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Miniature Circuit Breaker (MCB). Not to be confused with a Residual Current Device (RCD).

The MCB trips (cutting off the downstream electrical load) whenever the current exceeds the rating, based on a tripping curve. Different tripping curves are denoted by the letter prefixing the current rating. B curve types illustrated. Dimmers are often fitted with C curve breakers to prevent nuisance tripping from the inrush current into a cold lamp (this can be reduced by preheat) or the large surges created by certain lamp types when they blow.

Other specification options include:

• Mounting (DIN-rail is the norm)
• Breaking Capacity (6kA and 10kA are now common while early designs were only capable of 3kA or so)
• Number of poles:
  • Single Pole (illustrated)
  • Single Pole with Neutral Disconnect
  • Double Pole
  • Three Phase
  • Three Phase + Neutral
• Thermal or Thermal-Magnetic sensing

The thermal trip characteristic protects against long-term overcurrent faults. The magnetic trip characteristic provides a fast disconnect in the case of a short-circuit, the current at which this operates is denoted by the letter prefixing the current rating. B type trip at between three and five times the rated current and are generally reserved for domestic and very light commercial use. C type trip between five and ten times the rated current and are used for everything from motors to fluorescent lighting. D type trip between ten and twenty times the rated current and are used for very inductive loads such as very large motors and welding equipment. The resulting response curve of time against current is usually given in the MCBs datasheet.

BS EN 60898-1:2003+A1:2004 - Circuit Breakers for overcurrent protection for household and similar installations - Part 1: Circuit-breakers for a.c. operation,:

• Standard ranges of instantaneous tripping:

Type B - Above 3 In up to and including 5 In

Type C - Above 5 In up to and including 10 In

Type D - Above 10 In up to and including 20 In

• Discussion