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Thread: Solar Photovoltaic Disconnect

  1. #1
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    Solar Photovoltaic Disconnect

    Inspector called me about a solar sytem he is looking at for final. It has the standard DC disconnect installed in the inverter, which is installed in the garage.

    His fire department wants a disconnect outside. Solar company said no problem. But that only disconnects the DC conductors at the inverter. It leaves the DC conductors running inside the house from the roof to the garage inverter energized whether the disconnect is open or not.

    The DC conductors run from the Array on the roof, penetrate the roof, then go into a 4 x 4 inch metal j-box mounted on the truss. They splice there. From there the go down inside the house in flexible metal conduit to the inverter in the garage. Fire also wants the DC conductors from the Array to the inverter in the garage to be disconnect from outside. Fire does not want any possible DC energized in the home once they open the disconnect on the outside of the home.

    The inspector wanted to know if they made any type of product that could do this. I suggested a normally-closed relay at the existing 4x4 metal box in the attic that would open when the disconnect was opened.

    After talking with the inspector, I got confused as to being able to use an AC relay to hold DC conductors closed. Is this possible? Any other possibliities?

    Thanks in advance for any light you may be able to shed on this.

    EDIT: for clarification.
    Last edited by Energize; 05-18-08 at 12:19 AM.


    Faith is like electricity. You can't see it, but you can see the light.



  2. #2
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    Energize:

    First, if this array is not fused outside at the entry to the roof I would suggest that it should be.

    An appropriate contactor might work, but a mechanically operated disconnect on the outside of the roof at the entry point would be a more reliable disconnect means. This might be located at the roof edge and include a permanent vertical rod for operation. See
    http://www.solar-catalog.com/disconnects.html .
    Whether a modification of the handle for connection of a rod would be allowed would need to be determined. If not the fire department could access it with a ladder.

    Contactors.

    It would need to be able to break a DC arc.

    My initial reaction to normally closed was wrong, and I still think so.

    You want a switch that will open in an emergency condition. If you lose a way to get power to a NC relay, then it can not be opened. For example: the fire burns up the cable to the relay coil.

    Whether AC or DC is used to control a relay has nothing to do with what is switched by the relay. Or whether you are controlling air or oil with a solenoid valve.

    I might suggest a DC coil relay powered from the backup batteries of the system. However, for comparable relays with the same physical size coil and same pull-in force the AC coil will run cooler, and therefore longer life. So an AC coil relay may be a better choice.

    .

  3. #3
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    Thank you GAR for the information.

    Quote Originally Posted by gar
    First, if this array is not fused outside at the entry to the roof I would suggest that it should be.
    You are correct, the array is not fused outside, but the inspector who called me about this is disconnect issue is aware of that.



    Quote Originally Posted by gar
    You want a switch that will open in an emergency condition. For example: the fire burns up the cable to the relay coil.
    Quote Originally Posted by gar
    If you lose a way to get power to a NC relay, then it can not be opened.
    I do not know much about relays, but I thought that is what normally closed meant - the relay was closed (and therefore held the circuit closed) when energized, but if the relay lost power for whatever reason, the relay would then disconnect and therfore open the circuit it was controlling. For instance with your example, if the fire burns up the cable to the relay coil, then the relay would lose power and open. What am I missing?

    I am not trying to be aurgementative, just trying to to understand.

    My very first suggestion to the inspector was to have the inverter mounted on the roof. The inverter (inspector tells me) has a DC disconnect built into it. But the inspector says his Planning Department will not allow this "unsightly appearance" on the roofs of residential tract homes.

    Thanks again for your time and help!


    Faith is like electricity. You can't see it, but you can see the light.



  4. #4
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    Did the inspector and or fire department even take a look at Article 690? :-?

    No outdoor (or closest to the point of entrance) disconnect? 690.13(C)(1)?

    No overcurrent protection? 690.9?

    No sign 690.17?

    If push comes to shove you could come up with a control circuit to do what the FD wants but IMO you should not have to.

  5. #5
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    Energize:

    A normally closed relay is one that is closed when de-energized.

    http://en.wikipedia.org/wiki/Relay
    http://en.wikipedia.org/wiki/Relay#Pole_.26_Throw

    .

  6. #6
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    Solar/photo voltaic and Generator systems have different restrictions/requirements from jurisdiction to jurisdiction. Sometime it is very confusing for EIs as to which jurisdiction wants what.


    With that said, barring the NEC, local jurisdictions will have something to say about the install. What about interrupting your run through the house and install the conductors in a raceway outside down to the grade level with a disconnect located somewhere in the vicinity of the equipment? I know your situation may not permit this, but I have seen this type of installation.




    Here are two pictures of what I am trying to describe.




    Instructor, Industry Advocate

  7. #7
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    Just to offer clarification on side issues:

    • You CANNOT switch DC with a device that is rated for AC only. DC requires a much longer arc path to interrupt it. The contactor MUST be rated for carrying DC current. This is not to be confused with having a DC coil, that is a separate issue. Many devices carry both AC and DC current ratings, but you will almost always see that the DC ratings are lower than the AC ratings, so ALWAYS read the technical data on any device that you are applying to DC. ASS-u-me nothing.
    • It is becoming more common for solar array mfrs to provide DC contactors for this purpose, but it increases the cost and someone else is always willing to sell their systems cheaper without them, leaving the installer or homeowner to deal with the consequences later. While it is therefore possible for you to retrofit them, I would suggest, as it appears you are the EC and not the solar system provider, that you insist that the solar provider take care of it and cover it under their warranty.

  8. #8
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    Quote Originally Posted by Jraef
    Just to offer clarification on side issues:
    • You CANNOT switch DC with a device that is rated for AC only. DC requires a much longer arc path to interrupt it. The contactor MUST be rated for carrying DC current. This is not to be confused with having a DC coil, that is a separate issue. Many devices carry both AC and DC current ratings, but you will almost always see that the DC ratings are lower than the AC ratings, so ALWAYS read the technical data on any device that you are applying to DC. ASS-u-me nothing.
    • It is becoming more common for solar array mfrs to provide DC contactors for this purpose, but it increases the cost and someone else is always willing to sell their systems cheaper without them, leaving the installer or homeowner to deal with the consequences later. While it is therefore possible for you to retrofit them, I would suggest, as it appears you are the EC and not the solar system provider, that you insist that the solar provider take care of it and cover it under their warranty.
    Thanks to everyone for the replies.

    I am neither the EC nor the Solar porvider. I have nothing to do with this, other than a strong relationship with the inspector who values my judgement and suggestions. But my suggestions always come with lots of research and confidence in the accuracy of my suggestions. I never answer him off the cuff, and solar is not an area I am very familiar with - nor is he.

    I am just trying to find something that will have the dc conductors disconnect at the point they enter the house. Something he can suggest to the Solar people as a reasonable solution. He is a very good inspector, and he is asking for my help. I want to get my suggestion(s) right, that is why I am here. His city planners will not allow the inverter disconnect on the roof, nor will they allow the exposed conduit to be ran on the exterior of the homes. No sense argueing why planning won't allow it, it is what it is. Solar company wants to mount the inverter in the garage and just have the conductors remain energized from the roof to the inverter. The inspector is trying to work around it, and I am trying to help, because we respect each other and our professions.

    Maybe I mispoke. Perhaps a normally-OPEN relay would work? The reason I am hung up on the relay answer is that is the only thing that keeps coming to mind. Maybe my focus is too narrow, and I am hoping to get other's views and suggestions.

    Thanks, Guys, I really appreciate insights.


    Faith is like electricity. You can't see it, but you can see the light.



  9. #9
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    Energize:

    It appears that you will have to put a relay inside the attic from your description. If so I would suggest that at the control point you have a voltage measuring means across the DC bus so that you can verify that the relay actually dropped out. This might be some redundant LED indicators.

    .

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