Protection Coordination and TCC: What Time-Current Curves Can Prove

A time-current curve (TCC) shows how a protective device is expected to operate over current and time. Overlaying upstream and downstream curves is a powerful coordination check, but a clean-looking graph is not automatically proof of total selectivity, certified cascading or equipment SCCR.

1. Reading a time-current curve

The horizontal axis is current, often shown as amperes or multiples of rated current. The vertical axis is operating or clearing time. Real device curves are usually tolerance bands, not infinitely thin lines.

A positive margin at one current point is useful, but selectivity must be assessed across the relevant current range and against the manufacturer's rules for the actual device combination.

2. Current, time, energy and logic selectivity

• Current-based: upstream pickup thresholds are set above downstream thresholds.
• Time-based: upstream operation is intentionally delayed so the downstream device clears first.
• Energy-based: current-limiting behavior and let-through energy are used to establish coordination in high-current regions.
• Logic-based: communicating protective devices coordinate using interlocking or zone-selective logic.

Total selectivity applies through the defined fault-current range; partial selectivity applies only up to a stated selectivity limit. The available short-circuit current at the actual installation point therefore matters.

3. Worked example: timing margin at one fault level

Assume an illustrative 3 kA fault at a downstream board. Manufacturer curves for the selected devices show the downstream protective device clearing between 0.04 s and 0.07 s, while the upstream device operates between 0.20 s and 0.35 s at the same current.

At 3 kA only, the tolerance bands have a 0.13 s minimum separation in this illustrative example. That does not establish total selectivity: the curves must still be checked at lower overload currents and higher short-circuit currents, including the instantaneous/current-limiting region.

Open FaultChain Protection Analysis

4. Assumptions and limits: what a TCC does not prove by itself

• Breaking capacity: verify that each protective device can interrupt the available fault current at its location.
• Equipment SCCR: an assembly rating may depend on certified combinations and cannot be inferred from curve spacing alone.
• Current-limiting let-through: use manufacturer peak-current and total-clearing I²t data for the actual prospective fault current.
• Cascading/back-up protection: rely on tested or manufacturer-published coordination data where the design uses such a combination.
• All source modes: grid and generator operation can produce very different fault levels and must both be checked where applicable.

5. Practical coordination workflow

1. Calculate maximum and minimum prospective fault currents at the relevant nodes.
2. Verify device interrupting/breaking ratings and equipment withstand/SCCR independently.
3. Overlay the actual manufacturer TCC bands for downstream and upstream devices.
4. Check overload, short-circuit and earth-fault regions required by the project.
5. Review instantaneous/current-limiting behavior with manufacturer selectivity or let-through data.
6. Repeat for alternate sources and operating modes.
7. Record the source/revision of every curve, setting and coordination table used.

Adjacent checks

Start with Transformer Short-Circuit Current and Short-Circuit Current vs SCCR. Use the I²t Let-Through Energy guide when current-limiting protection changes the downstream thermal question.

Engineering references

Schneider Electric Electrical Installation Guide — Coordination between circuit breakers and the current Selectivity, Cascading and Coordination Guide 2026. The worked timing values above are intentionally illustrative and are not manufacturer device data.