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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?

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  • 0

    Having just read this thread, I have decided that everyone is mad! Why are you all discussing whether TT installations (of a fairly usual kind) are safe or not? There are hundreds of thousands of them in the country, mainly in rural and garden locations, many with additional risks on farms, and they are not considered dangerous in any way. But because this is a greenhouse, it suddenly is, and the bonding is the most important issue? Very strange. Realistically farms are never fully compliant for example, simply because there is often a lot of metalwork, (what exactly is structural, certainly the Bull pen) and bonding every bit in case an extension lead is used near it (oh, you hadn't thought of that one, very common) there must be an awful lot of dead farm workers (there are not, those there are are usually arguments with heavy and very powerful machinery, or livestock). This discussion is pointless and there is no definitive answer, and in a TT system there is assumed to be sufficient protection from a 30mA RCD to prevent deaths however contact is achieved. So whether the frame is bonded or not doesn't matter (except possibly to pedants and rule followers) by the definition of TT itself! The question as to whether a single RCD provides adequate protection due to some failure rate is irrelevant, that has been decided by BS7671! If you ever use an extension cable near a piece of Earthed metalwork there must be a risk of an Earth potential difference, true this is not BS7671 territory but surely has a significant bearing on the discussion? Perhaps cables more than 1m long on appliances should be banned? Possibly everything should be class 2? As I said, everyone appears mad.

  • 0 in reply to David Stone

    I have just had another idea. As we now have 30mA RCD protection on just about everything in houses, is there any need for Earth conductors at all? Discuss....

  • 0 in reply to David Stone

    Well David I can certainly see your point on this bit.

    If an RCD works correctly then for it to fire up the earth connection itself is not actually required.

    Just to add that yes I myself was guilty of the idea (I think it was OMS that raised it) ,that say the incoming earth was not at or near earth potential then the RCD would not clear it. The actual example I think was an existing lighting circuit which could at the time, theoretically, have a 5 second disconnection time for a general lighting circuit and the adding of a RCD at the end of that circuit to protect a bathroom light would not mitigate the raised earthwire potential during the time the original circuit OPD took to disconnect the fault. I must admit that I had not considered, till then, that the cpc potential could be say 120V above actual earth for the duration of that earthfault clearing irrespective of that RCD being connected to the bathroom subcircuit. I think it was AJJ that pointed out that the same rule applies to other distribution circuits too.

    So a two wire (or 4 wire) RCD correctly functioning would actually become more hazardous if an earth reference is added that is capable of reaching a voltage beyond general earth reference for up to 5 seconds in a correctly functioning circuit.

  • 0 in reply to David Stone

    Well if you go and stay in  South America for a bit you will see class 0 with or without RCD as the normal installation method, along with 'protection omitted' for no obvious reason sometimes.

    In reality a world of 2 pin 16A plugs and no CPCs is not that much more dangerous and it does eliminate a whole class of 'exported fault voltage' type problems

    You will also see showers with twisted joints in the cubicle that are out of reach to short people, and that is good enough as tall people are sensible enough not to touch them.

    After the initial surprise, if not shock, it sinks in that this is not the leading cause of death there, though most people know someone who got a shock once, in general folk are far more careful which compensates.

    Collectively we worry too much;-)

    Mike

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  • 0 in reply to David Stone

    Well if you go and stay in  South America for a bit you will see class 0 with or without RCD as the normal installation method, along with 'protection omitted' for no obvious reason sometimes.

    In reality a world of 2 pin 16A plugs and no CPCs is not that much more dangerous and it does eliminate a whole class of 'exported fault voltage' type problems

    You will also see showers with twisted joints in the cubicle that are out of reach to short people, and that is good enough as tall people are sensible enough not to touch them.

    After the initial surprise, if not shock, it sinks in that this is not the leading cause of death there, though most people know someone who got a shock once, in general folk are far more careful which compensates.

    Collectively we worry too much;-)

    Mike

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