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  1. #46
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    Thanks once again, it looks as though your persistance is getting places.

    Denn

  2. #47
    BobL is online now Member: Blue and white apron brigade
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    Could you kindly tell me what this is, its' purpose & how it is supposed to function?

    Lets say you have an live angle grinder sitting on your metal bench and a bit of steel falls across the cable exposing it and making the table live. If the table is not grounded the table can sit their live - someone touches it and they're in trouble. If the table is grounded the breakers will trip and problem can be fixed. I agree the best way to earth the bench is to the electrical system but that legally requires getting a sparky in whereas I though my idea wouldn't. I also already agreed in my previous post that legally all earthing must be done by a sparky - I have no argument there.

    If you want to muck around with electricity, be it on your own head. There is only one correct, legal & EFFECTIVE way to earth "extraneous conductive parts" (metallic benches etc) & it is NOT how you would do it (with spikes etc).

    FWIW I've been working and trouble shooting in labs that use up to 20,000 V, and teaching university level electromagnetism for 30 years. I am very familiar with earthing principles having dealt with a series of persistent ground loop problems in the labs I work in. When the sparkies couldn't fix it I organised them to install a series of 1" thick copper earth straps running around our labs that were connected to a big copper mains water junction underground outside our lab ie a big spike in the ground. Since then we have had absolutely no problems. When sparkies come to our labs to fix something I often have to work with them because they don't always understand what's involved. I pay close attention to regulations because of OHS and legal ramficiations but the regulations are sometimes not the only way to do things effectively and do not cover all aspects of safety especially in our special laboratories.

  3. #48
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    Quote Originally Posted by BobL View Post
    Lets say you have an live angle grinder sitting on your metal bench and a bit of steel falls across the cable exposing it and making the table live. If the table is not grounded the table can sit their live - someone touches it and they're in trouble. If the table is grounded the breakers will trip and problem can be fixed. I agree the best way to earth the bench is to the electrical system but that legally requires getting a sparky in whereas I though my idea wouldn't. I also already agreed in my previous post that legally all earthing must be done by a sparky - I have no argument there.

    In the above situation I am presuming that although I can't legally attach the bench to the earth on the electrical system, surely I can bolt that same bench to the shed structure or extend an existing bench; both of which are allready earthed - that seems to me to be an obvious way around the legalities. but would that introduce further complications? (apart from the obvious one of making sure that any such connections were secure and as close to the point where the shed is grounded as possible).

    Denn

  4. #49
    BobL is online now Member: Blue and white apron brigade
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    Quote Originally Posted by dennford View Post
    In the above situation I am presuming that although I can't legally attach the bench to the earth on the electrical system, surely I can bolt that same bench to the shed structure or extend an existing bench; both of which are allready earthed - that seems to me to be an obvious way around the legalities. but would that introduce further complications? (apart from the obvious one of making sure that any such connections were secure and as close to the point where the shed is grounded as possible).

    Denn
    The official or legal line is you should leave it all to the sparky.

    In practice my sources tell me this;
    - that if anything looks like an earth (ie a cable or a metal bar that connects a bench to a spike in the ground, or another device connected to an approved earth ) you should get it tested and signed off by a sparky. If you modify or expand an existing bench attached to an officially approved earth you should also get that tested. If anything goes wrong and there's no approved paperwork, court appearances and fines could result.

    - if your shed has steel poles and they happen to run into the ground (and provide an effective ground) and you weld your bench to those poles primarily for support and not for electrical safety purposes then no approval is required. The primary purpose of the welded connection is support and the fact that it provides some form of earthing is incidental. If it's a workplace and something goes pear shaped and you end up in court you might still be held liable for not providing an adequate earthing of a work environment but you are unlikley to be liable for installing an unapproved earth.

    This situation clearly demonstrates how, no matter how detailed regulations are they can't cover every situation. I've seen attempts to write regulations that try to do that and it is impossible. Deep fundamental knowledge of a physical phenomenon will always save more lives than regulations. Unfortunately few people have that depth of understanding which is why regulations are very useful for these people and especially to cover their behinds in court.

  5. #50
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    Quote Originally Posted by dennford View Post
    In the above situation I am presuming that although I can't legally attach the bench to the earth on the electrical system, surely I can bolt that same bench to the shed structure or extend an existing bench; both of which are allready earthed - that seems to me to be an obvious way around the legalities. but would that introduce further complications? (apart from the obvious one of making sure that any such connections were secure and as close to the point where the shed is grounded as possible).

    Denn
    I don't see any real problem with your idea. The primary thing to remember with all earthing is that the impedance (resistance) from the earthed equipment back to earthing electrode is within accepted tolerances, according to the type of protective devices used. The secondary thing to remember is that an effective earthing system will not permit dangerous voltages to be present on any conductive parts, for longer than the required times. A dangerous voltage is considered to be;

    1] above 50v R.M.S. (AC) and;
    2] above 120v "ripple free" D.C.

    Generally, a protective device (circuit breaker, fuse) must clear a fault within 0.4 seconds. Longer time periods can apply for other types of circuits. Of course, the best way to protect yourself is to use an RCD (20mS at 30mA), which should be tested monthly by pressing the "press to test" button.
    “I do not think there is any thrill that can go through the human heart like that felt by the inventor as he sees some creation of the brain unfolding to success... Such emotions make a man forget food, sleep, friends, love, everything.” - Nikola Tesla.

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