Reading a circuit breaker trip curve

Every miniature circuit breaker has two ratings printed on the front: the current (16 A) and the curve letter (C). The current gets all the attention. The letter is what decides whether the breaker holds or trips, and it is the reason two 16 A breakers behave completely differently on the same motor.

A trip curve plots current on the horizontal axis and time on the vertical axis, both logarithmic. Reading it takes about a minute once you know which part of the line does what.

Two protection elements in one device

A thermal-magnetic breaker has two independent mechanisms:

• Thermal element - a bimetal strip heated by the load current. It protects against overload, and it is deliberately slow: a 20 % overload should run for a while, a 100 % overload should not.
• Magnetic element - a solenoid that trips instantly above a current threshold. It protects against short circuit.

On the curve, the thermal element is the sloping band on the left, and the magnetic element is the vertical wall on the right. Between them is a region where the breaker may trip or may not, depending on tolerances and temperature - that ambiguity is normal and it is why the curve is drawn as a band, not a line.

What the letters mean

For breakers to IEC 60898-1 (the household and light commercial standard), the letter sets where the magnetic trip sits:

The gap between B and D is a factor of four. That is why "upgrade the breaker" is not a fix: replacing a C16 with a B16 on a motor circuit makes the nuisance tripping worse, not better.

What the thermal band guarantees

IEC 60898-1 sets three test points that every MCB must satisfy, and they are the numbers worth remembering:

• 1.13 In - must not trip within one hour (the conventional non-tripping current)
• 1.45 In - must trip within one hour
• 2.55 In - must trip between 1 s and 60 s for In up to 32 A

So a C16 carrying 18 A (1.13 In) should hold for at least an hour, while 23 A (1.45 In) must trip inside an hour. If a breaker trips at 1.1 In on a warm day, the bimetal is either degraded or the enclosure is too hot - ambient temperature shifts the whole thermal band, and a row of breakers side by side derates each other.

Why a 3 kW motor trips a C16

A 3 kW motor at 400 V draws about 6.5 A full load. A 16 A C-curve breaker looks generous. But direct-on-line starting draws 6-8 times full load for a few hundred milliseconds - roughly 45 A, or 2.8 x In.

On the curve, 2.8 x In sits in the thermal band, not the magnetic wall. The trip time there is seconds to minutes, so the breaker usually survives the start. The failures happen with:

• Larger motors: 5.5 kW gives ~11 A full load and 70 A starting, which is 4.4 x In on a C16 - closer to the magnetic wall, and inside the tolerance band of a B-curve device.
• Frequent starts: the bimetal does not fully cool between cycles, so each start begins from a hotter strip.
• High ambient: a panel at 50 °C moves the effective trip point down by 10-15 %.

The correct fix is a motor circuit breaker with a magnetic-only setting above the starting current, plus a separate overload relay set to the motor's full-load current. A motor circuit breaker is designed for exactly this: the thermal element is replaced by a Class 10 overload characteristic matched to the motor, and the magnetic trip is set at 13 x In by default.

MCCB curves are not letters

Above 125 A the rules change. A moulded case circuit breaker follows IEC 60947-2 and its curve is described by settings, not a letter:

• L (long time) - Ir as a fraction of In (typically 0.4-1.0) and a time delay tr. This is the overload protection.
• S (short time) - Isd as a multiple of Ir, with a short delay tsd. This is the first stage of short-circuit protection, and it is optional.
• I (instantaneous) - Ii, the current above which the breaker trips with no intentional delay.

An electronic trip unit lets you move each of these independently. That flexibility is what makes selectivity possible: an upstream 250 A breaker with a 0.3 s short-time delay will ride through a downstream fault that the 63 A breaker clears in 20 ms.

Reading the curve for selectivity

Two breakers in series are selective when only the downstream one trips. Two ways to get there:

1. Current selectivity - the upstream instantaneous setting is above the maximum fault current the downstream device can pass. Simple, but needs a large ratio between ratings.
2. Time selectivity - the upstream device has a deliberate short-time delay that outlasts the downstream clearing time. This is what electronic trip units are for, and it is why an air circuit breaker at the main board can sit at 0.4 s while everything downstream clears faster.

Manufacturers publish selectivity tables for specific pairs. Using the curves alone works for a first estimate, but the tables account for tolerances and for the dynamic behaviour during the first half-cycle - worth checking before commissioning.

The five numbers to read off any curve

When you only have the data sheet and two minutes:

1. In - the rating, and whether it is at 30 °C or 40 °C ambient.
2. Magnetic threshold - the vertical wall, as a multiple of In.
3. Thermal test points - 1.13 In and 1.45 In if it is an MCB.
4. Breaking capacity - Icn in kA, which is a separate rating and has nothing to do with the curve.
5. Derating - the multiplier for your enclosure temperature and for ganged installation.

The curve tells you what the breaker does. The breaking capacity tells you whether it survives doing it.