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Safest way to switch 240VAC

Discussion in 'General Electronics Discussion' started by jackorocko, Jan 13, 2012.

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  1. jackorocko

    jackorocko

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    Apr 4, 2010
    I have a generator I bought for my home. I want to be able to hook it up to the breaker box so that when the power goes out I just head out and turn on the generator.

    My first thought was to use a relay to connect the generator to the breaker box when the power went out. But, this got me to thinking about safety. I want to make sure that my generator doesn't stay on and connected when the power comes back. Plus I would like some redundancy.

    If I did this, I would move the circuits I wanted out of the primary breaker box and put them in another separate box and just control the power to that whole box. This is why a relay would be nice because I isolate the two power sources. Would an RC circuit or some other form of delay circuit be prudent since sometimes the power flickers and I don't want it to switch back til the power is stable. To be honest, 5 minutes sounds like a good delay, but that is not possible with an RC circuit. So I would have to step up to a 4060 or something similar.

    Anyone know of a better way?
     
    Last edited: Jan 13, 2012
  2. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    If you use a changeover relay with ´the common contact(s) connected to your home's installation, the break contact to your supplier's power line, the make contact to your generator I see no problem. If you drive the coil by a voltage derived from the supplier's power, then the relay will switch to the generator automatically when the power goes out.
    Due to the changeover relay there will never be a direct contact between the supplier's power line and the generator.

    Adding a delay circuit is a good idea.

    Also mind your power requirements. A typical relay used for household appliances will switch 5 A - 8 A, not more. If you are going to supply the whole home, this may not be sufficient.In that case you may need a more powerful air gap switch (contactor).

    Harald
     
  3. duke37

    duke37

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    Jan 9, 2011
    You should look at the regulations concerning this. The supply company do not look kindly on power being supplied from the wrong end. You probably need an approved manual change over switch.
     
  4. jackorocko

    jackorocko

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    Apr 4, 2010
    I actually know of a working system like this near to where I live. It is used in a pump house to provide water to some 400 people when the power drops. The generator starts automatically and from what I seen there is a simple relay/switching device that sits between the generator and breaker box. The question is, when does the switching take place? When the power drops out from the elec. co or when the power comes on and off from the generator? I might have to go take a closer look at that other system to see exactly how it is wired up.
     
  5. jackorocko

    jackorocko

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    Apr 4, 2010
    This is what I had first thought of as a possible solution. The break then make operation of the relay would make sure the two power lines where isolated at all times and that is what I want. safety!!
     
  6. daddles

    daddles

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    Jun 10, 2011
    If you live in the US, I strongly suggest you talk to a professional electrician. While there's nothing technically demanding about the application (e.g., conceptually, it just needs a Form C switch), you need to realize the danger you may put some linesman into in a power failure. If you accidentally backfeed the power company's transformer, you could expose someone to deadly voltages. Yes, they probably have standard procedures that clamp the line to the ground before working on it, but I sure wouldn't want to find out that my installation was faulty and that I injured or killed someone. You could rightly be brought up on criminal charges. As someone else mentioned, there are likely both approved hardware and installation procedures for this, as many people have installed generators in their homes.

    Oh, also get an electrical permit to do the work. Then it will be inspected and approved. If there's a fire later that's caused by your modification, your insurance company will rightfully not cover your losses if it was done without a permit.
     
  7. jackorocko

    jackorocko

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    Apr 4, 2010
    Well I wish it was that easy to get an answer, but I called the electric company and asked them flat out, does work on my side of the meter require inspection? They told me "We don't know, you will need to call an inspector." So I call the inspector and he says "Is it a new installation?", "No", "Then it won't need to be inspected."
    As for the insurance company. They just needed some information from me. One of those questions was how up to date was the electric. I told them that I had been working on it myself. As when I moved in I rewired all the rooms that had old style tub and knob wiring. They told me that was great and approved my application for insurance. No inspection or any visit from them, just my word on the matter.

    So with that said I am more then happy to do the work myself. If no one cares about whether it needs inspecting and my insurance co. could careless that I do the work. Then I am not gonna worry about it either. I am going out of my way by asking this question here, because I care more then "they" do.

    I have done plenty of residential wiring that has been inspected and approved before, so I feel very comfortable doing the work.
     
    Last edited: Jan 13, 2012
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    I would recommend that you have a chat to people that supply diesel generators and UPSs for commercial buildings.

    There are devices made to perform this function. The smallest one may be suitable for you.

    At the very least, reading their blurb and specs may give you some ideas about the best way to solve this problem (i.e. there may be some aspects you -- and we -- have not considered).
     
  9. alfa88

    alfa88

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    Dec 1, 2010
    Transfer Switch

    What you need is a transfer switch. Basically it is a relay with the coil connected to the utility's mains. If power goes out contacts drop to feed in alternate source (generator, UPS).
    Now here's a scenario where you don't want to connect a generator straight in. Suppose a line man is up there working on a fault. You wouldn't want him getting zapped by your generator.
    Get a licensed Electrician.
     
  10. jackorocko

    jackorocko

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    Apr 4, 2010
    I got to thinking about what everyone here has said about the guy outside working on the power line. But, what is the difference between me and my generator and a a solar/wind generator back feeding power to the grid?

    These energy systems don't stop working just because the grid goes out, so how does the lineman protect themselves from these sorts of situations? Seems to me their procedures must account for this and if so, then that should account for my system as well.

    I am now convinced more then ever that I should probably buy a UL listed transfer switch that is certified to do the work I am asking, but I will be installing it myself!
     
  11. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Yes they do...
     
  12. alfa88

    alfa88

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    Dec 1, 2010
    Just as I stated before. The transfer switch (relay coil) is powered up with the utility's AC. If that power goes out it will isolate your generation from the grid. At the same time your generation will be connect to you and only you..
     
  13. jackorocko

    jackorocko

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    Apr 4, 2010
    That wouldn't make a whole lot of sense unless you are talking that they disconnect themselves from the grid when the grid power goes out. If that is what you meant, then I guess I understand since most commercial inverters as I understand it produce power in sync with the grid to sell it back to the company.
     
  14. jackorocko

    jackorocko

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    Apr 4, 2010
    yeah that is the idea, I am not looking to power the neighborhood. scratch that, diode won't work with AC. :(

    I am not looking to power my whole house, just some necessities I can't do without. Power for heat, refrigerator, and my network/femtocell so I can have emergency access
     
    Last edited: Jan 14, 2012
  15. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    They don't disconnect themselves from the grid, they shut down.

    They have anti-islanding to prevent them from (even as a group) continuing to supply power if the mains has failed. I think mine will stop within a second if the mains voltage exceeds certain limits, the mains frequency exceeds certain limits, or the mains impedance increases beyond a certain limit.
     
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