Overcurrent Relays - Basics, Function, types of Overcurrent relay


What is an overcurrent relay?

Overcurrent relays are relays that work based on the increase in current that passes through them and can also be based on the specified time setting. Time measurement is related to security coordination issues. Overcurrent relays are simple, inexpensive and work faster.

Overcurrent protection is very different from overload protection, which usually uses relays operating in the time associated at a certain level with the thermal capability of the device to be protected.

While overcurrent protection is directed entirely at the interruption cleaning process, even with the arrangement it usually adopts some degree of overload current achieved. Therefore, the maximum load current must be known to determine the current minimum ratio for maximum loads high enough to enable overcurrent relays.

Functions of Overcurrent relay:

  • Short circuit protection and overload safety

  • Main safety of radial system distribution and sub-transmission networks

  • Reserve backup generators, power transformers, transmission lines.

Operation principle of Overcurrent relay:

The first step is to read the current device. The current of the device is reduced by a CT and then digitized by an A / D converter. The magnitude of the fundamental component can be estimated from current samples by using various methods of parameter estimation.

These methods will be discussed in more detail at later conferences. In the next step, the input current is compared to the reference current. If the sign of is positive, then it indicates the possibility of a failure. The I / Iref ratio is also known as the Plug Setting Multiplier (PSM)

Types of Overcurrent relay:

Instantaneous overcurrent relay:


R : Relay
CT : Current transformator
TC : Triping Coil
A : Alarm

The operating time of an instantaneous relay is of the order of a few milliseconds. This Overcurrent relays are the simplest type of overcurrent relay where the relay working period is starting when the relay is pick-up until the completion of the relay work is very short or without time delay. This current relay is used for the safety of large short circuits (high sets) so that tripping time in large fault currents of the relay will work instantly.

Time delayed Definite Time Relay:


Definite Time overcurrent relays are types of overcurrent relays where the relay period begins to pick-up until the completion of the relay work can be extended with a certain value and does not depend on the amount of current working (depending on the size of the setting, the working time of the relay is determined by the time setting)

In general, the setting range for relay Definite time is between 0.9 - 1.8 In while for the operating time from 0.1 seconds to 4 seconds.

Inverse definite minimum time (IDMT) Relay:

Inverse definite time relay with current time characteristics inversely are types of overcurrent relays where the term of the relay starts pick-up until the completion of the relay work depends on the amount of current passing through the coil, the relay has an inverse nature for the current value and working time.

The current relay is more inversely divided into 4, namely standard inverse, very inverse, extremely inverse and longtime inverse.

The relationship between current and time is shown by the following equation:



t: time in seconds
I: Flow interference
Is: Setting flow
TMS: time multiplier setting.

K and alpha for each large characteristic as in the table below:

Characteristics K Alpha
Standard Inverse 0.14 0.02
Very Inverse 13.5 1.0
Extremely Inverse 80.0 2.0
Long Time Inverse 120.0 1.0