Loading documentation...

Overcurrent Protection

Configure IDMT relay time-current characteristics and coordination.

What This Tab Does

The Overcurrent tab lets you configure multi-stage IDMT (Inverse Definite Minimum Time) relay characteristics and evaluate them against measured fault currents from your COMTRADE recording. It answers the question: "Which protection stage would have operated, and how fast?"

You can configure unlimited phase overcurrent and earth fault stages, each with its own curve type (IEC or IEEE), pickup current, and time multiplier. The tab provides two views: a TCC chart (Time-Current Characteristic log-log plot) showing curves and operating points, and an RMS Trend chart showing per-phase current over time with pickup thresholds and event markers.

Getting Started

Access the Overcurrent tab by clicking the shield icon in the left sidebar. The tab requires 3-phase current channels (IA, IB, IC) -- voltage channels are not needed. Configure relay settings (stages, trip source) via the gear icon. Switch between the TCC and RMS Trend chart using the toggle in the header. Use the playback slider to animate operating points through time.

Overcurrent Protection

2TCCRMS Trend3PhaseEF

FTC

437 / 999 ms1x

6A:7200 A→ S3(1.4×Is)INSTB:2500 A→ S2(2.5×Is)0.30sC:400 A→ S1(4.0×Is)4.20s73I₀:80 A→E1(2.67×Is)1.2s7Trip: 'TRIP SIGNAL' at 45 ms

8PASSActual 42 ms · Expected INST · S3 on A at 7200 A

Header & Readout Controls

1Title — identifies the Overcurrent Protection tab.

2Chart mode toggle \u2014 segmented control switching between TCC (log-log time\u2013current characteristic) and RMS Trend (time-domain RMS plot).

3Phase / EF sub-toggle \u2014 switches between phase overcurrent and earth fault curve views (visible in TCC mode only). “Phase” shows S1\u2013S6 stages; “EF” shows E1\u2013E4 stages.

4Settings gear — opens the settings sidebar for stage pickup/TMS/curve-type configuration, channel overrides, and trip source selection.

5Playback slider — play/pause, reset, and scrub through the recording. Markers show fault inception (F), trip event (T), and clearance (C). Operating points update in real-time as the slider moves.

6Per-phase operating point \u2014 color-coded dot, phase label, RMS current at the slider position, dominant stage badge, current multiple (I_r = I / I_s), and operating time. A dash (—) indicates no stage is picked up.

7Trip status — right-aligned indicator showing the trip source and time (digital channel assertion, current clearance, or "Not configured" / "No event detected").

8Relay validation — compares actual trip time (from trip detection) against expected time (from the dominant stage curve at peak fault current). Shows PASS (green), SLOW (amber), or FAST (red) with actual/expected times, stage, phase, fault current, and error percentage.

Settings Reference

All settings are accessible from the gear icon in the Overcurrent tab header. Settings are auto-saved per recording and restored when you reopen the same file.

Current Channels1

IaIA (A)(auto)

IbIB (A)(auto)

IcIC (A)(auto)

Phase Stages2

S13

CurveStandard Inverse (SI)

Is100A

TMS1.0

S24

CurveDefinite Time (DT)

Is1000A

TMS0.3

CurveDefinite Time (DT)

Is5000A

TMS0

5remove|6add

Earth Fault Stages7

CurveStandard Inverse (SI)

Is30A

TMS1.0

Trip Detection8

NoneCurrent-based clearanceDigital channel

52A9

1Current channels — auto-detected (green dots) or manually overridden (blue dots). "Override" button enables manual channel selection.

2Phase stages — ordered array of stage cards (S1, S2, S3…). Each has colour-coded badge, curve type, pickup current, and TMS.

3Stage badge — coloured label identifying the stage. S1 = green, S2 = blue, S3 = purple, etc.

4Curve selector — dropdown: SI, VI, EI, LTI, DT (IEC), or IEEE MI / VI / EI.

5Remove button — X icon that deletes this stage from the array.

6Add Stage — dashed button that appends a new SI stage with default pickup and TMS values.

7Earth fault stages — separate array (E1 = orange, E2 = cyan…). Same structure as phase stages but for residual/ground current.

8Trip detection — three radio options: None, Current-based clearance detection, or Digital channel assignment.

9Digital channel dropdown — lists all digital channels from the loaded COMTRADE file for breaker status mapping.

Phase Overcurrent Stages

Each phase stage (S1, S2, S3, ...) independently evaluates phase currents IA, IB, and IC. For each phase, Detego selects the fastest stage that picks up (the stage with the shortest operating time). Use the Add Stage button to add stages; there is no fixed limit. Stages are color-coded: S1 (green), S2 (blue), S3 (purple), S4 (red), S5 (amber), S6 (cyan).

Parameter	Unit	Default (S1 / S2)	Description
Curve	--	SI / DT	IDMT curve type. IEC: Standard Inverse (SI), Very Inverse (VI), Extremely Inverse (EI), Long-Time Inverse (LTI), Definite Time (DT). IEEE: Moderately Inverse, Very Inverse, Extremely Inverse.
Is	A	100 / 1000	Pickup current in primary amps. The stage begins timing when the measured RMS current exceeds this value.
TMS	--	1.0 / 0	Time Multiplier Setting. For IDMT curves, scales the operating time linearly. For DT curves, this is the trip time in seconds (set to 0 for instantaneous).

Modelling Instantaneous Elements

An instantaneous element is simply a DT stage with TMS = 0 and a high pickup current. For example, S2 with Curve = DT, Is = 1000 A, TMS = 0 models a traditional

I>>

instantaneous element. On the TCC chart, this appears as a horizontal line at the bottom of the plot labelled "INST".

Earth Fault Stages

Earth fault stages (E1, E2, E3, ...) operate on the residual (zero-sequence) current $3I_0 = I_A + I_B + I_C$ . They work identically to phase stages but evaluate the residual current instead. When no EF stages are configured, the earth fault element is disabled.

EF stages are color-coded differently from phase stages: E1 (orange), E2 (cyan), E3 (pink), E4 (lime). This visual separation helps distinguish phase and earth fault curves on the TCC chart.

Earth Fault Pickup

Earth fault pickup is typically set much lower than phase pickup because earth fault current is limited by system grounding impedance (neutral earthing resistor, Petersen coil, etc.). In a resistance-grounded system, earth fault current may be limited to a few hundred amps, requiring EF pickup of 10-30% of phase pickup.

Channel Detection

Detego automatically detects 3-phase current channels from the COMTRADE metadata using multiple strategies:

Phase field -- Uses the ph field in the CFG header (A/B/C or 1/2/3)
Name patterns -- Matches names like IA/IB/IC, I_L1/I_L2/I_L3, PHASE 1/2/3, or standalone A/B/C
Unit detection -- Identifies current channels by unit (A, kA, mA) or names starting with "I"

If auto-detection fails, you can manually override channel assignments using the Channel Override selector in the settings panel. A green dot indicates auto-detected channels; a blue dot indicates manually assigned channels.

Trip Detection

Detego does not inherently know when the relay issued its trip command -- you must select the trip source:

None -- No trip detection. Operating points are calculated but no actual trip time or event marker is shown.
Current-based clearance -- Uses the built-in fault clearance detection algorithm (RMS drop detection). Suitable when no digital channels are available.
Digital channel -- Select a specific digital channel from the COMTRADE recording (e.g., "52A", "TRIP", "CB1"). The trip time is the first state transition from the channel's normal state. This is the most accurate method.

Choosing a Digital Channel

Look for channels named "TRIP", "52A" (circuit breaker auxiliary contact), "CB1", or "POC Trip" (phase overcurrent trip). The normal state is defined in the COMTRADE CFG file -- Detego automatically uses it when detecting transitions.

Reading the TCC Chart

The TCC (Time-Current Characteristic) chart is a log-log plot showing operating time (Y-axis, seconds) versus absolute current in amps (X-axis). It is the standard visualization tool for overcurrent relay coordination studies.

Chart Elements

Element	Visual	Description
Phase stage curves	Solid coloured lines (S1, S2, ...)	Each stage's IDMT curve starts at its pickup current and extends rightward. Clipped where a faster stage dominates.
EF stage curves	Dashed coloured lines (E1, E2, ...)	Earth fault stage curves, shown on the same plot but visually distinct with dashed lines.
DT horizontal lines	Flat horizontal segments	Definite-time stages appear as flat lines at the configured TMS time. Labelled "INST" for TMS = 0.
Vertical connectors	Dotted vertical lines	Join the slower stage time to the faster stage time at the transition current (composite curve connections).
Phase operating points	Phase-coloured circles	Measured fault current for each phase plotted on the fastest-tripping stage curve. Labelled with the operating time or "INST".
EF operating points	Diamond markers	Residual current plotted on the fastest EF stage curve.

Phase vs EF View Toggle

Use the "Phase" and "EF" toggle buttons in the TCC header to switch between phase overcurrent curves and earth fault curves. This keeps the chart uncluttered when both types are configured.

How to Read the Chart

To determine the expected relay operating time for a given fault:

Find the fault current on the X-axis
Draw a vertical line upward -- the first curve it intersects is the fastest stage that picks up
Read the operating time on the Y-axis at that intersection point
Operating point markers do this automatically for the measured fault current

Reading the RMS Trend Chart

The RMS Trend chart is a time-domain view showing per-phase RMS current over the recording duration, overlaid with protection thresholds and event markers.

Current Traces

Phase RMS traces (solid lines, phase colours) -- Sliding one-cycle RMS for IA (red/L1), IB (yellow/L2), IC (blue/L3).
3I0 trace (dashed cyan) -- Residual current RMS. Rises sharply during earth faults; stays near zero during balanced faults.

Pickup Thresholds

Horizontal lines show each stage's pickup current. Phase stage thresholds are dashed lines in stage colours; EF stage thresholds are dotted lines in EF stage colours. When a current trace crosses above a threshold line, that stage picks up.

Event Markers

Marker	Colour	Meaning
Fault inception	Orange (dashed)	Auto-detected time when fault current first rises above pre-fault level.
Trip event	Red (dashed)	Trip time from the configured trip source. Only shown when trip detection is enabled.
Fault clearance	Green (dashed)	Auto-detected time when fault current drops back to pre-fault level.
Cursor	Gray (dashed)	Current cursor position. Click on the chart to set or move the cursor. Operating points are re-evaluated at the cursor time.

Time Playback

A playback slider sits between the header and the chart area. It lets you scrub through the recording or animate it in real time. As the slider moves, all operating points on the TCC chart, the readout bar values, and the RMS trend cursor update simultaneously.

Play / Pause — Animates through the recording at configurable speed (click the speed label to cycle: 0.5x, 1x, 2x, 4x, 8x).
Reset — Returns the slider to the start of the recording.
Scrub — Drag the slider handle to jump to any point in the recording. Click anywhere on the track to snap to that time.
Markers — Coloured markers on the slider track show key events: F (orange) = fault inception, T (red) = trip event, C (green) = fault clearance.

Watch the Dots Move

Play the slider while viewing the TCC chart. The per-phase operating point dots will travel along the curves as the fault current changes over time — low current before the fault (dots high on the curve), high current during the fault (dots drop toward the bottom), and back down after clearance.

Relay Validation

When a trip source is configured and a fault is detected, Detego automatically compares the actual relay operating time against the expected operating time from the configured IDMT curve. The result is shown as a coloured verdict bar below the readout.

How It Works

Actual time = trip event time − fault inception time (in milliseconds).
Expected time = operating time of the dominant stage at the fault current measured 2 cycles after inception. The dominant stage is the highest-pickup stage that has picked up — matching real relay coordination where higher stages take precedence.
Fault current is measured at 2 power-frequency cycles after fault inception. This captures the steady-state fault level after initial transients have settled.

Verdicts

Verdict	Colour	Meaning
PASS	Green	Actual trip time is within tolerance of the expected time. For IDMT stages: ±10% or ±30 ms (whichever is larger). For instantaneous DT (TMS = 0): actual ≤ 80 ms.
SLOW	Amber	Relay operated slower than expected. Possible causes: CT saturation, incorrect relay settings, or mechanical delay.
FAST	Red	Relay operated faster than expected. Possible causes: a different protection element operated first, or a relay setting discrepancy.

Verdict Display

The verdict bar shows a complete breakdown:

Actual — measured operating time in milliseconds
Expected — calculated operating time from the dominant stage curve (or "INST" for instantaneous)
Stage & phase — which stage and phase determined the expected time (e.g. "S2 on A")
Fault current — the RMS current used for the expected time calculation
Error % — percentage deviation from expected (for non-instantaneous stages)

When No Verdict Appears

The validation bar is hidden when trip detection is set to "None", no fault is detected, no trip event is found, or no stage picks up at the fault current level.

Readout Bar

The readout bar at the bottom of the Overcurrent tab provides a compact numerical summary of the current operating state.

Per-Phase Indicators

Field	Description
Phase dot	Colour-coded indicator (A = red, B = yellow, C = blue, 3I0 = purple)
RMS current	Measured RMS current at the measurement time (primary amps)
Fastest stage	Badge showing which stage trips first (e.g. "S1", "S2", "E1")
Ir	Current multiple for the fastest stage
Operating time	Computed trip time in seconds, or "INST" for instantaneous, or "--" if no stage picks up

Trip Status

The right side of the readout bar shows the trip status based on the configured trip source:

"Not configured" -- Trip detection is set to None
"No event detected" -- Trip source is configured but no transition was found
"[Channel name] at X ms" -- Digital channel trip detected
"Current clearance at X ms" -- Current-based clearance detected

Curve Selection Guide

Choosing the right IDMT curve is a key coordination decision. The following table summarises when each curve type is most appropriate:

Scenario	Recommended Curve	Rationale
General radial feeder	IEC SI	Simplest coordination; tolerant of CT errors and fault level variations
Variable fault level	IEC VI	Steeper curve gives better discrimination when fault current varies with distance
Fuse coordination	IEC EI or IEEE EI	Steep characteristic closely matches fuse melting curves
Thermal overload	IEC LTI	Very slow curve allows sustained overloads without nuisance tripping
High-set instantaneous	DT (TMS = 0)	Zero-delay clearance for close-up faults with very high current
Timed backup	DT (TMS = 0.3-0.5)	Fixed-time backup stage for reliable coordination with downstream relays
US distribution practice	IEEE MI / VI / EI	IEEE curves are standard in North American utility practice

Engineering Applications

Radial Feeder Coordination

In radial distribution networks, overcurrent relays are graded in series from the source to the load. Each downstream relay has a lower TMS than the upstream relay, so faults are cleared by the nearest upstream relay with minimal delay. Use Detego to verify that the actual trip time matches the calculated operating time.

Relay Performance Verification

Detego automates relay performance verification through the Relay Validation feature. Configure the correct relay settings (stages, pickup, TMS) and select a trip source (digital channel), then check the verdict bar:

PASS — Relay operated within the expected tolerance. Settings match the recorded behaviour.
SLOW — Relay took longer than the curve predicts. Investigate CT saturation, incorrect relay settings, or relay malfunction.
FAST — Relay operated faster than expected. A different protection element may have operated first, or settings may be mismatched.
No verdict — Trip detection not configured, no fault detected, or no stage picks up. Check settings and waveform.

Limitations & Notes

Static evaluation -- Detego evaluates operating points at a single time instant. Real relays integrate current over time using the IDMT energy integral.
No reset modelling -- When fault current drops below pickup during the relay timing period, Detego does not model the relay reset characteristic.
No directional element -- Overcurrent protection in Detego is non-directional. For directional overcurrent analysis, use the Directional Assessment tab alongside the Overcurrent tab.
CT saturation not compensated -- If the CT saturates during the fault, the measured current waveform is already distorted. Detego computes RMS from the available data without compensation.

Tips

Set up a 3-stage configuration: S1 (SI, low pickup, TMS = 0.3), S2 (DT, high pickup, TMS = 0), and an EF stage (SI, low pickup) for earth faults.
Use the RMS Trend view to visually confirm that fault current exceeds the expected pickup thresholds.
Check the Harmonics tab for elevated 2nd harmonic content if you suspect CT saturation is affecting the overcurrent measurement.
Use the playback slider to scrub through time and watch operating points move along the TCC curves. The validation verdict updates independently using the peak fault current.

How it works

For the mathematical theory behind IDMT curves, IEC 60255 and IEEE C37.112 equations, grading margin calculations, and coordination principles, see Theory: Overcurrent Protection.

PreviousDistance Protection NextFault Location