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Bonding in greenhouse

Alan B

On an eicr

I have a greenhouse connected to a sub DB in an outhouse, which is on a TT earth. Circuit is protected by upstream 30mA and 100mA RCD's.

I am debating in my mind if the frame of the greenhouse should be bonded to the single socket in the green house.

I would consider the frame as an extraneous part and therefore first reaction is to bond it.

But then thinking about it. bonding would reduce the risk of shock inside the greenhouse in the case of the socket became live but increase the risk of shock outside the greenhouse if the socket and greenhouse became live.

On balance I am tempted to go with not bonding, what are your thoughts?

Parents
  • 0

    Building or not?

    If red houses are made of red bricks, and blue houses are made of blue bricks, what are green houses made of? Try it next time that you see a 5 year old!

    So if a greenhouse is a type of house, and a house is a building, is a greenhouse not a building?

    Whilst "house" is usually associated with a dwelling, it might refer to a church (house of God), part of a zoo (e.g. reptile house), part of a farm (e.g. cow house), parliament, etc.

    So I think that a greenhouse is a building.

  • 0 in reply to Chris Pearson

    So we will bond it to the earth then Chris, blindly following an illogical mis-interpreted regulation.  That is in this case an earth rod used as a main Earthing terminal. Round and round in circles like a teddy bear......The possibly earthy greenhouse frame will be bonded to an TT earth terminal at an unsuitable metalclad socket, in a damp or humid greenhouse, that is connected to an earth rod.

    And how does that actually improve safety?

    Z.

  • 0 in reply to Zoomup

    And while I am still in angry mode....if equipotential bonding is designed to keep exposed-conductive-parts and extraneous-conductive-parts at SUBSTANTIALLY THE SAME POTENTIAL (page 35) how are those parts not already so. 

    Greenhouse frame insulated from earth=safe.

    Greenhouse frame earthy and TT earthed class one earthed appliances in greenhouse TT earthed then they are substantially at the same potential.

    No bonding required.

    Z.

  • 0 in reply to Zoomup
    Zoomup said:
    So we will bond it to the earth then Chris, blindly following an illogical mis-interpreted regulation.  That is in this case an earth rod used as a main Earthing terminal. Round and round in circles like a teddy bear......The possibly earthy greenhouse frame will be bonded to an TT earth terminal at an unsuitable metalclad socket, in a damp or humid greenhouse, that is connected to an earth rod.

    Only if the 'earth rod' (or other electrode) is in the range of influence of the frame (and vice-versa). If the TT earth electrode is remote (more than a few m separation below ground), the assumption they are at the same potential is not always true.

    Zoomup said:
    how are those parts not already so. 

    Not just faults in the installation, but ground potential rise from conditions outside the installation.

    Zoomup said:
    No bonding required.

    I would agree ... if and only if the earth electrode for the TT earthing arrangement is part of, or very close to the greenhouse extraneous-conductive-parts.

    Otherwise, a potential can develop from things extraneous to the installation (such as PME or HV ground potential rise).

    Even 3.5 m separation below ground can create a voltage potential that will give you "tingles", and more than that, shock risk increases.

    I agree there are cases where it's less likely a ground potential rise will occur.

  • 0 in reply to gkenyon

    Graham. It is extremely rare for me to do a TT install (almost never) , however I have decided to put in two earth rods and verify each on its own then connect together (around my neck of the woods I would usually expect around 40 to 65 ohms on a single 4` rod).  The two rods to be about 8` apart therefore outside each others sphere of influence.

    My reasons for two rods :-

    1/ Redundancy can`t hurt.

    2/ Lowers total Ra - not to half the value of one rod but quite a good difference though.

    Is there some merit to my reasoning or am I barking mad?

    I would welcome your comments on my reasoning (as an approx first thought then if say each rod were to be 100 ohms to the rest of the planet as measured at the antipodes then 80 ohms might be just local and the other 20 ohms to the rest of the earth. Therefore two 100 rods have 80 ohms/2 = 40 ohms total and the other 20 ohms takes it to around 60 ohms with two rods. Very rufly speaking of course                           

  • 0 in reply to gkenyon

    Zoomup The greenhouse in question is only something like 1.8m by 2.5m, this is a good general discussion as I probably look at 4 or 5 small domestic greenhouses with power per year.

    Most have plastic, normally weather proof sockets, don't think I have ever found one with the frame bonded.

    The ones I work on are normally in rural locations with supply coming from a DB in a garage or shed, from experience I don't trust RCD's to be reliable once they get past about 20 years old in these damp environments and it's unusual for the occupiers to test the RCD's. 

    In these installations at least having a source of ground potential rise anywhere near the earth rod or greenhouse is fairly unlikely.

    I think having operation RCD's and a good installation with a plastic socket is the key to safety. If the greenhouse is in contact with earth there are different risks for bonding or not but overall the risk is probably similar. I think if I was installing I would bond as it's following the basic interpretation of the regulations. For existing installs I would note on the certificate that it's not bonded, not sure i would code it as a C3.

    Out of interest I am also going to call NAPIT technical tomorrow to see what answer they give me.

  • 0 in reply to ebee

    Hi : 

    A general consideration (noted in BS 7430) is that any portion of an earth electrode which is not 600 mm deep is at risk of drying out or freezing.

    Assuming top of rods are below ground level in a suitable pit, this may turn 4' rods into 3' or 2' rods in dry summers, or freezing conditions in winter.

    Given that, your redundancy seems like a reasonably good idea. It may not be necessary with other types of electrode (e.g. earth mat or tape buried at least 600 mm deep) or other configurations.

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  • 0 in reply to ebee

    Hi : 

    A general consideration (noted in BS 7430) is that any portion of an earth electrode which is not 600 mm deep is at risk of drying out or freezing.

    Assuming top of rods are below ground level in a suitable pit, this may turn 4' rods into 3' or 2' rods in dry summers, or freezing conditions in winter.

    Given that, your redundancy seems like a reasonably good idea. It may not be necessary with other types of electrode (e.g. earth mat or tape buried at least 600 mm deep) or other configurations.

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