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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So, high current will operate overcurrent relay faster than lower ones. Instantaneous Over Current Relay Construction and working principle of instantaneous over current relay is quite simple.

When the current in the coil crosses a preset value, the attractive force becomes enough to pull the iron piece towards the magnetic core, and consequently, the no contacts get overcurdent. OverCurrent Relay gives protection against: As per IEC, the time of operation of any Inverse relay can be calculated from the formula given below.

From the picture, it is clear that there is some definite time after which the Relay will operate.

Types and Applications Of Overcurrent Relay (part 1)

Instantaneous Over-current relay is employed where the impedance between the source and the Relay is small as compared with the impedance of the section to be provided. Neil Govender Nov 03, Subscribe to our mailing list and get interesting stuff and updates to your email inbox.

October 27, at 4: April 29, at 3: The directionality of current flow can overcurrrent identified using voltage as a reference of direction.

The other terminology used for overcurrent is hi-set and low-set. Superb andriod app thanks a lot one of the best apps in the world …knowledge treasure. Current limiting can be considered as another overcurrent protective device rating, although not all overcurrent protective devices are required to have this characteristic.


Any deviation from this must indicate an abnormal current path.

Over Current Relay Working Principle Types

As well as protecting their own ovdrcurrent areas, the protective zones can overlap into other areas. IDMT is low set. But there is always an inherent time delay which we cannot avoid practically. April 29, at 5: As a result, the moving element starts moving to change the contact position in the relay. Modern relays may contain more than one stage of protection each stage includes each own current and time setting.

If the fault is severe, it will clear the fault faster. In this type of relays, operating time is inversely changed with current. Earth fault current is always obercurrent than phase fault current in magnitude. Based on the inverseness it has three different types: These are the types of overcurrent relay: Delcev Feb 01, Leave a Reply Cancel reply Your email address will not be published.

September 25, at Premium Membership Get access to premium electrical guides, technical idmtt and much more! It has a limitation that it can be applied only ovecurrent simple and non costly equipments. But when the current through the coil increases, the magnetic effect increases, and after a certain level of current, the deflecting force generated by the magnetic effect of the coil, crosses the restraining force.

Shashidhara TM May 21, Unit type schemes protect a specific area of the system, i. Time graded overcurrent protection Current graded overcurrent protection Distance or Impedance Protection.

If the interrupting rating is not properly selected, a serious hazard for equipment and personnel will exist. The most simple of these schemes measures current and incorporates an inverse time characteristic into the protection operation to allow protection nearer to the fault to overcurrent first.

Why have seperate relays when a single relay can be used to implement multiple elements such as protection against ground faults, as well as ac instantaneous overcurrent?


Inverse definite minimum time IDMT over-current Relay is one in which the operating time is approximately inversely proportional to the fault current near pick-up value and then becomes constant above the pick-up value of the relay.

There is no intentional time delay applied. Separate earth fault relay provided makes earth fault protection faster and more sensitive.

Over Current Relay and Its Characteristics | Electrical Concepts

From this phenomenon, it is clear that from trick value to certain range of faulty level, an inverse time relay shows specific inverse characteristic. For feeder protection, there would be more than one overcurrent relay to protect different sections of the feeder. Following are the important features of an Instantaneous Over-current Relay: Muhammad Usman Jan 17, September 25, at 6: We refer the pre-set value of current in the relay coil as pickup setting current.

As the relay current does not increase further, there would not be any further reduction in time of operation in the relay. These relsy relays need to coordinate with each other such that the relay nearest fault operates first. The non unit type protection system includes following schemes: Extremely Inverse Relay is one in which CT saturation occur still at a later stage as compared with Very Inverse Relay and hence the characteristic remain inverse up to a larger range of fault current.

The uncertainty of the operating time and the necessary operating time may require a grading margin of 0. It is also clear that the time of operation at Pick-up value is nearly very high and as the fault current increases the time of operation decreases maintaining some definite time.