What do you mean by open circuit logic, open CT detection? What type of relay is Performing the Bus protection.
I'm just going to assume when you say open circuit logic, that you are referring to open CT detection.
In a normal Bus Differential scheme without the use of open CT detection logic, if one were to lift a CT wire or if a CT wire were to come out of a lug.......etc, the Bus differential would operate and clear the bus right. Well Open CT detection logic allows the relay to distinguish an actual differential and trip on a fault with in the zone or to remain in service and alarm on an open CT.
Here is how it works:
When the CT circuit opens up, there is a spike or increase in "operating current" and a corresponding dip or decrease of the same magnitude in "restraint current" The increase in operate current and decrease in restraint current should sum to zero. Every relay is a little different but essentially the relay is saying "Is the operate current above X per unit? Is the restraint current below X per unit? Is the vector Sum of restraint and operate current very small (close to 0 zero)? If so, I declare an Open CT"
