automatic shutdown circuit for reversed polarity shore power hookup?

Discussion in 'OnBoard Electronics & Controls' started by sdowney717, Jun 22, 2013.

  1. sdowney717
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    sdowney717 Senior Member

    My situation is I have an automatic relay system controlling shore-gen-inverter power using twin cascading 4 pole relays.
    Boat has twin 30 amp 120 vac power cords.
    Only one power cord is powering the 4 pole relay.

    Works great!

    Basically, when my shore power is plugged in, the 4 pole 35 amp relay energizes and the boat gets shore power. ( I do have an on - off toggle switch to also kill shore power)

    So my question is, is there an easy way to prevent the relay coil from energizing if the polarity of the shore power is reversed? As in a hot neutral wire.

    The coil is 120 vac and gets its power from the shore power cord.
    This way I would not need a reverse polarity alarm or light. I have not hooked those up yet.
    So if polarity was reversed with a hot neutral wire, the boat would not have any AC power at all.

    I dont know how much of a problem this ever is at a marinas. I have never seen it. If some idiot wires the shore cord wrong it would happen.
     
  2. Mark Cat
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    Mark Cat Senior Member

    I use isolation transformers on shore power input, so single phase 120 VAC reverse polarity is not a problem. However, if polarity is reversed, the captain must OK its use.

    To answer your question, I do not think there is an easy way. But, then again the system you are using is very unique.

    The 120 VAC H-N reverse problem is usually a problem with the dock power pedestal and not the off-the-shelf shore power cords.

    All for now,

    Mark Cat
     
  3. CDK
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    CDK retired engineer

    In your relay box, after the GFI put a 10K resistor between protective ground (= ship's ground) and the terminal you expect to be neutral. As long as the polarity is normal, the resistor carries no or very little current, with reversed polarity the current through the resistor will trip the GFI.
     
  4. sdowney717
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    sdowney717 Senior Member

    Yes, that is a good idea.

    But, I dont have GFCI on the shore power breakers.
    My GFCI is in my distribution panel that sends power to the outlets. So the power coming into the panel is not GFCI.

    I did not install GFCI 30 amp breakers as I read they may nuisance trip the entire AC system with their low 5 milliamp trip rating.

    Power flow is

    shore cords
    receptacles
    Twin 30 amp double pole breakers in a box.
    Relay box with twin 4 pole cascading relays to switch all poles from grid-gen-inverter power
    QO 8 breaker distribution panel

    I thought about some kind of diode in the line supplying power to the coil to block flow one way, pass the other way, but then the voltage would be too low to energize the coil?
     
  5. sdowney717
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    sdowney717 Senior Member

    I thought of another way to do this using a small AC SPDT relay.

    If neutral is hot, then use a small relay that is energized when neutral to ground has power. The relay opens when energized breaking the circuit to the large 35 amp 4 pole relay's coil.

    When neutral to ground has zero power, relay will remain closed which will let large 4 pole relay coil be powered on so power flows into boat.

    Can that be done in solid state triac or somesuch easier than using a small relay?

    I have relays on the brain because they are so easy and reliable.

    Doing this you could still have a buzzer or light come on if polarity is reversed and the boat would not be getting any power.
    The old original system, with reversed polarity, a buzzer and light came on and the boat still gets power.

    here on ebay is a relay that could work. 16 amps with 120vac coil SPDT
    http://www.ebay.com/itm/New-120VAC-...488?pt=LH_DefaultDomain_0&hash=item3a7b99be68

    I have lots of free 12v SPDT automotive relays. But dropping 120vac to 12vdc, is there an easy way to do that?
     
  6. CDK
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    CDK retired engineer

    OK, put a small transformer 120:12V and a rectifier between ground and neutral to power a 12V relay that cuts off current to the large relays if polarity is wrong.
     
  7. sdowney717
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    sdowney717 Senior Member

    yes that will work.
    I dont know if I have a transformer.

    Or maybe I do have one of those! I have a lot of those small black power converters that turn 120 vac into 6 -9- 12 - 18vdc etc... Like used for phones, routers, games etc...

    That would work I think. Interesting idea?
    I need to give some thought to this.
     
  8. Mark Cat
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    Mark Cat Senior Member

    In the approach to shore power mis-wiring, do not exclude a high resistance or open grounding (green) conductor.

    Mark Cat
     
  9. sdowney717
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    sdowney717 Senior Member

    yes, but not sure how to automatically test for that.
    what you suggest is marina has neutral and hot reversed AND green is disconnected.

    I suppose an ideal test would be to plug in a special device to test the power before you plugged in the boat.

    As the boat was originally wired, the polarity alarm simply sounded if neutral to ground was energized with power.

    I plan to reconnect the alarm and also design this device so that my boat power stays off if polarity is reversed, still this is assuming an intact ground wire exists.

    Sometimes things do get a little extreme.

    My idea is create an outlet box with outlet. Outlet terminals connect to neutral and ground wire.

    Then plug in the black power converter. Send the 12vdc back into the outlet box and connect to a 12 vdc relay SPDT.
    The relay if energized due to reverse polarity situation (assuming ground is intact) kills the power to my 4 pole 35 amp grid power relay which disconnects the boat from shore power, like pulling the plug!

    Then if you wish to test this functionality, unplug the black converter and plug it into a 120vac power source. That should shut down the boat AC system.
     
  10. Mark Cat
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    Mark Cat Senior Member

    Yes, this is what I suggest. With the addition of: corrosion and failure to bond.

    Best regards,

    Mark Cat
     
  11. sdowney717
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    sdowney717 Senior Member

    yes, ok, I suppose doing the test I suggested as in plugging my special device in using normal polarity would test the ground wire. Then it would be hot to ground which activates relay and boat power shuts down.

    I suppose a way to do this would involve maybe a 4 pole on-off type switch. It would need to

    connect forward polarity-disconnect all-connect reverse polarity and perhaps use an LED and alarm and relay. Something could be constructed.
    I start out simple and think and then add on layers to get to where I want to go.

    Perhaps the increased resistance not as much of a problem, too much resistance and the relay wont have enough power to work. Of course a relay is a very low power device. To really test for low resistance, you need something that can use a lot of power, like an electric motor. This gets quite complex.
    Run a motor from hot to ground and see how it performs would be a good test of a ground circuit wire. A breaker is not going to be a good test. You may have enough resistance for current to flow and still not trip the breaker.

    You can see I like to think out loud:)
     
  12. sdowney717
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    sdowney717 Senior Member

    I was thinking or over thinking, sometimes hard to tell.

    If your putting in a circuit that joins the neutral-ground in any fashion, then is this not against the idea of bonding neutral to ground?

    If I put the transformer in between neutral - ground it does have a fairly high resistance running through a transformer, does it make a difference regarding ground looping?

    What though about a light bulb joining neutral to ground? That has a low resistance.

    Also, would you fuse or breaker protect a polarity test circuit? I was thinking of a 10 amp small breaker in the neutral line.

    Would you fuse or breaker protect a test feature? Like the test feature I was thinking of an on-on spring switch. One way paths neutral to transformer, other paths the hot wire to transformer. Default has it set to neutral. Conceivably you would need 2 fuses.

    Do I know what I am talking about, or do I not have a clue?

    reading here
    http://www.cruisersforum.com/forums/f14/reverse-polarity-ac-1180.html

    Says the reverse polarity circuit needs a minimum 25,000 ohm impedance. Otherwise, the ground-neutral connection allows too much current to flow onto the ground wire. So maybe, officially the idea of using a relay is out as the relay coil resistance wont be that high.

    Boat has a light and a buzzer. The buzzer has a label on it saying reverse polarity alarm.
    My little reverse polarity light has high resistance. My reverse polarity metal rectangular buzzer may not.
    These are original to the boat from 1970.
     
  13. sdowney717
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    sdowney717 Senior Member

    I have a picture of some parts I scrounged up.
    2 10 amp circuit breakers
    1 SPDT 12 volt relay
    1 transformer input 120vac output 6 vac
    (not showing the small 120vac to 12 vdc power converters)

    [​IMG]

    Using my battery charger set on 6 vdc, the 12 vdc relay positively clicks.
    Using the transformer, the relay will not engage.
    Is this because not enough ac volts? or not enough transformer power?
    or it needs rectifying to dcv?
    primary side .292K ohms
    secondary side 5.5 ohms

    I still have three 120vac to 12 vdc black boxes converters I can use.

    My idea is to put the reverse detection circuit after the main breaker and before the 4 pole power relay.
    Do you think it would matter if I dont follow the 25,000 ohm rule on this circuit?
    Wont it be safer to have it working showing reversed phases and also automatically kill the power to the boat vs having the ground and neutral bonded with less than 25,000 ohms basically right at the main breakers?

    I measured the black brick ohms. It is about 300 ohms.
    So if the transformer joins neutral to ground, you normally only have the power voltage returning on the neutral line and now potentially some running out onto the ground wire through the transformer. 300 ohms will create a voltage drop and put power back onto ground and most will run back on the ground wire to the better neutral-ground bond on shore somewhere, but some may run on another path back to the source.

    So what is the risk of a shock using 300 ohms in that circuit?
    How many volts?
    Perhaps a good test is try it at home, grab the wire return out of the transformer and also hold the ground and see if you feel a shock, also check with wet salty fingers.:eek:

    Ok, I tried it by using 2 fingers and my fingers were the return on the primary side with no ground and yes with salty fingers you get a decent tingle! So if your ground failed, it does become a shock hazard and that explains the 25,000 ohm rule.
    So maybe there is another solid state circuit you can design to also use a relay. For now I will give up unless someone comes along with an idea.
     
  14. CDK
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    CDK retired engineer

    You seem to have missed the word "rectifier" in post #6.:rolleyes:
     

  15. sdowney717
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    sdowney717 Senior Member

    Your saying

    line -- rectifier-- primary winding transformer -- line ?

    Or is the rectifier going on the transformer output?

    What about this 25,000 ohm rule?

    I measures my light and it meets that.
    The buzzer measures .5K, 500 ohms.
    So then it is considered an illegal device?

    If so, what buzzer is considered acceptable?

    [​IMG]
     
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