Earthing Cable Sizes

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benotype Commentator Join Date: Nov 2009 Posts: 89	Earthing Cable Sizes 01/19/2011 11:21 AM
	I am handling the earthing of power station and the size of he recommended copper conductor is 150mm2, but right now i am out on 150, i am only left with 120mm2.The earthing is a mat with 7m spacing between earth conductors.I am actually thinking of using the 120mm2 and then reducing the spacing, but my boss thinks i should use two 120mm2 combined together.personally, i think that wont work.Please, can anyone help me with brilliant technical ideas? The earthing design is "IEEE Standard 80-2000".Thank u sirs
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TonyS Guru Join Date: Aug 2010 Location: Resting under the Major Oak Posts: 4347 Good Answers: 181	#1 Re: Earthing cable sizes 01/19/2011 11:51 AM
	Sick to the specification You leave yourself open to litigation if you don't. __________________ The reports of my death are greatly exaggerated.
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PeterT Guru Join Date: May 2010 Location: Liverpool, NY Posts: 961 Good Answers: 131	#2 Re: Earthing cable sizes 01/19/2011 12:47 PM
	Reducing the spacing of the conductors in the grid won't do anything for the fault current capacity of the conductors themselves. The size of the conductors needs to be chosen for their thermal capacity to carry the possible fault current. How was the 150mm2 cable size chosen? IEEE Std 80-2000 doesn't give metric cable sizes. In fact, in section 9.4, it references as an example (item c) #4/0 AWG copper cable in a "typical" ground grid, and that is a very commonly used cable size in America for that application. 150mm2 is equivalent to 300kcmil, and even 120mm2 is larger than #4/0. If I were designing it, I would verify that 120mm2 is adequate thermally for handling your fault current. See section 11 of IEEE Std 80-2000, Tables 3 through 5 for the ultimate current carrying capabilities of various sizes of conductors, based on fault current, X/R ratio, and length of time that protection is expected to take to clear the fault. The tables even give metric equivalent of the AWG cable sizes. __________________ To get the right answers, first you need to ask the right questions.
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KRW43 Associate Join Date: Feb 2009 Posts: 34 Good Answers: 4	#3 Re: Earthing Cable Sizes 01/19/2011 10:52 PM
	Design it using 1 size and 1 size only. Choose that size according to the fault levels, thermal capacity, potential rise (step & touch etc) & the risk. If that means buying extra 150mm concuctor then do it - don't do half a job you will find it will bite you in years to come. __________________ Murphy was an optimist....
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kadex Active Contributor Join Date: Sep 2007 Posts: 13	#4 Re: Earthing Cable Sizes 01/20/2011 6:28 AM
	cable sizing is all about current carrying capacity and faulty current analysis, if 150mm2 will do the job, engineering wise 120mm2 will over do it. 120mm2 is ok, go ahead.
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SAK Commentator Join Date: Aug 2008 Location: Tanzania- East Africa Posts: 71	#5 Re: Earthing Cable Sizes 01/20/2011 10:00 AM
	spacing will not help,,maintain the distance of electrodes. you could go for 120 X 2 or 95 x 2 alternatively always safe to maintain the specs. you could buy the 150mm sq. SAK __________________ A wise person will understand more from a Foolish question than what a Fool will understand from a Wise Answer
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PWSlack Guru Join Date: Jan 2007 Location: In the bothy, 7 chains down the line from Dodman's Lane level crossing, in the nation formerly known as Great Britain. Kettle's on. Posts: 32175 Good Answers: 839	#6 Re: Earthing Cable Sizes 01/20/2011 4:23 PM
	Someone hasn't bought enough 150mm². Someone is trying to do the job on the cheap. Someone is going to get his/her backside bitten... __________________ "Did you get my e-mail?" - "The biggest problem in communication is the illusion that it has taken place" - George Bernard Shaw, 1856
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kadex (1); KRW43 (1); PeterT (1); PWSlack (1); SAK (1); TonyS (1)

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