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Protection - Overload

What do we mean by a fault?
Overload
Operating condition in an electrically undamaged circuit which
causes an current to flow in excess of the full load current
Example: Starting condition during DOL start
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If this type of fault continues indefinitely because of an anomolous
operating condition., damage begins to occur creating……….
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Protection – Short Circuit

What do we mean by a fault?
Short Circuit
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Operating condition in an electrically damaged circuit where there is an
accidental or intentional connection by a relatively low resistance
between two points of a circuit which are normally at different
voltages
This type of fault can generate high current flows, arcing and fire if
not cleared quickly
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01-04-06 -
An explanation of the terms
BS EN 60947 standard
for Industrial Switchgear
&
BS EN60898 for
Domestic Circuit
Breakera
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The ‘British Standards’

BS EN 60947 (Industrial switchgear)

BS EN 60898 (Domestic circuit breakers)
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BS EN 60947 Which section ?

EN 60947-2 for Circuit breakers

EN 60947-3 for Switches

EN 60947- 4-1 for Motor protection

EN 60947-6-1 for Auto changeover
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BS EN60947-2

Icu - Ultimate Short Circuit breaking capacity
 O - 3 min - CO ALL at full fault level
 Breaker must be intact and be able to pass insulation resistance test at the
end of the test
 Breaker does not need to able to carry full rated current continuously
without overheating after test

Ics - Rated duty Short Circuit Breaking capacity
 Often expressed as a % of Icu
 O - 3 min - CO - 3min - CO
 The test may be at fault level lower than Icu value
 Breaker must be intact and be able to pass insulation resistance test at the
end of the test
 Breaker MUST be able to carry full rated current continuously without
overheating after test

Note:
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O test begins with breaker closed before current applied
CO test requires breaker to close onto fault current
BS EN60947-2

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
Icw - Rated Short-time Withstand current

Preferred rating - 1 sec

Limits are

12x In or 5kA whichever is greater for In <= 2500A

30kA for In > 2500A
Utilisation Category

A type - Instantanous type

B type - Selective short circuit

B type MUST have withstand (Icw)rating defined
Icm Rated duty Short Circuit Making Capacity
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BS EN60947-2

It is recommended that circuit breakers are examined for
contact and arc chute damage after every fault
clearance

Icu/Ics values mean breakers can be put back into
temporary service after fault clearance
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Definitiions within BE EN 60947

In – Nominal current rating

Iu – Rated uninterrupted current

Ue – Operational Voltage
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AC/DC Distribution
Which mcb curve?

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
BS EN 60898 B type

General purpose lighting & power

Magnetic setting 3 - 5x In
BS EN 60898 C type

Industrial application with high surges

Magnetic setting 5 - 10x In
Different devices for DC applications above 60V
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