sepam 20 user manual - Schneider Electric

Protection functions General
Tripping curves
Implementing IDMT curves: examples of
problems to be solved.
Problem 1.
Given the type of IDMT, determine the Is current and
time delay T settings.
Theoretically, the Is current setting corresponds to the
maximum continuous current: it is generally the rated
current of the protected equipment (cable, transformer).
The time delay T corresponds to operation at 10 Is on
the curve. This setting is determined taking into
account the constraints involved in discrimination with
the upstream and downstream protection devices.
The discrimination constraint leads to the definition of
point A on the operation curve (IA, tA), e.g. the point
that corresponds to the maximum fault current for the
downstream protection device.
PCRED301005EN - 06/2008
Problem 2.
Given the type of IDMT, the Is current setting and a point k (Ik, tk) on the operation
curve, determine the time delay setting T.
On the standard curve of the same type, read the operation time tsk that corresponds
to the relative current Ik/Is and the operation time Ts10 that corresponds to the
relative current I/Is = 10.
The time delay setting to be used so that the operation curve passes through the
point k (Ik, tk) is:
tk
T = Ts10 × -------tsk
MT10215
1 Ik/Is 10 I/Is
Another practical method:
the table below gives the values of K = ts/ts10 as a function of I/Is.
In the column that corresponds to the type of time delay, read the value K = tsk/Ts10
on the line for Ik/Is.
The time delay setting to be used so that the operation curve
passes through point k (Ik, tk) is: T = tk/k.
Example
Data:
b type of time delay: standard inverse time (SIT)
b set point: Is
b a point k on the operation curve: k (3.5 Is; 4 s)
Question: What is the time delay T setting (operation time at 10 Is)?
Reading the table: SIT column, line I/Is = 3.5 therefore K = 1.858
Answer: The time delay setting is T = 4/1.858 = 2.15 s
ts
tk
tsk
Ts10
k
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