What is a time-current coordination (TCC) curve?

A time-current coordination curve plots the operating time of a protective device against current on a log-log chart. It is used to compare relay and device characteristics so engineers can check how devices respond to fault current and coordinate with each other.

What can I plot with this tool?

You can plot overcurrent relay curves and device characteristics on a log-log time-current chart and overlay conductor and equipment damage curves to compare device response against fault current levels.

Does it check selective coordination?

It helps you visualize and check selective coordination between protective devices by showing their curves together and the visual coordination margins. Final coordination decisions remain the responsibility of the engineer.

What protection tasks does it support?

It supports overcurrent coordination studies, settings checks, and verifying selectivity and damage-curve clearance during coordination and review work.

Curve Type	A	B	P	Description
SEL U1	1.5	1	2	Standard SEL curve with moderate inverse characteristics.
SEL U2	1.8	1	2	Moderately faster SEL curve for quicker operation.
SEL U3	2.0	1	2	SEL curve with faster response to high fault currents.
SEL U4	2.5	1	2	SEL curve for systems requiring faster high fault response.
SEL U5	3.0	1	2	Highly sensitive SEL curve for maximum fault current response.
IEEE Very Inverse (VI)	19.61	0.491	2	Defined by IEEE C37.112, suitable for heavily loaded systems.
IEEE Extremely Inverse (EI)	28.2	0.1217	2	Steeper curve for systems requiring highly sensitive protection.
IEEE Moderately Inverse (MI)	0.0515	0.114	0.02	Standard IEEE curve for balanced inverse fault response.
IEC Standard Inverse (C1)	0.14	0	0.02	Standard IEC curve for typical inverse-time characteristics.
IEC Very Inverse (C2)	13.5	0	1	IEC curve for faster fault current response.
IEC Extremely Inverse (C3)	80	0	2	Highly steep IEC curve for sensitive fault conditions.
IEC Long-Time Inverse (C4)	120	0	1	Gradual IEC curve for overload conditions.
IEC Short-Time Inverse (C5)	0.05	0	0.04	Rapid IEC curve for high transient fault conditions.
US U1 (Moderately Inverse)	0.0226	0.0104	0.02	Moderate response curve for typical inverse-time applications.
US U2 (Inverse)	0.180	5.95	2	Inverse curve with faster operation at higher fault currents.
US U3 (Very Inverse)	0.0963	3.88	2	Very steep curve for heavy fault current applications.
US U4 (Extremely Inverse)	0.02434	5.64	2	Extremely steep curve for sensitive fault current response.
US U5 (Short-Time Inverse)	0.00262	0.00342	0.02	Short-time inverse curve for rapid fault clearance.
UK Long-Time Inverse (LTI)	120	0	1	Used in UK systems for gradual overload protection.
UK Reset (R)	45900	0	5.6	UK-specific curve for highly steep reset conditions.

Curve Type

Description

SEL U1

1.5

Standard SEL curve with moderate inverse characteristics.

SEL U2

1.8

Moderately faster SEL curve for quicker operation.

SEL U3

2.0

SEL curve with faster response to high fault currents.

SEL U4

2.5

SEL curve for systems requiring faster high fault response.

SEL U5

3.0

Highly sensitive SEL curve for maximum fault current response.

IEEE Very Inverse (VI)

19.61

0.491

Defined by IEEE C37.112, suitable for heavily loaded systems.

IEEE Extremely Inverse (EI)

28.2

0.1217

Steeper curve for systems requiring highly sensitive protection.

IEEE Moderately Inverse (MI)

0.0515

0.114

0.02

Standard IEEE curve for balanced inverse fault response.

IEC Standard Inverse (C1)

0.14

0.02

Standard IEC curve for typical inverse-time characteristics.

IEC Very Inverse (C2)

13.5

IEC curve for faster fault current response.

IEC Extremely Inverse (C3)

Highly steep IEC curve for sensitive fault conditions.

IEC Long-Time Inverse (C4)

120

Gradual IEC curve for overload conditions.

IEC Short-Time Inverse (C5)

0.05

0.04

Rapid IEC curve for high transient fault conditions.

US U1 (Moderately Inverse)

0.0226

0.0104

0.02

Moderate response curve for typical inverse-time applications.

US U2 (Inverse)

0.180

5.95

Inverse curve with faster operation at higher fault currents.

US U3 (Very Inverse)

0.0963

3.88

Very steep curve for heavy fault current applications.

US U4 (Extremely Inverse)

0.02434

5.64

Extremely steep curve for sensitive fault current response.

US U5 (Short-Time Inverse)

0.00262

0.00342

0.02

Short-time inverse curve for rapid fault clearance.

UK Long-Time Inverse (LTI)

120

Used in UK systems for gradual overload protection.

UK Reset (R)

45900

5.6

UK-specific curve for highly steep reset conditions.

TCC Coordination - Time-Current Curve Study Tool

This in-browser time-current coordination (TCC) study tool plots overcurrent relay curves and device characteristics on a log-log time-current chart. It overlays conductor and equipment damage curves and helps you check selective coordination between protective devices. Everything runs in your browser, so you can iterate on settings without installing software.

Who it is for

Protection and system engineers, coordination study engineers, and commissioning teams who need a fast way to draw and review overcurrent coordination.

Inputs and outputs

Inputs: relay and device curve parameters, pickup and time-dial settings, and fault current levels.
Outputs: a TCC plot with relay and device curves, damage curves, and visual coordination margins.

Typical use cases

Overcurrent coordination studies.
Settings checks for pickup and time-dial values.
Verifying selectivity between upstream and downstream devices.
Checking damage-curve clearance for conductors and equipment.

What it does not do

It does not replace the relay manufacturer's setting software or a stamped coordination study. All settings must be confirmed against manufacturer data, applicable standards, and engineering judgment.

Privacy: This tool runs entirely in your browser. The values and settings you enter are processed locally on your computer and are not uploaded to a server.

Related tools

Frequently asked questions

What is a time-current coordination (TCC) curve?: A time-current coordination curve plots the operating time of a protective device against current on a log-log chart. It is used to compare relay and device characteristics so engineers can check how devices respond to fault current and coordinate with each other.
What can I plot with this tool?: You can plot overcurrent relay curves and device characteristics on a log-log time-current chart and overlay conductor and equipment damage curves to compare device response against fault current levels.
Does it check selective coordination?: It helps you visualize and check selective coordination between protective devices by showing their curves together and the visual coordination margins. Final coordination decisions remain the responsibility of the engineer.
Does this replace relay manufacturer software?: No. It does not replace the relay manufacturer's setting software or a stamped coordination study. All settings must be confirmed against manufacturer data, applicable standards, and engineering judgment.
Is my data uploaded to a server?: No. This tool runs entirely in your browser. The values and settings you enter are processed locally on your computer and are not uploaded to a server.
What protection tasks does it support?: It supports overcurrent coordination studies, settings checks, and verifying selectivity and damage-curve clearance during coordination and review work.

HV1 curve
Pickup (A, primary)
TD	5.000
50/DT	Enable Pickup (A) Time (s)
HV2 curve
Pickup (A, primary)
TD	2.379
50/DT	Enable Pickup (A) Time (s)
MV curve
Pickup (A, primary)
TD	0.392
50/DT	Enable Pickup (A) Time (s)

Full Coordination Results

Relay Settings

Operating Points & Coordination

Curve Equations

Calculated Values

Primary

Load currents

Fault currents

Pick-up currents

Plug Multiplier Setting

Time Multiplier Setting

Secondary

Load currents

Fault currents

Pick-up currents

Plug Multiplier Setting

Time Multiplier Setting

System Parameters

Overcurrent Relay Curve Constants

TCC Coordination - Time-Current Curve Study Tool

Who it is for

Inputs and outputs

Typical use cases

What it does not do

Related tools

Frequently asked questions