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Thread: Proposal for Defined Instantaneous Trip circuit breakers

  1. #1
    Join Date
    Nov 2005
    Berlin, MA

    Lightbulb Proposal for Defined Instantaneous Trip circuit breakers

    2008 NEC Proposal 10-55 consists of 3 new text additions to article 240, labeled A, B, and C here. They should be considered together.
    The text below differs in format from the text originally proposed. The text below is being offered to the CMP as Recommended Committee Action Text in the comment phase.

    1.) NEC Section/Paragraph: Article 240
    2.) Proposal Recommends: [new text]
    3.) Proposal: [proposed new wording]

    [Add new definition to 240.2 Definitions.]
    Defined Instantaneous Trip (DIT) Circuit Breaker. A circuit breaker that opens within 1 cycle of being subjected to an overcurrent equal to 11 times its rated current or greater.

    [Note: included in the formal proposal is additional text poposed as revision to UL 489. This text was drafted by UL in 1993 in conjunction with a previous proposal made at that time and has been revised by the submitter for use with this proposal.
    4.) Substantiation: This new definition in 240.2 will define a threshold current level at which an overcurrent condition is no longer recognized by a breaker as an overload, but a fault, and a specified clearing time for any overcurrent above the defined threshold. Despite widespread public expectation to the contrary, UL 489, to which standard circuit breakers are designed, requires no specified response to parallel 120-volt branch circuit-level fault current. As a result, breaker response to short-circuits or ground-faults at 120-volt outlets is inconsistent, resulting in many 15A/120-volt outlets with voltage-drop greater than 5% being protected by an inverse-time, thermal-only response. Data recently gathered using a test instrument that conducts a time-controlled short circuit from an outlet and measures the breaker’s response time to the short-circuit test indicates that 57% of the 1017 outlet tests gathered in the sample had no instantaneous response protection at all, only much slower thermal protection. Since in non-metallic cable wiring systems the equipment grounding conductor is the same gage as the circuit conductors, it is likely that the available fault current on the equipment grounding conductor is similar in magnitude to the available short-circuit current, resulting in the inability of circuit breakers in many instances to provide an effective magnetic (instantaneous response) to a fault condition. Article 250.4(A)(5) requires the installation of a low-impedance equipment grounding conductor “to facilitate the operation of an overcurrent protective device” in response to fault current. Because of this lack of standardization of breaker magnetic response, even though an equipment-grounding conductor is installed in full compliance with article 250, the intent of 250.4(A)(5) is often not met.

    Also, given continuing uncertainty surrounding cause and origin of electrical fires, particularly in residential infrastructure, and the limitations of other technologies to address all of the possible causes of electrical ignition, (AFCIs are ineffective in detecting non-arcing but potentially hot fault conditions), thermal responses to fault-conditions should be eliminated to the greatest extent possible to reduce the risk of ignition from a fault.

    Some have said that this proposal belongs in the standards process, not in the NEC. The NEC often stipulates specific product requirements pertaining to certain performance requirements. Chapters 3 and 4 are full of such requirements which are stipulated in the NEC and implemented through the standards process.

    1.) NEC Section/Paragraph: Article 240
    2.) Proposal Recommends: [new text]
    3.) Proposal: [proposed new wording]
    240.87 Circuit Breakers Installed in Dwelling Units. 120/240-volt circuit breakers rated 50 amperes or less installed in dwelling units to meet the requirements of 210.20 shall be of the DIT type.

    Exception No. 1: Use of non-DIT type circuit breakers in compliance with 240.4 shall be permissible at the discretion of the AHJ on circuits protecting specific loads with high inrush current or motor starting current where a DIT breaker can be shown to nuisance trip.

    Exception No. 2: Where 240.3 or 240.4(G) applies.

    4.) Substantiation: A new article 240.87 will limit the installation requirement of a DIT breaker to dwelling units, with exceptions for certain hi-inrush loads, specific conductor applications and where other overcurrent protection articles apply. The reason for the limitation to dwelling units is that evidence exists that instantaneous clearing at 11 times rated current will not produce an unacceptable level of nuisance tripping in the residential environment. The exceptions allow for practical considerations of specific circumstances of installation already recognized by the Code to continue to be recognized, and for exception to be made for specific appliances with exceptionally high inrush current. It should be noted 240.87 is intended to include ALL 120/240 circuit breakers rated 50A or less installed in dwelling units are included in the requirement, including AFCI and GFCI circuit breakers.

    1.) NEC Section/Paragraph: Article 240
    2.) Proposal Recommends: [new text]
    3.) Proposal: [proposed new wording]
    [add new marking requirement (F) to 240.83 Marking]
    240.83(F) Circuit breakers installed in accordance with 240.87 shall be marked with the letters “DIT” in accordance with 240.83(A).

    4.) Substantiation: This labeling requirement will permit easy recognition after installation of a standard circuit breaker that meets the DIT performance requirement from one that does not.

    Your comments and/or suggestions would be appreciated.

    Paul Keleher, submitter of proposal 10-55

  2. #2
    Join Date
    Feb 2003
    There is a large range in the magnetic trip points for the brands of breakers that are currently available. These trip points range from 8 times the handle rating to 40 times the handle rating. Most installers assume that all of the UL listed breakers provide approximately the same protection, but with the huge range in the trip points for short circuits and ground faults, the various listed breakers do not provide equal protection. The acceptance of this proposal would change this by requiring the magnetic trip point to be 11 times the handle rating or less. There is no real reason not to accept this change as it provides better protection at no increase in costs.
    Don, Illinois
    Ego is the anesthesia that deadens the pain of stupidity. Dr. Rick Rigsby
    (All code citations are 2017 unless otherwise noted)

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