Electrical Engineering (EE) Exam > Electrical Engineering (EE) Questions > If for an IDMT relay with a plug setting of 5...
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If for an IDMT relay with a plug setting of 50% and a CT ration of 400/5, the current is 3000 A, then the plug setting multiplier would be:
- a)7.5
- b)15.0
- c)18.75
- d)37.5
Correct answer is option 'B'. Can you explain this answer?
Verified Answer
If for an IDMT relay with a plug setting of 50% and a CT ration of 400...
Concept:
Plug setting multiplier:
Plug setting multiplier is the ratio of fault current and the product of plug setting and CT ratio.
Plug setting multiplier 
Calculation:
Plug setting = relay setting × secondary CT current
= 0.5 × 5 = 2.5
CT ratio = 400/5 = 80
Plug setting multiplier 
Most Upvoted Answer
If for an IDMT relay with a plug setting of 50% and a CT ration of 400...
Plug Setting Multiplier for IDMT Relay
Given:
- Plug setting = 50%
- CT ratio = 400/5
- Current = 3000 A
The plug setting multiplier is used to determine the actual operating current at which the IDMT (Inverse Definite Minimum Time) relay will trip. It is calculated by multiplying the actual current with the inverse of the plug setting.
Formula:
Plug Setting Multiplier (PSM) = 100 / Plug Setting
Calculation:
1. Plug Setting = 50%
Plug Setting Multiplier (PSM) = 100 / 50% = 2
2. CT Ratio = 400/5
Primary Current (I_p) = Secondary Current (I_s) x CT Ratio
I_p = 3000 A x (400/5) = 240,000 A
3. Actual Operating Current (I_actual) = I_p / PSM
I_actual = 240,000 A / 2 = 120,000 A
Therefore, the plug setting multiplier for the given IDMT relay is 2.
Explanation:
- The plug setting of an IDMT relay determines the current level at which the relay starts to operate and initiate tripping.
- In this case, the plug setting is given as 50%, which means the relay will start to operate when the current exceeds 50% of the full load current.
- The CT ratio is provided as 400/5, indicating that the secondary current is 5 times lesser than the primary current. This ratio is used to convert the secondary current to the primary current.
- The actual current is given as 3000 A, which is the secondary current.
- Using the CT ratio, the primary current is calculated as 240,000 A.
- The plug setting multiplier is then calculated by taking the inverse of the plug setting, which is 2 in this case.
- Finally, to determine the actual operating current at which the relay will trip, the primary current is divided by the plug setting multiplier, resulting in 120,000 A.
- Hence, the correct answer is option 'B' (15.0).
Given:
- Plug setting = 50%
- CT ratio = 400/5
- Current = 3000 A
The plug setting multiplier is used to determine the actual operating current at which the IDMT (Inverse Definite Minimum Time) relay will trip. It is calculated by multiplying the actual current with the inverse of the plug setting.
Formula:
Plug Setting Multiplier (PSM) = 100 / Plug Setting
Calculation:
1. Plug Setting = 50%
Plug Setting Multiplier (PSM) = 100 / 50% = 2
2. CT Ratio = 400/5
Primary Current (I_p) = Secondary Current (I_s) x CT Ratio
I_p = 3000 A x (400/5) = 240,000 A
3. Actual Operating Current (I_actual) = I_p / PSM
I_actual = 240,000 A / 2 = 120,000 A
Therefore, the plug setting multiplier for the given IDMT relay is 2.
Explanation:
- The plug setting of an IDMT relay determines the current level at which the relay starts to operate and initiate tripping.
- In this case, the plug setting is given as 50%, which means the relay will start to operate when the current exceeds 50% of the full load current.
- The CT ratio is provided as 400/5, indicating that the secondary current is 5 times lesser than the primary current. This ratio is used to convert the secondary current to the primary current.
- The actual current is given as 3000 A, which is the secondary current.
- Using the CT ratio, the primary current is calculated as 240,000 A.
- The plug setting multiplier is then calculated by taking the inverse of the plug setting, which is 2 in this case.
- Finally, to determine the actual operating current at which the relay will trip, the primary current is divided by the plug setting multiplier, resulting in 120,000 A.
- Hence, the correct answer is option 'B' (15.0).
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