Breakers on a TCC
Thermal-magnetic regions and LSI(G) settings on LVPCBs.
Breakers come in two flavors that draw very different shapes on a TCC.
The first you’ve already met — the thermal-magnetic MCCB from Lesson 1, with a single inverse-time long-time region and a vertical magnetic drop. Cheap, ubiquitous, and tuneable only within a narrow band.
The second is the Low-Voltage Power Circuit Breaker (LVPCB) with an electronic-trip unit. Instead of a single curve, it gives you a staircase of independently-adjustable regions — and the ability to move each one without affecting the others.
This lesson is about reading that staircase.
Thermal-magnetic recap
A TM MCCB has exactly two elements doing the protection work:
- A bimetallic thermal element that responds to long-duration overload. Its trip time is roughly I²t — the long sloping band on the upper part of the curve.
- A magnetic actuator that responds to instantaneous current. Its pickup is the vertical drop; below it, the breaker waits; above it, the breaker goes immediately.
You can shift the long-time band a little (within the breaker’s trip-curve tolerance) and you can set the magnetic pickup in adjustable-trip versions, but that’s it. Two knobs, two regions.
The LVPCB advantage
An electronic-trip LVPCB gives you up to four independent regions on one device:
- L — Long-time. Same role as the thermal element, but tuneable in both pickup (where it starts) and delay (how slow it is).
- S — Short-time. A new middle region. Lets the breaker wait on currents above the L pickup band but below an instantaneous threshold. This is the region that makes selective coordination possible between LVPCBs in series.
- I — Instantaneous. Same role as the magnetic element. On many LVPCBs the instantaneous can be turned off entirely when coordination needs every cycle of delay the device can give.
- G — Ground-fault. A residual-current trip, on its own pickup and delay. Not drawn on a phase TCC. (We’ll cover ground-fault curves later in the broad-scope expansion.)
Each element traces a separate segment of the device’s overall curve, and each one moves independently as you turn its dial.
Drag the dials
Below is a single LVPCB with sensor rating, L pickup + delay, S pickup + delay, and I pickup all exposed. Move them one at a time and watch what each slider does to the staircase.
Here’s what each control does to the curve:
- L pickup — slides the entire long-time region left or right. The L pickup is set as a fraction of the sensor rating; at 0.8× a 1600 A sensor, the breaker won’t start its long-time trip until current exceeds 1280 A.
- L delay (@ 6× pickup) — pulls the long-time band up or down in time. A larger delay means the breaker tolerates overload longer before tripping. The convention is to specify the delay at 6× the L pickup current.
- S pickup — moves the short-time band horizontally. The S band starts at S pickup and ends at I pickup (or extends to the right edge if I is off).
- S delay — sets the flat shelf height for fixed-time short-time. This is the region where you buy coordination room.
- S I²t in — toggles the short-time band from flat to sloped (I²t-shaped). The sloped version trades coordination room for more conservative damage-curve protection at lower current levels inside the S band.
- I pickup — moves the final vertical drop. Drag the slider all the way to 0 (OFF) to disable instantaneous entirely — the S band then runs to the right edge of the plot. Real LVPCBs have a fixed “instantaneous override” you can’t disable, but it’s far enough to the right that it doesn’t change coordination plots.
Read the staircase
For any current you read off the x-axis, the breaker’s clearing time is set by whichever region that current falls into:
| Current range | Region | Trip set by |
|---|---|---|
| Below L pickup | (no trip) | — |
| L pickup → S pickup | Long-time | L delay and L pickup |
| S pickup → I pickup | Short-time | S delay (flat or sloped) |
| Above I pickup | Instantaneous | I pickup |
When you read a coordinated curve for a feeder LVPCB, the L band covers normal overload, the S band gives you a small window of intentional delay to let downstream devices clear faults first, and I provides the fast tripping for severe near-bus faults. Tune each slider for the role it plays.
A note on “INST OFF”
On the very largest LVPCBs serving 480 V switchgear — typically those backed by transformers downstream — protection engineers will intentionally set instantaneous to OFF. This forces the breaker to wait for the short-time delay even on the worst available faults, which buys the downstream device the maximum possible time to clear.
This is only safe when the breaker’s short-time withstand rating covers the available fault — i.e., the breaker can physically survive the bolted-fault current for the full short-time delay without damage. Read this off the breaker nameplate; don’t guess.
Looking ahead
You can now read a TCC for the three device types that cover almost all low-voltage protection work: thermal-magnetic MCCBs, fuses, and electronic-trip LVPCBs.
Module 2 starts with the question that motivated this whole tutorial — what is selective coordination, and why does it matter? — and from there we’ll put two and three devices on the same plot and tune their curves so that the right one opens, and only the right one.