Square D
MCBs Technical Data
Definition of Curves (BS EN60898)
Curve B Protection of generators, long
cables, heating loads, tungsten lighting and persons (on TN and IT
neutral systems), residential. Overloads
Standard thermal protection. Short Circuits
Fixed magnetic curve B (Im between 3 and 5 In)
Curve C Protection of cables supplying loads
with relatively high switching surges e.g. discharge lamps and some
motors. Overloads
Standard thermal protection. Short Circuits
Fixed magnetic curve C (Im between 5 and 10 In)
Curve D Protection of cables supplying
loads with high switch on surges. Overloads
Standard thermal protection. Short Circuits
Fixed magnetic curve D (Im between 10 and 20 In)
External overload protection using a bi-metallic element provides
close overload protection for cables. Magnetic
protection using a solenoid means a fast operation during a phase/
neutral or phase / earth short circuit. It reduces electrical stress
in the system and helps meet the disconnection times for electric
shock protection. MCB Construction
For each rating of MCB (10 amp, 20 amp, 40 amp etc)
three distinctly different types of MCB are specified in BS EN 60898
- types B, C and D. The difference between these types relates to
the instantaneous tripping characteristics. The levels of over
current required for instantaneous tripping for the three main types
of MCB are shown in Table 51. Table 2 lists the fault loop impedance
values that correspond with the current required to ensure tripping
of MCBs within the requirements specified in Chapter 41 “Protection
Against Electric Shock” in the Wiring Rules. The selection of a
suitable type of MCB is an important factor in the design of a
circuit to ensure protection against electric shock, i.e. to ensure
that the protective device trips within the required time in the
event of a fault to earth. The required conditions will be met if
the circuit loop impedance is within the maximum permitted level, as
shown in Table 52. MCBs for industrial and commercial use normally
have a breaking capacity in excess of 10kA (KQ MCB = 15kA) to BS EN
60947-1.
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Technical information associated with:
Square D - MCBs Technical Data
Square D - MCBs Technical Data Part 2
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