MCCB
Breaker
A moulded-case circuit breaker: the industrial big brother of the MCB. Handles 100–1600 A frames, adjustable trip settings, and can interrupt very large fault currents in panels and machine feeders.
Schematic symbol
Switch (SPST)
How mccb appears in a circuit diagram.
Types & variants
Key specs
Frame / rated current (In)
AThe breaker's size class and continuous rating.
Breaking capacity (Icu)
kALargest fault it can safely interrupt — must exceed the panel's fault level.
Trip settings (Ir, Im)
×InAdjustable thermal (overload) and magnetic (short-circuit) thresholds.
Poles
3P for motors/feeders, 4P where neutral is switched.
Laws that govern it
- Joule's law of heatingH = I² × R × t
Adjustable thermal settings are an I²t curve you tune to the cable and load you are actually protecting.
- Ampère's circuital law∮ B · dl = μ₀ × I(enc)
The magnetic trip responds to the fault current's field, giving instantaneous clearing above the set multiple.
Breaking capacity (kA) is about arc interruption — how much fault energy the arc chamber can quench without the contacts welding.
Each law is stated in full — with its diagram, variables and worked meaning — on the electrical laws page.
How it compares
- MCB vs MCCB
Side by side with MCB (Miniature Circuit Breaker) — specs, variants and which one the job calls for.
Markings
Nameplate: frame size, In, Icu/Ics at voltage, trip curve/settings dials on electronic units.
Standard values
Frames: 100/250/400/630 A; breaking capacities 25–70 kA typical.
How to choose
This is design-engineer territory: rated current above load with margin, breaking capacity above prospective fault current, and trip settings coordinated with upstream/downstream devices. For homes, an MCB + RCCB is the right tool instead.
Pinout & package
Line in top, load out bottom (unless marked reversible), panel-mounted with phase barriers.
Example circuits
- Main incomer of a distribution panel
- Feeder protection for a 30 kW motor via starter
- Sub-panel isolation
Common failures
Welded or pitted contacts after clearing big faults, nuisance trips from wrong settings or heat, mechanism stiffening in old units — MCCBs that have cleared a major fault should be inspected/replaced.
How to test
For electricians: with power OFF, continuity through each pole (closed = ~0 Ω, open = infinite) and insulation resistance between poles. Trip-curve verification needs a primary-injection test set — not a DIY job.
Substitutes
Like-for-like on frame, In, Icu and trip curve, coordinated with the installation design. Never downgrade breaking capacity.