Protection & TCC curves

What it does

A time–current characteristic (TCC) plot shows each protective device that can see a given fault, on one log-log chart. The horizontal axis is current. The vertical axis is operating time.

Grading means this: arrange the device curves so that, for each possible fault, the device nearest to the fault operates first. Each device upstream waits.

Phasor draws each curve from the device settings, not from a traced image. When you change a time multiplier, the plot changes immediately. Phasor calculates the grading margins again at the same time.

Words used on this page

Supported device characteristics

• IEC 60255 inverse-time curves — standard inverse, very inverse, extremely inverse and long-time inverse, with the standard constants and a settable time multiplier.
• IEEE C37.112 curves — moderately inverse, very inverse and extremely inverse.
• Definite time — a pickup and a fixed delay.
• Instantaneous elements, with any transient blocking.
• Fuse curves — the minimum melting band and the total clearing band, from manufacturer data.
• LV breaker trip units — the long-time, short-time and instantaneous segments, with I²t on or off.

TCC plot with a downstream-to-upstream ladder, fault window, and coordination check

How to grade a scheme

1. Run a short circuit study first. Grading has no meaning without the current range in which the devices must operate. You need two values at each device: the maximum fault current (does the instantaneous element reach too far?) and the minimum fault current (does the inverse element operate at all?).
2. Assemble the device chain. Select a path from a fault location back to the source. Each device in series on that path belongs on one plot.
3. Set the pickup from the load current, not from the fault current. The inverse element must be above the maximum continuous load and above the expected inrush. It must also be below the minimum fault current. If these two limits cross, the problem is in the network, not in the settings.
4. Grade from the far end toward the source. Start at the device farthest from the source. Add a margin at each stage.
5. Check the margin at the current where it matters. That current is the maximum fault current at the downstream device. There, the two curves are nearest to each other.
6. Check the thermal limits. Put the cable and transformer damage curves on the plot. A scheme that protects the switchgear but not the cable is not complete.

Grading margins

The margin between two curves must cover the operating time of the downstream device, plus real physical delays:

The sum is usually 0.3–0.4 s between electromechanical relays, and 0.2–0.3 s between modern numerical relays.

Phasor calculates the real margin at each current. It marks each point where the margin is below the value that you set for the project. You do not read the margin from the plot by eye.

Inputs and outputs

Inputs

Outputs

Notes

Grade the earth-fault scheme separately. Phase elements and earth elements have their own curves, their own pickups and their own fault levels. Do not grade the phase scheme and assume that the earth scheme is also correct. That assumption often lets a high-impedance fault stay uncleared.

Earth-fault loop impedance is the LV equivalent of a minimum-current check. A final circuit must disconnect within the required time. For this, the total loop impedance Z_s must be low enough. Then the fault current reaches the disconnection threshold of the device. Phasor calculates Z_s along the path. It compares the result with the required disconnection time.