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Topic Title: Path of earth loop impedance
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Created On: 19 July 2012 07:56 AM
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 19 July 2012 07:56 AM
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mickygall

Posts: 4
Joined: 30 June 2012

Every diagram i have seen shows the fault on the live side of the load what would happen if the fault to earth occured on the neutral side of the load
 19 July 2012 08:18 AM
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perspicacious

Posts: 7235
Joined: 18 April 2006

"Every diagram i have seen shows the fault on the live side of the load what would happen if the fault to earth occured on the neutral side of the load"

Draw out a simple "loop" circuit using a load of say 1 kW and conductors of 0.1 Ohms and put your "fault" of 0.01 Ohms into the circuit as conventionally drawn and mark on the current in each conductor.
Then redraw, but put the fault at your chosen position and re-mark the new currents and compare how much current now flows through the overcurrent protective device at the "start" of your circuit.

Regards

BOD
 19 July 2012 08:20 AM
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impvan

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The current would split two ways, depending on the resistances (impedances) of the neutral path and the earth path. The size of the fault current would be limited, because it has to flow through your load first.

Try to think of this in terms of water flow and pressure.
 19 July 2012 09:19 AM
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ebee

Posts: 5702
Joined: 02 December 2004

In a Non RCD protected circuit you would probably never know because there is not much difference.

In a RCD protected circuit the RCD should trip and , if the RCD is double pole, it would disconnect the fault.

-------------------------
Regards,
Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik
 19 July 2012 11:19 AM
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KFH

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In every diagram I see the earth fault loop is always via the MET to the earth conductor, stake or PEN. None show the flow through alternative earth paths due to bonding, extraneous conductive parts etc. Nothing to do with the OP but just something which bugs me.
 19 July 2012 01:16 PM
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AJJewsbury

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Every diagram i have seen shows the fault on the live side of the load what would happen if the fault to earth occured on the neutral side of the load

These will be diagrams showing how to automatically disconnect a fault to mitigate the risk of electric shock. As neutral is normally fairly close to earth potential (and difference just due to voltage drop along N - perhaps a dozen volts at most), there's negligible risk of shock from a N to earth fault, so it's not a problem that needs addressing from a shock point of view.
- Andy.
 03 August 2012 10:46 AM
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mickygall

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Joined: 30 June 2012

"Every diagram i have seen shows the fault on the live side of the load what would happen if the fault to earth occured on the neutral side of the load"

Thanks for the replies however i guess what I'm trying to say is because earth & neutral are almost the same potential would you be aware of a earth to neutral fault or simply, a live to earth fault will trip the MCB as will a live to neutral fault, what happens if the neutral is in contact with earth
 03 August 2012 10:53 AM
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ebee

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As I said earlier

"In a Non RCD protected circuit you would probably never know because there is not much difference. "

-------------------------
Regards,
Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik
 06 August 2012 03:06 PM
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AJJewsbury

Posts: 11462
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what happens if the neutral is in contact with earth

The N current will split between the N and PE conductors. Overcurrent devices (fuses, MCBs) won't notice that. RCDs usually would and trip.

There is a danger with a undetected/uncorrected N-PE fault - if the N then goes open circuit all the N current is then carried by the PE conductor. If PE is smaller than the live conductors then (e.g. as with most T&E cables) there's a risk of it overheating and a possible fire hazard. If the PE-N short is on a small circuit (say lighting), but the N break is significantly further upstream (e.g. a faulty main switch) then a very large N current (e.g. that for the entire installation) could be trying to find its way back via the c.p.c. of the circuit with the N-PE fault - e.g. a 1mm2 c.p.c. - giving a significant fire risk.

- Andy.
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