Inverse-Time Overcurrent Relay Setting (IEC 60255-151)

Inverse-Time Overcurrent Relay Setting (IEC 60255-151) | Online Free Tool | Elysia Tools

Tool usage guide

Learn when to use this tool, what it supports, and how real users apply it.

Key facts

Category: Math & Numbers
Input types: select, number
Output type: json
Sample coverage: 4
API ready: Yes

Overview

Calculate the operating time of inverse-time overcurrent protection relays according to the IEC 60255-151 standard. This calculator supports Standard Inverse (SI), Very Inverse (VI), Extremely Inverse (EI), and Long-Time Inverse (LTI) curves, allowing electrical engineers to determine trip times and generate T-I characteristic data points using pickup current, fault current, and time multiplier settings.

When to use

Designing electrical protection coordination schemes for power distribution networks.
Verifying relay trip times during commissioning or periodic maintenance testing.
Generating T-I (Time-Current) curve data points to plot protection characteristics in engineering reports.

How it works

1Select the IEC 60255-151 curve type (Standard, Very, Extremely, or Long-Time Inverse) to define the curve constants k and alpha.
2Input the primary pickup current (I_s), the applied fault current (I), and the time multiplier setting (TMS).
3Specify the number of curve sample points and decimal precision to generate the detailed T-I characteristic dataset.
4Run the calculation to obtain the exact relay operating time and the sampled curve points in JSON format.

Use cases

Calculating the trip time of a feeder relay under a specific short-circuit fault current.
Determining the grading margin between upstream and downstream relays to prevent nuisance tripping.
Generating data points to model relay characteristics in power system simulation software.

Examples

1. Standard Inverse Relay Trip Time Calculation

Protection Engineer

Background

An engineer needs to verify the trip time of a feeder protection relay configured with a Standard Inverse (SI) curve during a 500 A fault.

Problem

Determine the exact operating time when the pickup current is set to 100 A and the TMS is 0.2.

How to use

Select 'Standard Inverse (SI)' as the curve type, set the pickup current to 100 A, the time multiplier to 0.2, and the fault current to 500 A.

{
  "curveType": "SI",
  "pickupCurrent": 100,
  "timeMultiplier": 0.2,
  "faultCurrent": 500,
  "curvePoints": 21,
  "decimalPlaces": 4
}

Outcome

The relay operating time is calculated as approximately 0.8559 seconds, and a 21-point T-I curve dataset is generated.

2. Extremely Inverse Relay Coordination Check

Substation Commissioning Technician

Background

A technician is testing an Extremely Inverse (EI) relay protecting a transformer, which must trip rapidly under high fault currents.

Problem

Calculate the trip time for a 1000 A fault with a pickup current of 100 A and a TMS of 0.3.

How to use

FAQ

What is the formula used for the calculation?

It uses the IEC 60255-151 standard formula: t = TMS * (k / ((I / I_s)^alpha - 1)), where I is the fault current and I_s is the pickup current.

What curve types are supported by this calculator?

It supports Standard Inverse (SI), Very Inverse (VI), Extremely Inverse (EI), and Long-Time Inverse (LTI) curves.

Why does the calculator require the fault current to exceed the pickup current?

An overcurrent relay only initiates its timing sequence when the fault current (I) is strictly greater than the pickup current (I_s).

What is the typical range for the Time Multiplier Setting (TMS)?

The TMS typically ranges from 0.05 to 1.0, which scales the overall operating time of the relay.

What output format does the calculator provide?

It outputs a JSON object containing the calculated operating time, input parameters, and a sampled array of points along the T-I curve.

{
  "curveType": "EI",
  "pickupCurrent": 100,
  "timeMultiplier": 0.3,
  "faultCurrent": 1000,
  "curvePoints": 21,
  "decimalPlaces": 4
}

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Continue with connected tools and hubs

Learn when to use this tool, what it supports, and how real users apply it.

Key facts

Overview

When to use

How it works

Use cases

Examples

1. Standard Inverse Relay Trip Time Calculation

2. Extremely Inverse Relay Coordination Check

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