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MCB principle, MCB/MCCB/RCCB, HRC cut-off

Short answer

MCB uses bimetal for overload, solenoid for short circuit, arc chute quenching. MCB (≤125A, 6-10kA) for final circuits; MCCB (≤2500A, adjustable) for feeders; RCCB detects earth leakage via CT for shock protection. HRC fuse cut-off: melts before peak, limiting let-through current and energy, enabling high breaking capacity (80-120 kA).

Explain the operating principle of MCB. What are the differences between MCB, MCCB and RCCB? What are the cut-off current characteristics of HRC fuse.

Operating principle of MCB

A miniature circuit breaker trips automatically on overload and short circuit:

  • Thermal trip: load current heats a bimetal strip. On a sustained overload it bends and releases the trip latch after a time inversely related to the current.
  • Magnetic trip: a solenoid coil carries the current. A short-circuit current (several times rated, e.g. 5–10 × In for C curve) pulls the plunger instantly and trips the latch.
  • The contacts open and the arc is driven into an arc chute, where it is split and extinguished. The MCB can be reset by its lever.

Differences between MCB, MCCB and RCCB

PointMCBMCCBRCCB
Full nameMiniature CBMoulded case CBResidual current CB
Protects againstOverload, short circuitOverload, short circuit (some earth fault)Earth leakage, shock
Current ratingUp to about 100–125 AAbout 16 A to 2500 A16–100 A
Breaking capacity6–10 kA (up to 25 kA)16–200 kANot for short circuit
Trip settingFixedOften adjustableFixed sensitivity (30, 100, 300 mA)
PrincipleBimetal + solenoidThermal-magnetic or electronicCore-balance CT senses I(L) − I(N)
UseFinal circuitsMain feeders, motorsWith MCB for shock protection

Cut-off current characteristic of HRC fuse

When a short circuit occurs, the prospective current would rise to a very high peak. An HRC fuse melts during the first rising part of the current wave, before the first peak, so the current is cut off at a much lower value.

 i  prospective current
 ^      .-.
 |     /   \
 |    / .   \
 |   / /\    \      cut-off
 |  / /  \    \ ← current (actual)
 | / /    \    \
 |/_/______\____\______> t
   |←pre-arcing→|←arcing→|
     total operating time
  • Pre-arcing time: from fault start until the element melts.
  • Arcing time: from melting until the arc is quenched by the quartz sand.
  • Cut-off current: the peak current actually reached, much less than the prospective peak.
  • The cut-off characteristic is a graph of cut-off current against prospective current for each fuse rating. It shows that the HRC fuse is current limiting: the let-through energy (I²t) and electromagnetic forces are very small, which protects cables and switchgear.

✓ Answer checked by Claude Opus 5.5 (high effort)

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