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TYPE CLASSIFICATION
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BSEN60898 specifies different tripping characteristics for different types of
MCB, depending on the level of overload current required to make the MCB
trip out in less than 100 milliseconds. Crabtree Polestar and Loadstar MCBs
are available as types B, C or D, enabling installation designers to choose an
MCB with a characteristic closely matched to the circuit requirement.
C50 range is available as type C only.
FAULT LEVELS
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Regulation 432–02–01 of BS7671 requires that a device providing protection
against overload currents and fault currents shall be capable of breaking any
overcurrent up to and including the prospective fault current at the point
where the device is installed.
In domestic situations this could be as high as 16kA, in industrial situations it
could be even higher.
According to regulation 434–03–01, the prospective fault current can be
higher than the breaking capacity of the protective device if another
protective device having the necessary breaking capacity is installed on the
supply side.
This means that MCBs can be backed up by devices of greater capacity such
as HRC fuses.
Crabtree Polestar Type B & C, Polestar Type D and Loadstar ‘H’ Types B, C & D,
Loadstar ‘F’ Types B, C & D and C50 MCBs can protect installations with
prospective currents up to 16kA, 10kA, 6kA and 4.5kA respectively without
the need for back-up devices.
When providing back-up protection, consideration must be given to
discrimination. Discrimination is said to occur when the device nearest the
fault operates first.
AMBIENT TEMPERATURE CONSIDERATIONS
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Polestar and Loadstar MCBs are calibrated to meet the requirements of
BS EN 60898, 30°C Ref Calibration Temperature. At other temperatures the
following rating factors should be used:
At 60°C 0.9
At 20°C 1.0
At 0°C 1.1
Adjacent thermal-magnetic MCBs should not be continuously loaded at or
near their nominal rated currents when mounted in enclosures. It is good
engineering practice to either apply generous de-rating factors or make
provision for adequate free air between devices. In common with other
manufacturers, we recommend a 66% diversity factor is applied to the MCB
nominal rated current where it is intended to load adjacent MCBs
continuously (in excess of 1 hour).
The C50 Range of MCBs meet the requirements of BS EN 60898 for 30°C Ref
calibration temperature, but are virtually unaffected by ambient temperature
variations over their working range from below freezing to 55°C
MCBs – GENERAL
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Crabtree Polestar, Loadstar and C50 range of MCBs comply fully with BSEN60898 and therefore enable the
requirements of BS7671, formerly the IEE Wiring Regulations to be met.
INDIRECT SHOCK RISK PROTECTION
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BS7671, requires that measures are taken to protect against the risk of
electric shock, which can be the result of contact with live parts.
MCBs can be used in conjunction with earthed equipotential bonding to
achieve the required disconnection times of 0.4 seconds for 230V socket
outlets and 5 seconds for circuits supplying fixed equipment (Regulation
413–02–08).
This regulation, together with tables 41B1, 41B2 and 41D,
specifies the maximum permissible earth fault loop impedance which is
allowed using various protective devices. For unspecified ratings the formula
of Regulation 413-02-08 should be used.
EARTH FAULT LOOP IMPEDANCES (Z
S
)
TO GIVE COMPLIANCE WITH
BS7671 REGULATION 413-02-08 AT 230V
Maximum earth fault loop impedance in ohms for circuits supplying socket
outlets (also fixed equipment in bathrooms)
RATINGS
DEVICE
5
A 10A 15A 20A 30A 40A 45A 50A 60A
Rewireable Fuse BS3036 10.0
2.67 1.85 1.14
0.62
Cartridge Fuse BS1361 10.9
3.43 1.78 1.20
0.60
Type C MCB BSEN60898 4.80 2.40 1.60 1.20 0.80 0.60 0.53 0.48 0.40
RENARD SERIES OF RATINGS
DEVICE
6
A 10A 16A 20A 32A 40A 50A 63A
HRC fuse BS88
8.89 5.33 2.82 1.85 1.09 0.86 0.63 –
Type B MCB BSEN60898 8 4.8 3 2.4 1.5 1.2 0.96 0.76
Type C MCB BSEN60898 4 2.4 1.5 1.2 0.75 0.60 0.48 0.38
Type D MCB BSEN60898 2.0 1.2 0.75 0.6 0.38 0.3 0.24 0.19
At these values of loop impedance fuses will operate within 0.4 seconds and
MCBs will operate in 0.1 second.
The loop impedances given in the tables should not be exceeded when the
conductors are at their normal operating temperatures. If the conductors are
at a different temperature when tested, the reading should be adjusted
accordingly.
TECHN I CAL DATA
MCB s
212
RMS PROSPECTIVE CURRENT AS A MULTIPLE OF RATED CURRENT
OPERATING TIME IN SECONDS
2
HRS
1
HR
1,000
100
10
1.0
0.1
0.01
0.001
1
3 5 10 20
100
1,000
Type
B
Type
C
Type
D
POLESTAR & LOADSTAR MCBs