Category: Square D MCBs Technical Data
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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)
MCB Construction
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 thedisconnection times for electric shock
protection.
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. Theselection 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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Square D
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