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  1. hightower
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    hightower Electrician's Arms

    Location:
    Durham
    Wiring a couple containers up, refrigeration units. External to factory unit, about 10m from incomer. TNCS supply, thinking we should be dropping a rod in at the containers, but additional to TNCS earth so not separating. Can anyone advise?
     
  2. Leesparkykent
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    Leesparkykent You Rock Gmes Staff Member Trusted Advisor

    Location:
    Kent
    I would rod it separately.
     
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  3. hightower
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    hightower Electrician's Arms

    Location:
    Durham
    That would then require RCD protection though
     
  4. Leesparkykent
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    Leesparkykent You Rock Gmes Staff Member Trusted Advisor

    Location:
    Kent
    Is there not a board with RCD in the container?
     
  5. hightower
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    hightower Electrician's Arms

    Location:
    Durham
    No, the lights are rcd protected but not the fridge part. Just our worry is if it trips over a weekend that's a lot of stock lost so that's why trying to avoid RCD protection
     
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  6. westward10
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    westward10 In echoed steps I walk across an empty dream. Electrician's Arms

    Location:
    Northamptonshire
    Doubt it has a board they are just big fridges or freezers. You may want to avoid rcds because if they trip and no one is aware a lot of loss/money maybe at stake.
     
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  7. Leesparkykent
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    Leesparkykent You Rock Gmes Staff Member Trusted Advisor

    Location:
    Kent
    I just see containers when skimming through.... yes defiantly best not having it on an RCD.
     
  8. hightower
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    hightower Electrician's Arms

    Location:
    Durham
    Thanks Westward. So with that in mind do you think it would be sufficient to rod alongside the supply earth
     
  9. westward10
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    westward10 In echoed steps I walk across an empty dream. Electrician's Arms

    Location:
    Northamptonshire
    There is a remote possibility of someone sensing a potential difference between the containers and true earth but I would say this is highly unlikely unless the containers are somehow not in contact with the ground.
     
  10. hightower
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    hightower Electrician's Arms

    Location:
    Durham
    So you wouldn't even rod it?
     
  11. westward10
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    westward10 In echoed steps I walk across an empty dream. Electrician's Arms

    Location:
    Northamptonshire
    Are they sitting on concrete or touching the ground.
     
  12. Ian1981
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    Ian1981 Electrician's Arms

    Location:
    North east
    How are these fridges connected in?
    Are they A standard plug into a socket or are they hard wired in to an outlet/isolator?
     
  13. hightower
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    hightower Electrician's Arms

    Location:
    Durham
    They sit directly on the ground which will be concrete. They are wired through a rotary isolator and 32A commando
     
  14. Ian1981
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    Ian1981 Electrician's Arms

    Location:
    North east
    It's interesting that then the 18th edition comes into play then all socket outlets upto 32 amp require 30mA rcd protection (unless that changes after consideration during the draft).
    In the mean time your okay not to have rcd protected unless required by other means as you will know.
     
  15. hightower
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    hightower Electrician's Arms

    Location:
    Durham
    Yes Ian, I understand the requirements for RCD . My question is based around do I rod it or not, and if I do should I separate it from the TNCS earth
     
  16. Ian1981
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    Ian1981 Electrician's Arms

    Location:
    North east
    If you rod it then yes I'd separate it from the supply earth to eliminate any possible potential differences that may appear.
    its a steel container is it?
    Sounds just like a separate building(container) so tncs can be used and steel frame can be bonded to MET
     
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  17. westward10
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    westward10 In echoed steps I walk across an empty dream. Electrician's Arms

    Location:
    Northamptonshire
    Does it require a main protective bond. If you rod it and separate the earth you are in back to the rcd scenario which you need to avoid.
     
  18. hightower
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    hightower Electrician's Arms

    Location:
    Durham
    No bonding required
     
  19. westward10
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    westward10 In echoed steps I walk across an empty dream. Electrician's Arms

    Location:
    Northamptonshire
    If there are no exposed metal parts I don't see the need for a rod.
     
  20. Ian1981
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    Ian1981 Electrician's Arms

    Location:
    North east
    happy days!
     
  21. hightower
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    hightower Electrician's Arms

    Location:
    Durham
    Thanks for your help everyone. On phone at minute so will respond properly when i get on a computer.
     
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  22. Richard Burns
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    Richard Burns Trusted Advisor

    Location:
    Cambridgeshire
    Business Name:
    Richard Burns
    If you were going to install a rod it would have to be a very good rod connected to the MET of the installation and the cost would be prohibitive unless you have very good soil as the resistance would likely need to be in the region of one ohm. The container should be bonded to the MET as well.
    Placing a rod at the containers will reduce potential differences at the container but the rod may take the installation current on an incoming broken neutral.
     
  23. hightower
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    hightower Electrician's Arms

    Location:
    Durham
    Okay, on a computer so I'll try and put all the relevant details in a single post.

    Two containers that are insulated and have refrigeration units, to be installed about 20m away from a factory unit in the factory car park. Supply is TN-C-S so my initial thoughts were stop the supply earth at the isolators and have an RCD to protect with a rodded earth. But then my thoughts were we need to avoid putting it on an RCD so there is less chance of the units tripping out the feed and the stock within going bad.

    However, the units being big metal things got me thinking what happens if the DNO loses a neutral. That's why I thought about rodding it alongside the supply earth, but I'm not really sure what that'll achieve. I think the electrical supply is totally separate from the unit, as in Class II, however, is the unit still classed as an extraneous conductive part and therefore needs bonding?
     
  24. Wilko
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    Wilko Electrician's Arms

    Location:
    Berkshire
    Business Name:
    Wilko Electrics
    Happy to be shown the error of my ways, but if it's to be an extended TNCS then I'd be inclined to bond the metal container back to main building met.
     
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  25. hightower
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    hightower Electrician's Arms

    Location:
    Durham
    ^^^ That's exactly what I'm thinking as well @Wilko@Wilko
     
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  26. hightower
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    hightower Electrician's Arms

    Location:
    Durham
    But TN-C-S means it's gonna be a 10mm. Don't suppose anyone has a table of SWA and their armour CSAs (in copper equiv.) to see if the armour can be used? Otherwise guess it's a case of getting the calculator out.
     
  27. hightower
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    hightower Electrician's Arms

    Location:
    Durham
    According to this table:

    http://www.askthetrades.co.uk/hosted_images/Armour CSA.pdf

    6mm 4-core has a copper equiv. CSA of 16.8mm so my thoughts are I can take a 10mm cable straight from the feed terminated in the isolator and bolt it on to the metal frame, and use the armour as my bonding return to the MET?
     
  28. Leesparkykent
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    Leesparkykent You Rock Gmes Staff Member Trusted Advisor

    Location:
    Kent
    no if another metal is used for bonding then it needs to offer at least equivilant conductance. The carbon content of steel varies so you have to use the guidance in GN8 which I believe is in the region of 8.5. So if 10mm copper is deemed adequate then the steel armour would need to have a cross sectional area of at least 85mm. It would be better to use 5core SWA or run in a seperate bonding conductor. Remember if the seperate bonding conductor is to be buried you would have to use 16mm.
     
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  29. hightower
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    hightower Electrician's Arms

    Location:
    Durham
    So what are the copper equivalent columns in that link referring to then? Reason we've used 4-core is because it's what the factory owner had lying around and he's on a cost saving exercise. Will have to have a look at pulling in a separate conductor.
     
  30. Leesparkykent
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    Leesparkykent You Rock Gmes Staff Member Trusted Advisor

    Location:
    Kent
    They are for earthing not bonding.
     
  31. hightower
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    hightower Electrician's Arms

    Location:
    Durham
    Why should they be different though?
     
  32. Ian1981
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    Ian1981 Electrician's Arms

    Location:
    North east
    tables will give you the minimum CSA of steel deemed to comply with table 54.7 for armour to be used as a circuit protective conductor which is not the same as a protective bonding conductor.
    Like lee says you would need the armour to be at least 85mm steel to offer the same conductivity of 10mm copper using the 8.5 ratio.
     
  33. hightower
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    hightower Electrician's Arms

    Location:
    Durham
    Thanks but why are they different. Why is a steel to copper equivalency that's good for a CPC not good enough for bonding?
     
  34. Ian1981
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    Ian1981 Electrician's Arms

    Location:
    North east
    Because for a cpc the Adiabatic equation can also be used.
    You often find table 54.7 gives an over sized cpc than what is needed.
    Bonding requirements have no wiggle room I'm afraid
     
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  35. Leesparkykent
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    Leesparkykent You Rock Gmes Staff Member Trusted Advisor

    Location:
    Kent
    544.1.1 and the * under table 54.8 both say about the bonding conductor being copper or a cross-sectional area affording equivilant conductance in other metals. GN8 has a section on it but I don't have it around me ATM. Probably to do with limiting touch voltages under fault conditions etc.
     
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  36. Wilko
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    Wilko Electrician's Arms

    Location:
    Berkshire
    Business Name:
    Wilko Electrics
    Man did I struggle with this one ... The SWA table gives k numbers suitable for use in the adiabatic equation. I haven't got it open, but I recall the factor is about 2.2. For bonding it's about voltage rise so it's the resistivity that's important and as LSK has said the factor is about 8.5. So armour is normally ok for CPC but rarely enough for bonding.
     
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  37. Flanders
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    Flanders Regular EF Member

    Location:
    Tamworth
    There was a site i was on the other day with metal porta cabin's, but the earth had been exported from the pme , i was looking at using the structural steel as the earth sorce to the cabins , i spoke to nic tech , and said they could not see a problem doing that way ,not sure if that somthing you can do with your job.
     
  38. hightower
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    hightower Electrician's Arms

    Location:
    Durham
    Cheers guys. I've just had a read of the relevant section of GN8 but it didn't offer any reasoning behind the 8.5 factor and the difference between a CPC and bonding conductor in terms of copper equivalency.

    @Wilko@Wilko thanks, that answer makes a lot of sense (I think). Appreciate you all taking the effort to explain it, not being one to accept 'cos that's he way it's always been'
     
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  39. westward10
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    westward10 In echoed steps I walk across an empty dream. Electrician's Arms

    Location:
    Northamptonshire
    For the armour to be adequate for a 10.0 bonding conductor the minimum four core cable size would be 50.0. Are 10.0 bonds the required size for the factory supply.
     
  40. davesparks
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    davesparks Trusted Advisor

    Location:
    guildford
    They are for situations where you need to know the copper equivalent CSA, this is not used for bonding.

    For bonding you need equivalent conductance, this is a different physical property. A factor of 8.5 is generally accepted as being the ratio of conductance of copper versus steel. So for a 10mm copper bond you need 85mm steel.
     
  41. davesparks
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    davesparks Trusted Advisor

    Location:
    guildford
    A CPC's size is based on the temperature rise in the time it takes the ocpd to operate when a particular fault current flow. So this is looking at the ability of the conductor to survive a high current for a short length of time.

    Bonding is sized considering its ability to safely handle a sustained flow of current.

    Also it's worth noting that the table you linked to above is incorrect and has been for many many years.
     
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  42. Leesparkykent
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    Leesparkykent You Rock Gmes Staff Member Trusted Advisor

    Location:
    Kent
    Theres an echo in here .......
     
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  43. Ian1981
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    Ian1981 Electrician's Arms

    Location:
    North east
    Gn8 also gives warning to using the armour as a combined cpc/protective conductor as with tncs diverted neutral currents can raise the temperature of the cable meaning a increased sized conductors maybe needed for your ccc.
    It's up to the designer whether this may happen or not.
     
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  44. Ian1981
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    Ian1981 Electrician's Arms

    Location:
    North east
    Or rather raise the temp of the armour I should say
     
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