Post on 25-Jan-2016
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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
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
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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BS EN 60947 standard for Industrial Switchgear
&BS EN60898 for Domestic Circuit
BreakeraAn explanation of the terms
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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: O test begins with breaker closed before current applied
CO test requires breaker to close onto fault current
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BS EN60947-2
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 DistributionWhich mcb curve?
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