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Spud28

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Morning, so just finishing off the install to a shepard hut which is located in a field. The construction is steel chassis, plyboard floor, timber stud clad with corrugated sheets. The corrugated iron sheets which clad the unit are plastic coated and there is no continuity between each sheet when tested, and 4meg to the met. TT at supply and therefore TT at the hut. The chassis is earthed but the 'box' that will be used for accomodation is insulated from the main chassis as it is sat on wood. Im thinking that i should bond diasy chain style each corrugated sheet and connect back to MET. Im worried that someone touching the metalwork could provide the earth path in a fault. Its going to be a ballache to do and potentially look a little ugly. What test would you do to satisfy exactly what is needed / or not. Thanks for taking time to read.
 
The test results you give do not match with a physical possibility.
If each sheet is 4MΩ to the MET then testing between each sheet must give a maximum of 8MΩ even if the maximum resistance points are widely separated because they are both "connected" to the MET.
From practical perspective the corrugated sheets should not show a contact with the MET as the only way this could happen is via the timber structure which unless it is very wet could not give that sort of reading.
However electrically the resistance is above the 22,000Ω required to characterise the sheets as not an extraneous conductive part, therefore bonding should not be required. However care should be taken on installing the accessories that an earth connection is not made to the corrugated sheets, if only to prevent a large surface area earth.
 
Good shout. I'm wondering of I meggered the chassis and continuity tested the sheets. Thinking further I remember meggerring between the coach screws used to fix the sheets which I'm sure may have insulation to the sheet in the form of a foam rubber. I need to recheck ins res between sheets. But you think if less than 23k dont worry. I'm wondering more as to just the earth path from the sheets should a fault occur. I need a Pen and paper lol
 
I think if the resistance is greater than 23K you do not need to bond, not less than.
If a fault occurs the sheets are not connected to any part of the electrical installation so will provide no path.
If the fault is a live part contacting the sheet and someone touching the sheet, this is an unlikely occurrence but would be protected against by the presence of the RCD protection, the plastic coating would also mitigate the effect.
 
Just a quick update, spoke to technical and they advised to bond.
no real reasoning but 'if in doubt - bond!'.
 
Just a quick update, spoke to technical and they advised to bond.
no real reasoning but 'if in doubt - bond!'.

Would they say to bond metal window frames for the same reason?
Does Cockburn work for their technical help line?
 
pretty much mirror my thoughts on this... ive always wondered the merits of making a metal object live under fault conditions.
However, in this instance the hut represents a floading metal box in the middle of a field. under a fault im thinking it can sit there quite happily with no reference to earth until some person or animal touches it and gets a zap until rcd operates. If its bonded then the rcd, in principle, will have more time to act as the mechanism will start as soon as the fault occurs.
WRT the window frames, i guess the chances of a live cable coming into contact with a frame is unlikely due to cable runs etc. with the hut, in contrast - there are lots of cables run through the studwork so maybe a mouse or something could gnaw away etc.
This is a TT system so the pfc etc is very low, i'm relying pretty much on rcds to give adequate disconnection.
Thanks for looking and replying
 

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