What is the PSM of a relay having a pick up current of 5 A, when used with a 400/5 current transformer if the fault current in the circuit is 4000 A?

Concept

The Plug Setting Multiplier is calculated as:

\(PSM={Fault \space current \space referred \space to \space secondary \over Relay \space pick-up \space current }\)

Fault current on secondary = \({Fault \space Current \space in \space Primary ​ \over CT \space ratio \space Primary \space side }\times CT \space  Ratio \space Secondary \space Side\)

Calculation

Given, I_pick-up = 5 A

Current Transformer (CT) ratio: 400/5. This means that when 400 A flows in the primary circuit, 5 A flows in the secondary.

Fault Current in the Primary Circuit: 4000 A

Fault current on secondary =  \({4000\over 400}\times 5 = 50 \space A\)

\(PSM={50\over 5}=10\)