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Topic Title: Agricultural Location
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Created On: 23 September 2013 12:09 PM
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 23 September 2013 12:09 PM
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Finnley

Posts: 3
Joined: 23 September 2013

Hi

The installation is a Farm outbuilding(cattle shed), TT, lighting, power, flooring to be taken as 0 v potential.

Im after some advice with regard to reg 705.415.2.1, if we have a large metallic structure (cattle shed) and the floor is Terafirma how can this reg be applied. I accept that that the steel frame work would probably tie the voltage down under fault conditions, but surely a risk of a votage gradient appearing around the base of the frame work exists, would it not be better to isolate instead of introducing voltages where there is no equipotential zone?

Thanks
 23 September 2013 12:58 PM
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AJJewsbury

Posts: 11539
Joined: 13 August 2003

would it not be better to isolate instead of introducing voltages where there is no equipotential zone?

Isolate the metal frame from the electrical earth?

I know where you're coming from - I've had a similar debate about not bonding the metal frame of a domestic greenhouse - i.e. treating it all as "outside", on the basis that the soil and hence any metal staging etc will be at true earth, so defeating any attempt at an equipotential zone, while most greenhouse electrical equipment will be class 2 anyway - allowing all the metalwork to remain at true earth potential under almost all circumstances.

I guess part of the counter argument is what happens if there's contact between any exposed-conductive-parts and the frame -
If you do have class 1 (earthed) equipment, there's always the risk of that equipment coming into contact with with the frame (either deliberately, e.g. metal fittings being fixed to the building fabric or just class 1 "portable appliances" touching it accidentally). At that point not only is the separation idea defeated, but there's the risk of small c.p.c.s being called into service as main bonding conductors and perhaps being asked to carry much larger currents - with could be a fire risk (especially in an agricultural situation). TTing with a local rod probably mitigates that risk somewhat (compared with say being part of a large TN-S system, which the regs also have to cover), especially if there are no other extraneous-conductive-parts (e.g. water supply); but on the other hand, there's a risk from step voltages around the rod itself (especially with large 4-legged animals) and bonding the frame probably reduced that.

I think it's one of those, there are problems if you do and problems if you don't situations.

- Andy.
 23 September 2013 02:52 PM
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Parsley

Posts: 1032
Joined: 04 November 2004

Originally posted by: Finnley

Hi



The installation is a Farm outbuilding(cattle shed), TT, lighting, power, flooring to be taken as 0 v potential.



Im after some advice with regard to reg 705.415.2.1, if we have a large metallic structure (cattle shed) and the floor is Terafirma how can this reg be applied. I accept that that the steel frame work would probably tie the voltage down under fault conditions, but surely a risk of a votage gradient appearing around the base of the frame work exists, would it not be better to isolate instead of introducing voltages where there is no equipotential zone?



Thanks


It's why an earthed metallic grid in the floor is recommended with extensive supplementary bonding to assist with keeping voltage differences low.
If the floor isn't bonded PME is not allowed and TT must be used.

I'm not sure how you could isolate the system from earth if there's anything fixed to the earthy floor.

Regards
 23 September 2013 03:15 PM
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Finnley

Posts: 3
Joined: 23 September 2013

Hi

Unfortunately these are existing units, the supply is pme, no ill effects as yet though it will soon be TT, my understanding is PME is not advisiable where no grid exists.

There are three outbuildings similar construction, upright steel stanchions joined via timber, so no connection between stanchions. I think I will connect all the steel stanchions and then connect the three outbuildings to create a large elctrode system, hopefully the Ra will be low enough to limit the touch voltage.

Regards
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