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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