Miniature Circuit Breakers Selection Guide

On a miniature circuit breaker, the curve letter sets the magnetic tripping threshold: B 3-5, C 5-10, D 10-20 In; check tripping curve and breaking capacity.

This guide explains what to look for when choosing products in the Miniature Circuit Breakers category.

What the tripping curves B, C and D mean

The curve letter gives the threshold of the magnetic release, which trips instantly in a short circuit, as a multiple of the rated current (In); the limits of the thermal release, which operates on overload, are the same for the B, C and D curves. According to IEC/EN 60898-1:

• B curve: the threshold is between 3 In and 5 In. It is used in circuits with low inrush current, such as lighting and socket circuits in homes and offices.
• C curve: the threshold is between 5 In and 10 In. It is for loads that draw a brief high current when switched on, such as motorized equipment and lamp groups.
• D curve: the threshold is between 10 In and 20 In; it is for loads with a very high inrush current, such as transformers. The product data in this category includes the B and C curves.

The device does not trip instantaneously below the lower limit, and trips in less than 0.1 seconds from the upper limit: for a 16 A device, these limits are 48 A and 80 A on the B curve and 80 A and 160 A on the C curve. For this reason, at the same rated current, the B curve allows a higher loop impedance than the C curve, that is, a longer line.

What to look at when choosing a miniature circuit breaker

1. Rated current: according to the overload rule in IEC 60364-4-43, the design current of the line must not exceed the rated current of the breaker, and the rated current of the breaker must not exceed the current carrying capacity of the cable. For a preliminary assessment you can use the Current and Fuse Calculation from Active Power and Cable Current Carrying Capacity Calculation tools; the exact value is determined in the project.
2. Number of poles: 1P trips a single phase conductor, while 3P trips all three phases together. In 2P and 4P models all poles are protected; in 1P+N and 3P+N models the neutral is switched together with the phase, but overcurrent protection is on the phase poles only.
3. Breaking capacity: it must not be lower than the short-circuit current calculated at the point where the device is connected.
4. Ambient and arrangement: the thermal tripping of devices with B, C and D curves is set for a reference ambient temperature of 30 °C. In a hotter enclosure, and for devices mounted side by side without gaps and fully loaded, the continuously carried current drops; the derating factor is taken from the manufacturer's catalog. ABB gives 0.77 for eight adjacent devices in the S 200 series.

How to read the label

• C16: the curve letter and the rated current in amperes.
• The number in the rectangle (4500, 6000, 10000): the rated short-circuit capacity (Icn) in amperes according to IEC/EN 60898-1.
• The number in the square: the energy limiting class, with a value from 1 to 3. Class 3 is the class in which the energy allowed to pass through to the cable in a short circuit (I²t) is lowest.

IEC/EN 60898-1 is for building installation devices to be used by untrained persons. IEC/EN 60947-2 covers circuit breakers to be used by trained personnel and gives the breaking capacity as two values: ultimate (Icu) and service (Ics). If the product data lists both standards together, read the kA value of each separately.

Miniature circuit breakers for DC circuits

Because direct current does not pass through zero, extinguishing the arc is harder than with alternating current; for this reason a device's AC and DC values are given separately. Among the series in the category made for use on direct current are ABB S 200 M UC (the models whose names end in UC), Siemens 5SY5, Schneider Electric C60H-DC and the ABB S800PV-SP listed as Solar DC Miniature Circuit Breaker. The SP value in the tripping characteristic filter belongs to this last series.

According to the ABB catalog, UC (universal current) means AC and DC use; the S 200 M UC is used up to 220 V DC with a single pole and up to 440 V DC when two poles are connected in series. Because it contains permanent magnets that help extinguish the arc, polarity (+/−) must be observed when connecting. For every DC device, check the system voltage, the series connection diagram and the polarity markings in the manufacturer's data.

Protections a miniature circuit breaker does not cover

• Earth leakage: Residual Current Devices or Miniature Circuit Breakers with Residual Current Protection, which combine the two functions in a single housing.
• Motor overload: the tripping current of a miniature circuit breaker cannot be adjusted; Motor Protection Circuit Breakers, whose thermal setting is made according to the motor's rated current, are a separate product.
• Above 125 A and adjustable tripping: Molded Case Circuit Breakers.

Brands in this category

• ABB: FMSL (4.5 kA), FMS, SH 200 and S 200 (6 kA), SH 200 M and S 200 M (10 kA) series, and S 800 for high currents. The product name is read in the order series, number of poles, curve and current: S 203 M-C 16 is a three-pole, C-curve, 16 A device.
• Schneider Electric: Easy9 (3 kA and 4.5 kA), Easy9 Pro (6 kA and 10 kA) and Acti9 series; in Acti9 the iC60N is listed as 6 kA and the iC60H as 10 kA.
• Siemens: 5SL3 (4.5 kA), 5SL6 (6 kA) and 5SL4 (10 kA) series, and 5SP4 for high currents. In the order code, the digit after the series gives the pole arrangement and the digit after the hyphen gives the curve (6 = B, 7 = C): 5SL6116-7 is a single-pole, 16 A, C-curve device.
• Chint: NXB-63S (4.5 kA) and, for high currents, DZ-158 (10 kA) series.

Ordering notes

• Products are sold per piece. You can reach a product directly by typing the manufacturer order code (such as 5SL3116-6YA or EZ9F14116) into the search.
• Some products are listed as made to order; for these, the lead time is confirmed before the order.
• Fuse Boxes and MCB Panels are chosen by number of modules. One module is 18 mm; up to 63 A, a single-pole miniature circuit breaker takes one module and a three-pole one takes three. 1P+N models are one or two modules depending on the series, and at 80 A and above the pole width can rise to 1.5 modules; verify the dimension from the product data.
• MCB busbars are in the Busbars and Distribution Blocks category.

Frequently asked questions

What is the difference between a B curve and a C curve miniature circuit breaker?

Overload behavior is the same on both curves; the difference is in the threshold of instantaneous tripping in a short circuit. According to IEC/EN 60898-1, this threshold is 3 to 5 times the rated current on the B curve and 5 to 10 times on the C curve. A B curve may trip unnecessarily on a load that draws high current when switched on. A C curve requires a higher short-circuit current for instantaneous tripping; on long lines this condition is verified by project calculation.

Does a 16 A miniature circuit breaker trip at exactly 16 A?

No. According to IEC/EN 60898-1, a device with B, C and D curves up to 63 A must carry 1.13 times its rated current for one hour without tripping at the 30 °C reference temperature, and must trip within one hour at 1.45 times. For a 16 A device these limits are about 18.1 A and 23.2 A. At currents between the two values the device may or may not trip within an hour; in a hot enclosure and in side-by-side mounting the limits shift downward.

What do 4.5 kA, 6 kA and 10 kA on a miniature circuit breaker mean?

This value is the breaking capacity: the largest short-circuit current the device can interrupt. According to IEC/EN 60898-1 it is called Icn and is written in amperes in a rectangle on the front of the device (4500, 6000, 10000). The selected value must not be lower than the short-circuit current calculated at the point where the breaker will be installed. This current depends on the transformer power and on the length and cross-section of the supply line; it is calculated in the project.

Can a higher-amp breaker be installed in place of one that trips frequently?

The rated current is chosen not only according to the load but also according to the cable. According to the rule in IEC 60364-4-43, the rated current of the breaker must not exceed the current carrying capacity of the cable; otherwise the cable is left unprotected against overload. If the breaker trips often, first examine the load on the line, whether the trip is thermal or magnetic, and the curve type; the rated current should be changed only if the cable cross-section is suitable and in accordance with the project.

Can an AC miniature circuit breaker be used in DC circuits such as solar panels?

Only within the DC voltage and connection diagram that the manufacturer specifies for that device. Because direct current does not pass through zero, the arc is harder to extinguish and DC values are defined separately from AC values. For example, for the standard S 200 series, ABB gives a limit of 72 V DC, or 125 V DC with two poles connected in series. In photovoltaic lines, models made for DC are chosen according to the system voltage; on a device with a polarity marking, the connection direction must be observed.

What is the difference between a miniature circuit breaker and a molded case circuit breaker?

A miniature circuit breaker is a modular device mounted on a rail, with tripping values fixed at the factory; IEC/EN 60898-1 limits these devices to 125 A and 25 kA. A molded case circuit breaker is a circuit breaker made according to IEC/EN 60947-2, reaching higher currents and breaking capacities, and in many models its tripping can be adjusted. Typically a molded case circuit breaker is used at the main incomer of the enclosure and on large outgoing feeders, and miniature circuit breakers on the final circuits.