A miniature circuit breaker (MCB) protects a low-voltage circuit against overload and short circuit. Its ampere rating tells you the continuous-current limit, while the curve letter tells you how much short-duration current the breaker will tolerate before its magnetic mechanism trips. The curve is therefore a load-matching parameter, not a quality grade.

How B, C and D curves differ

For example, a C16 MCB enters its magnetic trip region between about 80 A and 160 A. A D16 enters between about 160 A and 320 A. This does not mean that a C16 can carry 80 A continuously: the thermal element still trips on sustained overload according to its inverse-time characteristic.

Decode a real PL10 model

The Eaton Moeller xPole PL10 family is published in B, C and D curves with 1-, 2-, 3- and 4-pole configurations. The listed PL10 range uses a 10 kA rated switching capacity under IEC/EN 60898-1; always confirm the exact catalog page and the installation standard for the market where the breaker will be used.

Complete selection method

1. Determine design current Ib from the connected load and duty cycle.

2. Select conductor size and installation method, then confirm the cable current-carrying capacity Iz after temperature, grouping and enclosure derating.

3. Choose breaker rated current In so the normal design relationship is satisfied: Ib <= In <= Iz, subject to the applicable wiring rules.

4. Obtain the load's measured or manufacturer-stated inrush current and duration. Select B, C or D so normal start-up remains below the nuisance-trip region.

5. Calculate or measure prospective short-circuit current and earth-fault loop impedance. Confirm the minimum fault current will operate the chosen curve within the required disconnection time.

6. Confirm breaking capacity (for example 6 kA or 10 kA) is not lower than the available prospective fault current.

7. Select pole count, neutral switching arrangement, isolation requirement and accessories such as auxiliary contacts, shunt trip or undervoltage release.

8. Check discrimination or selectivity with upstream and downstream protective devices using manufacturer tables or time-current curves.

Common mistakes

• Moving from C to D only to stop nuisance tripping, without confirming that the higher magnetic threshold can still clear the worst-case fault.

• Choosing a 32 A breaker because the load is 32 A while ignoring conductor derating inside a warm, crowded enclosure.

• Assuming every breaker carrying the same B, C or D letter has the same breaking capacity, accessories or certification.

• Using a domestic IEC 60898-1 device where the project specification requires an industrial IEC 60947-2 device.

Frequently Asked Questions

Which MCB curve is the best general-purpose choice?

Type C is a common starting point for commercial and light-industrial circuits because it tolerates moderate inrush. It is not automatically correct; cable protection, loop impedance and the actual load profile still govern the final choice.

Can I replace a C16 with a D16?

Only after verifying the fault-current and disconnection-time requirements. The current rating remains 16 A, but the D curve needs substantially more instantaneous current to trip magnetically.

What is the difference between PL10-C32 and PL10-C32/3?

PL10-C32 is the one-pole 32 A C-curve model; /3 denotes a three-pole version. A /2 or /4 suffix identifies two or four poles.

Are Type K and Type Z the same as B, C and D?

No. Z is a more sensitive curve used for electronic or semiconductor loads; K is commonly used for certain motor and inductive applications. Availability and exact bands depend on the manufacturer and applicable standard.

Bottom line

Select the curve from the load's inrush and the circuit's minimum fault current, then select the exact product model from current, poles, breaking capacity and standard. In the Eaton PL10 range, PL10-B16, PL10-C16 and PL10-D16 share the same 16 A rating but are not interchangeable engineering decisions.

Need a PL10 B-, C- or D-curve model checked against your load and pole configuration? Send ASBEAM the system voltage, load current, inrush data, pole count and available fault current for a model-level review.

Technical references

• ASBEAM - Eaton PL10 B/C/D model range

• ASBEAM - Eaton PL10-C32 product page