Inverse Time Current Characteristic

[Mohammed Ahmed]

Both attached curves are different with regards to inverse and extremely inverse time characteristics for circuit breaker? which one is correct? noting that the first picture was extracted from Zach practice exam and I get the other graph from external source.

 

[RedRaider2020]

The slope of the curves are the same. The time dial settings will move the curves up and down on the graph and are different. You can make the curves operate at the same time for a given current level by adjusting the time dial. The Definite time is incorrect on Zach's graph. Definite time should be horizontal with no curve for a given time.

The curves in this picture are all the same just with different time dial settings.

 

[Mohammed Ahmed]

The slope of the curves are the same. The time dial settings will move the curves up and down on the graph and are different. You can make the curves operate at the same time for a given current level by adjusting the time dial. The Definite time is incorrect on Zach's graph. Definite time should be horizontal with no curve for a given time.

The curves in this picture are all the same just with different time dial settings.
View attachment 21093

So, for extremely inverse curve the breaker will take less time to operate for the same amount of current, correct?

 

[RedRaider2020]

Yes, since the slope is steeper. Normally you are trying to match the curves with an equipment damage curve and there may also be curves from other relays that you have to coordinate with all in one graph.
The settings that you can control on the relay will be the pickup, curve and time dial. If you select the wrong curve the lines will intersect and you won't be properly protecting your equipment or your breakers won't coordinate their tripping in the proper order.

Here are the settings from an actual relay. This is extra information that you won't need to remember for the exam. If it's a new microprocessor relay then you can set the torque control which controls if the relay is allowed to operate or not. Here the torque control is set to 32GF meaning it will only operate for a forward fault. You can also set it to 1 so that they relay is not torque controlled. The EM reset sets if you want the reset time delay to operate similar to an old electromechanical relay or to operate instantly. The operate quantity is set to operate on 3PH Zero Sequence Line current only.