Inverter RCD

Discussion in 'Electricians' Talk' started by dask, Jul 25, 2021.

  1. dask

    dask New Member

    Hi

    Bought this inverter: https://www.amazon.co.uk/gp/product/B076P9PGX3/ref=ask_ql_qh_dp_hza

    I am trying to understand what sort of protection against shock is required. I have seen some discussions around floating ground, that RCD will not work without earth etc.

    To explain I did the following:
    1. - I watched explanation around RCD's and:
    a) if the electricity goes through my body to ground then my understanding is that it would work
    b) I am not sure if this will work if I let's say electricity will flow only through my body, not to ground, so I will become part of the circuit (same will come back through my body)?
    c) furthermore interesting subject is as well, in case of powering the circuit from battery you will not be shocked until you hold both poles as electricity will not flow due to imbalance inside of the battery?
    2. I asked the supplier and the reply is:
    "The unit has two internal fuses, and if you were to use the car lighter socket, then the car circuit will be fused as well (often 10amp). Presumably anything you plugged into it will have a fused plug (or certainly should do) so just check the fuse is the correct rating. So theoretically you've got 3 separate fail-safes, so I think you would not require an RCD."

    I guess they don't understand what fuses are about? Yes they could work (too late) but the electricity needed to stop my heart I guess is much less than fuse rating.

    3. It can definitely kill you following this:
    https://electronics.stackexchange.c...-from-a-120v-300w-ups-enough-to-kill-a-person

    So my question still stands, how to protect yourself from electrical shock if you use inverter? Is there any device I can just plug in to inverter which will work like RCD? Then in home installation, if I hold hot and then touch cold - my understanding is that it is same case - so RCD will not work (with assumption nothing goes to earth so no imbalance?).

    Let's say the scenario is that I have socket from car/battery on wet ground where I am standing. Sorry for all confusion :) I am just trying to understand.
     
    Last edited: Jul 25, 2021
  2. MGW

    MGW Screwfix Select

    It does depend what you are using the inverter for, with for example a shaver socket you have line 1 and line 2 and no earth and the isolation transformer has no reference to earth, but the point is it only powers a single item so touching line 1 and line 2 at the same time is very unlikely.

    However if you have two or more items powered on one line 1 could go to earth and the other line 2 could come into contact with you, so there is a possibility you touch both line 1 and line 2, so you need some protection, there are rules for using a supply without an earth called an IT supply, but in the main we don't use IT supplies except for shaver sockets, one line is connected to earth and then it is called neutral, and we can use a RCD.

    However an inverter can have a DC component, so may need a type B RCD which are rather expensive, or a device to detect DC and auto disconnect so can use type A, but unlikely you can use the type AC used in most homes.

    So 300 watt is not that much, and it seems unlikely you will be powering multi devices from such a small output, so personally I would use it as a IT supply.

    The official guidance gets complex this is part of it
    but wait for other answers and tell us what the inverter does.
     
  3. dask

    dask New Member

    Ok, let me explain exactly what I plan to do with it. The idea is to play some music at remote location, we hired a private land and we are going to be playing live music there. So to achieve that I have 12v car battery - I am planning to plug into inverter mixer, piano and two amps. Amps are using around 50w each, so I will be using around 150w of power in total from it. Piano and mixer are going through power supplies which change the voltage to safe values - that is my understanding, however amps run on 220v. If it gets wet due to gear used it will not going to happen, however if amp develops an issue and somebody touches the metal case - that is more of my concern. Multiple devices yes, but I am wondering how inverters are so widely available (the shelf in go outdoors was empty last time I was there) so people are buying them.
     
    Last edited: Jul 25, 2021
  4. The Happy Builder

    The Happy Builder Screwfix Select

    It is like trying to put a RCD on the output of a shaver socket in a bathroom, pointless.

    Like the shaver socket that inverter should only supply one appliance plugged directly into it, it should not supply a distribution circuit with multiple socket outlets or appliances.

    Safety is provided by electrical isolation and it’s earth free, trying to earth it will make it dangerous rather than improving safety.

    You need a “Portable Appliance Testing” regime, before you use the inverter and the appliance it is supplying you need to do a user visual inspection and check that they are not damaged, also you should stick to using double or all insulated appliances.

    The danger with the supply from the inverter comes from touching live and neutral at the same time, as a RCD will not protect direct contact between live and neutral it doesn’t achieve what you might think it would.
     
  5. The Happy Builder

    The Happy Builder Screwfix Select

    Then you need four inverters, one for each of the appliances to maintain electrical separation.

    Is all of the equipment double insulated and have you had it Portable Appliance Tested assessed for safety, which for all insulated double insulated equipment is a through visual inspection?
     
  6. sparky steve

    sparky steve Screwfix Select

    My understanding is as, MGW has already touched upon.

    Professional setups tend to have an isolated source known as IT, and must have some kind of earth fault detection, but for a temporary setup this isn't considered necessary. You need a special isolation fault detector made for IT mains setups.

    If you're using a decent quality inverter, the risk of electric shock is lower than your normal setup at home, because the inverter is supposed to provide a floating, (isolated from any kind of earth or other reference potential) output. So if you touch one of the output pins no matter if it's labeled live or neutral, no current will flow - no electric shock. Same for a possibly defective piece of equipment that shorts one of the mains input wires to its metallic case - no current flow - no electric shock due to the isolated output of the inverter.

    So, a second fault, the other wire has to be shorted to a "real" earth has to occur while the first fault exists to make your setup dangerous. This is why professional setups that have an isolated source must have some kind of earth fault detection. For a temp setup this isn't considered necessary. Any kind of RCD won't do anything useful in such a setup, you'd need a special isolation fault detector made for IT mains setups.
     
  7. MGW

    MGW Screwfix Select

    The class II or class III equipment is not a problem, either no earth anyway, or extra low voltage, so the problem is the amplifiers.

    Sinking an earth rod is one option, but it can make it more dangerous rather than less, what we are looking as is the birds landing on power lines, even with 33,000 volts they are not harmed, in USA they has to increase the distance between the two lines as bald eagles were touching both due to their wing span.

    Best option is an inverter for each appliance. However it depends on if there is some official with a bee in his bonnet, I have had it where some inspector has insisted on some really daft things.
     
  8. The Happy Builder

    The Happy Builder Screwfix Select

    If you sink an earth rod and connect it to the neutral you are creating a danger where none existed.
     
  9. Comlec

    Comlec Screwfix Select

    Plug in your inverter, connect your equipment and enjoy your gig.

    All will be fine.

    I use and inverter in my van to provide ‘mains’ when I out and about as it is a perfectly safe setup. Sometimes these inverters have an “earth” terminal but these are designed to be connected to the vehicle chassis not true earth.
     
  10. dask

    dask New Member

    Thanks for all replies. So I guess best solution is inverter per appliance and it is not extremely dangerous in general I guess. My idea is that I will probably set a small tent with speakers in it and other gear so this will prevent most of human interactions with it just in case. I am now though very confused about earthing and flow of electricity. So to put it in some ordered form:
    1. As far I understand till now, electricity will not flow if I connect to earth in example any of terminals of the battery, the reasoning is that the battery as whole is balanced and electrons will not flow to earth as the is no return path. From this I understand there must be a closed circuit for it to go anywhere and kinda fix imbalance between A and B.
    2. So following point 1 how the heck is earth taking charge from any appliance? Earth is neutral so why would that accept any electricity gathered on case, it should be same principle as with battery. I understand that neutral can be linked to earth but still it will not go due to resistance?
    3. If there must be a closed circuit and earth is neutral (balanced) , why it accepts lightning in example (or this is another beast?).
    4. In general electricity will flow if there is potential difference and enough small resistance to allow the flow. So maybe you don't actually need closed circuit but enough difference (voltage) and small resistance - this would explain battery example as voltage is small (? though what about this closed circuit theory etc. it should still discharge if earth is taking electricity), will explain case to earth as 220v could be enough and as it is directly plugged resistance would be low, and lightning is ok as well as it can break resistance of air with super high voltage?

    Thanks
     
    Last edited: Jul 27, 2021
  11. The Happy Builder

    The Happy Builder Screwfix Select

    You’re trying to over think this.

    I said that earthing the neutral would introduce a danger where none existed.

    The Health and Safety executive say regards small generators:

    “30. Floating systems, particularly when limited in extent and under close supervision, do not introduce possible consequential hazards which may arise from deliberately earthing an initially separated single-phase generator winding.”

    https://www.hse.gov.uk/foi/internalops/ocs/400-499/oc482_2.htm

    So, ensure the inverters are in good condition, avoid using extension leads, don’t create a distribution system using extension leads with multiple socket outlets and run multiple items of equipment from the same inverter, use double or all insulated equipment and make sure the equipment is in good condition.

    Also do not expect a RCD to protect you or anyone else.
     
  12. The Happy Builder

    The Happy Builder Screwfix Select

    and if you really want to confuse yourself which leg of the telephone network is earthed?
     

Share This Page