Infrastructure Engineering · Lesson 18 of 24 · 25 minutes

Trace protection zones and the full trip chain

Protection is not a green status field at one current. It is a chain from the actual fault through measurement, logic, DC supply and interruption. Use a declared inverse-time curve to learn how a calculation behaves, then inspect what that curve cannot establish about CT saturation, electrical zones or selectivity.

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Learning objectives

Evaluate the stated inverse-time teaching model.
Distinguish a nominal timing margin from full coordination.
Identify CT-defined zone boundaries and end-to-end trip evidence.

Editorial derivation

Evaluate the model, not a product

For current multiple M greater than 1, the declared curve is trip time = 0.14 × TMS / (M^0.02 − 1) seconds. M = 10 and TMS = 0.1 produce approximately 0.297 s; M = 5 and TMS = 0.2 produce approximately 0.856 s. The difference is about 0.559 s, but breaker interruption, tolerances, CT behaviour and other active functions still matter. As M approaches 1 from above, the model time increases sharply.

Illustrative assumption

Separate thermal and measurement screens

The adiabatic conductor screen is area ≥ fault current × sqrt(clearing time) / k. At 20 kA, 0.2 s and illustrative k = 143, the area is approximately 62.55 mm² before other selection checks. A resistive CT burden model gives VA = secondary current² × loop resistance: at 2 Ω, 1 A gives 2 VA while 5 A gives 50 VA. Neither scalar screen establishes complete cable suitability or freedom from CT saturation.

Unresolved

Draw the actual zone boundary

Protection zones follow CT locations, conductors, breakers, trip matrices and backup logic. A length-overlap comparison alone cannot establish coverage. The manuscript’s workbook is explicitly a set of constructed examples, not approved operating settings. Incident energy also depends on the adopted model, geometry, source state, current and clearing behaviour. Do not replace missing evidence with a universal arc-flash threshold, clearance time or safety certificate.

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.

For M > 1: t(s) = 0.14 × TMS / expm1(0.02 × ln M). At M ≤ 1, no finite trip result is available in this model.

Declared starting inputs

{
  "multiple": 10,
  "tms": 0.1
}

Worked default — unrounded educational outputs
Quantity	Value	Unit
tripS	0.297059862418842	s or null outside trip domain
status	model-evaluated	curve state

Challenge

What happens near the pickup boundary?

{
  "multiple": 2
}

The declared trip time rises to approximately 1.0029 s. At M ≤ 1 this equation is outside its finite trip domain; do not display a misleading zero-second trip.

Worked challenge
Quantity	Value	Unit
tripS	1.002902702004684	s or null outside trip domain
status	model-evaluated	curve state

CT-defined zones over a breaker cutaway — static equivalent

Reveal CT1, CB1, CT2, cable, CT3 and CB2.
Overlay zone boundaries tied to the CT positions, not decorative distance bands.
Select a fault location and illuminate its documented detection/trip chain.
Expose missing coverage or degraded trip DC without producing a safety approval.

Zone-membership table, trip-matrix narrative and numerical curve values.

Inspect fault locations with buttons as well as pointer selection; change M and TMS in a labelled teaching-curve view.

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

Assessment and answer criteria

6.1 · Evaluate the declared inverse curve at M = 2, TMS = 0.1.

Read answer and marking criterion

Approximately 1.0029 s.

Criterion: Use the stated exponent and retain seconds.

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

6.2 · Find the conductor screen for 10 kA, 0.5 s and k = 143.

Read answer and marking criterion

49.45 mm² before other selection checks.

Criterion: Use amperes, seconds and the declared k units.

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

6.3 · Describe a breaker outside both CT-defined protection zones.

Read answer and marking criterion

Show the uncovered conductor or breaker segment and identify the missing detection or backup tripping arrangement.

Criterion: Electrical connectivity, not an overlap distance, controls the answer.

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 6, PDF pp.30–33. PDF page 1 includes the cover. manuscript-derived; not independent source verification.

No relay settings for real equipment, live-work guidance or incident-energy calculator.
Teaching curves must not be represented as a verified manufacturer implementation.

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.