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  • Earthing a metal enclosure?

    Discussion in 'The main mechanical design forum' started by brads, Apr 29, 2013.

    1. brads

      brads Active Member

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      Please can somebody with a bit of electrical knowledge advise on the requirements for earthing a metal enclosure?

      This enclosure will contain some electrical equipment, some of which is 230v. All of the internal components are earthed in their own right and do not need to be put inside and kind of enclosure, such as, there will be a control box with its own plastic enclosure and its own internal earthing. Therefore my argument is that there is no need earth the outer metal enclosure. But I have been told that as I have put a metal enclosure round the entire product that means it must be earthed.

      My customer has given me a description of how the enclosure needs to be earthed. He said all accessible metal parts, other than actuating members, of in-line cord, free-standing and independently mounted controls of class 0I and class I which may become live in the event of an insulation fault, shall be permanently and reliably connected to and earthing terminal or termination within the control, or to the earthing contact of an equipment inlet.

      There is a lot there I’m not familiar with such as what is;

      1. An actuating member?
      2. In-line cord?
      3. Class OI?
      4. Class I?
      5. An insulation fault? (Does this mean a partly severed wire?)

      All of the metalwork is painted does this change anything? Such as if I can’t put a multi-meter on it to see if I have continuity between the part and the earth bar, then does none of the above apply? I have been told (I can’t remember who by) that I would need to scratch off a little bit of paint on each part to carry out the test. This seems ridiculous to me as we reject any metal work that has scratches on it (even tiny ones!).

      Any help would be appreciated.
       
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    3. Michael Ross

      Michael Ross Well-Known Member

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      This is all the purview of licensed electricians. This is safety related.

      If you are not licensed, you need to get a licensed electrician to review it. Most of us here are not licensed, it is outside our expertise and we should probably not comment on it, and you should probably not follow what we say.

      There an excellent book called The National Electrical Code Workbook. You need access to this at a minimum to make decisions about this stuff. But without licensure, you are not qualified. You cannot suss this out on your own. The answers to your question can be found in the book - not wiki, not in a mechanical design forum.

      The paint changes nothing. You get this wrong, someone dies. The code is written and required, because people have died. You do not get to second guess this stuff.

      Can I be clearer?
       
    4. ChrisW

      ChrisW Well-Known Member

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      I've just posted this in response to a similar query about conformance. Contact this company and they will advise you as to the standards and practices you require. It will save 1) a death 2) you doing time!

      http://www.tuv-uk.com/uk

      An alternative can be found here for specifically electrical standards, they used to offer a free handbook.

      http://www.pilz.com/en-GB

      Good luck.
       
    5. brads

      brads Active Member

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      Thanks for the advice, that pilz looks useful, I haven’t found that handbook yet.

      Before my original post we had already discussed this with a qualified electrician (in UK we have a qualification called Part P). However he advised that this was outside of an electricians remit. He suggested that to find the answers to the questions in my original post I should contact a Mechanical Engineer (that’s me unfortunately). He advised that when installing this equipment there are no tests that an electrician would carry out as a matter of course or due to any law. He advised there is a test he carries out in commercial premises and rented accommodation called a PAT (Portable Appliance Test) which sounds like this may be suitable, however this test is only carried out on portable appliances, as my product is not portable and will not have a plug the PAT test definitely does not apply. He gave the enclosure a once over with a multi-meter and said it all looked fine.

      As advised in your responses I will be contacting one of the companies for a bit more information. I was expecting a few more responses of people who had come across this problem in the past?
       
    6. ChrisW

      ChrisW Well-Known Member

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      While I have undertaken this type of testing in the past, it was long ago and military application so the relevant standards will be different, however, the practical details were 1) the metal enclosure, door and bolted panels were fitted with a stud-welded earth bonding stud. 2) Earthing cable was attached to each stud to ensure adequate earthing of metal parts which could reasonably become detached from the chassis. 3) a bonding test was applied by passing a current (in my case 5A) between each metal part to ensure continuity.

      A PAT tester may be suitable as they induce an earth fault and measure the instantaneous current to ground. A regular electrician will not know the methods though as it is quite different to domestic wiring. A commercial or industrial electrician may be more conversant with the regulations.

      If your customer is a control panel builder can they carry out the approval testing on your behalf and certify the earthing as unless you are going to install the earth bonding cables yourself, final bonding tests can't be done. Sorry I can only be a bit vague without knowing the full details.

      with respect to the Pilz handbook, try the contact details on this page: http://www.machinesafety.co.uk/machinery-safety-guide/
      You can phone and request a copy or register and log-in to download it from their website.
       
    7. Michael Ross

      Michael Ross Well-Known Member

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      I think you need a second licensed electrical opinion. Maybe here in the US the license indicates more understanding? Hard to say. The folks I know who hold a license are pretty sharp and thorough. They won't hesitate to get out the code and look it up either. If someone tells you something and you think it is not right, ask them to show you where it is in the code. If they brush you off, go find a better person to talk to.

      If you want people to be safe you want to have a very low impedance path to ground for any metallic component or structure. You can't get this all the time; but you need to know that if you do not, you have increased the chances for someone to get hurt.

      This is true even for things that have only isolated electrical equipment or no electrical equipment at all attached to it - for example, in the US and piping networks must be grounded even if there is no pump attached to it. If you have dielectric joints you have to bond both sides or jumper across them. It must be bonded with wire sufficient to carry the entire shorted current of the system. We have 200 Amp breakers at our service panel, and so our plumbing is bonded to it with 6GA wire or larger quch can handle the 200 Amps. The reason is that if the network gets energized at some unseen location you want the current going to ground not waiting for you to touch it. If the bonding can carry the whole 200A, then the breaker will be able to trip. If the bonding cannot carry the whole 200A, then in a fault situation the network might be hot and no one knows until it is too late.

      I can't comment on painted enclosures, I am not licensed and I have no idea what UK code is like, but you better follow it. I am pretty sure you are allowed to have cast aluminum that is not painted at all, so I don't see the issue. If you bond it with heavy wire to a good ground that should make you happy yourself, but if it doesn't meet code then fix it. You will be liable if you don't. Probably you can't do it yourself, or without a licensed electrician to do it, or at very least to sign off on it.

      I can tell you that I have learned to take grounding very seriously. I do data acquisition and the electrical noise that motors and drives make can be very troublesome. You want everything grounded so there is less than an Ohm of resistance to ground throughout the system - including conduit if you can do it. I think the code is pretty slack on this 30 Ohms? I wouldn't like that. If I saw a ground that high I would fix it.

      Let me be clear, I am not a qualified, licensed electrician.
       

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