Step And Touch Voltage Screen Calculator

A step and touch voltage screen converts an assumed ground-fault current and grounding-grid resistance into a preliminary grid potential rise (GPR), then applies entered step and touch fractions to produce comparison voltages.

The calculator produces five primary electrical values:

• Grid potential rise
• Step voltage screen
• Touch voltage screen
• Step margin
• Touch margin

These values can support an early grounding review for a service, substation, switchgear lineup, generator installation, utility-interconnected equipment, or other system where a line-to-ground or ground-fault condition may energize a grounding electrode system. The output is useful for identifying when an assumed fault condition should receive deeper grounding-system engineering review before conductor routing, grounding-grid layout, equipment bonding details, or site construction decisions are finalized.

Grid Potential Rise

Grid potential rise is the voltage developed by the entered grounding system resistance when the entered fault current flows to earth.

\(\displaystyle \text{Grid Potential Rise} = \text{Fault current} \times \text{Grid resistance}\)

The calculator uses these interface fields:

Input	Electrical use
Fault current (A)	Assumed current flowing into the grounding system during the evaluated fault condition
Grid resistance (ohm)	Assumed resistance from the grounding grid or electrode system to remote earth
Step fraction (%)	Entered percentage of grid potential rise used for the step-voltage screen
Touch fraction (%)	Entered percentage of grid potential rise used for the touch-voltage screen
Entered step limit (V)	Project or review comparison limit for the calculated step-voltage screen
Entered touch limit (V)	Project or review comparison limit for the calculated touch-voltage screen

A higher fault-current assumption raises GPR in direct proportion. A higher grid resistance does the same. For example, reducing grid resistance from 0.40 ohm to 0.20 ohm cuts the calculated GPR in half when the assumed fault current remains unchanged.

GPR is not automatically the voltage a person experiences. Actual exposure depends on the geometry of the grounding system, local surface gradients, equipment bonding, conductor paths, soil conditions, accessible locations, footwear, surface material, and the fault-clearing condition.

Step and Touch Voltage Screens

The worksheet applies the entered fractions to calculated grid potential rise:

\(\displaystyle \text{Step Voltage Screen} = \text{Grid Potential Rise} \times \left(\frac{\text{Step fraction}}{100}\right)\)

\(\displaystyle \text{Touch Voltage Screen} = \text{Grid Potential Rise} \times \left(\frac{\text{Touch fraction}}{100}\right)\)

A step voltage condition represents the voltage difference that may exist between two points on the earth’s surface separated by a person’s step. A touch voltage condition represents the voltage difference between a grounded or bonded accessible object and the earth at the person’s location.

The calculator does not derive either fraction from soil resistivity, grounding-grid geometry, fault duration, surface-layer resistivity, body-current criteria, or a specific grounding standard. The values in Step fraction (%) and Touch fraction (%) are project assumptions entered for screening purposes.

The entered limits are also comparison values rather than values calculated from the other inputs. The calculator compares its calculated screen voltage to each entered limit and reports a margin and status.

Margin and Status Logic

The worksheet calculates each margin as:

\(\displaystyle \text{Step Margin} = \text{Entered step limit} - \text{Step voltage screen}\)

\(\displaystyle \text{Touch Margin} = \text{Entered touch limit} - \text{Touch voltage screen}\)

A positive margin indicates that the applicable voltage screen is below the entered comparison limit. A negative margin indicates that the screen exceeds the entered limit and should be reviewed under the project’s grounding design basis.

Output	Meaning
Grid potential rise	Calculated voltage rise of the assumed grid or electrode system
Step voltage screen	GPR multiplied by Step fraction (%)
Touch voltage screen	GPR multiplied by Touch fraction (%)
Step margin	Entered step limit (V) less Step voltage screen
Touch margin	Entered touch limit (V) less Touch voltage screen
Step status	Comparison result against Entered step limit (V)
Touch status	Comparison result against Entered touch limit (V)

A “Within entered limit” status means the arithmetic result does not exceed the limit entered into that worksheet field. A “review” status means the calculated screen exceeds the entered comparison limit.

Calculation Example

Using the entered values:

Field	Entered value
Fault current (A)	1,000 A
Grid resistance (ohm)	0.40 ohm
Step fraction (%)	20%
Touch fraction (%)	50%
Entered step limit (V)	100 V
Entered touch limit (V)	100 V

First, calculate grid potential rise:

\(\displaystyle 1{,}000\text{ A} \times 0.40\text{ ohm} = 400\text{ V}\)

\(\displaystyle \text{Grid potential rise} = 400\text{ V}\)

Apply the Step fraction (%):

\(\displaystyle 400\text{ V} \times 0.20 = 80\text{ V}\)

\(\displaystyle \text{Step voltage screen} = 80\text{ V}\)

Apply the Touch fraction (%):

\(\displaystyle 400\text{ V} \times 0.50 = 200\text{ V}\)

\(\displaystyle \text{Touch voltage screen} = 200\text{ V}\)

Calculate the comparison margins:

\(\displaystyle 100\text{ V} - 80\text{ V} = 20\text{ V}\)

\(\displaystyle \text{Step margin} = 20\text{ V}\)

\(\displaystyle 100\text{ V} - 200\text{ V} = -100\text{ V}\)

\(\displaystyle \text{Touch margin} = -100\text{ V}\)

The worksheet result is:

Result	Value
Grid potential rise	400 V
Step voltage screen	80 V
Touch voltage screen	200 V
Step margin	20 V
Touch margin	-100 V
Step status	Within entered limit
Touch status	review

The step screen is 20 V below the entered 100 V step limit. The touch screen is 100 V above the entered 100 V touch limit, producing a negative touch margin and a review status.

Grounding Design Boundary

This calculation is an arithmetic screen based on entered fault current, grid resistance, fractions, and comparison limits. It does not establish a safe work limit, personnel protection method, field measurement result, grounding-grid design, or code-compliance conclusion.

A completed grounding assessment may require verification of the actual available ground-fault current, clearing time, grounding and bonding conductor paths, electrode and grid configuration, soil resistivity and layering, transferred-potential exposure, surface conditions, accessible equipment, utility grounding conditions, and local AHJ or project requirements.

Do not use the screen result by itself to select grounding electrode conductors, equipment grounding conductors, bonding jumpers, grounding-grid conductor size, electrode spacing, or protective equipment settings. Those decisions require the applicable electrical design basis and the actual installation conditions.