Infrastructure Engineering · Lesson 19 of 24 · 25 minutes

Inspect grounding, distortion and control-power loops

Low resistance, low distortion and a nominal battery voltage each answer only a narrowly defined question. This lesson turns those single numbers into explicit boundaries. Follow the ground current, compare harmonic denominators, and include the return conductor in the trip circuit before drawing any conclusion about service.

Run this lesson interactively

Learning objectives

Distinguish ground-potential rise from touch/step assessment.
Calculate THD and TDD using different denominators.
Evaluate a complete DC loop and explain frequency-change consequences.

Illustrative assumption

Grounding has several functions

Protective bonding, source-neutral grounding, lightning protection and signal/EMC interfaces are related but not interchangeable. In the deliberately simplified resistive model, 2 kA entering a 0.5 Ω grounding system gives 1,000 V ground-potential rise. It does not automatically give the voltage across a person. Soil structure, geometry, current distribution, bonding, surface layer, duration and the applicable criterion remain necessary for a safety assessment.

Editorial derivation

Choose the correct distortion denominator

THD uses aggregate harmonic RMS current divided by fundamental RMS current. TDD uses the same numerator divided by a declared reference maximum-demand current. With 30 A harmonics, 100 A fundamental and 300 A reference, THD is 30% and TDD is 10%. Those different percentages describe the same numerator. A generic five-percent statement without a metric, operating state and boundary is not a universal conformity rule.

Editorial derivation

Close the return path and preserve physical duty

A two-wire DC route has both outgoing and return conductors. At 10 A, one-way length 100 m and resistance 0.003 Ω/m, the conductor drop is 6 V before contacts and other elements. The minimum loaded source voltage and receiving coil requirement still govern. Frequency changes are also physical changes: at unchanged voltage, 60-to-50 Hz increases ideal transformer flux by 20%; rotating-equipment affinity relations have their own stated assumptions.

Predict → run → explain

Read the declared boundary and units. Predict the selected result before calculating.
Run the starting case and inspect the ledger, assumptions and model limits.
Apply the challenge, run again, and explain the difference. Export only the last completed inputs and results.

THD(%) = 100 × harmonic A/fundamental A; TDD(%) = 100 × harmonic A/reference A; GPR(V) = ground A × ground Ω; DC drop(V) = 2 × DC A × one-way m × Ω/m.

Declared starting inputs

{
  "harmonicA": 30,
  "fundamentalA": 100,
  "referenceA": 300,
  "groundA": 2000,
  "groundOhm": 0.5,
  "dcA": 10,
  "oneWayM": 100,
  "ohmPerM": 0.003
}

Worked default — unrounded educational outputs
Quantity	Value	Unit
thdPercent	30	%
tddPercent	10	%
groundPotentialRiseV	1000	V
dcDropV	6	V DC

Challenge

What changes if the fundamental current falls while harmonics and reference demand stay fixed?

{
  "fundamentalA": 50
}

THD rises to 60%; TDD stays at 10%. This demonstrates denominator sensitivity, not a new measured harmonic event.

Worked challenge
Quantity	Value	Unit
thdPercent	60	%
tddPercent	10	%
groundPotentialRiseV	1000	V
dcDropV	6	V DC

The hidden return path — static equivalent

Separate neutral grounding, protective bonding and signal interfaces.
Show ground-potential rise without depicting a safe human exposure.
Swap THD and TDD denominator labels while keeping harmonic bars fixed.
Trace both halves of the DC circuit and reveal the remote-voltage loss.

Metric definitions, denominators, ground-current assumptions and outgoing/return conductor table.

Change the fundamental denominator and DC route length independently; inspect source versus receiving-device voltage.

Narrative steps are not physical seconds. Essential states and quantities remain available without animation.

Assessment and answer criteria

7.1 · Compute THD and TDD for 20 A harmonics, 80 A fundamental and 200 A reference.

Read answer and marking criterion

25% THD and 10% TDD.

Criterion: Identify both denominators.

Source: DCIE-R1 Appendix A, PDF pp.59-61

7.2 · Find the ideal flux ratio for a 400 V/60 Hz winding used at 380 V/50 Hz.

Read answer and marking criterion

(380/50)/(400/60) = 1.14, or 14% higher ideal flux.

Criterion: Do not label this equipment approval or a prediction of failure.

Source: DCIE-R1 Appendix A, PDF pp.59-61

7.3 · Why are two earth conductors not independent when both use one severed bonding point?

Read answer and marking criterion

The common connection disables both intended paths.

Criterion: Identify the shared node and initiating failure.

Source: DCIE-R1 Appendix A, PDF pp.59-61

Source and model boundaries

Data Centre Infrastructure Engineering, reference manuscript revision 1 (DCIE-R1), 11 October 2026, chapter 7, PDF pp.34–37. PDF page 1 includes the cover. manuscript-derived; not independent source verification.

No touch/step-voltage certification or jurisdictional harmonic limits.
Frequency adaptation and DC operating voltages remain equipment-specific.

Original educational adaptation. The private PDF and missing archive are not public downloads. Source statement, Editorial derivation, Illustrative assumption and Unresolved remain distinct.

Result schema: infrastructure-screen-1; content 2.0.0; model 1.0.0. Raw inputs, outputs, units, assumptions, source locators and canonical hashes. No supported in-browser Python verification protocol exists for this result family.