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Blue Skies Thread, Rewiring an entire school in General; Cool. I've now asked our site manager about how our site is grounded, and he's gonna ask the sparkies we ...
  1. #31

    localzuk's Avatar
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    Cool. I've now asked our site manager about how our site is grounded, and he's gonna ask the sparkies we usually use.

    I'm gonna have a wild guess but say that all our buildings have their own grounding plates. Fingers crossed they come back and say 'oh yes, we grounded them all to the same point.' Ooh look, pigs flying through the sky!

    I'm not going to risk anything here, just seeing what options are. It is looking more and more like we're going to end up sticking with at least 1 cabinet in each building, which sucks as it increases 'backbone' costs by from one fibre and associated adapters to 3.

  2. #32

    localzuk's Avatar
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    Ok, I've spoken to our sparkies and they've said that the site doesn't have a 'grounding point' of its own, but instead is grounded only by the mains cable that feeds the school back to the nearest grid transformer.

    Does this sound possible?

  3. #33

    HarryMonkey's Avatar
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    You have potential problems with the buildings being fed from two different phases. If you had a piece of live equipment that became faulty and fed back through its network cable to a different building on a different phase, you end up shorting across the phases and putting 415v through your 3 pin plugs. You should never use copper to join buildings on different phases.

  4. #34

    localzuk's Avatar
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    Quote Originally Posted by stevenewman View Post
    You have potential problems with the buildings being fed from two different phases. If you had a piece of live equipment that became faulty and fed back through its network cable to a different building on a different phase, you end up shorting across the phases and putting 415v through your 3 pin plugs. You should never use copper to join buildings on different phases.
    What if a building is fed by more than one phase on its own. ie. we use one phase for one half of the main building, and a different one for the other half? We also have network cables crossing that boundary at the moment, within the building, so the situation you describe above could exist in any building using 2 phases and comms gear?

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    plexer's Avatar
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    Yes it could and thus connectin those lives bits of comms equipment with copper cables presents a possible path.

    However the main danger is normally wall sockets, we had a new feed to our new ict workshop and ict suite and there was a single existing socket left in the workshop that ran back to the nearest consumer unit, as they were on different phases there was a potential of 415v so if I stuck a screwdrive in both of them at the same time it would really hurt. Because of this the existing socket had to be removed.

    Ben

  6. #36

    HarryMonkey's Avatar
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    Comms kit and electrical phasing can be a nightmare that's why I've always made sure that I don't use copper to join buildings. I would recommend getting in a networking company to give you a quote and tell them about your power and they will suggest the best course of action.

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    tonyd's Avatar
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    There is also an enviromental and physical concerns, that is an awfuly large amount of copper that you are considering pulling across your site. I've never seen a bundle of 300 CAT5 (or surely 6) but I'd bet it would be thicker than any drill bit you can get at B&Q! Nobody is going to want to even thing of pulling that number of cables down ducts, let alone have them swinging between buildings. Some nice lenghts of OM3 fibre between buildings make so much more sense from every standpoint I can think of...

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    Quote Originally Posted by stevenewman View Post
    You have potential problems with the buildings being fed from two different phases. If you had a piece of live equipment that became faulty and fed back through its network cable to a different building on a different phase, you end up shorting across the phases and putting 415v through your 3 pin plugs. You should never use copper to join buildings on different phases.
    What realistically are the chances of this happening? Surely the data cable is isolated from the power supply, otherwise even the possibility of just one phase of mains at 240V across the data cable would be enough to fry any kit connected?

  9. #39

    localzuk's Avatar
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    Quote Originally Posted by tonyd View Post
    There is also an enviromental and physical concerns, that is an awfuly large amount of copper that you are considering pulling across your site. I've never seen a bundle of 300 CAT5 (or surely 6) but I'd bet it would be thicker than any drill bit you can get at B&Q! Nobody is going to want to even thing of pulling that number of cables down ducts, let alone have them swinging between buildings. Some nice lenghts of OM3 fibre between buildings make so much more sense from every standpoint I can think of...
    There wouldn't be 300 in one duct - each secondary building would have about 25 maximum. The rest are spread out throughout the main building.

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    Quote Originally Posted by tonyd View Post
    There is also an enviromental and physical concerns, that is an awfuly large amount of copper that you are considering pulling across your site. I've never seen a bundle of 300 CAT5 (or surely 6) but I'd bet it would be thicker than any drill bit you can get at B&Q! Nobody is going to want to even thing of pulling that number of cables down ducts, let alone have them swinging between buildings. Some nice lenghts of OM3 fibre between buildings make so much more sense from every standpoint I can think of...
    Yes they do if sticking with satellite cabs running fibre as the backbone between them but the OP is trying to remove these by running the wall data points back to one location and not a satellite cab.

    Ben

  11. #41

    SYNACK's Avatar
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    Quote Originally Posted by localzuk View Post
    Ok, I've spoken to our sparkies and they've said that the site doesn't have a 'grounding point' of its own, but instead is grounded only by the mains cable that feeds the school back to the nearest grid transformer.

    Does this sound possible?
    TBH it sounds dangerous but most of your UK electrical systems sound dangerous from my prospective given that all of your wireing stuff was designed during a copper shortage and so you end up needing stuff like fuses on plugs. If it is how it is usually done then your probably good. The differing phases should not be so much of a concern as long as your grounds are sorted. The primary danger is potential difference across the cable due to it being used as a grounding path. Personally I'd be pushing for fibre but I mirror the others suggestions to talk to a professional (read expencive) company about this in a consulting capacity. There are plenty of cowboys that will string cat5 through a drainage pipe if you tell them too, you want to talk to one of the propper professional companies that actually knows about this indepth.

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    Quote Originally Posted by plexer View Post
    Yes it could and thus connectin those lives bits of comms equipment with copper cables presents a possible path.

    However the main danger is normally wall sockets, we had a new feed to our new ict workshop and ict suite and there was a single existing socket left in the workshop that ran back to the nearest consumer unit, as they were on different phases there was a potential of 415v so if I stuck a screwdrive in both of them at the same time it would really hurt. Because of this the existing socket had to be removed.

    Ben
    As opposed to mildly hurt if you just stuck a screwdriver in one of them??

  13. #43

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    That's the general idea yes.

    Ben

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    mpe
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    Quote Originally Posted by localzuk View Post
    Ok, I've spoken to our sparkies and they've said that the site doesn't have a 'grounding point' of its own, but instead is grounded only by the mains cable that feeds the school back to the nearest grid transformer.
    Yes and it also means that the entire site only has one "grounding point"

  15. #45
    mpe
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    Quote Originally Posted by seacider View Post
    The problem is earth potential.Data is sent at 5v to earth. If a different earth source is used in different building there could be a voltage difference between the two earths.This would make the data invisible. I have seen it when I was in the RAF, stopped a radar from being useful. Until we bonded the earths to a common. We had a difference of 30v.
    Coaxial and STP have an earth connection. UTP dosn't, an earth connection isn't even required for PoE.

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