Over-current and Earth Fault Protection

v    Introduction

As the fault impedance is less than load impedance, the fault current is more than load current. If a short circuit occurs the circuit impedance is reduced to a low value and therefore a fault is accompanied by large current.

Over-current protection is that protection in which the relay picks up when the magnitude of current exceeds the pickup level.

 The basic element in Over-current protection is an Over-current relay.

The Over-current relays are connected to the system, normally by means of CT's.

Over-current relaying has following types:

1.      High speed Over-current protection.

2.      Definite time Over-current protection.

3.      Inverse minimum time Over-current protection.

4.      Directional Over-current protection (of above types).


Over-current protection includes the protection from overloads. This is most widely used protection. Overloading of a machine or equipment generally) means the machine is taking more current than its rated current. Hence with overloading, there is an associated temperature rise. The permissible temperature rise has a limit based on insulation class and material problems.
Over-current protection of overloads is generally provided by thermal relays.


Over-current protection includes short-circuit protection. Short circuits a be phase faults, earth faults or winding faults. Short-circuit currents are generally several times (5 to 20) full load current. Hence fast fault clearance is always desirable on short-circuits.


When a machine is protected by differential protection, the over-current is provided in addition as a back-up and in some cases to protect the machine from sustained through fault.

Several protective devices are used for over-current protection these include:

1.      Fuses

2.      Circuit-breakers fitted with overloaded coils or tripped by over-current relays.

3.      Series connected trip coils operating switching devices.

4.      Over-current relays in conjunction with current transformers.


The primary requirements of over-current protection are:

  • The protection should not operate for starting currents, permissible over-current, and current surges. To achieve this, the time delay is provided (in case of inverse relays). If time delay cannot be permitted, high-set instantaneous relaying is used.

  • The protection should be coordinated with neighboring over-current protections so as to discriminate.


v          Applications of Over-current Protection

Over-current protection has a wide range of applications. It can be applied where there is an abrupt difference between fault current within the protected section and that outside the protected section and these magnitudes are almost constant.


The over-current protection is provided for the following:

v                 Motor Protection
Over-current protection is the basic type of protection used against overloads and short-circuits in stator windings of motors. Inverse time and instantaneous phase and ground over-current relays can be employed for motors above 1200 H.P. For small/medium size motors where cost of CT's and protective relays is not economically justified, thermal relays and HRC fuses are employed, thermal relays used for overload protection and HRC fuses for short-circuit protection.

v                 Transformer Protection
Transformers are provided with over-current protection against faults, only, when the cost of differential relaying cannot be justified. However, over-current relays are provided in addition to differential relays to take care of through faults. Temperature indicators and alarms are always provided for large transformers.

Small transformers below 500 kVA installed in distribution system are generally protected by drop-out fuses, as the cost of relays plus circuit-breakers is not generally justified Line Protection.

The lines (feeders) can be protected by

(1)               Instantaneous over-current relays.

(2)               Inverse time over-current relays.

(3)               Directional over-current relay.

Lines can be protected by impedance or carrier current protection also.


Protection of Utility Equipment

The furnaces, industrial installations commercial, industrial and domestic equipment are all provided with over-current protection.


v    Relays used in Over-current Protection

The choice of relay for over-current protection depends upon the Time / current characteristic and other features desired. The following relays are used.

1.      For instantaneous over-current protection. Attracted armature type, moving iron type,
      permanent  magnet moving coil type and static.

2.      For inverse time characteristic.  Electromagnetic induction type, permanent magnet
       moving coil type and static.

3.      Directional over-current protection. Double actuating quantity induction relay
      with directional feature.
4.      Static over-current relays.
5.      HRC fuses, drop out fuses, etc. are used in low voltage medium voltage
       and high voltage distribution systems, generally up to 11 kV.

6.      Thermal relays are used widely for over-current protection.
Not: Now Digital Numerical Relay you can used for all types


v     Characteristics of relay units for over current protection

There is a wide variety of relay-units. These are classified according to their type and characteristics. The major characteristic includes:

           1. Definite characteristic

           2. Inverse characteristic

           3. Extremely Inverse
Very Inverse


In definite characteristic, the time of operation is almost definite i.e.

I 0 * T = K


          I = Current in relay coil

          T = Relay lime

          K = Constant.

In inverse characteristic, time is inversely proportional to current i.e.

I 1 * T = K

In more inverse characteristic


I n * T = K


Where n can be between 2 to 8 the choice depends on discrimination desired.

Instantaneous relays are those which have no intentional time lag sod which operate in less than 0.1 second, usually less than 0.08 second. As suck they are not instantaneous in real sense.


The relays which are not instantaneous are called Time Delay Relay'. Such relays are provided with delaying means such as drag magnet, dash poss. bellows, escape mechanisms, back-stop arrangement, etc.


The operating time of a relay for a particular setting and magnitude actuating quantity can be known from the characteristics supplied by the manufacturer. The typical characteristics are shown in (Fig. 1)


An inverse curve is one in which the operating time; becomes less as the magnitude of the actuating quantity is increased. However for higher magnitudes of actuating quantity the time is constant. Definite time curve is one in which operating time is little affected by magnitude of actuating current. However even definite time relay has a characteristic which is slightly inverse

The characteristic with definite minimum time and of inverse type is also called Inverse Definite Minimum Time (IDMT) characteristics (Fig.1).

(Fig.1)  Inverse Definite Minimum Time (IDMT) characteristics