Reading a TCC
Axes, log-log scale, and the regions of the plot.
A Time-Current Curve (TCC) is the protective-device engineer’s primary diagnostic tool. It says, for every fault current a device might see, how long the device will take to open the circuit. Everything in selective coordination — every overlap, every miscoordination call — ultimately reduces to comparing curves on one of these plots.
This lesson teaches you to read them.
The axes
A TCC has two axes:
- The horizontal axis is current, in amperes. It runs from a small fraction of a device’s continuous rating all the way up to the available short-circuit current at the device’s bus — usually four to five decades wide.
- The vertical axis is time, in seconds. It runs from one or two power-frequency cycles at the bottom (~0.01 s) up to many minutes at the top.
Both axes are logarithmic. A linear axis would compress everything useful into a sliver of the plot, because faults can happen at five amps or at fifty thousand amps and we care about the device’s behavior across that entire range.
Read it like this: pick a current on the x-axis. Trace straight up. The curve tells you the longest time the device will take to interrupt that current. Anything to the right of the curve trips faster; anything to the left, slower (or not at all).
A real device on the plot
Below is a generic 400 A thermal-magnetic molded-case circuit breaker. Drag the sliders — the curve redraws in real time.
Three things to notice as you move the sliders:
- The long-time region — the curved part at the top — is the thermal element responding to overload. At 1.5× rating the breaker takes hundreds of seconds; at 6× rating it trips in single- digit seconds. The shape is roughly I²t: doubling the current shortens the trip time by four.
- The vertical drop is the instantaneous magnetic element. When current crosses the instantaneous pickup (typically 5–12× the rating), the breaker stops waiting and opens within one cycle.
- The plateau at the bottom is the device’s minimum clearing time — the floor set by mechanical contact-parting and arc-extinction. No matter how high the fault current goes, the breaker can’t open faster than that.
Four regions, one device
Every TCC for a protective device — fuse, breaker, relay — divides into the same four regions:
| Region | Sets clearing time by | Visible feature on the plot |
|---|---|---|
| Below pickup (~1.1× rating) | Device doesn’t see overload | Curve doesn’t extend below pickup |
| Long-time / overload | Thermal element (or fuse element heating) | Sloped band, sometimes I²t |
| Short-circuit | Magnetic element / fuse melting | Steep drop or vertical line |
| Floor | Mechanical opening + arc extinction | Horizontal plateau |
In later lessons we’ll add fuses (which trace a band rather than a single curve — min-melt and max-clear) and electronic-trip LVPCBs (which expose Long-time, Short-time, Instantaneous, and Ground-fault elements you can tune independently). The reading rules stay the same. The plot stays the same. Only the shape of the curve changes.
Try this
Before you move on:
- Slide Rating down to 100 A. The whole curve shifts left — the device is more sensitive, so the plot’s center of action moves toward lower currents.
- Push Instantaneous trip out to 12×. The vertical drop moves to the right. The device now waits longer before going to instantaneous — useful when there’s a downstream device you want to clear the fault first.
- Drop Long-time band shift to 0.6. The thermal region drops. The device trips sooner on overload, at the cost of nuisance tripping near continuous load.
Each of those slider moves is a real decision a protection engineer makes in the field. Coordination — which we’ll get to in Module 2 — is the art of making those decisions across an entire one-line so that the right device opens, and only the right device.