Do I need a RCD on an inverter?

Discussion in 'Tech/Mech General' started by RogerThat, Aug 2, 2016.

  1. RogerThat

    RogerThat Funster

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    Hi everyone,

    I'm debating buying an inverter but I'm a bit unsure about the RCD situation.

    My leisure battery is mounted under the drivers seat and if I go ahead I would bolt the inverter directly on to the rear of the drivers seat base frame. This means the wires to the battery would be really short, maybe 10" or less (not sure that has any bearing on anything?).

    I had thought that I would just use an extension lead from the inverter directly to the appliance in question.

    So for example, if I wanted to plug a desk-type fan in while wild camping, I'd run a normal household mains extension lead from the inverter to wherever I decided the fan needed to be (somewhere in the rear lounge probably).

    So apart from the obvious trip hazard of a trailing mains cable running from the cab to the rear lounge for example, would I also need to fit an RCD somewhere? Or would whatever fused protection the inverter offered be sufficient?

    I haven't yet decided on a size, make or model of inverter (suggestions welcomed by the way!), I'm just trying to assess the overall cost and any unforeseen issues that I might not have considered at this stage :)
     
  2. Techno

    Techno Funster Life Member

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  3. maxi77

    maxi77 Funster

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  4. Techno

    Techno Funster Life Member

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  5. hilldweller

    hilldweller Funster Life Member

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    The real question is..............

    Do inverters have a N-E bond or is the 240V just floating ?
     
  6. scotjimland

    scotjimland Funster Life Member

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    An RCD is not required.. it would only work if the Neutral at the inverter were grounded..
     
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  7. Techno

    Techno Funster Life Member

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    It varies but needs to be neutral earth bonded for an RCD
    Sterling pure sine units can be altered for either
     
  8. Techno

    Techno Funster Life Member

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    Easy to check, if the test button does not trip the device then it is a centre tapped inverter
     
  9. Techno

    Techno Funster Life Member

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    Mine was neutral earth out of the box as I believe most larger inverters will be
    [​IMG]
     
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  10. RogerThat

    RogerThat Funster

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    I'm still confused :unsure:

    Do I need a RCD?

    I don't intend running the extension lead outdoors and I don't use RCDs on extension leads that I have in my house.

    But maybe that's the point? I have a fusebox on the house, I don't have with an inverter?
     
  11. Techno

    Techno Funster Life Member

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    Read the inverter instruction book
     
  12. hilldweller

    hilldweller Funster Life Member

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    I don't on mine.

    "Fuse" is an entirely different matter, the inverter will shut itself down if overloaded.
     
  13. pyro

    pyro Funster

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    There's some really excellent advice given here, so I won't confuse things.

    Just to add that a well engineered true RMS (well, sinewave, anyway) will cost a whole lot more than a 12V fan, and a cheap low wattage double insulated hairdryer (look for 1970's 400W Moulinex ones on ebay) will work just as well on a £40 Maplins invertor as it would on a £500 quality job
     
  14. Robin McHood

    Robin McHood Funster

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    @RodgerThat An RCD protects you from electrocution a fuse protects the equipment and cable from damage caused by overloads, totally different forms of protection.
    But also what @Techno says above.
    It is possible to have a different form of RCD which doesn't require an earthed neutral. Instead it looks at the current in both the "power" legs and trips if there is a difference of more than 30mA.
    These are very common at sea as we tend not to have grounded neutrals at all, for reasons I wont bother confusing everyone with
     
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  15. scotjimland

    scotjimland Funster Life Member

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    can you explain what path is the leakage current is taking if there is no earthed neutral ?
     
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  16. Robin McHood

    Robin McHood Funster

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    It means the current is escaping to earth via another route. You just dont want that route to be you!

    Technically, on ships you don't earth neutrals to prevent spurious tripping, you might lose some vital equipment because of a relatively minor fault. So the neutral of distribution transformers, unlike ashore is never earthed down but left floating. Of course we still have to protect guys using portable mains powered equipment on deck, where electricity is particularly hazardous due to the presence of nice conductive seawater and sweat and loads of nice conductive steel around you:eek::eek:
     
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  17. RogerThat

    RogerThat Funster

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    As per original post, I haven't actually bought one yet, still scoping out my options (y)

    Thanks for all your help everyone, interesting subject! :)
     
  18. tonyidle

    tonyidle Funster

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    If neither line from the inverter is earthed then the only way to receive a shock from its output is by contact with both lines - a pretty unlikely scenario. Earthing one line so an RCD can be used is simply engineering circumstances where the risk of shock is increased so that the RCD is needed. Bit of a waste of time, money, and effort I would have thought?
     
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  19. Robin McHood

    Robin McHood Funster

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    Nah sorry @tonyidle doesn't work like that.
    The inverter output has a potential difference between each pole and ground, voltage is a measurement of that level. If you think of your ungrounded inverter output as two connected tanks of water above your head one for each pole.

    They each have a potential energy. As long as its in the tanks you don't get wet, make a small hole in either one of them and the water will seek equilibrium (ground) and you will get wet.
    The danger with any electrical circuit is a) voltage which provides the push and b) the amount of electricity that actually flows, this is the current. Its the current that kills not the voltage. The higher the voltage the more push and the higher amount of current that will flow. This is why personal protection RCDs are set to trip at 30mA as this is below the general heart stop current. The voltage is immaterial except in so far as insulation and sizing of internal components of the RCD is concerned.

    Those guys who come in contact with HV lines arent so much electrocuted as burned to death by the vast amounts of current being pushed through their bodies by the high potential, heating the salt water part of their body so that they are boiled/incinerated from the inside out. I do have some lovely illustrative photos if anyone is interested ..No? Mmm thought not.:D

    What you say sounds logical (y) but you are not taking into account the capacitive charge that exists between any electrical source and ground whether one side of it is connected to earth or not, and you seriously don't want to use yourself to earth that charge!

    On a larger scale it is what happens when you are struck by lightning, you act as the conduit between a high voltage source and earth potential even though the charged cloud is not earthed at any part of itself. The cloud exists at a potential difference of millions of volts to the ground. Harmless in and of itself but provide a conductive path to ground (ie a nice bag of thinly covered salt water = 1 human being) and its crispy fry time...:eek:

    Anything above 30mA current is enough to stop your heart and the only protection a human naturally has is his/her skin resistance. This can be in the order of 100 thousand ohms or higher with dry skin but get wet, or wear a ring, or have a cut and that resistance drops dramatically, the amount of current your body will pass rises dramatically.

    So for example in industry, generally you use stepdown transformers to take voltage from 220v down to 55-0-55v (centre-tapped to ground) which gives the 110v for actually operating the portable machinery but ensures that the operator will only be exposed to a 55v push, he'll still get a nasty shock and probably survive, but as I keep lecturing our guys at our safety standowns, the fall from the ladder as a result of the shock will still kill you....:D

    The UK distribution system is one of the most dangerous around as it always has a 230v push behind it and it was designed that way to make electricity transmission cheap, especially to rural areas, as it does away with one expensive copper wire wire because you can send the neutral and the ground connections, literally via the ground you stand on, instead of a copper wire. On board our domestic suplies are at 220v the actual push if you come in contact with either live pin and ground is 110v cutting the push in half compared to the UK supply.

    Here endeth RCD 101, Lesson 1 Chapter 1 ...may the force be with you....:):)
     
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  20. Robin McHood

    Robin McHood Funster

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    The fusebox won't protect you agaginst electrocution if anything you have attached to the inverter develops a fault. Its not there for that purpose. The fuse just stops things like cables overheating and catching fire when there is a fault, a 5A fuse for example will pass enough current to stop your heart about 150 times over, for the cost of it get an RCD & press the test button to make sure it works regularly!
     
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