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Topic Title: Measuring Ohms
Topic Summary: Metal work
Created On: 16 November 2012 11:15 AM
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 16 November 2012 11:15 AM
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Ampman

Posts: 963
Joined: 06 February 2006

morning all

Can someone please verify ,these results for me ,

I have been measuring resistances between the MET of a building and metal work within the building that has been fixed into the ground through rawl bolts ,

i am getting readings between 2.4 k ohms and 3.7 k ohms on resistance on my meter ,

now i know the reading im looking for is 22 k ohms but not sure weather my readings are below this or above ,

and if its below or above 22 k ohms should the metal work require bonding ,

thanks
 16 November 2012 11:48 AM
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AJJewsbury

Posts: 10977
Joined: 13 August 2003

22kΩ (+1kΩ presumed body resistance) comes from the resistance needed to limit shock current from a 230V to 10mA. If you want to be on the safe side of that, you'd be looking for a lower current - therefore a higher resistance. So you want 22kΩ OR MORE to void the need to bond.

i am getting readings between 2.4 k ohms and 3.7 k ohms on resistance on my meter

So they are extraneous-conductive-parts, so will need main bonding (possibly supplementary bonding too if they extend into and area that requires supp bonding).

- Andy.
 16 November 2012 02:52 PM
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leckie

Posts: 1605
Joined: 21 November 2008

If you have been carrying out a contract and some metalwork is installed, how would you assess the need to bond it or not prior to it being fitted? Things like rails, etc are often fitted later into the contract and it may not be possible to run bonding conductors at that stage of the contract due to damage to decor, etc.
 16 November 2012 03:22 PM
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AJJewsbury

Posts: 10977
Joined: 13 August 2003

how would you assess the need to bond it or not prior to it being fitted?

Tricky. I guess as a rule of thumb, if it's a modern building (with a plastic DPM under the floor and walls) then any metalwork (with its fixings) that's entirely above DPM level and entirely within the building, is unlikely to introduce a potential. I guess that'll cover most small hand-rails and the like.

Bigger stuff that might be fixed directly into the foundations (things like steel spiral staircases) would need a closer look at. If in doubt perhaps run a G/Y conductor 'just in case', only connecting it if final readings show it's necessary.

Given that this idea of testing for 22kΩ+ is comparatively new, what we're really missing is some comprehensive data on what resistances odd bits of metalwork really do have to earth in practice (ensuring false readings from nearby earthed exposed-conductive-parts are excluded).

- Andy.
 16 November 2012 05:58 PM
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OMS

Posts: 18943
Joined: 23 March 2004

Perhaps the other thing you need to consider is if the potential (earthy) that it introduces is already there anyway - if so, then bonding it may well be unneccessary

Regards

OMS

-------------------------
Failure is always an option
 18 November 2012 01:07 PM
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UKPN

Posts: 452
Joined: 17 January 2012

-----------the criteria is, has and always will be, (for PME),
if metalwork is in contact with the general mass of earth, and
even if there is a plastic incoming pipe before the copper tap
with water, and you can simultaneously touch metalwork connected to the neutral conductor, equipotential bonding is required.
well meaning guide books give theoretical advice, the rules are clear.
in fact, even if there is no electrical supply in the building/outbuilding but
a steel service ie water and a steel stanchion within reach they must be bonded if the steel service because in the event of a lost neutral the pipe
in the building will become live.

Regards
Statistics

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