Contents (6 sections)
1. Which device at which level
Quick answer
A low-voltage electrical system is usually divided into levels: the main switchboard receives power from the transformer or the grid, distribution boards split it out to each area, and final circuits supply lights, socket-outlets and machines. The LS Susol catalogue divides the manufacturer's circuit-breaker ranges according to exactly these levels, with the rated current and breaking capacity ranges of each type. This is LS's own division. Other manufacturers have different ranges, so the figures below are for guidance only; when choosing an item, read the catalogue for that exact item code.
| Position | Type | Rated current | Breaking capacity Icu | Ics (% Icu) | Selectivity category |
|---|---|---|---|---|---|
| Main switchboard | ACB | 630–6300 A | 65–150 kA | 100 % | B |
| Main switchboard | MCCB | 1000–1600 A | 50–150 kA | 75–100 % | A, B |
| Main switchboard, distribution board | MCCB | 16–800 A | 37–150 kA | 100 % | A |
| Distribution board | MCCB | 3–1200 A | 10–85 kA | 100 % | A |
| Final circuit | MCB | 1–125 A | 6–10 kA | 50–100 % | A |
Swipe sideways to see all columns.
Source: LS Susol MCCB catalogue (2011), page "Range of LS low voltage circuit breakers". Standards stated by the manufacturer: IEC 60947-2 for ACB and MCCB; IEC 60898 and IEC 60947-2 for MCB.
2. Four questions for each device
Whether it is an ACB, MCCB or MCB, every switching device must be checked against the same four questions. The first three derive from the overcurrent protection requirements of QCVN 12:2014/BXD (clause 2.6), the fourth from the protection against electric shock requirements (clauses 2.3, 2.4).
- What rated current? The overload protective device must satisfy IB ≤ In ≤ Iz and I2 ≤ 1.45 × Iz (QCVN 12:2014/BXD, clause 2.6.3.1); Iz is looked up in the cable current-carrying capacity tables. For how to do this for a specific circuit, see selecting an MCB by load current, or use the circuit-breaker selector.
- What breaking capacity? The breaking capacity shall not be less than the prospective short-circuit current at the point of installation, unless a device with sufficient breaking capacity is already installed on the supply side and the two devices are coordinated (clause 2.6.5.4). See selecting an MCCB by short-circuit current.
- What tripping characteristic? For an MCB it is curve B, C or D; for an MCCB or ACB it is a fixed thermal-magnetic trip unit or an adjustable electronic trip unit. See reading the B, C and D curves of circuit-breakers.
- Is residual current protection needed? Circuits supplying hand-held equipment must have a residual current device (RCD) with a rated residual operating current IΔn not exceeding 30 mA (clause 2.3.3.4). See how RCCB, RCBO and ELCB differ.
3. Main switchboard: ACB or MCCB
IEC 60947-2 classifies circuit-breakers by selectivity category. Category B has a rated short-time withstand current (Icw) and allows a short-circuit trip delay so that the downstream device trips first; this category is typically the air circuit-breaker (ACB) and some large MCCBs. Category A covers the remaining circuit-breakers, typically MCCBs and MCBs, which trip instantaneously on short circuit (Schneider Electrical Installation Guide, section "Selectivity categories").
The LS table above shows that both ACBs and large-frame MCCBs are used at the main switchboard. When to choose which, and when an automatic transfer switch is needed for a generator: see when to use an ACB and an ATS.
4. Final circuits: MCB and residual current protection
On final circuits, the MCB handles overload and short circuit, while detecting residual current is the job of the RCD (QCVN 12:2014/BXD, clause 1.4.24). For TT and TN-S earthing systems, the regulation requires an RCD for protection against faults to exposed conductive parts (clause 2.4.2.2). An RCCB provides only residual current protection and must therefore be paired with a circuit-breaker; an RCBO combines both functions in one device.
5. Coordination between levels
The levels of circuit-breakers must work together in two ways. Selectivity: when a fault occurs on a downstream circuit, only the circuit-breaker immediately upstream of the fault trips, and the rest of the system stays energised. Back-up protection (cascading): the upstream circuit-breaker can break the maximum short-circuit current, allowing downstream circuit-breakers with a lower breaking capacity, but only for combinations listed in the manufacturer's tables. Details in coordinating cables and circuit-breakers.
6. What to send to get a quotation for the right item code
Switching devices with the same rated current may differ in breaking capacity, number of poles and characteristic, so a line reading "MCCB 100 A" is not enough to select an item code. When sending a request, state the information below, or attach the switchboard schematic diagram. How to prepare a bill of quantities: preparing a BOQ to receive a quotation.
- Device type and number of poles (1P, 2P, 3P, 4P).
- Rated current In, or setting range for electronic trip units.
- Required breaking capacity and the associated voltage (e.g. Icu at 415 V).
- Characteristic B, C or D for MCBs; thermal-magnetic or electronic for MCCBs and ACBs.
- For RCDs: operating current IΔn (30 mA, 100 mA, 300 mA…) and type AC, A or B.
- Brand specified in the design documents, if any.
Sources and scope
- QCVN 12:2014/BXD clauses 1.4.24, 2.3.3.4, 2.4.2.2, 2.6.3.1, 2.6.5.4
- LS Susol MCCB catalogue (2011)
- Schneider Electric, Electrical Installation Guide, section "Other characteristics of a circuit-breaker" (accessed 14/10/2026). Compiled by Hoang Nhat
- not a substitute for design and calculation by an engineer