"I^s T" refers to the inverse time delay setting for ground fault protection. Whether it should be set to ON or OFF depends on your specific protection requirements and coordination study.

Understanding I^s T I^s T - Inverse Time

This setting introduces an inverse time characteristic to the ground fault trip curve. An inverse time characteristic means that the trip time decreases as the fault current increases.

When ON

When I^s T is enabled (ON), the ground fault trip time will be inversely proportional to the fault current. Higher fault currents will cause faster tripping.

When OFF

When I^s T is disabled (OFF), the ground fault trip time becomes definite (constant) after exceeding a certain current threshold determined by the instantaneous setting. This means the breaker trips at the defined time delay, regardless of the fault current magnitude above the pickup setting.

Why it's OFF by Default (Usually) Coordination

In many systems, the default is to have I^s T OFF for ground fault to allow for better coordination with upstream and downstream protective devices. This prevents nuisance tripping. If multiple devices have inverse time curves, it can be difficult to ensure the correct device trips first during a fault.

When to Consider Turning I^s T ON



Coordination Study

If a coordination study reveals that an inverse time setting is needed to provide faster tripping for specific types of faults while maintaining coordination, then I^s T might be enabled.

Specific Application Needs

In very specific applications, where faster tripping at higher ground fault levels is desired for equipment protection and stability, inverse time may be considered. For example, transformer protection.*

Selective Coordination

Inverse time delay can sometimes be used to help achieve selective coordination, especially with downstream devices that also have ground fault protection. However, it requires very careful analysis.

Recommendations1. Consult a Qualified Electrical Engineer

Determining the correct ground fault settings requires a coordination study by a qualified electrical engineer familiar with your specific electrical system. They will analyze fault current levels, upstream and downstream protection, and equipment damage curves to determine the appropriate settings.2.

Coordination Study is Essential

Never blindly enable or disable I^s T. A coordination study is critical to ensure proper protection and avoid nuisance tripping. The engineer will look at the Time-Current Curves (TCCs) to make sure that your ground fault protection is coordinated with other protective devices.3.

Understand the Consequences

Changing protection settings without a proper understanding of the system can lead to equipment damage, power outages, and safety hazards.

In Summary

The default setting of I^s T being OFF is common. It's not an indication of a problem but a safety precaution until the electrical system's coordination requirements are properly evaluated. Get professional help for setting your protection devices! Flag for review