User Tag List

Results 1 to 6 of 6

Thread: Enclosed lighting transformer disconnects for panels with large incoming feeders

  1. #1
    Join Date
    Aug 2018
    Location
    Evansville, IN
    Posts
    6
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)

    Enclosed lighting transformer disconnects for panels with large incoming feeders

    Has anyone had experience with lighting transformer disconnects like those from Daykin in terms of use in custom control panels that have large incoming feeders. I am conducting an arc flash analysis for a customer who has Daykin lighting disconnects in most of their custom panels. Sometimes they are mounted inside on the sub-panel while others are mounted outside to the roof of the cabinet. In one case I have a panel fed with 2 per phase 350mcm from a bus duct overhead. The total length of the incoming cables is no more than 10-15'. When the panel was buiilt they installed a Daykin lighting disconnect inside the panel and wired it per the manufacturers directions using 12 awg wire connected to the line side of the main disconnect.

    In this case the Daykin lighting disconnect is mounted inside of the panel which I believe allows it to comply with the NEC tap rule. (240.21(B(1)(4) states if the conductors leave the enclosure where the tap is made the capacity of the tap conductors is not less than one-tenth of the rating of the over-current device protecting the feeder conductors. With this setup the lighting disconnect results in an incident energy high enough to make the device "Extreme Danger". By being mounted inside the panel the panel itself then becomes "Extreme Danger".

    In other cases the Daykin lighting disconnects are mounted outside of the enclosure to the roof of the panel. In these cases since the tapped conductors leave the panel where the tap is made and they are less than one-tenth of the rating of the over-current device protecting the feeder conductors the wiring of the lighting disconnect does violate the tap rule.

    The engineer at Daykin did not seem to know much about the tap rule or these issues that result from wiring their device per their recommendations.

  2. #2
    Join Date
    Aug 2004
    Location
    Northern illinois
    Posts
    17,422
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    what difference does it make.

    you can't work live in the panel anyway. there is live voltage on the line side of the main disconnect even when it is turned off. so whatever the IE is when the main disconnect is closed is going to be the same when it is open.

    it is not the job of the daykin tech support people to apply their product to your situation, or to be experts in the NEC beyond what they need to know to design and support the xfmr disconnect itself.

    In any case, the design of control panels should follow the requirements of UL508a, not the NEC. The current version of UL508a does not require one to follow the tap rules. the newer version that will become effective fairly soon does require adherence to the tap rules.

    If the xfmr disconnect is part of the control panel it gets dispensation to follow the listing rules for the control panel. UL508a allows devices that are part of the control panel to be mounted on the outside surfaces of the enclosure.
    Last edited by petersonra; 09-26-18 at 01:51 PM.
    Bob

  3. #3
    Join Date
    Aug 2018
    Location
    Evansville, IN
    Posts
    6
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by petersonra View Post
    what difference does it make.

    you can't work live in the panel anyway. there is live voltage on the line side of the main disconnect even when it is turned off. so whatever the IE is when the main disconnect is closed is going to be the same when it is open.

    it is not the job of the daykin tech support people to apply their product to your situation, or to be experts in the NEC beyond what they need to know to design and support the xfmr disconnect itself.

    In any case, the design of control panels should follow the requirements of UL508a, not the NEC. The current version of UL508a does not require one to follow the tap rules. the newer version that will become effective fairly soon does require adherence to the tap rules.

    If the xfmr disconnect is part of the control panel it gets dispensation to follow the listing rules for the control panel. UL508a allows devices that are part of the control panel to be mounted on the outside surfaces of the enclosure.
    I am fine with what you've said. To your point let's say it is perfectly acceptable to mount this device to the exterior of the panel and say that allows it to squeak by and meet the requirements of the tap rule. With the large amount of IE due to the device being wired per the manufacturers recommendation the lighting disconnect becomes extreme danger and thus no one can plug a tool into it's duplex receptacle while energized. In my opinion, it is not a good idea to have available a 120VAC duplex receptacle on the outside of a panel that is considered to be extreme danger in terms of arc flash.

  4. #4
    Join Date
    Aug 2004
    Location
    Northern illinois
    Posts
    17,422
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by jmoore284@gmail.com View Post
    I am fine with what you've said. To your point let's say it is perfectly acceptable to mount this device to the exterior of the panel and say that allows it to squeak by and meet the requirements of the tap rule. With the large amount of IE due to the device being wired per the manufacturers recommendation the lighting disconnect becomes extreme danger and thus no one can plug a tool into it's duplex receptacle while energized. In my opinion, it is not a good idea to have available a 120VAC duplex receptacle on the outside of a panel that is considered to be extreme danger in terms of arc flash.
    what makes you believe that merely plugging into the receptacle creates a danger of arc flash?
    Bob

  5. #5
    Join Date
    Aug 2018
    Location
    Evansville, IN
    Posts
    6
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by petersonra View Post
    what makes you believe that merely plugging into the receptacle creates a danger of arc flash?
    Maybe you are right. Typically I analyze and label plugs if they themselves contain an arc flash hazard such as if they are 240V, 480V, etc. In this case the plug is being inserted into a 120V receptacle that happens to be mounted to a small enclosure containing 480V. After thinking about this further I agree that the arc flash hazard should not be considered an issue to prevent plugging a cord into the receptacle. The lighting disconnect enclosure will still have a high IE and resulting arc flash PPE level. This will only make more difficult the opening of the lighting disconnect or actuating the disconnect handle.

    Unfortunately for the panel I'm describing and a handful of others in the facility they do not comply with the tap rule and therefore should be corrected. The addition of current limiting fuses in the control panel to protect this device will both bring it in compliance with the tap rule and provide a much lower IE.

  6. #6
    Join Date
    Aug 2004
    Location
    Northern illinois
    Posts
    17,422
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by jmoore284@gmail.com View Post
    Maybe you are right. Typically I analyze and label plugs if they themselves contain an arc flash hazard such as if they are 240V, 480V, etc. In this case the plug is being inserted into a 120V receptacle that happens to be mounted to a small enclosure containing 480V. After thinking about this further I agree that the arc flash hazard should not be considered an issue to prevent plugging a cord into the receptacle. The lighting disconnect enclosure will still have a high IE and resulting arc flash PPE level. This will only make more difficult the opening of the lighting disconnect or actuating the disconnect handle.

    Unfortunately for the panel I'm describing and a handful of others in the facility they do not comply with the tap rule and therefore should be corrected. The addition of current limiting fuses in the control panel to protect this device will both bring it in compliance with the tap rule and provide a much lower IE.
    why would actuating the disconnect handle cause an arc flash?

    I am not even real sure you can trip the feeder breaker with the little bit of current you can get through a #12 wire. on the other hand, how much IE can you get through a #12 wire before it melts? you should crunch those numbers and see what you come up with.

    I would point out that even if you have a relatively low IE at the xfmr, you still can't work on it live, except for a few exceptions, so even if you can reduce the IE by adding CL fuses inside the control panel, I don't see that you have gained anything.
    Bob

Tags for this Thread

Posting Permissions

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