Isolate inverter from shorepower

Discussion in 'OnBoard Electronics & Controls' started by RoyHB, Apr 15, 2010.

  1. CDK
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    CDK retired engineer

    Frosty, that's why I wrote in post #12 that the other 2 contact groups on such a relay (Brown Bovary, AEG, Klockner etc) could be used to exclude certain devices from the inverter circuit.

    The relay is powered as long as there is shore power: wiring to these users must be connected to an NO contact. In a fully automatic, power saving setup, there should also be one contact set used to put the inverter in standby or sleep mode. Modern inverters have a modular socket (RJ-11) for that purpose.
     
  2. daiquiri
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    daiquiri Engineering and Design

    Hi CDK,
    I am not an expert in electrotechnics, so this is just a thinking out loud - don't you think a condenser should be inserted in your circuit, to dampen the transitory current harmonics when the relay switches between the shore power and the inverter? Something like in the attached pic.

    P.S.
    Thinked a bit about it - then I guess that (by the same logic) it would be needed for manual swithches too, but it is not being done - so I have probably said a nonsense here...?
     

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    Last edited: Apr 20, 2010
  3. capt littlelegs
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    capt littlelegs New Member

    The neutral would still need to be switched at the same time because one side of the inverter needs to be connected to earth to make it a neutral but the shore supply neutral must not be earthed on board. It gets interesting if the inverter has a centre tapped earth connection!
     
  4. CDK
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    CDK retired engineer

    A capacitor in an AC circuit is only used in combination with a resistor or inductance to suppress spark noise.
    In this case it would serve no purpose at all. These relays have an iron core with a copper short around it, its mass is large enough to avoid contact jitter. There are suppressor diodes in the coil housing to absorb the energy generated by the moving core.
     
  5. CDK
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    CDK retired engineer

    I do not recommend using the center tap on the inverter, nor would I connect one side of the inverter output to ground. Let the inverter supply float, appliances that are not double isolated have their own ground terminal connected to the ship's ground.
    A safety device like a ground fault interrupter is already standard equipment on a boat with shore power connection. Such a device compares the current through both wires and will trip as soon as there is a substantial difference between them.
     
  6. daiquiri
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    daiquiri Engineering and Design

    Thank you so much for this info. I didn't know this fact about copper shading rings, so your post has spurred me to make a research in internet on that subject. Learned a lots on that and some other electrical stuff in the past few hours, which made it worth the effort. ;)
    Thanks again.
     
  7. capt littlelegs
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    capt littlelegs New Member

    That's true but unless the inverter is earthed on one side and wired through, no interrupter can work. If it's floating, one fault to earth will not be detected, a second fault elsewhere also won't be detected and can leave it potentionally live to earth and reliant on fuses or trips to break the circuit quickly enough to avoid shock, the inverter case will also be live and unprotected even if centre tapped at half volts. Centre tapped inverters can usually be modified to a neutral/earth configeration.
     
  8. CDK
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    CDK retired engineer

    The word "ground fault interrupter" is somewhat misleading: it needs no ground connection to work. For ease of wiring there are ground terminals on both input and output but the device is isolated from them. The device contains a double coil where L and N current runs in opposite directions, so the resultant field is always zero if there is no current leak. An unbalance above a fixed threshold makes it pull a small wedge from under the spring-loaded contacts.
    The ones with 50 mA threshold are purely mechanical, the more sensitive types have a handful of electronic components.

    The inverter case is connected to 12 or 24 V ground; most smaller ones are internally wired that way, the big ones just have a ground terminal.
     
  9. Frosty

    Frosty Previous Member

    I don't use Earth. I have but 1 wire in and 1 wire out. Most if not all my electrical s are Thai 2 pin plugs from electric drill to Tvs and DVD's.

    I have a ring main with appropriate cut out switch.

    The only thing I have with an earth is the microwave. Which if wired would be live if my next door neighbor had a prob.

    I don't want to be my next door neighbors sacrificial anode.

    Connecting neutral to earth blows marinas earth leakage trips and IF the marinas earth is connected, or not broken you may get some protection.

    Where is your earth when anchored off with gen running.
     
  10. capt littlelegs
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    capt littlelegs New Member

    The interrupter doesn't need an earth connection that is right but in order for it to work the supply neutral must be earthed before the input otherwise there is nothing to return to as a fault/leak and create an imbalance!

    The incoming shore supply has that earth/neutral connection at source and at various points in the transmission so the inverter/generator must replicate the same circuit onboard with its own earth/neutral connection in order for an interrupter (a residual current circuit breaker in the UK) or a fuse/trip to work.

    The inverter earth connection does vary but in my experience as you say the casing is usually connected to the output, at least in larger permanent installations. If you centre earth it without the neutral also being switched, the output will be short circuited on one side while using the shore supply, so you can't float it and earth it anyway and although the interrupter would then work with the inverter centre earthed it would also need fusing on both output lines so the circuit would then not be compatable with a earthed shore supply. They don't mix!

    That's why I recommend a neutral/earth that still must be switched to avoid the supply being neutral/earth connected on board which would interfer with the interrupter operation and in the event of a neutral fault, dangerously allow the supply to connect through the earth wire, this is against regulations in the UK and quite sure it is in the US. Gets complicated does it not!
     
  11. CDK
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    CDK retired engineer

    As long as there is no imbalance there is no danger!
    In a decently wired ship, anything with a metal surface is grounded to the hull, 12V negative, electrodes etc. What the exact potential between ground and neutral is, doesn't really matter as long as it does not drift away so far that isolation is breached.
    That doesn't happen: there is always a capacitive coupling in AC filters and a few resistors that will keep the voltage more or less centered. Even MOSFET inverters, with near perfect isolation between AC and DC, have a resistor between one output and DC negative to protect the control circuit against static electricity.

    I do not understand the last part about regulations. Shore power should be 2 leads only, the 3rd wire - if present - must be isolated to avoid galvanic processes.
     
  12. powerabout
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    powerabout Senior Member


    You should have a galvanic isolator and it also warns you that there is a current trying to run ..very common on US boats
    http://en.wikipedia.org/wiki/Galvanic_isolation
    from http://www.sterling-power.com/products-galvanic-why.htm
    In order for modern boat builders to comply with modern CE standards such as EN ISO 13297 they must fit the shore earth wire to your boat’s bonding system which is also connected to the hull / anodes / fuel tanks / engine blocks / shafts / propellers / stern tubes / rudders / rudder glands / water intakes / etc. This ensures that any 230V mains faults will operate the R.C.B on the boat in order to save your life.
    Same rule in just about the whole world. The US is changing its rules to bring them into line with this as they have trouble exporting boats
     
  13. capt littlelegs
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    capt littlelegs New Member

    What you are saying is if there is no imbalance i.e. no fault, there is no danger! What then if a fault in an appliance develops, how will it be detected and safely cleared if the inverter is not earthed?

    Electricticity regulations determine for very good reasons that an earth is always required with an AC supply, any perceived galvanic action is not a reason for the dangerous act of disconnecting an earth. If galvanic action is a problem then an onshore isolation transformer can be used.
     
  14. capt littlelegs
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    capt littlelegs New Member

    They are common but unsafe as the diodes usually cannot carry the fault current and fail leaving no earth at all. AFAIA this is still being looked into by the ABYC who strangely make boat electrical codes rather than the NEC!

    In the UK they are allowed now but have to be tested to a set standard, there's no evidence this is happening yet and the same old imported stuff is being sold!
     

  15. capt littlelegs
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    capt littlelegs New Member

     
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