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Thread: 24 vdc control - 480 vac power single enclosure -work in control side while energized

  1. #1
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    24 vdc control - 480 vac power single enclosure -work in control side while energized

    We want to design a multi- door control panel that has the 480 segregated to one side of the control panel behind one of the doors and use the other door for access to the 24 vdc control circuits.

    What kind of barrier do we need between the left and right side of the backplane so that we can keep the 480 door closed but be able to work without PPE on the 24 vdc side. Cat 5 cables and some relay contacts need to run between the two sides of the enclosure.

    Assume there is 10 cals of energy at the panel's disconnect.

  2. #2
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    It is not hard to make a barrier to protect against voltage exposure.
    It is potentially very hard to make a barrier to contain an arc flash in one section from exploding out another section. The arc gas pressure can exert a lot of force!
    You might try the logic that you can prevent what you are doing on the lower voltage side from causing an arc event on the higher voltage side, but I am not sure what level of engineering and testing OSHA would require to accept that.

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    Thank you for your reply. Enclosures are not rated for calorie level so I don't believe that should be an issue. The panels will have a sufficient SCCR.

  4. #4
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    Quote Originally Posted by Dan Kissel View Post
    Thank you for your reply. Enclosures are not rated for calorie level so I don't believe that should be an issue. The panels will have a sufficient SCCR.
    I am confused on your statement about not rated for calorie level. The calorie (incident energy) level is determine by analysis of the system the panel is installed in so there is no rating that can be applied to the cabinet. The incident energy level at that cabinet would determine the level of PPE required to work in the cabinet.

    How do you know the panels will have sufficient SCCR? Are you going to design and test the panels to have a 65kA rating? I would think that rating would be sufficient for a vast majority of facilities.

  5. #5
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    if it was me, I would get two separate enclosures and just butt them up against each other. Then you don't have to worry about it.
    Bob

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    Quote Originally Posted by petersonra View Post
    if it was me, I would get two separate enclosures and just butt them up against each other. Then you don't have to worry about it.
    How do you pass conductors from one to the other without also allowing blast pressure / plasma flash through the same openings?

    There are lots of options available now from the major enclosure manufacturers to address this, using what can be generically referred to as "power isolation enclosures" where the 480V main disconnect is in a separate structure, either inside of the main cabinet or on the side, but the power passes through the walls via a terminal block with connectors on both sides, but no air passing through a hole. So with that, you can open the main disconnect and although its line terminals will still be hot, they are sequestered in their own separate cabinet.

    Hoffman's version

    Saginaw's version
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  7. #7
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    If you are not working on the side that has the arc blast potential how is there any risk of Arc blast in the control cabinet? In any case how far away do you have to be for the risk to be limited enough that it would be safe? A mile?
    Bob

  8. #8
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    With reference to my question. The 480 side will have motor starters, VFDs, etc. The two cabinet solution is really just a "punt" on the issue. There would be conduit gong between the two cabinets. When the 480 door is closed, there is very little chance of a fault or shock hazard happening because of an accidental tool drop or wring problem.

    I want one enclosure so that I can test it before the panel is shipped. I don't want the double cost of handling, re-connecting and then retesting. Is a simple barrier with holes as needed sufficient to protect me while I work in the 24 vdc side without any PPE?
    Last edited by Dan Kissel; 09-29-17 at 07:08 PM.

  9. #9
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    Quote Originally Posted by Dan Kissel View Post
    With reference to my question. The 480 side will have motor starters, VFDs, etc. The two cabinet solution is really just a "punt" on the issue. There would be conduit gong between the two cabinets. When the 480 door is closed, there is very little chance of a fault or shock hazard happening because of an accidental tool drop or wring problem.

    I want one enclosure so that I can test it before the panel is shipped. I don't want the double cost of handling, re-connecting and then retesting. Is a simple barrier with holes as needed sufficient to protect me while I work in the 24 vdc side without any PPE?
    I don't see how it is a "punt" You can ship it connected together as a single unit.

    I am not a fan of the DIY barriers. Most people make them so they are not all the "safe". If the 480 and 110 Volt stuff is completely contained within a separate enclosure it is hard to be a whole lot safer when working with the 24 VDC stuff as long as you do not open the door to the 480 Volt side. As long as the door to the 480 Volt enclosure is closed, I don't see how working in the 24 VDC introduces a risk more than if you just walk by the 480 V enclosure.
    Bob

  10. #10
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    Quote Originally Posted by petersonra View Post
    If you are not working on the side that has the arc blast potential how is there any risk of Arc blast in the control cabinet? In any case how far away do you have to be for the risk to be limited enough that it would be safe? A mile?
    Plasma doesn't know it is not supposed to travel...

    How big is the opening between the cabinets? Is 1" OK? If so, how about 4"? Then 8"? Where is the line drawn? The answer is, where the blast energy that gets through is in excess of 1.2 cal/cm2. But how is one supposed to know how much energy will pass through an opening, and who is going to take responsibility for that number?

    I know for sure in MCCs, ALL air spaces are considered "common". So when you open one bucket door, even if the stabs are REMOVED from the bus (as some mfrs now offer), technically you are still in the "Arc Flash Boundary" because an arc flash in another section somewhere could potentially blow out into your face.
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