1 and half circuit breaker bus bar differential protection calculation
A very specific and technical question!
To calculate the bus bar differential protection for a 1.5 circuit breaker bus bar, we'll need to follow a step-by-step process. Here's the calculation:
Assumptions:
- The bus bar is a single, solid, and continuous conductor.
- The circuit breakers are identical and have the same current rating.
- The bus bar is not divided into sections or compartments.
- The differential protection is based on the principle of comparing the current in the bus bar with the sum of the currents in the circuit breakers.
Calculation:
- Determine the bus bar current rating: This is the maximum current that the bus bar is designed to carry. Let's assume it's 4000 A.
- Determine the circuit breaker current rating: This is the maximum current that each circuit breaker is designed to interrupt. Let's assume it's 2500 A.
- Calculate the bus bar current per circuit breaker: Since there are 1.5 circuit breakers, we need to calculate the bus bar current per circuit breaker. Let's assume the bus bar current is evenly distributed among the circuit breakers.
Bus bar current per circuit breaker = Bus bar current rating / Number of circuit breakers = 4000 A / 1.5 = 2667 A per circuit breaker 4. Calculate the differential current: This is the difference between the bus bar current and the sum of the currents in the circuit breakers.
Differential current = Bus bar current per circuit breaker - (Number of circuit breakers * Circuit breaker current rating) = 2667 A - (1.5 * 2500 A) = 2667 A - 3750 A = -1083 A (negative value indicates a fault) 5. Calculate the differential current percentage: This is the percentage of the bus bar current rating that the differential current represents.
Differential current percentage = (Differential current / Bus bar current rating) * 100 = (-1083 A / 4000 A) * 100 = -27.08% (negative value indicates a fault) 6. Set the differential protection setting: The differential protection setting is typically set to a value that is slightly higher than the differential current percentage calculated in step 5. Let's assume the setting is 30% of the bus bar current rating.
Differential protection setting = 30% of bus bar current rating = 0.3 * 4000 A = 1200 A
In this example, the differential protection setting is 1200 A, which is higher than the calculated differential current of -1083 A. This means that the differential protection will trip the circuit breaker when a fault occurs.
Conclusion:
The calculation shows that the differential protection setting for a 1.5 circuit breaker bus bar is 1200 A, which is higher than the calculated differential current of -1083 A. This ensures that the differential protection will trip the circuit breaker when a fault occurs, providing reliable protection for the bus bar and the connected circuit breakers.