A Molded Case Circuit Breaker (MCCB) is the workhorse of low-voltage protection. Inside one rugged moulded housing it carries two completely different tripping mechanisms — a slow thermal trip for overloads and a fast magnetic trip for short circuits — and the Time-Current Characteristic (TCC) curve is simply the picture of how those two mechanisms share the work. This post explains both operations and how to read the TCC curve.
What an MCCB is
An MCCB provides heavy-duty low-voltage protection — up to 1,600 A, with breaking capacities of 16–100 kA — inside a rugged moulded insulating housing made of thermoset polyester resin. Both trip settings are adjustable: the thermal (long-time) pickup Ir = 0.4–1.0 × In and the magnetic (instantaneous) pickup Im = 2–10 × In. A spring-toggle mechanism and an arc chute complete the breaker.
The TCC curve
Figure 2.1: MCCB Time-Current Characteristic (TCC) curve. Left: the sloping inverse-time thermal region. Right: the flat instantaneous magnetic region below 20 ms. (Diagram redrawn for this post; schematic, not to scale — magnetic pickup shown at 5 × In, mid-range.)
1. Thermal operation — the long-time (overload) region
Under a sustained overload (about 1.05 to 6 × Ir), current heats a bimetallic strip (or, in electronic-trip MCCBs, an I²t sensor). The strip deflects in proportion to I²t, and when the deflection is enough it unlatches the toggle mechanism and opens the breaker.
On the log-log TCC curve this is the upper inverse-time sloping segment: the higher the overload current, the faster the trip. A small overload (just above pickup) may take minutes; a 6 × Ir overload trips in about a second. That shape is deliberate — it lets a motor draw its starting current briefly while still protecting cables and equipment against a genuine sustained overload.
2. Magnetic operation — the instantaneous (short-circuit) region
Above the adjustable magnetic pickup threshold, a solenoid electromagnet pulls its armature in instantaneously and unlatches the toggle mechanism within 10–20 ms. There is no intentional delay: a short circuit must be cleared before it can damage anything.
On the TCC curve this is the vertical drop followed by the flat horizontal bottom region: once current passes Im, tripping time stops depending on current and stays at roughly 10–20 ms, however large the fault current is.
How to read the curve — a worked example
Take an MCCB rated In = 100 A, set to Ir = 80 A (0.8 × In) and Im = 500 A (5 × In):
- 90 A (1.1 × Ir) — just above pickup: the bimetal heats slowly; the breaker trips after a long delay, or not at all if the load drops first.
- 300 A (3.75 × Ir) — deep in the thermal region: inverse-time trip, in seconds.
- 800 A (8 × In, above Im) — magnetic region: the solenoid acts in 10–20 ms, regardless of exactly how large the current is.
Key points to remember
- An MCCB has two trips in one box: thermal (bimetal, I²t, slow, inverse-time) for overloads and magnetic (solenoid, fast, 10–20 ms) for short circuits.
- Thermal pickup Ir is adjustable from 0.4 to 1.0 × In; magnetic pickup Im is adjustable from 2 to 10 × In.
- On the log-log TCC curve, the thermal region slopes down (higher current, faster trip) and the magnetic region is flat at 10–20 ms.
- Ratings to remember: up to 1,600 A, breaking capacity 16–100 kA, moulded thermoset polyester resin housing.
Based on Power System Protection & Switchgear, Paper II, Q2(a), Exam 11 June 2026.