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TECHN I CAL DATA
MCB s
213
POLESTAR
Polestar MCBs are of the thermal-magnetic current limiting type.
There are 3 distinct modes of operation:
1
Small overload conditions
Small overload currents are detected by the use of a thermo-metal, which
deflects at a rate in proportion to the size of the overload. The thermo-metal
moves against a latching system which releases the contacts, allowing them
to open under spring pressure.
2
Large overload conditions
If the overload current reaches a predetermined level (which depends on the
current rating and type classification of the MCB), then the current in the coil
produces a magnetic field in the solenoid which is strong enough to pull in
the armature and operate the latching mechanism. Again the contacts open
under spring pressure.
3
Short circuit conditions
If the fault current is of a high enough level, not only does the solenoid trip
the mechanism, it forces the contacts apart very rapidly in a process known as
‘
hammer trip’.
Under these conditions as the contacts separate an arc is drawn between
them. The combination of magnetic fields in the MCB and the flow of the
current in the arc acts to push the arc along the runners and into the arc
chamber where it is quickly extinguished. The rapid opening of the contacts
and extinction of the arc give a total operating time that is typically 3.5–5
milliseconds.
C50
The C50 MCB employs the hydraulic/magnetic principle, the heart of which is
a hermetically sealed tube filled with silicone fluid and containing a closely-
fitting iron slug. In normal load conditions, the magnetic pull from the trip coil
is unable to overcome the restoring force of the time delay spring, and the
iron slug remains at the far end of the tube.
When an overload occurs, the magnetic field pulls the slug through the tube,
the speed of travel being governed by the magnitude of the current.
As the slug nears the end of the tube, the gap in the magnetic circuit is
reduced, so increasing the pull on the armature until it moves and the breaker
is tripped.
If a large overload or short circuit occurs, the magnetic field generated is much
greater and the armature moves without having to wait for the slug to reach
the end of the tube. In this way instantaneous tripping occurs.
INSTALLATION CONDITIONS
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When used in Crabtree distribution boards and consumer units, Polestar and
C50 MCBs are mounted on specially-designed rails for ease of installation.
Polestar MCBs are also suitable for use in custom built panels, where they
should be mounted on standard 35mm top hat rail to BS 5584: 1978 EN50022
giving a projection within the standard 70mm.
Arc runner
15
plate
arc chamber
Arc runner
Thermo-metal
element
Calibration
mechanism
25
mm
2
max
tunnel
terminal
Moving
contact
Latch
mechanism
Fixed
contact
Operating
handle
Coil
assembly
Trip
bar
25
mm
2
max
’
Jaw‘ terminal
DIN rail
mounting
Fixed
contact
Main
spring
Trip
lever
Trip
spring
Moving
contact
hinge pin
Catch
plate
Armature
Armature
spring
Pole
piece
Moving
contact
Stop pin
Knee joint
hinge
Toggle
links
Sealing
wire
hole
Trip lever
re-set pin
Catch
spring
Time delay
device
Magnet
yoke
Overload
coil
Cable
clamp
Busbar
clamping
screw
Arc grid
plates
MCBs – POLESTAR & C50
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METHOD OF OPERATION
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