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Understanding IEC 60947-2 (Circuit Breaker) Choice Criteria

Updated on June 9, 2015

Circuit Breakers

Introduction

IEC 60947-2 is a subset of IEC 60947 for industrial application. Basically IEC 60947 consists of 7 parts which are:

  • IEC 60947-1 Part 1 : General rules
  • IEC 60947-2 Part 2 : Circuit breaker
  • IEC 60947-3 Part 3 : Switch disconnectors
  • IEC 60947-4-1 Part 4 : Contactors
  • IEC 60497-5-1 Part 5 : Control circuit devices
  • IEC 60497-6-1 Part 6 : Multifunction devices (bypass switch, transfer switch)
  • IEC 60947-7-1 Part 7 : Auxiliary materials

2.0 Rated Values

2.1 Rated Uninterrupted Current Iu

Rated uninterrupted current, Iu, is the current that the circuit breaker can carry uninterrupted for an infinite period of time. It is deferent from rated current, In, which is the current that the thermomagnetic or electronic trip unit. In is lower or equal to Iu.

Factors that affecting Iu are temperature and altitude. An increase in temperature will decrease the Iu of the circuit breaker. Besides that, an increase in altitude also will decrease the Iu of the circuit breaker.

2.2 Rated Operational Voltage Ue

Rate operational voltage, Ue, is the maximum voltage that can applied continuously to the circuit breaker without harming the circuit breaker.

When the Ue, of the circuit breaker increase its breaking capacity decreases. The altitude will also affect the Ue of the circuit breaker. Higher altitude will decreases the Ue of the circuit breaker.

3.0 Limit Values

3.1 Rated Ultimate Short Circuit Breaking Capacity, ICU

Rated ultimate short circuit breaking capacity, ICU, is the maximum breaking capacity of the circuit breaker. The circuit breaker meeting the ICU does not guarantee to be able to carry its rated current continuously after the fault.

Test sequence for ICU: O – 3 min – CO

The terminal of the close circuit breaker is intentionally short using cable / copper bar, then the rated operating voltage, Ue, is input into the circuit breaker. The circuit breaker tripped (open). Wait for 3 minutes then when the Ue applied to the circuit breaker and maintain the fault, the circuit breaker is forced to closed and allow to tripped (open) again.

(Caution: This test will be tested using automated machine please do not try to test the circuit breaker using your hand, it might not pass the test and you will lose your arm permanently)

After passing the O – 3 min – CO, the circuit breaker is test for:

  • Dielectric withstand at 2 x Ue
  • Verification of overload release at 2.5 x I1

3.2 Rated Service Short Circuit Breaking Capacity, ICS

Rated service short circuit breaking capacity, ICS, is the breaking capacitor which the circuit breaker met and still able to carry its rated current continuously. ICS can be less than or equal to ICU and usually expressed as a percentage of ICU. (example ICS = 25% ICU)

Test sequence for ICS: O – 3min – CO – 3min – CO

The test for ICS is roughly the same with ICU only that it is required to go for process two twice.

(Caution: This test will be tested using automated machine please do not try to test the circuit breaker using your hand, it might not pass the test and you will lose your arm permanently)

After passing O – 3 min – CO – 3 min – CO, the circuit breaker is test for:

  • Dielectric withstand at 2 x Ue
  • Verification at temperature rise at Iu
  • Verification of overload release at 1.45 x I1
  • Verification of the electrical life

3.3 When to use ICU or ICS

Now we know the different between ICU and ICS. To save cost we not necessary use ICU as the breaking capacity of the circuit breaker for all the application.

Use ICU when:

  • Continuity of service is not the fundamental requirement (allow the maintaining team to change the circuit breaker)
  • For motor protection
  • When there is only single load (outgoing of a distribution board)
  • Where maintenance work is easily carried out
  • When the circuit breaker rating is common in the market.

Use ICS when:

  • Continuity of service is a fundamental requirement (data centre, oil and gas, power station)
  • At the upstream (main switchboard)
  • Where maintenance work is difficult
  • Difficult to manage spare breaker (ACB)

3.4 Rated Short – Time Withstand Current, ICW

Rated short – time withstand current, ICW, is usually found in category B circuit breaker (ACB). ICW is the short circuit current the circuit breaker can withstand for a specific time, usually 1s or 3s. However 1s is recommended because by withstand the high current for a long period of time it might damage the more expensive equipment such as transformer, genset.

3.5 Rated Short – Circuit Making Capacity, ICM

Rated short – circuit making capacity, ICM, is the making capacity of the circuit breaker which the prescribed conditions according to a specified test sequence include the capability of the circuit breaker to make the peak current corresponding to that rated capacity at the appropriate applied voltage.

It is always necessary to verify that : Icm ≥ Ipeak

Category B Circuit Breaker Time-Current Curve

Source

4.0 Insulation Value

4.1 Rated Insulation Voltage, Ui

Rated insulation voltage, Ui, is the rated insulation voltage of a circuit breaker is the value of voltage to which dielectric test and creepage distances are referred.

It shall be always verified that : Ue ≤ Ui

4.2 Rated Impulse Withstand Voltage, Uimp

Rated impulse withstand voltage, Uimp, is the peak value of an impulse voltage of prescribed form and polarity which the circuit breaker is capable of withstanding without failure under specified conditions of test and to which the values of clearances are referred.

It shall be always verified that : Uimp ≥ transient overvoltage in the plant

Conclusion

With this article, hopefully you guys is able gain some understanding on the value printed on a circuit breaker data sheet and have a better selection of the circuit breaker.

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