Time-Current Curve Part 2 – Discrimination

In Part 1, I have discussed the basics of using excel in plotting time-current curves.

In this tutorial, I will be presenting an example of a possible discrimination curve using the different curve types based on IEC 60255 (BS 142).

  1. Standard Inverse (Normal Inverse) - this is the most commonly used curve for over-current protection.
  2. Very Inverse - used where fault current decreases significantly between protection points.
  3. Extremely Inverse - used when coordinating with fuses.

IDMT Equation:

t = time, seconds
TMS = time multiplier setting
alpha=curve contant (per IEC 60255)
beta=curve contant (per IEC 60255)
I = current
I_o=relay current plugin

alpha - beta Constants Values
Curve Type alpha beta
Standard Inverse 0.02 0.14
Very Inverse 1 13.5
Extremely Inverse 2 80

When doing relay coordination, take note that "?Inverse Over Current Relay" operating time is inversely proportional to the fault current which means it will have faster operating times at higher fault levels and for faults nearer to the source.

I have attached the excel in creating this curves but before downloading the excel file, please take time to read this instructions on using the worksheet.

  1. The worksheet was created using Excel 2010 saved on compatibility mode with Excel 97-2003. There might be some functionality lost during the process due to backward compatibility.
  2. Curve type can be selected using a drop down list.
  3. This worksheets could be used for a three relays.
  4. Curves on the graph:
    1. Fault Current
    2. Supply relay
    3. Clearance curve 1 between supply relay and relay 1 down stream.
    4. Relay 1 curve
    5. Clearance curve 2 between supply relay and relay 1 down stream.
    6. Relay 2 curve.
  5. Downstream relay currents are plotted on the graph converted into the supply voltage base.
  6. The worksheet have been formatted for easy printing.

Please take time to read the Disclaimer before downloading.

  • Lim Khuan

    Dear Sir,
    Great information and knowledge, will learn from your turtorial and share with my fellow colleague…thanks again…

    Lim Khuan

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