting summary is:

Result	Value
Test-point count	3 points
Average resistance	8.4 ohm
Minimum resistance	8.2 ohm
Maximum resistance	8.6 ohm
Reading spread	0.4 ohm
Spread vs average	4.7619%

The 0.4-ohm spread is calculated as (8.6 – 8.2). The percentage spread is \(0.4 \div 8.4 \times 100\), producing 4.7619%.

Test Voltage and Test Current

Test current (A) and Measured test voltage (V) provide optional documentation for an instrument-based voltage/current resistance measurement. Enter a measured test voltage only when it is paired with the applicable test current.

These fields should reflect actual instrument readings and the testing procedure used. The resistance summary shown by the calculator is based on the entered Measured resistance values. Do not substitute a calculated value for an instrument reading unless the governing procedure and test equipment instructions permit that approach.

Reference Resistance

Use Entered reference resistance (ohm) to document an applicable project, procedure, or manufacturer reference value. Entering 0 leaves the comparison unassigned.

A reference resistance is not automatically an acceptance limit. Acceptance depends on the installation’s governing requirements, the grounding arrangement being tested, the authority having jurisdiction (AHJ) where applicable, equipment manufacturer instructions, and the project specification. Ground resistance requirements can differ substantially between service grounding, separately derived systems, lightning protection, communications grounding, utility work, and specialty installations.

Field Verification Limits

Earth electrode resistance is affected by soil resistivity, moisture, temperature, electrode depth and length, parallel metallic paths, bonding connections, test-lead routing, and the actual geometry of the test setup. A stable-looking group of readings does not by itself prove that the measurement was obtained with proper probe placement or that all parallel paths were isolated as required by the selected procedure.

Use the calculator to preserve the measured values and quantify their variation. A qualified person must separately verify the test method, instrument condition, electrode configuration, applicable installation requirements, and any code, AHJ, owner, engineer, or manufacturer acceptance criteria.