MKA IDMT Overcurrent Relay User’s. Guide. A BRIEF OVERVIEW a b c d h g f e a – Auxiliary power supply indicator b – High-set start/trip status indicator. OVERCURRENT RELAY WITH IDMT. ○ Test and reset facility. ○ Convenient C. T. connections. ○ Double pole double throw 10A VAC Relay Output. Setting calculation is separated for overcurrent and eart fault relay with input parameters: Name of Feeder, Load Current: (IL) in Amps, Min Fault.

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We refer the pre-set value of current in the relay coil as pickup setting current.

Over Current Relay and Its Characteristics | Electrical Concepts

But when the CT becomes saturated, there would not be a further proportional increase of CT secondary current with increased system current. Please suggest us to reduce these frequent tripping of relay, what will be the characteristics of relay to be set definite time, normal inverse or very long overurrent etc.

Inverse time over current relay or simply inverse OC relay is again subdivided as inverse definite minimum time IDMTgelay inverse timeextremely inverse time over current relay or OC relay. IDMT is low set. Uzair Dec 10, In relxy type of relays, operating time is inversely changed with current.

The other terminology used for overcurrent is hi-set and low-set. These ratings include voltage, ampere and interrupting rating. September 25, at 6: The voltage rating of the over-current protective device must be at least equal to or greater than the circuit voltage. This site uses Akismet to reduce spam. This happens so as the CT saturation occurs at a later stage but as soon as CT saturation occur there will not be any flux change and hence the current output of CT will become zero and hence the time of operation will nearly become constant.


Overcurrent relays can be used to protect practically any power system elements, i. The content is copyrighted to EEP and may not be reproduced on other websites. But after this level of fault, the CT becomes saturated and relay current does not increase further with ovecurrent faulty level of the system. Protection schemes can be divided into two major groups: From the picture, it is clear that there is some definite time after which the Relay will operate.

Thus, it has a time setting adjustment and pickup adjustment. Over-current relays can be used to protect practically any power system elements, i.

Hence, the characteristic is inverse in the initial part, which tends to a definite minimum operating time as the current becomes very high.

Leave a Comment Click here to cancel reply. The overcurrent protection is a bigger concept So that the overload protection can be considered as a subset of overcurrent protection. The general practice is to employ a set of two or three overcurrent relays and a separate overcurrent relay for single line to ground fault. Earth fault current is always less than phase fault current in magnitude.

Why is the last point in distribution protection an advantage? Here generally a magnetic core is wound by a current coil. Could you please agree to put myself in your group that can help me to follow all comments from all engineers and also i can send my question on it.

As the current in the system increases, the secondary current of the current transformer is increased proportionally. As the relay current does not increase further, there would not be any further reduction in time of operation in the relay.

Modern relays may contain more than one stage of protection each stage includes each own current and time setting. When the power system is not radial source on one side of the linean overcurrent relay may not be able to provide adequate protection.


As well as protecting their own designated areas, the protective zones can overlap into other areas.

Ibrahim Kurdi Feb 19, Tech Power System Control ,B. To achieve this, the time delay is provided in case of inverse relays.

As a result, the moving element starts moving to change the contact position in the relay. Any deviation from this must indicate an abnormal current path. Instantaneous Over-current Relay is one in which no intentional time delay is provided for the operation.

Over Current Relay Working Principle Types

Inverse time over-current Relay is one in which the time of actuation of Relay decreases as the fault current increases. Three conditions must be satisfied for its operation: Following are the important features of an Instantaneous Over-current Relay: Time graded overcurrent protection Current graded overcurrent protection Ogercurrent or Impedance Protection. Therefore, relay connected for earth fault protection is different from those for phase to phase fault protection.

So, high current will operate overcurrent relay faster than lower ones.

Suppose the pick-up current for an IDMT relay is set at 0. Premium Membership Get access to premium electrical guides, technical articles and much more! Depending on the time of operation of overfurrent, they are categorized as follows: Why have seperate relays when a single relay can be used to implement rflay elements such as protection against ground faults, as well as ac instantaneous overcurrent?

The non-unit schemes, while also intended to protect specific areas, have no fixed boundaries. The more the fault current the lesser will be the time of operation of the Relay. Mathieu Lachane Jun 24, Based on the inverseness it has three different types: