ECN Electrical Forum - Discussion Forums for Electricians, Inspectors and Related Professionals

ECN Shout Chat
Top Posters(30 Days)
Admin 19
Recent Posts
fuse rejectors
by NORCAL. 03/29/17 01:44 AM
Old decora style outlets
by NORCAL. 03/29/17 01:41 AM
NFPA stuff
by gfretwell. 03/28/17 08:08 PM
Anyone hiring inspectors?
by HotLine1. 03/27/17 08:03 AM
ESA Arc flash course
by TheShockDoctors. 03/24/17 10:15 AM
New in the Gallery:
SE cable question
Popular Topics(Views)
231,751 Are you busy
166,656 Re: Forum
160,801 Need opinion
Who's Online Now
0 registered members (), 78 guests, and 9 spiders.
Key: Admin, Global Mod, Mod
Previous Thread
Next Thread
Print Thread
Rate This Thread
#34424 - 02/11/04 08:35 AM Why a continuous ground?  
Big A  Offline
Member
Joined: Jan 2003
Posts: 46
Lynchburg, VA
Does anybody know why a system has to have a continuous ground? I understand why you would need it in things that were close together but I don't understand why if I have a feed on one side of the street, and a metal cabinet on the other side of the street, these must have a continuous ground. Seperate grounds are not allowed.

Any reasons why?


Work Gear for Electricians and the Trades

#34425 - 02/11/04 08:50 AM Re: Why a continuous ground?  
Attic Rat  Offline
Member
Joined: Dec 2003
Posts: 524
Bergen Co.,N.J. USA
...So that it can never be disconnected,or the lugs,bugs,etc..become loose and create a high resistance to ground.
AR


.."if it ain't fixed,don't break it...call a Licensed Electrician"

#34426 - 02/11/04 09:22 AM Re: Why a continuous ground?  
earlydean  Offline
Member
Joined: Dec 2003
Posts: 751
Griswold, CT, USA
To facilitate the operation of the OC device you need a low impedance fault path back to the OC device.


Earl

#34427 - 02/11/04 03:08 PM Re: Why a continuous ground?  
Big A  Offline
Member
Joined: Jan 2003
Posts: 46
Lynchburg, VA
So the OCP will not function with a seperate ground? I guess that makes sense, but wouldn't the difference of potential between the two grounds have to be some huge (say 1 meg ohm) difference?


#34428 - 02/11/04 03:25 PM Re: Why a continuous ground?  
winnie  Offline
Member
Joined: Sep 2003
Posts: 649
boston, ma
Lets say, for example, that the grounding electrode on that remote box had a resistance of 25 ohms. Let us further stipulate that the grounding electrode system on the main panel has a resistance of 15 ohms. You get a dead 'hot to ground' short on the remote box, but you don't have any equipment grounding conductor in the feed to the box. Finally, presume that this is a single phase 120V to ground circuit.

Net result: 3A of current flowing through the ground. The remote box is sitting at 75V to ground, and the circuit breaker won't trip. All you've done is created a shock hazard and wasted electricity heating the earth.

Note: 'Resistance to ground' of a grounding electrode is something of a made up number. You presume that the earth as a whole is a perfect conductor, and then attribute any resistance to current flow to the grounding electrode. But in reality the earth is a real conductor with real resistance, and the distance between your electrodes will matter. Imagine that your two electrodes were driven into the ground parallel to each other and 1cm apart. Clearly the resistance between the electrodes will be lower than the overall resistance to earth. But the approximation is good for electrodes spaced significantly greater than their size.

-Jon



Member Spotlight
richard
richard
L.I. New York
Posts: 94
Joined: August 2003
Show All Member Profiles 
Featured:

2017 NEC and Related
2017 NEC
Now Available!

Shout Box
Powered by UBB.threads™ PHP Forum Software 7.6.0
Page Time: 0.014s Queries: 14 (0.003s) Memory: 0.7659 MB (Peak: 0.9050 MB) Zlib enabled. Server Time: 2017-03-29 19:05:57 UTC