Page 1 of 3 123 LastLast
Results 1 to 10 of 25

Thread: E-Stop in motor controls

  1. #1
    Join Date
    Feb 2007
    Location
    Colorado
    Posts
    1

    E-Stop in motor controls


    What does the NFPA say about the use of the E-Stop circuit when it comes to breaking the power to a three phase motor? If we have a motor controller that has dynamic breaking, does the code allow the E-stop circuit to apply dynamic braking and then interrupt the power to the motor with a contactor? Is the contactor interrupt before or after the motor controller? Does the E-stop circuit require a contactor interrupt to the motor or is the motor controller sufficient?

  2. #2
    Join Date
    May 2004
    Posts
    19

    Post NFPA 79 is a good start

    NFPA 79

    Stop Categories:

    Category 0 = an uncontrolled stop by immediately removing power to the machine actuators

    Category 1 = a controlled stop with power to the machine actuators available to achieve the stop then remove power when the stop is achieved.

    Category 2 = a controlled stop with power left available to the machine actuators.

    Each machine shall be equipped with a Category 0 Stop.

    Category 0,1, and/or 2 stops shall be provided where indicated by an analysis of the risk assessment and the functional requirements of the machine.

    The emergency stop shall function as either a Category 0 or a Category 1 Stop. The choice of the category of the emergency stop shall be determined by the risk assessment of the machine.

    10.7.5 Local Operation of the supply Disconnecting Means to Effect Emergency Stop.

    10.7.5.1 The supply disconnecting means shall be permitted to be locally operated to serve the function of emergency stop as follows:
    (1) Where it is readily accessible to the operator
    (2) Where it is of the type described in 5.3.2(1), 5.3.2(2), or 5.3.2(3)

    5.3.2
    (1) A listed motor circuit switch (switch disconnector) rated in horsepower.
    (2) A listed, branch circuit rated molded case circuit breaker
    (3) A listed molded case switch.

  3. #3
    Join Date
    May 2004
    Posts
    19

    Post NFPA 79 one more note

    9.2.5.4.2.2. Emergency switching off shall be accomplished by disconnecting the incoming supply circuit of the machine effecting a Category 0 Stop. Where the machine cannot tolerate the Category 0 stop, it shall be necessary to provide other protection (e.g., against direct contact), so that emergency switching off is not necessary.

  4. #4
    Join Date
    Feb 2003
    Location
    Wisconsin
    Posts
    8,752
    The NEC (NFPA70) does not address the specifics of an E-Stop operation.

    However, NFPA79 (Electrical Standard for Industrial Machinery) does. In most jurisdictions NFPA79 is not mandatory.
    Just because you can, doesn't mean you should.

  5. #5
    Join Date
    May 2004
    Posts
    19

    Post OSHA and the "General Duty Clause"

    What about OSHA's General Duty Clause?

    SEC. 5. Duties
    (a) Each employer --


    (1) shall furnish to each of his employees employment and a place of employment which are free from recognized hazards that are causing or are likely to cause death or serious physical harm to his employees;

    (2) shall comply with occupational safety and health standards promulgated under this Act.

    29 USC 654
    (b) Each employee shall comply with occupational safety and health standards and all rules, regulations, and orders issued pursuant to this Act which are applicable to his own actions and conduct.

    Carl Van Tilburg
    Electrical Engineer, PE

  6. #6
    Join Date
    Feb 2003
    Location
    Wisconsin
    Posts
    8,752
    The Category 0 stop in NFPA79 9.2.5.3.1 corresponds with all of the different NEC requirements for a loadbreak "disconnecting means".

    However NFPA79 9.2.5.3.2 does not require that a Category 0 stop be the only method by which a piece of equipment can be E-stopped.

    So, while OHSA could cite NFPA79 as a promulgated standard, there is nothing in NFPA79 that limits which E-stop functions may be used.
    Just because you can, doesn't mean you should.

  7. #7
    Join Date
    Feb 2003
    Location
    Massachusetts
    Posts
    48,725

    Smile

    Jim am I understanding this correctly?

    The NEC required disconnecting means fulfills NFPA 79s Each machine shall be equipped with a Category 0 Stop.

    And now that we have a category 0 stop all the other E-stops could be Category 1?

    It happens that in the last month I have been involved adding safety switches on equipment. The way these are set up if you where to open a guard the motor will be shut down but dynamic braking comes on.

  8. #8
    Join Date
    Feb 2003
    Location
    Wisconsin
    Posts
    8,752
    Quote Originally Posted by iwire
    Jim am I understanding this correctly?

    The NEC required disconnecting means fulfills NFPA 79s Each machine shall be equipped with a Category 0 Stop.

    And now that we have a category 0 stop all the other E-stops could be Category 1?

    It happens that in the last month I have been involved adding safety switches on equipment. The way these are set up if you where to open a guard the motor will be shut down but dynamic braking comes on.
    A little clarification: While NFPA79 Chapter 9 is for control circuits, 9.2.5.4.2.2 allows a power circuit disconnect instead. An NEC disconnect (i.e. 670.4(B)) must also remove all other machine power also (ie. hydraulics, steam, and air). So if the machine is not "self-contained" an NEC disconnect only may not be suffcient.

    A risk analysis (9.2.5.4.1.4) must be performed when deciding on which type of E-stop control circuit to use.
    Just because you can, doesn't mean you should.

  9. #9
    Join Date
    Feb 2003
    Location
    Massachusetts
    Posts
    48,725
    Thanks Jim.

    Quote Originally Posted by jim dungar
    A risk analysis (9.2.5.4.1.4) must be performed when deciding on which type of E-stop control circuit to use.
    I am sure this factory has done what is required, I am just helping with the installation.

    It seems their risk assessment is that absolutely everything is a risk, they buy brand new equipment and add all kinds of guards all with dual channel safety switches going back to a safety relay.

    It's been interesting and fun doing something different.

  10. #10
    Join Date
    May 2004
    Posts
    19

    Ansi

    Also, watch "Incorporation by Reference" of OSHA.

    ANSI has many standards referenced to by OSHA.

    Actually, the first standard for the safe use of machinery was ANSI B11.1-1926.

    ANSI B11 standards cover applications at the machine level.

    UL produces standards for specific safety items or devices.

    e.g.

    ANSI B11.19 "Performance Criteria for Safeguarding". This is where the redundancy comes from in the safety circuits. The probability of two or more failing is less than one failing (i.e. single point of failure).

    Carl Van Tilburg
    Electrical Engineer, PE

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •