Substation Grounding Grid Measurement and Touch Voltage Validation Project

This project produces a substation grounding-grid measurement and touch-voltage validation package. The deliverable is an energization and handover file: test boundary, measured ground resistance, fault-current split, ground potential rise, touch and step voltage checks, bonding corrections, relay clearing-time review, acceptance gates, and release decision.

The project is not a general explanation of grounding and not a failure case. It is a commissioning workflow for deciding whether an installed substation grounding system is safe enough to energize and hand over. A low ground-resistance value by itself is not enough. The installed system must also control local touch voltage, step voltage, bonding continuity, transferred potential, surface-layer assumptions, and fault-clearing time.

Project Objective

Validate a medium-voltage substation grounding grid before unrestricted energization.

The final package must include:

• one-line and grounding boundary;
• approved fault-current and current-split basis;
• ground-resistance measurement method and result;
• touch and step voltage measurement plan;
• ground potential rise calculation;
• clearing-time adjustment to allowable touch and step limits;
• bonding and surface-layer inspection;
• corrective action and retest evidence;
• energization release recommendation.

Engineering Scenario

An industrial site is commissioning a 13.8 kV outdoor substation that feeds a 480 V process distribution system. The grounding grid includes buried copper conductors, ground rods, transformer tank bonds, switchgear bonds, fence bonds, cable-shield terminations, a control cabinet, a personnel gate, and a crushed-rock surface layer.

The civil installation is complete, but the electrical team must prove that the as-built grounding system matches the safety assumptions in the grounding study. The protection settings are also being verified because clearing time affects allowable touch and step voltage.

Use the following project basis:

Item	Value
maximum line-to-ground fault current at the yard bus, I_{LG}	8.0\ \text{kA}
grid current split factor, S_g	0.55
measured grounding-grid resistance, R_g	0.42\ \Omega
design clearing-time basis, t_0	0.50\ \text{s}
validated relay clearing time, t_c	0.62\ \text{s}
touch-voltage limit at t_0, E_{touch,0}	720\ \text{V}
step-voltage limit at t_0, E_{step,0}	1800\ \text{V}
measurement uncertainty allowance	5\%

The touch and step limits must come from the project-approved electrical safety method. This project uses the values above only to demonstrate the engineering workflow.

System Boundary

The validation boundary includes:

1. substation grounding grid and ground rods;
2. transformer tank, neutral and cable shields;
3. switchgear enclosure and control cabinet;
4. perimeter fence, gate posts and gate latch;
5. exposed cable tray, metallic conduit and structural steel;
6. surface-layer condition near accessible points;
7. relay settings and breaker clearing path for the studied ground fault.

The boundary excludes downstream building distribution panels except where they can transfer potential into the yard through cable shields, equipment grounding conductors, control cables, communications cables, pipes or structural steel.

Measurement and Inspection Plan

Use a measurement plan that records both electrical values and installed details.

Work item	Purpose	Required record
continuity and bonding checks	prove exposed metal is intentionally bonded	point list, resistance or continuity result, photo
fall-of-potential or approved grid-resistance test	verify R_g assumption	method, probe layout, current, voltage, result
low-current touch-potential injection	locate high transfer factors	point map, injected current, scaled factor
step-potential survey	verify accessible walking paths	point spacing, scaled result, surface condition
surface-layer inspection	confirm crushed-rock depth and coverage	thickness checks, repair notes, photos
protection timing verification	confirm fault duration basis	relay setting file, trip simulation, breaker time
as-built review	close drawing and field differences	redlines, sign-off, open actions

Do not treat a spreadsheet of numbers as enough. Grounding validation depends on location, bonding detail, measurement setup, access condition and protection timing.

Ground Potential Rise Calculation

The grid current is:

Substitute:

Ground potential rise is:

Use kiloamperes and ohms:

Therefore:

Engineering Comment

The grid resistance is low, but the ground potential rise is still high enough to require a touch-voltage review. Grid resistance is a global value. Touch voltage is a local exposure value that depends on where the person stands, what metal is touched, surface-layer condition, bonding and fault duration.

Clearing-Time Adjustment

The approved touch and step limits were stated at:

The validated clearing time is:

Use a first-pass time adjustment:

Adjusted touch-voltage limit:

Adjusted step-voltage limit:

Engineering Comment

The slower clearing time reduces the allowable voltage. If the relay file, breaker condition, or source mode changes, the grounding validation must be rechecked. A grounding study based on a faster trip time is not valid evidence for a slower installed clearing time.

Touch and Step Measurement Results

Low-current injection is used to estimate transfer factors. The measured factor is scaled by the calculated ground potential rise:

where k is the measured transfer factor at the location.

Before Correction

Location	Measured factor	Scaled voltage	Acceptance
personnel gate latch touch	0.37	684\ \text{V}	fail
control cabinet touch	0.24	444\ \text{V}	pass
transformer tank touch	0.29	536\ \text{V}	pass
switchgear door touch	0.21	388\ \text{V}	pass
worst step path	0.16	296\ \text{V}	pass

Gate latch touch voltage:

Compare with the adjusted limit:

The gate latch fails before correction.

Worst step voltage:

Compare:

The step-voltage screen passes with large margin.

Engineering Comment

The failure is localized. The grid resistance test alone would not have found it. The measurement points show that the personnel gate is the controlling location because the latch can sit at a different potential from the standing surface during a ground fault.

Corrective Actions

Apply corrections before energization:

1. install a flexible bonding jumper across the gate hinge and latch-side post;
2. bond the fence section to the grid at the gate bay;
3. replace thin or contaminated surface rock near the gate with the specified crushed-rock depth;
4. remove paint and corrosion at bonding lugs before termination;
5. torque and mark bonding hardware;
6. update as-built drawings and inspection records;
7. repeat touch-potential measurement at the corrected gate.

The correction must reduce both electrical impedance and local exposure geometry. Bonding a gate without restoring the surface layer may still leave a weak field condition.

Retest and Acceptance

After correction, the gate latch transfer factor is:

Retested touch voltage:

Apply the 5 percent measurement uncertainty allowance conservatively:

Compare with the adjusted limit:

The corrected gate touch-voltage screen passes.

Engineering Comment

The uncertainty allowance is important because the measured result is being used for release. If the retested value were close to the limit, the decision should require stronger evidence, a larger correction, shorter clearing time, access restriction or independent review.

Release Gates

Use these acceptance gates:

Gate	Acceptance rule
grounding-grid resistance	measured method and result match study basis or are reviewed
touch voltage	all accessible touch points pass adjusted limit with uncertainty allowance
step voltage	accessible walking paths pass adjusted limit
clearing time	relay and breaker timing match the validation basis
bonding	every exposed metal item in the boundary has verified intentional bonding
surface layer	crushed-rock depth and coverage match the safety basis
transferred potential	fences, gates, pipes, shields and control cables are reviewed
documentation	as-built drawings, test records and open actions are complete

An energization release should fail if any gate is missing. Missing evidence is not the same as passing evidence.

Final Recommendation

Release the substation for energization only after the corrected gate measurement and documentation are accepted.

For this worked scenario:

• the measured grounding-grid resistance supports the study basis;
• calculated ground potential rise is 1848\ \text{V};
• clearing-time adjustment reduces the touch-voltage limit to 647\ \text{V};
• the gate latch fails before correction at 684\ \text{V};
• bonding and surface-layer corrections reduce the decision value to 427\ \text{V};
• step voltage passes with substantial margin;
• release is conditional on retaining the relay clearing-time setting and the corrected as-built bonding details.

If the source fault current, current split, relay delay, breaker condition, fence layout, crushed-rock condition or grounding connections change, the grounding validation should be reopened.

Final Deliverable

The completed package should contain:

1. one-line diagram and grounding boundary;
2. fault-current and current-split basis;
3. grid-resistance measurement report;
4. touch and step voltage point map;
5. scaled voltage calculations and adjusted limits;
6. bonding checklist with photos;
7. surface-layer inspection record;
8. relay clearing-time evidence;
9. corrective-action and retest records;
10. energization release decision and open-action register.

Limitations

This project uses simplified screening equations and example limits. Real projects must follow the applicable electrical safety standard, utility requirements, owner procedures, soil model, body-current assumptions, surface-layer correction method, seasonal soil conditions, test-equipment limitations and jurisdictional rules.

Measurement quality matters. Probe placement, injected current, lead routing, stray currents, soil moisture, nearby buried metal, grid geometry, cable shields and surface condition can all affect the result. A validation package should state method limitations and require engineering review when readings are near limits.

Common Mistakes

Common grounding-validation mistakes include:

• accepting a low resistance number without touch-voltage measurement;
• using design clearing time after installed relay settings have changed;
• testing only the substation yard and ignoring fence gates or transferred potential;
• missing cable shield, conduit, pipe or control-cable bonds;
• assuming crushed rock is present and effective without inspection;
• failing to retest after a bonding correction;
• releasing energization with open grounding as-built discrepancies;
• treating a case-study calculation as a substitute for the approved standard.

The engineering standard is practical: the installed grounding system must be measured at the locations where people can touch metal, scaled to the credible fault and clearing time, corrected where it fails, and documented before energization.