close and latch capability of high voltage power circuit breakers

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  • 8/13/2019 Close and Latch Capability of High Voltage Power Circuit Breakers

    1/2Powered by Safety

    Powell Technical Brief #2 - Revised

    December 29, 2006

    In 2000, ANSI Standard C37.06, which identies the typical recommended rating for circuit breakers above 1000VAC,changed from the 1987 standard. This Technical Brief describes the importance of understanding the change toclose and latch capability as it relates to the circuit breaker and switchgear manufactured by Powell Industries.

    The close and latch capability of a circuit breaker is the ability of the breaker to be closed onto a faultedsystem and stay closed to allow the fault to be cleared down stream. The close and latch rating allows the faultedportion of the electrical system to be isolated, de-energizing the smallest portion of the electrical system as dictatedby system selectivity. For the circuit breaker to pass the close and latch type test, the breaker must be designed toovercome the mechanical forces acting on the breaker mechanism, primary disconnect and racking the systemwhen the rst major loop of current passes through the equipment. The breaker closing mechanism must bestrong enough to stay latched once given the command to close and the tripping opening mechanism must be ofsufficient force to break any welding that may have formed in the moving contacts due to the energy of the fault.

    Prior to 1987, close and latch capability was given in an asymmetrical rms value dened as the symmetricalinterrupting rating multiplied by 1.6. It referred to the current at the rst major loop of current passes throughthe circuit breaker in the cycle after closing. In 1987, the standard was changed to an expression of crestvalue dened by multiplying the maximum symmetrical interrupting current by 2.7. (See the gure below.)

    Figure - Latch and Close Rating in Crest and Asymmetrical rms for a Circuit Breaker rated for 50kA SymmetricalInterrupting at Contact Part

    Close and Latch Capability of High Voltage Power Circuit Breakers

    135kA crest Latch & Close

    80kA rms Asym Latch & Close

    Load Current

  • 8/13/2019 Close and Latch Capability of High Voltage Power Circuit Breakers

    2/2Powered by Safety Powell Electrical Systems, Inc.PO Box 12818

    Houston, TX 77217

    Tel: 713.944.6900 Fax: [email protected] Powell Industries, Inc. All rights reserved.

    05/2008 Rev 0Publication No. 03084

    In 2000, the latch and close rating was changed again as a compromise with the IEC standard which had been basedon a 2.5 multiplier for many years. The IEC standard body agreed to increase the test value for IEC breakers to coincidewith the 2.6 multiplier for 60Hz equipment.

    Powell Industries opted to disregard the 2000 change to the standard and continues to manufacture equipment con-sistent with the higher test values given by the 2.7 multiplier. Powell feels the stricter standards are more consistentwith the high x/r ratios in which its customers apply Powell equipment.

    Given the changes to C37.06 over the years and considering the variety of standards quoted in specications, it isimportant to use the correct test values for the circuit breakers being applied. Table 1 below correlates the variousvalues. Note the Powell standard values equal to the 1987 Edition.

    Table - Closing and Latching Capacity Test ValuesRated

    MaximumVoltage

    (kV)

    MVA Class1979 Standard

    MVA Class1987 Standard

    K = 12000 Standard

    K = 1Powell Standard

    Short Circuitat Max Volts

    (kA)

    Latch & Close(Asym rms)

    (kA)

    Short Circuitat Max Volts

    (kA)

    Latch &Close (Crest)

    (kA)

    Short Circuitat Max Volts

    (kA)

    Latch &Close (Crest)

    (kA)

    Short Circuitat Max Volts

    (kA)

    Latch & Close(Crest)

    (kA)

    4.76 29 (250 MVA) 58 29 97 31.5 82

    40 104 36 97

    4.76 41 (350 MVA) 78 41 132

    4.76 50 130 50 135

    4.76 63 164 63 170

    15.0 18 (500 MVA) 37 18 62

    25 65 25 65

    15.0 28 (750 MVA) 58 28 97 36 97

    31.5 82

    36 97

    15.0 37 (1000 MVA) 77 37 130 40 104

    15.0 60 130 50 135

    15.0 63 164 63 170

    * K=1 mean interrupting current is not a function of the operating voltage

    Please contact us at 713-944-6900 or [email protected] if you have questions or need assistance with informationcontained in this Technical Brief.

    Powell Technical Brief #2 was originally authored by Baldwin Bridges and issued on May 18, 1990. It has been revised andupdated by Jim Bowen and reissued.

    Jim Bowen Technical Director