The Definitive Guide to Circuit Breakers 2026: AC/DC Sizing

Introduction: New Challenges in Electrical Protection in 2026

The rise of residential and commercial photovoltaic installations, coupled with the widespread integration of battery energy storage systems (BESS) and hybrid inverters, is radically transforming the requirements for electrical protection. The circuit breaker is no longer simply a modern fuse: it has become a strategic safety component standardised in accordance with IEC 60898-1 (domestic) and IEC 60947-2 (industrial) standards, in a context where short-circuit currents can reach several kA and where direct current (DC) imposes specific physical constraints.

This comprehensive guide is aimed at installers, design offices and knowledgeable individuals who wish to gain an in-depth understanding of the sizing, selection and limitations of circuit breakers in AC (alternating current) and DC (direct current) installations, in accordance with French (NF C 15-100) and Belgian (RGIE) standards. We will also cover technological developments for 2026: smart circuit breakers, AFDD (Arc Fault Detection Device), solid-state circuit breakers (SSCB) and predictive maintenance.

Fundamental principle: A circuit breaker does not merely protect against overload; it must be capable of interrupting the maximum short-circuit current at any point in the installation, whilst ensuring selectivity to maintain service continuity.

1. What is a Circuit Breaker? Function and Internal Architecture

1.1 Definition and Role

A circuit breaker is a modular protective device capable of automatically interrupting an electrical circuit in the event of an overcurrent (overload or short circuit). Unlike a fuse, which is destroyed each time it trips, a circuit breaker can be manually reset and reused.

The circuit breaker protects two critical components:

• Electrical cables: prevents excessive heating that could lead to a fire
• Connected equipment: limits mechanical and thermal stress on devices (inverters, motors, transformers)

⚠️ Warning: A standard circuit breaker does NOT protect against:

• Earth leakage (indirect contact) → requires a 30 mAresidual current device
• Transient overvoltages (lightning, switching) → requires a surge arrester
• Prolonged undervoltages → requires a voltage monitoring relay
• Low-current series arcs (loose connection) → requires an AFDD

1.2 Internal Architecture: The Dual Thermal + Magnetic Mechanism

A thermal-magnetic circuit breaker combines two complementary protection mechanisms operating on different time scales:

1. Thermal Protection (Bimetal):
A bimetallic strip consisting of two metals with different coefficients of thermal expansion (often iron-nickel and brass) gradually deforms under the effect of the heat generated by an overload current (I = 1.1 to 1.5 ×_In). This deformation has a thermal inertia that mimics that of the copper cable it protects.

According to standard IEC 60898-1, the circuit breaker must not trip before 1 hour at 1.13 ×_In, but must trip in less than one hour at 1.45 ×_In. This tolerance range allows normal load variations to be absorbed without unwanted tripping. Tripping time: from a few seconds to several minutes depending on the current.

2. Magnetic Protection (Coil/Solenoid):
A coil generates an intense magnetic field during a short circuit (I ≥ 3 to 20 ×_In depending on the curve). The field activates a movable ferromagnetic core which instantly strikes the tripping mechanism. Tripping time: < 10 ms (well before the adiabatic heating I²t causes the conductors to melt).

1.3 The Thermodynamics of the Electric Arc: Why AC and DC Are Two Different Worlds

When a live circuit is opened, the circuit breaker’s contacts separate. The contact area decreases, increasing the current density until the metal melts locally. A conductive plasma forms:the electric arc, whose core temperature reaches 10,000 to 20,000°C. This arc maintains the flow of current through the ionised air.

Critical consequence: In DC, circuit breakers use magnetic arc chambers (Lorentz force) to stretch the arc towards flat splitters (splitting fins) which break it down into several arcs of cumulative voltage, until forced extinction. The electric arc that forms is extinguished by these metal plates, which split, cool and deionise the arc.

⚠️ Life-threatening danger: An AC circuit breaker used in DC without certification can sustain the arc indefinitely, causing a fire. Even at reduced voltage, the absence of a zero-crossing in DC prevents the arc from being extinguished.

1.4 Current Limitation and Energy Capacity (I²t)

Modern circuit breakers do more than simply open the circuit; they limit the peak value of the fault current. The contacts separate spontaneously under magnetic pressure (electrodynamic repulsive forces) even before the mechanism is released, introducing an arc voltage that opposes the rise in current.

Example: In the event of a presumed 20 kA short circuit, a limiting circuit breaker can reduce the actual current to a peak of 8 kA. This characteristic is crucial for cascading and the protection of sensitive downstream components. The selectivity tables provided by manufacturers (Schneider, ABB, Legrand, Hager) are based on this specific energy (A²s), not on the rated current.

2. Key Parameters of a Circuit Breaker

2.1 Rated Current (_In): Nominal Current

The rating or nominal current (_In) represents the maximum current that the circuit breaker can withstand continuously without tripping. Standard ratings are:

2.2 Number of Poles: 1P, 1P+N, 2P, 3P, 4P

The number of poles corresponds to the number of live conductors that the circuit breaker can trip simultaneously:

Rule of thumb: All hybrid inverters (AC-coupled) require a two-pole (2P) circuit breaker for single-phase systems or a four-pole (4P) circuit breaker for three-phase systems to ensure complete phase + neutral disconnection.

2.3 Breaking Capacity (PdC or_Icn/Icu)

The breaking capacity (in kA) represents the maximum short-circuit current that the circuit breaker can interrupt without being destroyed. It depends on the transformer’s power rating, the distance to the distribution board and the cable cross-section. This breaking capacity is defined by the breaking capacity (PdC) expressed in kA.

Formula for calculating the short-circuit current_Icc:

_{Icc_source} = U / (_Ztransfo +_{Zcâble_alimentation})
_{Icc_panel} =_{Icc_source} × [1 / (1 + (ρ ×_Lcable) / (_Scable × U))]

Example: 12 kVA single-phase meter (_Icc ≈ 5 kA) + 10 mm² copper cable over 15 m →_{Icc_panel} ≈ 3.2 kA → select PdC 4.5 kA (safety factor × 1.5).

2.4 Trip Curves: B, C, D (and K, Z, MA)

The trip curve defines the instantaneous (magnetic) current threshold at which the circuit breaker trips in less than 10 ms. It is determined by the_Im/In ratio. It is the setting of the magnetic core’s return spring that defines whether a circuit breaker is type B, C, D or K.

PV Recommendation: Use only C curves for single-phase and three-phase inverters. B curves may trip prematurely during start-up transients, and D curves do not provide sufficient protection for cables.

Focus: K-curve vs D-curve for Motors

Although D and K have similar magnetic thresholds (10–14 ×_In), they are not identical. The K curve, specific to standard IEC 60947-2, offers tighter thermal protection (tripping at 1.05–1.2 ×_In). It allows the motor’s starting peak (high magnetic) to pass but provides precise protection for the windings against a slight overload (precise thermal). The D curve, which is less precise thermally, is less suitable for expensive industrial motors where service life is critical.

Focus: Z-curve for Power Electronics and BMS

Semiconductors (thyristors, triacs, BMS MOSFETs) have very low thermal capacity and trip faster than a standard fuse. The Z curve (threshold 2.4–3.6 ×_In) offers the fastest possible response for a thermal-magnetic circuit breaker. It is particularly recommended for protecting battery management systems (BMS) where power MOSFETs can melt in a matter of milliseconds in the event of an overcurrent.

3. Regulatory Framework 2026: IEC 60898-1 vs IEC 60947-2

3.1 The Key Distinction: Residential vs Industrial

Confusion between these two standards leads to serious sizing errors. This is not a bureaucratic difference, but one of fundamental technical performance. It is vital for the engineer to know which standard to apply, as the performance ratings displayed on the product vary depending on the test standard.

Critical implication: A circuit breaker marked "C16" may be certified for 10 kA under IEC 60947-2 but only 6 kA under IEC 60898. Using it in an industrial installation with an_Icc of 9 kA is valid under 60947-2, but dangerous in a residential application if the RGIE required it. Conversely, using a purely residential circuit breaker (3 kA or 4.5 kA) in an industrial environment is a serious error, as it lacks the necessary robustness against pollution, humidity and intensive cycling.

3.2 Breaking Capacity:_Icu vs_Ics

_Icu (Ultimate Breaking Capacity):
This is the maximum short-circuit current that the circuit breaker can interrupt twice (O-t-CO cycle) whilst remaining safe for the operator, even if it is subsequently electrically destroyed.

_Ics (Service Breaking Capacity):
This is the current it can interrupt three times (O-t-CO-t-CO) and remain fully functional. In a factory or data centre,_Ics is required to be 100%_{of Icu}. In residential applications (60898),_Ics is often only 75% or 50% of_Icn.

3.3 The Belgian RGIE 2025-2026: Major Developments

The Belgian General Regulations on Electrical Installations incorporate new requirements specific to storage systems and fire safety:

Publicly Accessible Areas and Batteries (Book 1):
Any domestic or commercial battery installation located in an area accessible without prior authorisation (building lobby, shop) must meet strict criteria: locked cabinets, enhanced protection against mechanical impact (IK08 minimum), and specific signage. A documented risk assessment is now mandatory to justify the location and protective measures (ventilation, dedicated fire detection). The document detailing external influences must explicitly mention this equipment.

Protection of Bathrooms and Wet Areas:
RGIE 2025 refines the protection zones (0, 1, 2) in wet rooms. The 30 mA residual current device (RCD) remains the baseline, but the requirements for the protection rating (IP) of equipment, including distribution boards if they are located within these areas, have been tightened (minimum IPX4). This affects the choice of small distribution boards for hot tubs or instantaneous water heaters.

Arc Fault Detectors (AFDD):
Circuit breakers are unable to detect ‘series arcs’ (poor contact in a terminal, crushed cable) which generate heat without a significant increase in current. By 2026, the installationof AFDDs (Arc Fault Detection Devices) will become virtually mandatory for circuits supplying sleeping areas (dormitories, care homes), timber structures, or premises storing valuable goods. The AFDD analyses the electronic signature of the current (high frequencies > 10 kHz) to trip in the event of an arc (< 300 ms), preventing a fire well before a conventional circuit breaker.

3.4 DC Standards and Batteries: IEC 62619 and UL 1973

For storage systems, the IEC 62619 standard requires that the complete system (BMS + Contactor/Circuit Breaker) be tested to ensure that no flames or dangerous breakdowns occur, even in the event of a component failure. The American UL 1973 standard goes further by requiring thermal runaway tests on individual cells.

Golden rule: Never size a battery circuit breaker relying on the BMS to cut off the power. The circuit breaker must be capable of cutting off the total short-circuit current, even if the BMS MOSFETs melt in the closed position (fail-closed).

4. When Does a Circuit Breaker Trip? Practical Cases

4.1 Thermal Tripping (Overload)

Thermal tripping occurs when the current exceeds the rated current_In for a sufficient duration to heat the bimetallic strip. The tripping time is inversely proportional to the overcurrent according to a standardised curve in IEC 60898-1:

Practical example: A 20 A circuit protected by a 20 A C-curve circuit breaker supplies 5 radiators of 1000 W each (I = 5000 W ÷ 230 V = 21.7 A = 1.09 ×_In). The circuit breaker will not trip immediately, but if the ambient temperature rises (e.g. due to a poorly ventilated electrical cabinet), tripping may occur after 30–60 minutes.

4.2 Magnetic Tripping (Short Circuit)

Magnetic tripping is instantaneous (< 10 ms) as soon as the current reaches the_Im threshold defined by the curve:

• Curve B:_Im = 3–5 ×_In → tripping between 60 A and 100 A for a 20 A circuit breaker
• Curve C:_Im = 5–10 ×_In → tripping between 100 A and 200 A for a 20 A circuit breaker
• Curve D:_Im = 10–20 ×_In → tripping between 200 A and 400 A for a 20 A circuit breaker

Typical scenario: Direct phase-to-neutral contact (full short circuit) in an electrical socket → short-circuit current_Icc = 3000 A → all circuit breakers (B, C, D) trip instantly because_Icc >>_Im.

4.3 The Danger of a "Minimal" Short Circuit

If_Icc is between 3 ×_In and 5 ×_In (grey area between thermal and magnetic for curve B), tripping may take several seconds, with a risk of cable overheating and the initiation of a persistent arc. This is why the NF C 15-100 standard requires verification of the condition:

_{Icc_min} ≥ 5 ×_In (for curve C)

Critical case: C63 circuit breaker (magnetic threshold 5–10 ×_In, i.e. 315 A to 630 A max). If, due to excessive cable length (150 m for 10 mm²), the_Icc drops to 400 A, we are in the “uncertainty zone” of curve C. If the circuit breaker is set high, it will not recognise the short circuit as a magnetic fault. It will react thermally (overload), taking several seconds, or even tens of seconds, to trip. During this time, the cable is subjected to enormous I²t thermal stress, melting the insulation and posing a fire risk.

Solutions:

• Increase the cable cross-section (to reduce R and raise_Icc)
• Switch to Curve B (threshold 3–5 ×_In, i.e. 189A–315A). With a 400A fault, magnetic tripping is guaranteed

4.4 Cases where the Circuit Breaker DOES NOT Trip

It is crucial to understand that a standard thermal-magnetic circuit breaker does not detect or protect against:

• Earth leakage (indirect contact, insulation fault) → requires a 30 mA residualcurrent device (mandatory under NF C 15-100, Art. 531.2)
• Transient overvoltages (direct/indirect lightning strikes, inductive switching) → requires a type 1+2 or 2 surge arrester
• Prolonged undervoltage (mains voltage drop < 195 V for several minutes) → requires a voltage monitoring relay
• Low-current series arcs (loose connection, damaged insulation) → requires an AFDD (Arc Fault Detection Device) in accordance with NF C 15-100 Amendment A5 (2021)
• Non-hazardous overcurrent: if you draw 28 A on a circuit protected by a 32 A circuit breaker for hours, it will not trip (normal, correctly sized)

5. What Does a Circuit Breaker Actually Protect?

A thermal-magnetic circuit breaker provides three-fold protection:

1. Protection of cables against overheating:
The_In rating is selected based on the conductor cross-section according to the standardised correspondence table. If the current exceeds 1.45 ×_In, the circuit breaker must trip within one hour to prevent the PVC insulation from degrading (softening point: 70°C, critical point: 105°C).

2. Protection of equipment against electrodynamic stresses:
During a short circuit, the electromagnetic forces between conductors can reach several tonnes per metre. The circuit breaker limits the duration of the fault current (Joule integral I²t), thereby protecting terminal blocks, contactors and transformers.

3. Protection of the installation against electrical fires:
30% of domestic fires in Europe are caused by electrical faults (source: Electrical Safety First, 2023), mainly due to undetected overloads or faulty connections causing arcing.

What the circuit breaker does NOT protect against:

• People against electrocution (direct or indirect contact) → role of the 30 mA residual current device
• Equipment against common-mode surges (lightning, switching) → role of the surge protector
• Data and communication systems (EMC interference) → role of EMI/RFI filters and galvanically isolated inverters

6. AC Sizing: Calculation of Short-Circuit Current and Attenuation

6.1 Short-Circuit Current at the Source (Transformer)

All sizing begins with the calculation of the maximum symmetrical three-phase short-circuit current at the secondary terminals of the transformer. The fundamental formula is:

_{Isc_transfo} =_Sn / (√3 ×_Un ×_Ucc)

Where:
•_Sn: Apparent power of the transformer (kVA)
•_Un: Secondary composite voltage (400V or 410V no-load)
•_Ucc: Short-circuit voltage as a percentage (typically: 4% for_Sn < 630 kVA, 6% for_Sn ≥ 630 kVA)

6.2 Attenuation due to Cable Impedance (Point-by-Point Method)

As the distance from the transformer increases, the short-circuit current decreases because the cables introduce resistance (R) and reactance (X). This attenuation allows for the use of more economical circuit breakers at the end of the line, but introduces a new risk: the fault current may become too low to trip the magnetic mechanism.

Calculation formula:
_Isc =_Usimple /_Ztotal
Where_Ztotal = √((ΣR)² + (ΣX)²)

Parameters R and X (to be used in a fault situation):
• Copper resistivity (ρ): 0.0225 Ω·mm²/m (hot value, not at 20°C)
• Reactance (X): ≈ 0.08 mΩ/m for multi-core LV cables

Conclusion: After 50 m, the_Icc has dropped from 24 kA to 6.6 kA. A 10 kA branch circuit breaker can be installed (cheaper than a 25 kA one). However, it must be checked that this current does indeed trip the magnetic component of the selected curve.

Simplified formula (practical approach):
_{Icc_panel} =_{Icc_source} / [1 + (ρ × 2L) / (S × U × √3)]
Where_ρcopper = 0.023 Ω·mm²/m (at 90°C)

Rule of thumb: For residential PV installations (distance between main distribution board and inverter < 30 m), a 6 kA RCD is sufficient in 95% of cases.

7. Cable Cross-Section / Circuit Breaker Rating Correspondence Table

This table consolidates the requirements of the NF C 15-100 (France) and RGIE (Belgium) standards for domestic and commercial installations. It takes into account the reference ambient temperature (30°C), the installation method (type B: cables laid in open conduits), and a correction factor of 1 (no grouping).

Key points:

• These values are given for exposed copper cables (type B according to IEC 60364-5-52). For cables buried or in poorly ventilated attics, apply a correction factor (0.7 to 0.9)
• The standard stipulates: _In ≤_Iz (the circuit breaker must never exceed the cable’s current-carrying capacity)
• For three-phase installations, divide the total power by √3 × U × number of phases

8. Summary Table: Short-Circuit Current Attenuation (Copper)

This table provides an indicative estimate of_Icc in kA based on cable cross-section and length, assuming a main distribution board with infinite_Icc (simplified). Basis: 230/400V, ρ = 0.0225 Ω·mm²/m.

Note: Indicative values depending on the actual upstream_Isc. Always calculate using approved software such as Caneco, Ecodial, or Wattuneed calculators.

9. Standard Circuit Breaker vs. Residual Current Circuit Breaker (RCCB)

9.1 Fundamental Differences

9.2 Types of Residual Current Circuit Breakers and the PV

Strict PV rule (RGIE Art. 5.3.6.3, NF C 15-100 Amendment A5):

Any photovoltaic inverter connected to the grid MUST be protected by a type F (minimum) or type B (recommended if the inverter is transformerless) residual current device. The use of a type AC or A device constitutes a serious non-compliance, exposing users to the risk of undetected electrocution (DC leakage current > 6 mA).

⚠️ WARNING: An AC-type residual current device will NOT detect a current leak from a PV inverter, even if it is fatal (> 30 mA DC). Risk of undetected fatal electrocution.

10. The Photovoltaic (PV) Challenge: DC Sizing and the 1.56 Rule

10.1 Why Oversize PV Circuit Breakers?

Unlike a conventional load, a PV panel is a limited current source. Its short-circuit current (_{Isc_PV}) is barely higher than its operating current (_Impp). The problem is not to cut off a huge current, but to withstand the rated current continuously under high temperatures.

Standards (NEC and IEC guidelines) require double oversizing:

_{Circuit breaker} ≥_{Isc_PV} × 1.25 × 1.25 =_{Isc_PV} × 1.56

First factor 1.25 (Irradiance):
Although the panels are tested at 1000 W/m² (STC), phenomena such as the ‘cloud edge effect’ or reverberation (albedo on snow, reflection on a white façade) can temporarily raise the irradiance to 1250 W/m², increasing the generated current by 25%.

Second factor 1.25 (Continuous Operation):
Circuit breakers are tested to operate at 100% of their load for a limited time. In solar power systems, generation can continue for hours at full capacity. To prevent unwanted thermal tripping caused by the circuit breaker itself overheating (particularly in hot roof-mounted enclosures), this derating factor is applied.

10.2 The Life-Threatening Danger of DC Polarity

In DC, arc extinction depends on the direction of the current if the circuit breaker uses permanent magnets to blow out the arc (Lorentz force). If the current flows in the correct direction, the arc is pushed towards the arc-quenching chamber and is extinguished. If the current flows in the opposite direction, the arc is pushed back towards the circuit breaker mechanism or the enclosure wall. It does not extinguish, melts the plastic and causes an immediate fire.

In simple PV systems (string → inverter), the current flows in only one direction. However, in systems with batteries or meshed networks (multiple MPPTs, multiple inverters), the current can reverse.

Recommendation 2026: Use only non-polarised (or bidirectional) DC circuit breakers for battery applications and hybrid inverters. For simple PV strings, polarised circuit breakers are acceptable but require extreme care during wiring (marking +/-, photographs before commissioning).

10.3 Mandatory DC Certification: IEC 60947-2 Annex C

An AC-certified circuit breaker must NEVER be used in DC applications, even at reduced voltage. DC certification requires specific arc-quenching tests in accordancewith Annex C of the IEC 60947-2 standard. Markings to look for on the circuit breaker:

• DC symbol (straight line with a line underneath)
• Maximum DC voltage (e.g. 1000V DC, 1500V DC)
• DC breaking capacity (e.g. 6 kA @ 1000 V DC)
• Polarity marking (+/-) if the circuit breaker is polarised

Selection rule:

• _{In_DC} ≥ 1.25 ×_{Isc_chain} (safety margin per IEC 62548)
• Rated voltage ≥ 1.2 ×_{Voc_max} (at -10°C)
• Trip curve: gG curve (AC fuse) or specific DC curve (long time > 2 hours at 1.13 ×_In)

11. Batteries (BESS): The New Industrial Risk

The sizing of protection for lithium-ion batteries is undoubtedly the most complex and most underestimated aspect of modern installations. A battery behaves like a giant capacitor: it can release its energy almost instantly. Internal resistance is the only limiting factor.

11.1 Calculation of Battery Short-Circuit Current (_{Isc_batt})

Internal resistance (_Rint) depends on the chemistry and capacity:

• 100Ah LFP (LiFePO4) cell:_Rint ≈ 0.3–0.5 mΩ
• Complete 48V (16S) battery: Total pack resistance (cells + busbars + BMS MOSFETs) ≈ 10–20 mΩ

Calculation formula:
_{Isc_battery} =_Vbat /_Rint

For a 48V / 200Ah (9.6 kWh) battery with_Rint = 10 mΩ:
_{Isc_battery} = 48 V / 0.010 Ω = 4800 A

11.2 BMS vs Circuit Breaker Interaction: The "Race to Trip"

The BMS (Battery Management System) also protects against short circuits via its transistors (MOSFETs). This creates a "race condition" between the BMS and the circuit breaker.

Ideal Scenario:
The BMS detects the short circuit within 10 µs and opens the MOSFETs. The current is cut off before it rises too high. The circuit breaker remains inactive.

Catastrophic Scenario (Fail Closed):
The current rises so quickly (low inductance) that it passes through the MOSFETs’ “saturation zone”. The MOSFETs overheat and melt in the closed position (internal short circuit within the component) within a few milliseconds. The BMS is destroyed and no longer provides any protection.

The Role of the Circuit Breaker:
It is the ultimate protection. It must be rated to withstand the total_Isc without the aid of the BMS. You must never size the circuit breaker on the assumption that “the BMS will trip first”.

11.3 Number of Batteries in Parallel and Impact on_Isc

Conclusion: The short-circuit current increases in proportion to the number of batteries in parallel. From 4 modules onwards, standard modular circuit breakers are no longer sufficient. It is necessary to switch to industrial technologies (MCCBs) or specialised fuses.

⚠️ Warning: A DC circuit breaker alone is NOT sufficient to protect a lithium battery. The BMS and HRC fuses are mandatory additional protective measures.

12. Selectivity and Cascading: Optimising the Installation

12.1 Cascading

This is the technique of using a ‘robust’ upstream circuit breaker to protect a ‘weak’ downstream circuit breaker. If circuit breaker A (main distribution board) is a high-performance current limiter, it will only allow 10 kA peak current to pass during a presumed 25 kA fault. A circuit breaker B rated for 10 kA can therefore be installed downstream, even if the theoretical calculation gives 25 kA at that point.

This technique enables significant savings (30–40% on the cost of the switchboard). Please note: It places the responsibility on the designer and only works with devices from the same manufacturer, validated by laboratory test tables (provided by Schneider, ABB, Legrand, Hager).

12.2 Selectivity (Discrimination)

The objective: to ensure that ONLY the circuit breaker closest to the fault trips. Three methods exist:

Amperometric selectivity:
The upstream circuit breaker has a higher magnetic trip threshold than the downstream one. Works up to a certain_Isc, then loses selectivity.

Time-based selectivity:
The upstream circuit breaker has a time delay (0.1 to 0.3 s). The downstream circuit breaker trips instantly. Works for all_{Isc values}, but introduces a delay that is dangerous for personnel in the event of an upstream fault.

Energy-based selectivity (modern):
Electronic circuit breakers (e.g. Schneider’s Compact NSX, ABB’s Tmax) analyse the arc energy. If the upstream unit ‘detects’ that the energy is low (short circuit far downstream), it does not trip, knowing that the downstream unit will handle it. This allows for total selectivity even with very high_Isc values, which was impossible with older methods.

PV Recommendation: For an installation with an 8 kVA inverter (40 A circuit breaker) on a 63 A main supply, check that_Is ≥_{Icc_tableau} (typically_Is ≈ 6 kA for this combination).

12.3 Total Selectivity vs Partial Selectivity

Full Selectivity:
The downstream circuit breaker ALWAYS trips before the upstream circuit breaker, regardless of the fault current. Condition:

_{In_downstream} ≤ 0.4 ×_{In_upstream} (Schneider Electric rule of thumb)

Example: Main circuit breaker 63 A + branch circuit breakers 16 A → 16 A ≤ 0.4 × 63 A → total selectivity ensured

Partial Selectivity:
Selectivity is guaranteed up to a limit short-circuit current_Is, beyond which both circuit breakers trip. Manufacturers publish selectivity tables (Schneider Acti9, ABB S200, Hager MJ) indicating the value of_Is for each upstream/downstream combination.

13. Technology Outlook 2026: The Future of Circuit Breakers

13.1 Solid-State Circuit Breakers (SSCB)

The revolution expected between 2026 and 2030 is the replacement of mechanical contacts with semiconductors (silicon carbide – SiC or GaN).

Advantages:

• Speed: < 10 microseconds (compared to 10,000 µs for mechanical systems)
• Safety: Arc energy becomes negligible. No longer any risk of arc flash to personnel
• Wear: Infinite. Thousands of on-load switching operations can be performed without maintenance
• Intelligence: The circuit breaker acts as a network analyser (harmonics, cos φ) and can limit the current gently (soft fuse) without tripping abruptly

Current limitations: Cost still high (3–5 times that of thermomagnetic circuit breakers), thermal dissipation via conduction (MOSFET_RDSon resistance), conduction losses (0.1–0.3 W), maximum junction temperature 175°C (requires active cooling above 100 A), and standard certification pending.

13.2 AFDD (Arc Fault Detection Device)

Amendment A5 to NF C 15-100 (2021) introduces a requirement for AFDDs in certain high-risk premises (multi-occupancy dwellings, public access buildings). These devices detect series arcs (high impedance, undetected by conventional circuit breakers) through spectral analysis of the current (detection of frequencies > 10 kHz).

Principle: A series arc (loose connection, charred insulation) generates a current of only a few amperes (<_In), but with characteristic high-frequency harmonics. The AFDD trips in < 300 ms, preventing an electrical fire well before a conventional circuit breaker.

13.3 Smart Breakers and Predictive Maintenance

Connected circuit breakers (IoT) are becoming the norm in the commercial sector. They enable monitoring of contact wear (via the accumulated I²t integral) and prediction of the device’s end of life before failure. By 2026, this ‘circuit breaker health’ data will be natively integrated into the Centralised Technical Management (CTM) system.

Connected circuit breakers (Modbus RTU, KNX, MQTT protocols) incorporate sensors for temperature, RMS current, THD, and active/reactive power. They transmit data to cloud platforms (Schneider EcoStruxure, ABB Ability) for:

• Early fault detection (abnormal overheating, phase imbalance)
• Energy optimisation (identification of phantom loads)
• Automated regulatory compliance (traceability of trips in accordance with ISO 50001)

Modules such as the Shelly Pro 3EM or Schneider EcoStruxure solutions already enable this real-time monitoring with preventive alerts.

14. How to Choose Your Circuit Breaker: A 6-Step Method

1. Determine the equipment’s rated current:
I = P / (U × cos φ × √3) for three-phase
I = P / U for single-phase

2. Select the cable cross-section based on length and installation method (Table 7)

3. Select the circuit breaker rating_In ≤_Iz (maximum current rating of the cable)

4. Check the number of poles (minimum 1P+N, 2P recommended for single-phase PV, 4P mandatory for three-phase)

5. Calculate the short-circuit current_{Icc_min} and select a conductor cross-section (PdC) ≥ 1.5 ×_{Icc_max}

6. Select the curve: curve C for 95% of applications, curve B for long lines, curve D for motors

15. Practical Sizing Checklist

Before confirming your choice of circuit breaker, always check these 12 points:

1. Nominal load current (_In): Calculated, not estimated
2. Simultaneity factor: Do all loads operate at the same time?
3. Short-circuit current (_Icc) at the installation point: Calculated using the impedance method
4. Breaking capacity (_Icu or_Icn): Greater than_Icc
5. Applicable standard: IEC 60898-1 (residential) or IEC 60947-2 (industrial)?
6. Trip curve: B/C/D/K/Z depending on the load
7. Minimum_Icc verification: Does a clear fault at the end of the line trip the magnetic circuit breaker?
8. AC or DC certification: For PV/battery, require IEC 60947-2 Annex C
9. Polarity (if DC polarised): +/- marking observed
10. Ambient temperature: Derating if > 40°C (PV roof-mounted enclosure)
11. Selectivity: Coordination with upstream circuit breakers verified (manufacturer’s tables)
12. Certification: If used, validation by manufacturer’s certificate

FAQ: Frequently Asked Questions about Circuit Breakers

Can I replace a 16 A circuit breaker with a 20 A one if it trips frequently?

NO, it is dangerous and illegal. If the circuit breaker trips regularly, it means the circuit is overloaded. Increasing the rating without changing the cable cross-section exposes the circuit to excessive heating and a fire risk. Solution: