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Protection - Overload

Learn about electrical faults, including overload and short circuit conditions, and how to protect circuits. Includes explanations of BS.EN 60947 standards and different MCB curves.

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Protection - Overload

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  1. 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……….

  2. 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

  3. BS EN 60947 standard for Industrial Switchgear&BS EN60898 for Domestic Circuit Breakera An explanation of the terms

  4. The ‘British Standards’ • BS EN 60947 (Industrial switchgear) • BS EN 60898 (Domestic circuit breakers)

  5. 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

  6. 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

  7. 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

  8. 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

  9. Definitiions within BE EN 60947 • In – Nominal current rating • Iu – Rated uninterrupted current • Ue – Operational Voltage

  10. 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

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