Question

An IDMT relay is used to protect feeder through 500/1 CT. The relay has a PSM 100% and TMS = 0.4, relay characteristics are given for TMS = 1.

$\begin{array}{ccccccc}\text{PSM}& 2& 3& 5& 8& 10& 12\\ \text{Time (sec)}& 10& 6& 4.5& 3.2& 3& 2.5\end{array}$

If the fault current is 5000 A, then time of operation of relay will be_______sec.

• A. 1.2

Answer 1

The correct option is A 1.2
$\text{Given,}$ $I_F=5000A$

$\text{CT = 500/1}$

$\text{\% relay setting = 100}$

$I_{sec}\left(CT\right)=I_F\times \frac{I_2}{I_1}=5000\times \frac{1}{500}=10A$

$PSM=\frac{I_{se{c}_{(}CT)}}{I_{pick-up}}$

$=\frac{10}{\text{relay setting}\times I_{rated\left(relay\right)}}$

$=\frac{10}{1\times 1}=10$

Time of operation of relay, corresponds to PSM = 10 is 3 seconds (from characteristics table)

$\text{Time of operation of relay for TMS = 1 is 3 sec}$

$\text{Then time of operation of relay (for TMS = 0.4) is =}{T}_{op}$

$T_{op}=3\times TMS$

$T_{op}=3\times 0.4=1.2sec$