Variables
Description
What is this formula?
The Protection Coordination Time Interval (CTI) is the time margin between the operation of two protective devices arranged in series. It ensures that the downstream device clears the fault before the upstream device operates.
When to use it
Use this formula when coordinating relays, fuses, reclosers, and other protective devices in electrical distribution and transmission systems.
Example
A protection system has:
Upstream relay operating time = 0.85 s
Downstream relay operating time = 0.45 s
Formula:
CTI = tupstream − tdownstream
Substitution:
CTI = 0.85 − 0.45
Result:
CTI = 0.40 s
Applications
Relay coordination studies
Substation protection design
Distribution feeder protection
Fuse-relay coordination
Power system reliability analysis
Note
The required coordination interval depends on relay technology, breaker operating time, relay overshoot, CT errors, and utility standards. Typical coordination margins are often between 0.2 s and 0.5 s, although the exact value varies by application and protection philosophy.