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CIRCUIT PROTECTION
583
Sentry Technical
Miniature Circuit Breakers (MCBs)
3. Fault breaking capacity
All Sentry MCBs have a short circuit breaking capacity of 6,000A (M6).
For applications where the prospective fault current is in excess of this, a BS 1361,
100A (maximum) fuse should be used upstream of the MCB to provide a system
breaking capacity of 16,000A (in accordance with BS EN 60439-3).
4. Discrimination
A Sentry MCB consumer unit will normally be supplied via an HRC fuse. The HRC
in such instances will be the major device and remain unaffected by any fault
current which causes the MCB to operate.
The level of fault current up to which this can be assured is determined by
comparing the I
2
t characteristics of the two devices. Discrimination will theoretically
occur up to the level at which the value of the total operating I
2
t of the MCB is below
the minimum pre-arcing I
2
t of the fuse, although in practice, discrimination will be
achieved at higher levels than this.
5. Cable protection
The current carrying capacity of the cable should always exceed the current rating
of the MCB to prevent damage.
However, should this not be the case, a further calculation may show that the MCB
can still interrupt the current in a sufficiently short time to prevent overheating of the
cable insulation. Although this will prevent mechanical damage to the cables, further
overload protection should be provided by a separate device, e.g. a motor overload
relay.
In case of doubt please contact the MK Technical Sales and Service Department.
Dimensions (mm)
17.5
83
43.7
6.8 66
61
ON
SENTRY