Forum » General Discussion » what is the permissible Resistance of earthing rod as per BS7671
Topics: what is the permissible Resistance of earthing rod as per BS7671 on General Discussion
#1
Start by
ajaz ahed
09-26-2013 12:28 AM
what is the permissible Resistance of earthing rod as per BS7671
any one can tell me the clause of BS standard where they mention the maximum permissible value of the resistance for earthing rod.I try to find it in BS7671 but not cleared that who much it should be.
09-26-2013 03:26 AM
Top #2
Tony Cutner BSc CEng MIET
09-26-2013 03:26 AM
The BS does not care what the resistance is of a single earth rod, that's why you cannot find it.
The standards is performance related, so your loop impedance will typically need to be less than 0.35 ohms.
Your earthing arrangement- it might be in dry sand, rock, or moist clay, might need 3 or 30 copper rods to get the resistance low enough to operate safely the trip circuits.
I guess, that you are use to US standrds, in which they tell you to do things in a certain way- irrespective of circumstances.
You will find BS, the emphasis is on the performance, they do not tell you how to engineer it, you have to design, install, measure, confirm parameters are correct.
You will find forum on BS7671
http://www.theiet.org/forums/forum/messageview.cfm?catid=243&threadid=50467
Or best to buy the guidance notes, as they use plain English to describe the British Standards.
09-26-2013 05:34 AM
Top #3
Norman Chambers
09-26-2013 05:34 AM
Yes Tony. the The IEE onsite guild clearly states that THE LOOP IMPEDANCE READINGS MUST NOT BE GREATER THAN 200 OHMS for TT systems. This includes the rod. For the installation itself, TN-C-S 0.35; TN-S - 0.8
IEE ONSITE GUIDE Page 22 section 3.6.3.
09-26-2013 08:17 AM
Top #4
Norman Chambers
09-26-2013 08:17 AM
The NEC is a little vague. An additional rod is not required if reading are below 25 ohms. It does not give a max limit as the IEE do.
09-26-2013 11:02 AM
Top #5
Douglas Drummond
09-26-2013 11:02 AM
And in the USA, the practice is to use two rods, which removes the requirement to measure the ground resistance.
09-26-2013 01:59 PM
Top #6
Norman Chambers
09-26-2013 01:59 PM
Douglas that is true due to the soil type which has low resistivity in most developed areas.
09-26-2013 04:21 PM
Top #7
Tony Cutner BSc CEng MIET
09-26-2013 04:21 PM
Hi Douglas, and it's things like that, that creep me out. Just putting two rods in rock, is not as useful as soft clay.
So how does the NEC demonstrate it is a safe installation?
The BS/ IEC codes require a site measurement and compare to design figures before signing off it is a safe installation.
09-26-2013 06:35 PM
Top #8
Norman Chambers
09-26-2013 06:35 PM
Hi Douglas, What is the statistics on electrical fires in the US. Do you have current data?
09-26-2013 08:54 PM
Top #9
Norman Chambers
09-26-2013 08:54 PM
Adding to Tony's comments, I had an interview with a highly prominent person in the US electrical industry a few yrs ago. It was shocking to me when i was told that the reading is not necessarily important because of the protective bonding between the neutral and the earth will suffice.
So how do you justify safe connections to earth in any environments? From experience, if the rod or rods are too loose in the soil then it is unlikely for any reading to be achieved. possibly 600 to 1000 ohms.
