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Topic Title: Ground Fault Current Path for Multiple Sources
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Created On: 18 March 2014 03:26 AM
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 18 March 2014 03:26 AM
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jackdaniel

Posts: 69
Joined: 29 November 2012

Hi Guys,

I'm clear about single source ground fault current path, multiples sources or separate drive system still confusing me, please check the ground fault current path for multiple sources as per link, hopefully somebody can explain a bit for ground fault current path for multiple grounding involving multiple generators or transformers.

http://s23.postimg.org/i51xz4r...Fault_Current_Path.jpg
 18 March 2014 12:34 PM
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Zuiko

Posts: 521
Joined: 14 September 2010

There are two ways I can see that you can solve this.

But first, you must consider the earth return as a conductor like any other. This conductor will have an impedance, so when you sketch out your new circuit diagram, include the earth as a component with an impedence. This will allow you to solve the circuit using simple techniques. If the earth return is via the mass of the earth and not a metallic conductor, you will need to make some assumptions about the value of this impedence. At great distances this impedence can be large enough that you need sensitive earth fault detection, which operates on low currents for long times. Sometimes your assumptions may be bigger than what you can know, making the whole exercise an assumption! It's fine practice to use rules of thumb in these cases.

The first way to solve the circuit is to do a mesh analysis using Kirchoffs voltage law.

The second way, which may be simpler if you are clear about single sources, but takes longer, is to use superposition theorem (which sounds grander than it is). From your circuit diagram, remove all but one of the voltage sources, call it VsourceA, and solve the circuit (i.e. find the earth fault current, and call that i(efA). The earth fault current will be the current you find flowing in the component you added called earth impedence.

Now do the same, but solve for the other voltage source, call it VsourceB. You will have an answer for the earth fault current. Call that i(efB).

Now add them together i(earthfaulttotal) = i(efA) + i(efB)



However you solve it, you will find that parallel sources reduce the impedance of a fault and greatly increase fault current. On grid sites where a network is fed from multiple sources, you might often see a third large power transformer (with one breaker open), the impedance of which is large which limits fault current.

Edited: 19 March 2014 at 12:58 PM by Zuiko
 19 March 2014 12:39 AM
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jackdaniel

Posts: 69
Joined: 29 November 2012

Great explanation!, , however, how about the ground fault current flow in the diagram, is it correct?
 19 March 2014 12:55 PM
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Zuiko

Posts: 521
Joined: 14 September 2010

If you ask, does fault current flow from source to fault to source, then yes.

If the return is via the mass of earth, the route it takes will depend on the resistance of the mass of earth: electricity likes the path of least resistance. If you have lots of metalwork in the ground, (pipes, steel frames) it will like that. You will get parallel paths.


I was once told by an old engineer that on overhead line circuits, earth fault current flows in the earth directly underneath the line, following the route of the line back to source. I have never investigated this claim, and always taken it to be true because this guy was very knowledgable. In Australia, I saw quite a lot of SWER circuits, which use the mass of earth as the return path in normal operating conditions. It seems logical that the path is underneath the line rather than radiating across the desert!

Edited: 19 March 2014 at 03:52 PM by Zuiko
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