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Thread: Machine safety

  1. #1
    Join Date
    Apr 2008
    Posts
    365

    Machine safety

    A bit off topic....Looking for some help on a machine safety concern. I have is a mixer unit with several blades controlled with AB drives. When the mixing is done the blades are raised and needs to be scraped off. I want to make sure the operator can put the system in a safe state to do this task.

    Given that scenario I was looking at a couple of options.

    1. AB 750 drives have a safe torque feature that disables the IGT's in the drive thus preventing it from working. In reading the literature from AB is seem it's designed to exactly what I want. With that said being said... I'm old school & I don't feel comfortable relying on electronics to keep persons safe. I would like to see the power disconnected from the motor which brings me to option 2.

    2. Use a service disconnect at the motors, the operator would stop the motor and open the service disconnect. My question here is, does we need to verify the power has been removed via opening the door and taking a voltage reading? If so that will require an electrician and that was something I was hoping to avoid. I have looked at some of the Hazardous location disconnect where they have a viewing window where you can visually verify the stabs have disengaged. Is a visual verification sufficient?

    3. Do a standard lockout at the MCC. At our site this would require an electrician or a qualified person, typically training the operator to perform this task. I'm not a big fan of training operators to do electrical work.

    Just to reiterate, the lock out is for operations to clean down machine, no exposure to any live parts. Looking forward to the feedback from this esteemed forum.

  2. #2
    Join Date
    Apr 2009
    Location
    Williamsburg, VA
    Posts
    4,253
    Is there anything else associated with the mixer process, like a jet-wet blower/water solenoid valves for making polymer, or conveyor system? Multiple sources of power? If no and no, a local lockable disconnect should suffice. Locking out the MCC is not a difficult process for a non electrician, tho there's always the chance of locking out the wrong equipment or shutting down something that needs to be running.

    Since adding a local disconnect will slightly change the maintenance process, I'd also make sure the process for raising the blades has sufficient measures/procedures to lock them in place for cleaning so that they cant move/fall on the operator during cleaning, and if they need to rotated manually for cleaning, there is an approved procedure for doing that as well. Any other processes (automatic fill valves, etc.) for the tank should be isolated and LO/TO, and if it's a confined space and/or there is engulfment or hazardous atmospheric potential, now may be a good time to review those too.
    Electricians do it until it Hertz!

  3. #3
    Join Date
    Aug 2004
    Location
    Northern illinois
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    15,683
    The disconnect and then test for voltage applies when someone might be exposed to live voltage. The operator won't be exposed to live voltage in this case.
    Bob

  4. #4
    Join Date
    Dec 2012
    Location
    Placerville, CA, USA
    Posts
    18,461
    Quote Originally Posted by petersonra View Post
    The disconnect and then test for voltage applies when someone might be exposed to live voltage. The operator won't be exposed to live voltage in this case.
    The mechanical equivalent is to run, stop, disconnect and then try to run. Machine should not move.

  5. #5
    Join Date
    Sep 2013
    Location
    Brisbane, Australia
    Posts
    50
    In response to your options cornbread...

    1. Designing a safety rated control system using the safe torque off is certainly an option and arguably the safest option. It is also the most complex/costly option for you/your customer. The control system can often be designed to ensure that deliberate circumvention or careless use can not result in operator exposure.

    2. A local disconnect is certainly another option. It can be expensive/impractical depending on the size of the disconnect and operating environment. Plug and socket can also sometimes be used but often only for smaller disconnects. Visual separation of contacts is usually regarded as a valid verification of isolation method therefore preventing the need to do a voltage check. However, seeing the actual current carrying blades separate rather than an attached mechanical flag is often recommended for safety reasons but they can be hard to see even for electricians who know what they are looking for!

    3. Lockout at the MCC is the cheapest and most common method IMHO. It suffers from not being able to easily associate the isolation point with the motor which could lead to an instance of working on an energised motor! However, there are methods available to address this issue and allow voltage verification without direct voltmeter measurement ie operators don't need electrical training. Verifying using the try start method is generally acceptable by authorities but many large corporations have abandoned this approach as it can provide misleading results.

    JFletcher's comments about restraining the blades is also a good point. Continuing on this theme it might be worth considering if de-energizing using a disconnect results in the blades coming to rest in a timely fashion.

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