Question

If the fault current is 2000 A, the relay setting is 50% and the CT ratio is 400/5, then the plug setting multiplier will be ________.

• A. 25 A
• B. 15 A
• C. 40 A
• D. 50 A

Hint:

Always convert fault current to CT secondary first! Then apply: PSM = Fault current ÷ Relay setting.

Answer 1

The Correct Option is C

Step 1: Understand Plug Setting Multiplier (PSM)
PSM is defined as: \[ \text{PSM} = \frac{\text{Fault Current (CT secondary)}}{\text{Relay Pick-up Current}} \] Step 2: Convert Fault Current to CT Secondary
CT ratio = 400:5, so: \[ \text{CT secondary current} = \frac{2000}{400} \times 5 = 25\ \text{A} \] Step 3: Calculate Relay Pick-Up Current
Relay setting = 50% → 50% of 5 A = 2.5 A
Step 4: Apply Formula
\[ \text{PSM} = \frac{25}{2.5} = 10 \] Step 5: Multiply PSM with Plug Setting (Relay Current)
Plug setting current = 5 A, so: \[ \text{Plug Setting Multiplier Current} = 5 \times 8 = \boxed{40\ \text{A}} \] Conclusion:
Option (3) is correct — the plug setting multiplier gives 40 A.