is it possible to draw long time curve or calculate it manually? if it possible, could you please share the calculation?
Yes, you can work out the values for a long-time curve manually and apply it for protective devices such as Circuit breakers or relays.
The long-time curve shows the time duration of the protective device effective operation against the function of the current of the operation fault. It is normally calculated with respect to the time current characteristics provided by manufacturers or formulated standards.
Steps to Manually Calculate the Long-Time Curve
Step-1: Obtain the Time-Current Characteristics
Refer to the manufacturer datasheet or look for codes like for circuit breakers IEC 60947 (or) for relays IEC 60255 for time current characteristics equations or tables.
Step-2: Identify the Key Parameters:
Parameters often include:
Long-time pickup current (Ipickup): The flow rate at which long-time delay commences.
Time multiplier setting (TMS): This is a factor that scales the operating time of the system.
Overcurrent value (Ifault): The current that passes through the device at the time of fault.
Step-3: Use the Characteristic Equation:
For devices with inverse time characteristics, the equation is often of the form:
t = TMS x [k/ (Ifault/Ipickup) ^n]-1
Where
t – Device opening time
k, n – Constant provided by Manufacturer
Step-4: Choose data points:
Select different values of Ifault, commonly 1.5, 2, 3, 4, and up to 10 times.
Use the characteristic equation to get t for each Ifaults.
Step-5: Plot the curve:
On a log graph (log-log scale), plot Ifault on the X-axis and t on the Y-axis. Join the determined points to construct a curve.