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

Introduction: The New Challenges of Electrical Protection in 2026The boom in residential and commercial photovoltaic installations, coupled with the massive integration of battery storage systems (BESS) and hybrid inverters, is radically transforming electrical protection requirements. The circuit breaker is no longer a simple modern fuse: it is becoming a strategic safety element standardised to IEC 60898-1 (domestic) and IEC 60947-2 (industrial), 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 informed private individuals who wish to gain an in-depth understanding of the sizing, choice and limits of circuit breakers in AC (alternating current) and DC (direct current) installations, in compliance with French (NF C 15-100) and Belgian (RGIE) standards. We will also integrate the technological perspectives of 2026: intelligent circuit breakers, AFDD (Arc Fault Detection Device), solid state circuit breakers (SSCB) and predictive maintenance.Fundamental principle: A circuit breaker does not just protect against overloading; it must be able to cut off the maximum short-circuit current at any point in the installation, while guaranteeing selectivity to maintain continuity of service.1. What is a Circuit Breaker? Function and Internal Architecture1.1 Definition and RoleA circuit breaker is a modular protection 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 is tripped, a circuit breaker can be manually reset and reused.The circuit-breaker protects two critical elementsElectrical cables: prevents overheating that could lead to fire. Connected equipment: limits mechanical and thermal stress on appliances (inverters, motors, transformers).⚠️ Warning: A conventional circuit breaker does NOT protect against :Current leaks to earth (indirect contact) → requires a 30 mA differential circuit breakerTransient overvoltages (lightning, switching) → requires a lightning arresterProlonged undervoltage → requires a voltage monitoring relayLow current series arcs (loose connection) → requires an AFDD1.2 Internal architecture : The Dual Thermal + Magnetic MechanismA thermal-magnetic circuit breaker combines two complementary protection mechanisms operating on different time scales:1. Thermal protection (bimetallic strip):A metal bimetallic strip made of two metals with different coefficients of expansion (often iron-nickel and brass) gradually deforms under the effect of the heat released 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 IEC 60898-1, the circuit-breaker must not trip before 1 hour at 1.13 ×In, but must trip within one hour at 1.45 ×In. This tolerance zone allows normal load variations to be absorbed without nuisance 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 in the event of a short-circuit (I ≥ 3 to 20 ×In depending on the curve). The field activates a moving ferromagnetic core which instantly strikes the tripping mechanism. Tripping time: < 10 ms (well before the adiabatic heating I²t melts the conductors).1.3 The Thermodynamics of the Electric Arc: Why AC and DC Are Two Different WorldsWhen a circuit under load is opened, the circuit-breaker contacts separate. The contact area decreases, increasing the current density until the metal melts locally. A conductive plasma is formed: theelectric arc, whose core temperature reaches 10,000 to 20,000°C. This arc keeps the current flowing through the ionised air.ParamètreCourant Alternatif (AC)Courant Continu (DC)Passage par zéro2 fois par période (10 ms à 50 Hz)Aucun – arc stable et continuExtinction naturelleRégénération diélectrique à chaque zéroImpossible sans action mécaniqueTechnique de coupureExploitation du passage à zéroSoufflage magnétique + splitter platesTension d'arc requiseModérée (quelques dizaines de V)Élevée (doit dépasser tension source)Énergie I²tStandard2-3× supérieure (danger accru)Critical consequence: In DC, circuit breakers use magnetic breaking chambers (Lorentz force) to stretch the arc towards flat splitters (splitting fins) which cut it into several arcs of cumulative voltage, until forced extinction. The electric arc that forms is extinguished thanks to these metal plates, which split, cool and de-ionise the arc.⚠️ Mortal danger: An AC circuit breaker applied in DC without certification can maintain the arc indefinitely, causing a fire. Even at reduced voltage, the absence of a DC zero crossing prevents the arc from being extinguished.1.4 Current Limitation and Energy Passing (I²t)Modern circuit-breakers do more than simply open the circuit; they limit the peak value of the fault current. The contacts spontaneously move apart under magnetic pressure (electrodynamic repulsion forces) even before the mechanism is released, introducing an arcing voltage which opposes the rise in current.Example: In the event of an assumed short-circuit of 20 kA, a current-limiting circuit-breaker can reduce the actual current to 8 kA peak. This characteristic is crucial for cascading and protecting sensitive downstream components. The selectivity tables supplied by manufacturers (Schneider, ABB, Legrand, Hager) are based on this specific passing energy (A²s), not on the rated current.2. The Key Parameters of a Circuit Breaker2.1 Rating (In): Rated currentThe rating or rated current (In) represents the maximum current that the circuit-breaker can withstand continuously without tripping. The standard ratings are :Calibre (A)Applications Typiques10 ACircuits d'éclairage (max 8 points selon NF C 15-100)16 APrises de courant (max 8 prises), volets roulants20 ACircuits spécialisés (lave-linge, lave-vaisselle), 12 prises max25 AFour électrique, plaques de cuisson (si < 4600 W)32 AOnduleur monophasé 6-8 kVA, plaques induction, borne VE 3,7 kW40 AOnduleur monophasé 9-10 kVA, cuisinière électrique, borne VE 7,4 kW63 AArrivée générale (compteur monophasé), borne VE 11 kW, batteries BESS80 AArrivée triphasée, générateur de secours, batteries haute capacité PRACTICAL EXAMPLE Inverter WKS Circle Pro 6.5 kVARated power: 6500 W single-phase 230 V ACCalculation: I = 6500 W ÷ 230 V = 28.3 ARecommended circuit breaker: 32 A curve C 🔗 Discover the WKS Circle Pro 6.5 kVA PRACTICAL EXAMPLE Deye SUN-12K inverter (12 kVA three-phase)Rated power: 12 000 W three-phase 400 V ACCalculation: I = 12,000 W ÷ (√3 × 400 V) = 17.3 A per phaseRecommended circuit breaker: 25 A four-pole (3P+N) curve C 🔗 Discover the Deye SUN-12K 2.2 Number of poles: 1P, 1P+N, 2P, 3P, 4PThe number of poles corresponds to the number of active conductors that the circuit breaker can break simultaneously :TypeConfigurationApplications1P (unipolaire)Coupe la phase uniquementCircuits simples en monophasé (déconseillé en France, interdit dans certains cas RGIE)1P+NCoupe phase + neutre (neutre non protégé)Standard résidentiel France/Belgique pour prises et éclairage2P (bipolaire)Coupe phase + neutre (les deux protégés)Circuits spécialisés (chauffe-eau, VMC), arrivée générale, onduleur monophasé3P (tripolaire)Coupe les 3 phasesMoteurs triphasés, charges industrielles4P (tétrapolaire 3P+N)Coupe 3 phases + neutreOnduleurs triphasés, arrivée générale triphasée, conformité RGIE obligatoire en BelgiquePV rule: All hybrid inverters (coupled AC) require a two-pole circuit breaker (2P) in single-phase or a four-pole circuit breaker (4P ) in three-phase to ensure complete phase + neutral breaking.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 destruction. It depends on the power of the transformer, the distance from the switchboard and the cross-section of the cables. This breaking capacity is defined by the breaking capacity (PdC) expressed in kA.PdC (kA)NormeApplications3 kAIEC 60898-1Zones rurales éloignées (usage résidentiel limité)4,5 kAIEC 60898-1Standard résidentiel minimum (France, maisons individuelles)6 kAIEC 60898-1Standard recommandé installations PV (résidentiel + petit tertiaire)10 kAIEC 60898-1Immeubles collectifs, installations tertiaires15-25 kAIEC 60947-2Environnements industriels, BESS haute capacité36-50 kAIEC 60947-2Centrales électriques, data centers critiquesFormula for calculating the short-circuit current Icc:Icc_source = U / (Ztransfo + Zcâble_alimentation)Icc_tableau = Icc_source × [1 / (1 + (ρ × Lcâble) / (Scâble × U))]Example: 12 kVA single-phase meter (Icc ≈ 5 kA) + 10 mm² copper cable over 15 m → Icc_tableau ≈ 3.2 kA → choose PdC 4.5 kA (safety × 1.5).2.4 Trip curves: B, C, D (and K, Z, MA)The tripping curve defines the instantaneous (magnetic) current threshold above which the circuit-breaker cuts out in less than 10 ms. It is determined by the ratioIm /In. It is the setting of the magnetic core return spring which determines whether a circuit-breaker is type B, C, D or K.CourbePlage Magnétique (Im)Applications TypiquesAvantagesContraintesB3 à 5 × InGénérateurs, câbles très longs, charges résistives pures (chauffage, éclairage LED)Sécurité maximale, déclenchement garanti sur faible IccNe supporte aucun courant d'appel (inrush). Interdit pour moteurs/transfosC5 à 10 × InUsage universel : prises, onduleurs PV, moteurs standard, borne VECompromis universel, tolérance aux appels modérésPeut déclencher sur LED à fort condensateur d'entrée si circuit surchargéD10 à 20 × InMoteurs fort appel de courant (compresseurs, pompes), transformateurs, soudeuses, rayons X, PAC domestiquesLaisse passer les forts pics de démarrageNécessite Icc très élevé pour déclencher. Vérifier longueur câbleK10 à 14 × InMoteurs industriels (protection optimisée enroulements)Thermique serrée (1,05-1,2 × In) protège finement les enroulementsRéservée IEC 60947-2. Plus coûteuse que DZ2,4 à 3,6 × InSemi-conducteurs (thyristors, triacs), circuits de mesure, BMS sensiblesRéaction ultra-rapide pour composants à faible capacité thermiqueTrès sensible, déclenchements possibles sur transitoiresMA12 × In (fixe)Circuits moteurs avec relais thermique externe (pas de bilame)Pas de bilame = pas de redondance thermique. Coordination Type 2 parfaiteProtection thermique OBLIGATOIRE ailleurs (PTC moteur ou relais)PV recommendation: Only use C curves for single-phase and three-phase inverters. B curves can trip unexpectedly during start-up transients, and D curves do not provide sufficient protection for cables.Focus: K vs D curves for motorsAlthough D and K have similar magnetic thresholds (10-14 ×In), they are not identical. Curve K, specific to standard IEC 60947-2, has tighter thermal protection (tripping from 1.05-1.2 ×In). It allows the motor starting peak to pass through (high magnetic) but provides fine protection for the windings against a slight overload (precise thermal). Curve D, which is less thermally precise, is less suitable for expensive industrial motors where life counts.Focus: Z curve for power electronics and BMSSemiconductors (thyristors, triacs, MOSFETs in BMS) have a very low thermal capacity and blow 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 milliseconds in the event of an overcurrent.💡Additional resource: Wattuneed sizing calculators Access our impedance calculation and protection selection tools3. Standards Framework 2026: IEC 60898-1 vs IEC 60947-23.1 The Grand Distinction: Residential vs IndustrialConfusion 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, because the performance displayed on the product changes according to the test reference.CritèreIEC 60898-1 (Résidentiel)IEC 60947-2 (Industriel)Public ciblePersonnes non averties (grand public)Personnel qualifié / installateursCourbes disponiblesB, C, D uniquementB, C, D, K, Z, MA (étendue)Pouvoir de coupureIcn (nominal, 1 cycle O-CO)Icu (ultime) et Ics (service)Ics / IcuSouvent 50-75% seulement100% exigé en industrie critiqueMaintenanceNon maintenable (remplacement)Maintenable (contacts vérifiables)Robustesse environnementaleStandard (pollution, humidité limitée)Renforcée (IP élevé, cycles intensifs)Critical implication: A circuit breaker marked "C16" may be certified 10 kA to IEC 60947-2 but only 6 kA to 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 required by the RGIE. Conversely, using a purely residential circuit-breaker (3 kA or 4.5 kA) in an industrial environment is a serious mistake, as it does not have the required robustness against pollution, humidity and intensive cycles.3.2 Breaking capacity: Icu vs IcsIcu (Ultimate Breaking Capacity):This is the maximum short-circuit current that the circuit-breaker can interrupt twice (O-t-CO cycle) while remaining safe for the operator, even if it is subsequently destroyed electrically.Ics (Service breaking capacity):This is the current it can cut three times (O-t-CO-t-CO cycle) and remain perfectly functional. In a factory or data centre, Ics = 100% Icu is required. In residential applications (60898), Ics is often only 75% or 50% of Icn.3.3 The Belgian RGIE 2025-2026: Major changesThe Belgian General Regulations on Electrical Installations include new requirements specific to storage systems and fire safety:Places Accessible to the Public and Batteries (Book 1):Any domestic or tertiary battery installation located in an area accessible without prior authorisation (building lobby, shop) must meet strict criteria: locked cabinets, reinforced protection against mechanical shocks (IK08 minimum), and specific signage. A documented risk analysis becomes mandatory to justify the location and means of protection (ventilation, dedicated fire detection). The external influences document must explicitly mention this equipment.Protection of bathrooms and wet areas:The RGIE 2025 refines the protection volumes (0, 1, 2) in shower rooms. The 30 mA differential protection remains the basis, but the requirements on the protection index (IP) of the switchgear, including the divisional enclosures if they are in these volumes, are reinforced (IPX4 min). This has an impact on the choice of small distribution boards for jacuzzis or instantaneous water heaters.Arc detectors (AFDD):Thermal-magnetic circuit breakers are blind to "series arcs" (poor contact in a terminal, crushed cable) which generate heat without any significant increase in current. By 2026, the installation ofAFDDs (Arc Fault Detection Devices) will become virtually compulsory for circuits supplying sleeping quarters (dormitories, homes), wooden 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 the conventional circuit breaker.3.4 DC and battery standards: IEC 62619 and UL 1973For storage systems, IEC 62619 requires the complete system (BMS + Contactor/Circuit Breaker) to be tested to ensure that no dangerous flames or ruptures occur, even in the event of a component failure. The American standard UL 1973 goes further by requiring thermal runaway tests on individual cells.Golden rule: Never size a battery circuit breaker by relying on the BMS to cut the current. The circuit breaker must be able to cut 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? Case Studies4.1 Thermal trip (Overload)Thermal tripping occurs when the current exceeds the rated ratingIn for a sufficiently long time to heat up the bimetallic strip. The tripping time is inversely proportional to the overcurrent according to an IEC 60898-1 standard curve:Surintensité (I / In)Temps de Déclenchement Thermique1,13 × In> 1 heure (test normalisé température ambiante 30°C)1,45 × In< 1 heure (obligatoire selon IEC 60898-1)2,0 × In4 à 40 secondes (selon courbe et température ambiante)2,55 × In1 à 15 secondesExample: A 20 A circuit protected by a 20 A curve C 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 (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 theIm threshold defined by the curve :Curve B:Im = 3-5 ×In → tripping between 60 A and 100 A for a 20 A circuit-breakerCurve C:Im = 5-10 ×In → tripping between 100 A and 200 A for a 20 A circuit-breakerCurve D:Im = 10-20 ×In → tripping between 200 A and 400 A for a 20 A circuit breakerTypical scenario: Direct phase-neutral contact (open short-circuit) in an electrical socket → short-circuit current Icc = 3000 A → all circuit-breakers (B, C, D) trip instantaneously because Icc >>Im.4.3 The danger of a "minimal" short-circuitIf Icc is between 3 ×In and 5 ×In (grey zone between thermal and magnetic for curve B), tripping can take several seconds, with the risk of overheating the cable and persistent arcing. This is why standard NF C 15-100 requires verification of the condition :Icc_min ≥ 5 ×In (for curve C)Critical case: C63 circuit breaker (magnetic threshold 5-10 ×In, i.e. 315A to 630A max). If, because of excessive cable length (150 m in 10 mm²), the Icc falls to 400 A, you are in the "uncertainty zone" of curve C. If the circuit-breaker is set high, it will not see the short-circuit as a magnetic fault. It will react thermally (overload), taking several seconds or even tens of seconds to cut out. During this time, the cable undergoes enormous thermal stress I²t, melting the insulation and risking fire.Solutions :Increase the cable cross-section (to reduce R and increase 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 tripIt is crucial to understand that the conventional thermal-magnetic circuit breaker neither detects nor protects against:Current leakage to earth (indirect contact, insulation fault) → requires a 30 mA differential circuit breaker (mandatory NF C 15-100 art. 531.2)Transient overvoltages (direct/indirect lightning strikes, inductive switching) → requires a type 1+2 or 2 lightning arresterProlonged undervoltage (mains voltage drop < 195 V for several minutes) → requires a voltage monitoring relayLow 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-dangerous overload: if you draw 28 A on a circuit protected by a 32 A circuit breaker for hours, no tripping will occur (normal, correct sizing)🔧 ESSENTIAL SAFETYAC lightning arrester enclosure for PV installations - Type 2 protection, integrated 16-25A circuit breaker, IP65 waterproof ⚡ SEE TECHNICAL DETAILS 5. What does a circuit breaker actually protect?A thermal-magnetic circuit breaker provides threefold protection:1. Protection of cables against overheating :TheIn rating is chosen according to the conductor cross-section in accordance with the standardised table. If the current exceeds 1.45 ×In, the circuit-breaker must trip within an hour to prevent the PVC insulation from degrading (softening point: 70°C, critical point: 105°C).2. Protection of equipment against electrodynamic stress :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), thus protecting terminal blocks, contactors and transformers.3. Protection of the installation against electrical fires :30% of domestic fires in Europe are of electrical origin (source: Electrical Safety First, 2023), mostly caused by undetected overloads or faulty connections generating arcs.What circuit breakers do NOT protect :People against electrocution (direct or indirect contact) → role of the 30 mA differential.Equipment against common mode overvoltages (lightning, switching) → role of the lightning arresterData and communication systems (EMC disturbances) → role of EMI/RFI filters and galvanically isolated inverters6. AC sizing: Calculation of short-circuit current and attenuation6.1 Short-circuit current at the source (transformer)All sizing begins with the calculation of the maximum symmetrical three-phase short-circuit current at the transformer's secondary terminals. The basic formula is :Isc_transfo =Sn / (√3 ×Un × Ucc)Where :-Sn: Apparent transformer power (kVA)-Un: Secondary phase-to-phase voltage (400V or 410V no-load)- Ucc: Short-circuit voltage in % (typical: 4% forSn < 630 kVA, 6% forSn ≥ 630 kVA) CALCULATED EXAMPLE Plant supplied by 1000 kVA transformerData: 400V, Ucc = 6%.Calculation:In = 1,000,000 / (√3 × 400) = 1443 AIsc at terminals: 1443 / 0.06 ≈ 24,050 A (24 kA)Consequence: The main LV circuit breaker must have an Icu > 24 kA. A 10 kA or 15 kA model would explode, unable to contain the arc energy.6.2 Mitigation by cable impedance (point-by-point method)As you move away from the transformer, the short-circuit current decreases because the cables add resistance (R) and reactance (X). This attenuation makes it possible to use 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 magnet.Calculation formula :Isc =Usimple / ZtotalWhere Ztotal = √((ΣR)² + (ΣX)²)Parameters R and X (to be used in fault conditions) :- Copper resistivity (ρ): 0.0225 Ω-mm²/m (hot value, not at 20°C)- Reactance (X): ≈ 0.08 mΩ/m for LV multicore cablesÉlémentValeurCalculTGBT Icc initial24 kA-Câble 35 mm² × 50 m--R câble0,032 Ω0,0225 × 50 / 35X câble0,004 Ω0,08 × 10⁻³ × 50Z source (transfo)0,0096 Ω230V / 24000AZ total0,035 Ω√(0,032² + 0,0136²)Icc final (bout 50m)6,57 kA230 / 0,035Conclusion: After 50 m, the Icc has dropped from 24 kA to 6.6 kA. A 10 kA divisional circuit breaker can be installed (cheaper than a 25 kA). However, you need to check that this current actually triggers the magnetic of the chosen curve.Simplified formula (practical approach) :Icc_tableau = Icc_source / [1 + (ρ × 2L) / (S × U × √3)]Where ρcopper = 0.023 Ω-mm²/m (at 90°C)Section Cuivre (mm²)Longueur (m)Icc si Source = 10 kAPdC Minimal Requis10 mm²5 m6,8 kA10 kA10 mm²15 m3,2 kA4,5 kA16 mm²25 m2,9 kA4,5 kA25 mm²40 m2,4 kA3 kA (rural)Practical rule: For residential PV installations (distance general panel ↔ inverter < 30 m), a 6 kA PdC is sufficient in 95% of cases.🔧 RECOMMENDED TOOLShort-circuit current calculator and selectivity check ⚡ ACCESS SIZING TOOLS 7. The Cable Section/Circuit Breaker Rating Correspondence TableThis table consolidates the requirements of standards NF C 15-100 (France) and RGIE (Belgium) for domestic and tertiary installations. It takes into account the reference ambient temperature (30°C), the installation method (type B: exposed cables in trunking), and a correction factor of 1 (no grouping).Section Cuivre (mm²)Courant Admissible Iz (A)Disjoncteur Max (A)Applications Typiques1,5 mm²18 A16 AÉclairage (8 points max), prises (8 max), volets roulants2,5 mm²24 A20 APrises spécialisées (12 max), lave-linge, lave-vaisselle, sèche-linge4 mm²32 A25 AFour électrique indépendant, plaques de cuisson 5,7 kW6 mm²40 A32 AOnduleur PV monophasé 6-8 kVA, plaques induction, borne VE 3,7 kW10 mm²52 A40 AOnduleur PV monophasé 9-10 kVA, cuisinière, borne VE 7,4 kW16 mm²68 A63 AArrivée générale monophasé 12-15 kVA, batteries BESS 48V / 100Ah, borne VE 11 kW25 mm²89 A80 AArrivée triphasée 18 kVA, onduleur triphasé 15-20 kVA, générateur secoursCritical points:These values are given for exposed copper cables (type B according to IEC 60364-5-52). For cables laid underground or in poorly ventilated attics, apply a correction factor (0.7 to 0.9).The standard requires: In ≤ Iz (the circuit-breaker must never exceed the permissible cable current).For three-phase installations, divide the total power by √3 × U × number of phases.8. Summary table: Short-circuit current attenuation (copper)This table gives an indicative estimate of the Icc in kA as a function of the cable cross-section and length, starting from a low-voltage distribution board with infinite Icc (simplified). Base: 230/400V, ρ = 0.0225 Ω-mm²/m.Section (mm²)10 m20 m50 m100 m1,53,0 kA1,5 kA0,6 kA0,3 kA2,54,8 kA2,4 kA1,0 kA0,5 kA1016 kA9,0 kA4,0 kA2,0 kA35> 25 kA22 kA10 kA5,5 kANote: Indicative values depending on the actual upstreamIsc. Always calculate with approved software such as Caneco, Ecodial, or Wattuneed calculators.9. Classic circuit-breaker VS Differential circuit-breaker (DDR)9.1 Fundamental differencesCritèreDisjoncteur MagnétothermiqueDisjoncteur Différentiel (DDR)ProtectionSurcharge + court-circuitSurcharge + court-circuit + fuite de courantTechnologieBilame thermique + bobine magnétiqueBilame + bobine + tore détecteur différentielSeuil différentielNon applicable30 mA (domestique), 300 mA (industriel), 10 mA (salles d'eau)Protection des personnesNonOui (obligatoire NF C 15-100 art. 531.2)TypesCourbes B, C, D, K, ZAC, A, F, B (selon nature du courant de fuite)9.2 Types of Differential Circuit Breaker and PVTypeCourant Différ Different entiel DétectéApplicationsType ACCourant de fuite alternatif sinusoïdal purCircuits résistifs uniquement (chauffage, éclairage classique) INTERDIT pour PVType AAC + courant de fuite pulsé unidirectionnelCircuits avec électronique (plaques induction, lave-linge variateur) INSUFFISANT pour PVType FA + courant de fuite haute fréquence (jusqu'à 1 kHz)OBLIGATOIRE pour onduleurs PV monophasés (norme IEC 62606, NF C 15-100 art. 534.6.2)Type BF + courant de fuite DC lissé (> 6 mA) + haute fréquence (> 1 kHz)OBLIGATOIRE pour onduleurs sans transformateur d'isolation, bornes VE DC, batteries BESSStrict 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 differential (minimum) or type B (recommended if inverter without HF transformer). The use of type AC or A constitutes a serious non-conformity exposing the user to the risk of undetected electrocution (DC leakage current > 6 mA).⚠️ CAUTION: A type AC differential will NOT detect a current leakage from a PV inverter, even if it is fatal (> 30 mA DC). Risk of fatal undetected electrocution.10. The Photovoltaic (PV) Challenge: DC Sizing and the 1.56 Rule10.1 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 greater than its operating current (Impp). The problem is not to cut a huge current, but to withstand the rated current continuously at high temperatures.The standards (NEC and IEC guides) require double oversizing:Idisjoncteur ≥Isc_PV × 1.25 × 1.25 =Isc_PV × 1.56First factor 1.25 (Irradiance):Although the panels are tested at 1000 W/m² (STC), "cloud edge effect" or reverberation phenomena (albedo on snow, reflection on white facade) can raise the irradiance to 1250 W/m² temporarily, increasing the current produced by 25%.Second factor 1.25 (Continuous operation):Circuit breakers are tested to operate at 100% load for a limited time. In solar, production can last for hours at full capacity. To avoid untimely thermal tripping due to overheating of the circuit-breaker itself (especially in hot roof-mounted enclosures), this derating factor is applied. PRACTICAL EXAMPLE Photovoltaic string sizingString delivering:Isc = 10 ACalculation: 10 × 1.56 = 15.6 ACircuit breaker required: 16 A minimum (do not use 10A or 12A)10.2 The Deadly Danger of DC PolarityIn 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 extinguishing chamber and is cut off. 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 only flows in one direction. But in systems with batteries or meshed arrays (multiple MPPTs, multiple inverters), the current can reverse.Recommendation 2026: Use only non-polarised (or bi-directional) DC circuit breakers for battery applications and hybrid inverters. For single PV strings, polarised are acceptable but require extreme care in wiring (+/- marking, photos before commissioning).10.3 Mandatory DC certification: IEC 60947-2 Annex CAn AC certified circuit-breaker can NEVER be used in DC, even at reduced voltage. DC certification requires specific arcing tests in accordance withAnnex C of IEC 60947-2. Markings to look for on the circuit-breaker :DC symbol (straight line with line at bottom)Maximum DC voltage (e.g. 1000V DC, 1500V DC)DC breaking capacity (e.g. 6 kA @ 1000V DC)Polarity marking (+/-) if polarised circuit breakerSelection rule :In_DC ≥1.25 ×Isc_chain (normative safety margin IEC 62548)Rated voltage ≥ 1.2 ×Voc_max (at -10°C)Tripping curve: gG curve (AC fuse) or specific DC curve (long time > 2 hours at 1.13 ×In)💡 Technical support: Questions about DC sizing? Our technical team will help you to choose the right protection.11. Batteries (BESS): The New Industrial RiskThe sizing of protection for lithium-ion batteries is without doubt the most complex and underestimated aspect of modern installations. A battery behaves like a giant capacitor: it can release its energy almost instantaneously. Internal resistance is the only limit.11.1 Calculating the Battery Short-Circuit Current (Isc_batt)The internal resistance (Rint) depends on the chemistry and the capacity:LFP (LiFePO4) 100Ah cell: Rint ≈ 0.3 - 0.5 mΩFull 48V (16S) battery: Total pack resistance (cells + busbars + BMS MOSFETs) ≈ 10 - 20 mΩCalculation formula:Isc_battery = Vbat / RintFor a 48V / 200Ah battery (9.6 kWh) with Rint = 10 mΩ :Isc_battery = 48 V / 0.010 Ω = 4800 A10 kWh system (2 parallel 5 kWh modules)Voltage: 51.2 VModule resistance: 20 mΩEquivalent resistance (2 in //): 10 mΩCable resistance (2×1m in 35mm²): ≈ 1 mΩTotal fault loop resistance: 11 mΩ (0.011 Ω)Isc = 51.2 / 0.011 ≈ 4654 A (4.6 kA)This current is manageable by a DC 6 kA or 10 kA circuit breaker. CRITICAL TERTIARY CASE 50 kWh installation (10 modules in parallel)Equivalent resistance :Req = 2 mΩIsc ≈ 15 000 A to 20 000 A⚠️ A conventional modular circuit breaker (MCB) is out of order.You need to switch to an MCCB (Moulded Case Circuit Breaker) capable of breaking 36 kA or 50 kA DC, or use ultra-fast fuses (Class T or aR). Using an unsuitable circuit-breaker here guarantees that it will be welded or explode in the event of a fault.11.2 The BMS vs Circuit Breaker Interaction: The "Race to the BreakerThe 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 in 10 µs and opens the MOSFETs. The current is cut off before rising too high. The circuit breaker does not move.Catastrophic scenario (Fail Closed):The current rises so fast (low inductance) that it crosses the "saturation zone" of the MOSFETs. These overheat and melt in the closed position (internal short-circuit in the component) in a few milliseconds. The BMS is destroyed and no longer protects anything.The role of the circuit breaker :It is the ultimate protection. It must be sized to withstand the totalIsc without the help of the BMS. Never size the circuit breaker thinking "the BMS will cut in first".11.3 Number of Parallel Batteries and Impact onIscNombre de Modules en //Req (mΩ)Isc Estimé (A)Disjoncteur Requis1202 560MCB 6 kA DC2105 120MCB 6-10 kA DC4510 240MCCB 15 kA DC82,520 480MCCB 25-36 kA DC10225 600MCCB 36-50 kA DC ou Fusible aRConclusion: Short-circuit current increases in proportion to the number of batteries in parallel. From 4 modules upwards, standard modular circuit breakers are no longer sufficient. You need to switch to industrial technologies (MCCBs) or specialist fuses.⚠️ Warning: A DC circuit breaker alone is NOT enough to protect a lithium battery. The BMS and HRC fuses are compulsory additional protection.12. Selectivity and Filiation: Optimising the Installation12.1 CascadingThis is the art of using a "strong" upstream circuit breaker to protect a "weak" downstream circuit breaker. If the A circuit-breaker (main LV board) is a high-performance current limiter, it will only let through 10 kA peak in the event of 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 this point.This technique offers massive savings (30-40% on the cost of the switchboard). Warning: It is the responsibility of the designer and only works with equipment from the same manufacturer, validated by laboratory test tables (supplied by Schneider, ABB, Legrand, Hager).12.2 Selectivity (Discrimination)The aim: to ensure that ONLY the circuit-breaker closest to the fault trips. There are three methods:Amperometric selectivity :The upstream circuit-breaker has a higher magnetic threshold than the downstream circuit-breaker. Operates up to a certainIsc, then loses selectivity.Chronometric selectivity :The upstream circuit breaker has a time delay (0.1 to 0.3 s). The downstream circuit-breaker cuts out instantly. Works for allIsc, but introduces a dangerous delay for personnel in the event of an upstream fault.Energy selectivity (Modern) :Electronic circuit breakers (e.g. Schneider Compact NSX, ABB Tmax) analyse the arc energy. If the upstream circuit-breaker "senses" that the energy is low (short-circuit far downstream), it does not trip, knowing that the downstream circuit-breaker will take care of it. This allows total selectivity even with very highIsc, impossible with the old methods.PV recommendation: For an installation with an 8 kVA inverter (40 A circuit breaker) on a 63 A general feeder, check thatIs ≥ Icc_tableau (typicallyIs ≈ 6 kA for this combination).12.3 Total vs. partial selectivityTotal selectivity:The downstream circuit breaker ALWAYS trips before the upstream circuit breaker, whatever the fault current. Condition:In_aval ≤ 0.4 ×In_amont (Schneider Electric rule of thumb)Example: Main circuit-breaker 63 A + dparts 16 A → 16 A ≤ 0.4 × 63 A → total selectivity ensuredPartial selectivity:Selectivity is ensured up to a short-circuit current limitIs, above which both circuit-breakers trip. Manufacturers publish selectivity tables (Schneider Acti9, ABB S200, Hager MJ) indicating the value ofIs for each upstream/downstream combination.13. Technological Perspectives 2026: The Future of Switching13.1 Solid State Circuit Breakers (SSCB)The revolution expected for 2026-2030 is the disappearance of mechanical contacts in favour of semiconductors (Silicon Carbide - SiC or GaN).Advantages :Speed: < 10 microseconds (compared with 10,000 µs for mechanical contacts)Safety: The arc energy is negligible. No risk of flash arc for personnelWear: Infinite. Thousands of load interruptions can be made without maintenance.Intelligence: The circuit-breaker becomes a network analyser (harmonics, cos φ) and can limit the current gently (soft fuse) without cutting off suddenly.Current constraints: Cost still high (× 3-5 for magnetothermal), conduction heat dissipation (RDSon resistance of MOSFETs), conduction losses (0.1-0.3 W), max junction temperature 175°C (requires active cooling above 100 A), and standards certification in progress.13.2 AFDD (Arc Fault Detection Device)Amendment A5 to NF C 15-100 (2021) introduces the requirement for AFDDs in certain high-risk premises (collective housing, ERP). These devices detect series arcs (high impedance, not detected by conventional circuit breakers) using a spectral analysis of the current (detection of frequencies > 10 kHz).Principle: A series arc (loose connection, charred insulation) generates a current of just a few amperes (