Protection Trip Time & Time-Current Curve Calculator

MCB · IDMT Relay · Fault Current · TCC Plot

Select a protective device, enter its settings and a prospective fault current, and immediately see the time-current characteristic curve with the fault current plotted against it. Supports LV circuit breakers (Type B/C/D magnetic trip bands per IEC 60898-1) and IDMT overcurrent relays (IEC 60255 inverse-time curves). Circuit breaker results are shown as an operating-time range where the standard itself defines a tolerance band, not a fabricated exact figure.

Protective Device

Breaker Rating

Characteristic

Compare Against a Second Breaker (Device B)

Study Type

Fault Type

Fault Current

MAXIMUM OPERATING TIME

≤ 0.1 s

Trip Type: Magnetic

Fault current: 2 kA

Fault type: Three-Phase Fault

Rating: 32 A

Current multiple: 62.5× In

The magnetic (short-circuit) element governs at this current — it operates well before the slower thermal element. 0.1 s is the IEC 60898-1 guaranteed upper bound, not a measured point value.

Results for guidance only. Verify against current standards and manufacturer data.
Not a substitute for engineering judgement or a licensed professional.

Indeterminate trip bandGuaranteed instantaneousFault operating point

Methodology

LV circuit breaker (MCB) — fault/magnetic region: for Type D, the instantaneous trip region is modelled per IEC 60898-1 cl. 9.10.2, which defines a two-point test — a hold threshold (guaranteed no instantaneous trip) and a trip threshold (guaranteed instantaneous trip within 0.1s):

Type D Hold Threshold

I < 10\times I_n \;\Rightarrow\; \text{no instantaneous trip guaranteed}

Type D Trip Threshold

I \geq 20\times I_n \;\Rightarrow\; \text{instantaneous trip within } 0.1\ \mathrm{s}

Between these two thresholds, the standard does not define a single trip time — the result is shown as a range, not a fabricated precise figure.

Assumptions & Limitations

MCB magnetic trip bands are the IEC 60898-1 standard ranges — individual manufacturer product families may specify a narrower band (e.g. some Type D families use 10–14× In rather than the full 10–20× In standard range). Verify against the specific device's datasheet for coordination-critical work.
Calculated times are a protection-analysis aid — actual device behaviour should be confirmed against manufacturer TCC data
Engineering judgement and a full coordination study remain required for final protection settings

Related Calculators

Transformer Current & Fault Calculator

Calculate transformer primary and secondary currents, prospective fault current and fault level in kA and MVA.

→

Earth Fault Loop Impedance Calculator

Calculate earth fault loop impedance, prospective earth fault current and indicative protective device disconnection for Australian installations.

→

Per-Unit Fault Level Calculator

Convert network elements to per-unit, calculate three-phase fault levels at multiple buses. Full calculation shown.

→

Distribution Fault Study Builder

Build a radial network with utility, transformers and cables. Calculate three-phase fault levels at each bus.

→

AusElectrical.tools

Calculators About Contact Terms of Use Privacy Policy

Built with reference to AS/NZS 3000, AS/NZS 3008, AS/NZS 1359

Results for guidance only — not a substitute for engineering judgement

Does not reproduce copyrighted Standards Australia material. Always consult current standards for compliance.