How to bond Protective Earth on PCB

Discussion in 'The Projects Forum' started by jgrv, Jul 14, 2018.

  1. jgrv

    Thread Starter New Member

    Mar 2, 2018
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    Hi,

    Does anyone here know if there are any regulations or best practices regarding how protective earth wires should be fastened to connectors on a pcb to prevent it from coming lose? I.e. screw clamp terminal with ring tongue crimp, spring clamps, or something like this? In larger cabinets, regular earth bars are used, but what is normal to use on a PCB?

    I'm making a module contained within a small plastic enclosure, that will control a couple of solenoid valves etc. The valves' PE wire will go to this earth bonding point within the module.

    Many thanks!
     
    Last edited: Jul 14, 2018
  2. ericgibbs

    Moderator

    Jan 29, 2010
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    hi jgry,
    What voltage levels are being used on the PCB.?
    Is it mains Earth grounding.?
    E
     
  3. jgrv

    Thread Starter New Member

    Mar 2, 2018
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    HI, Eric.

    Yes, European mains 230VAC + PE, and a 24VDC transformer is present on the board.
     
  4. ericgibbs

    Moderator

    Jan 29, 2010
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    hi jgry,
    Sorry to ask so many questions, but it is important to know the purpose of the grounding.
    On the 230Vac mains to 24Vdc, transformer/bridge are you connecting the mains Earth wire to the transformers metal frame?
    If so. I usually connect any onboard Earthing points to the same screw point.
    One point to consider when using lock/spring washers in contact with the PCB track, is that the track should be well tinned.
    Even then, depending upon the environment contact corrosion will occur over time.

    Do you have a clear photo of the assembled PCB.

    Eric
     
  5. jgrv

    Thread Starter New Member

    Mar 2, 2018
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    Here is the board so far:
    https://drive.google.com/file/d/1xVnlqfm4C0Ua4XLGe8EUyM0lVWpzFQ6D/view?usp=drivesdk

    X1: Barrier terminal block for L, N and PE.
    F1: Fuse
    X4: Temporary earth terminal ( regular 4 way spring clamp terminal)
    PSU: Meanwell IRM-20-24.
    IC1: Linear regulator 24V to 5V.
    X5: Terminal block to supply power to separate controller board.

    The power supply unit does not have a metal frame, so it is not connected to earth PE. But the valves the module will control are metal and will control water flow in private homes. Need to connect it to PE to protect people touching the valves.
     
  6. ericgibbs

    Moderator

    Jan 29, 2010
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    hi,
    Is there any reason that the 0V [Gnd] of the power supply output is not connected directly to X4 [Earth] copper track.?
    Ref the separate Controller board, could you give details of that wiring.?

    Is this going to be a product for sale to household consumers, if so will be it be submitted for CE or any other Approval Standards.?
    E
     
  7. jgrv

    Thread Starter New Member

    Mar 2, 2018
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    Connecting PE and Signal ground is something I not have quite clear and would also appreciate some input on this. I tried to look this up, but I'm still a bit unsure whether signal ground and PE can be tied together or not.

    Where I am from, IT power network is very common, meaning there is no Neutral, only L1, L2, L3 and PE. Can you then be sure that the ground from the power supply is same potential as PE? Should they not be kept separate to prevent current flowing through the ground wire?

    The separate microcontroller board is driven by 5V, and uses mosfets for switching the 24V to the valves. The cables coming from the valves are connected to the controller board via spring clamp terminals, while the earth wire in the cable is connected to X4.

    Yes, it must be submitted for CE approval.

    Thanks!
     
  8. ericgibbs

    Moderator

    Jan 29, 2010
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    hi jgrv,
    I am not familiar with your countries mains supply specification.
    Could you post a link to a web information page, which covers your local mains supply, so that I can check it out.?

    E
     
  9. Hymie

    Member

    Mar 30, 2018
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    My understanding of an IT power distribution system is that they are isolated from earth - except that one point may be connected to earth through an impedance or a voltage limiter.

    If no neutral is provided, how is single phase equipment powered?
     
  10. Hymie

    Member

    Mar 30, 2018
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    Since you want to meet the applicable CE Directives for this product – have you identified the harmonised standard you will be using for the purposes of compliance with the Low Voltage Directive?
     
  11. jgrv

    Thread Starter New Member

    Mar 2, 2018
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    Hi

    Here is a schematic of IT earthing system:
    https://no.m.wikipedia.org/wiki/IT-nett#/media/Fil:IT-fordelingssystem_IMG.jpg

    Here, there is 230V between the phases, and around 130V between phase and PE. There is no N, like in typical TN network.

    I'm not sure if this has any impact on your grounding philosophy. I am a bit confused after reading on the internet about this. Some say keep PE and ground separate, some say use a capacitor between ground and earth and some say use a resistor between...
     
  12. Hymie

    Member

    Mar 30, 2018
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    To allow single phase equipment to be powered, there must be a neutral connection in the supply as shown in the IT power distribution shown below.

    IT power distribution with neutral.jpg
     
  13. ericgibbs

    Moderator

    Jan 29, 2010
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    hi,
    Thanks for link.
    In the UK, our Earth connection at the incoming distribution point is connected to Ground Earth by using the incoming cold water metal pipe work as well as the conductive outer covering of the incoming mains supply from the outside roadway, also at the central power generating station.

    Is your PE Earth grounded in a similar way.? ie: at the house incoming supply.?

    E
     
  14. jgrv

    Thread Starter New Member

    Mar 2, 2018
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    I have bought and read the applicable standards yet. Do they mention how protective earth and grounds should be bonded and connected? I suppose EN 50491 is applicable in my case.

    In TN you have 230V 50Hz between Line and Neutral. In IT you have 230V 50Hz between Line 1 and Line 2. So one phase equipment are actually powered by two phases.

    I'm not sure exactly how this is done but for newer installations I suppose it is done similarly.
     
  15. KeepItSimpleStupid

    Distinguished Member

    Mar 4, 2014
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    I'm going to add the following to this discussion:

    I started here

    https://lugsdirect.com/4-7-1-7-neutral-ground-bar-4-14AWG-wire-lug.htm

    and ended up:

    http://ihiconnectors.com/UL-Listed-Grounding-Lugs.htm




    and It's UL rather than CE.

    I guess the deal is use a listed terminal for grounding.

    Here https://www.edn.com/design/pc-board/4443239/Properly-ground-your-circuits is a brief article about grounding.

    Another blurb:

    http://www.interpower.com/ic/design...tion/More-Information-on-Terminal-Blocks.html

    A UL/CE http://www.deca.com.tw/pdf/EUROPE_PDF/ME130-500508.pdf
    block, whatever that means.

    https://www.eetimes.com/document.asp?doc_id=1272972
     
  16. MaxHeadRoom

    Expert

    Jul 18, 2013
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    What I currently do is extend the ground plane of any board that requires to be bonded to earth and use a star washer under the ring terminal and wire to the nearest service GND point.
    Or use a screw with star washer through the board ground plane to the aluminum chassis which in turn has been earth grounded.
    Max.
     
  17. MaxHeadRoom

    Expert

    Jul 18, 2013
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  18. Hymie

    Member

    Mar 30, 2018
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    Given that the device is to control water valves, I think that the applicable harmonised standard would be EN 60730-2-8, which is used in conjunction with EN 60730-1; but before spending any money obtaining the standards – you need to ensure that your product is with the standard’s scope.

    Whichever standard you select, it will specify the requirements re earthing methods/tests.
    That said, there is much commonality in earthing requirements between electrical safety standards.

    On your circuit board, you have indicated the protective earthing terminal with the letters ‘PE’ and terminals 1-4 with the circled earth symbol (which is used to indicate the protective earthing terminal).
    Therefore you should replace the letters ‘PE’ with the circled earth symbol and use an un-circled earth symbol (where you have used a circled earth symbol), which is used to indicate an earth bonding point.
    If you are then using these terminals (1-4) to provide earthing to other parts – this should provide adequate earthing, providing that the PCB track material has adequate thickness and the connecting earth wires have the required cross sectional area.

    Most electrical safety standards require that parts earthed for safety purposes can withstand a current of 2 times the rating of the protective device (fuse), with a minimum value of 25A applied to incoming mains. The test current is applied between the protective earthing terminal and the part required to be earthed, which results in maximum resistance of 0.1 ohm (based on the voltage/current).

    Your connection of the V- to ground/earth could be considered as a PELV circuit and these earth tracks not required to pass the above bond current test – however they would need to be separated from the mains circuit by double/reinforced insulation.

    In relation to the PCB earth terminals, there can be specified minimum screw thread sizes (based on the rated current) and aperture requirements, but otherwise providing they pass the tests set out in the safety standard, any common PCB screw terminals (attached to the PCB by soldering) should suffice.
     
  19. jgrv

    Thread Starter New Member

    Mar 2, 2018
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    Thank you for your detailed answer! I will look more into the applicable standards the coming week. But do you know if they usually specify if you shall connect them to screw terminals using any specific type of cable lugs?

    As mentioned earlier in this thread, about connecting the ground plane on the PCB together with PE. Does anyone know what the advantages/disadvantages of this are rather than just passing the PE straight through to the equipment you want to connect to PE?
     
  20. Hymie

    Member

    Mar 30, 2018
    462
    95
    Screw terminals are perfectly acceptable for all electrical connections to the PCB, screwing down onto stranded wire.

    Two things to consider of the screw connections (besides that mentioned in earlier posts):-

    1) It is considered that any one wire termination may fail, and that free wire should not create a hazard as a result of movement of the free wire. One way to comply with this requirement is to tie-wrap adjacent cable insulation close to their terminations, such that should a wire break free from its terminal its possible movement is limited.
    A crimp connection is considered to comply, providing two means of fixing (crimp on the wire and crimp on the cable insulation).

    2) It is considered that in attaching the wires to the terminals, one stranded wire (8mm in length) will not have been correctly inserted in the terminal. This 8mm free strand should not create a hazard through its movement (note that this free wire strand of 8mm is not bent though multiple angles – but left straight).


    Many designs flood the PCB plane with a 0V/earth (most claiming it for EMC purposes). Problems can occur if it is judged that the earth plane needs to meet the earth bond test (at 25A or more), due to its close proximity to the mains. If you flood the PCB with an earth plane, ensure that it has at least 5mm separation from mains circuits, and I’d recommend at least 1mm from other SELV circuit tracks to avoid possible shorts. I don’t see why you need to earth the secondary circuit in your design – you would probably be better off only earthing those parts needing to be earthed for safety purposes.

    You also appear to be using the PCB mounting points for earth connections, which is not good practice due to the PCB insulation within the earth connection.
     
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