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Fault loop impedance

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  • 21-07-2015 9:19pm
    #1
    grousedogtom
    Registered Users Posts: 463 ✭✭


    Hi
    I started a rewire on a house in Dublin yesterday morning before we started i noticed there was no Neutralising link from the esb.
    So i contacted the customer and informed him he would need to contact esb networks and inform them to install the link to there cut out.
    All Ok there.

    Next i checked the voltage between live and neutral getting almost 240 v , all ok ,then i checked the voltage between neutral and earth and i got 96 volts so i taught there was no earth rod either.

    I fitted a new earth rod and 10sq earth back to the board and this brought the n to e voltage down to zero.

    Next i carried out the fault loop impedance again expecting a reading of about 100 omhs but i got a reading of >999 omhs.

    Is this reading very high with a new earth rod is in place ?.

    Power is off at house at the moment until rewire is complete.


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    #2
    brightspark
    Registered Users Posts: 2,637 ✭✭✭brightspark


    Is the neutralising link present now?


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    #3
    grousedogtom
    Registered Users Posts: 463 ✭✭grousedogtom


    No
    Esb are due tomorrow to connect link


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    #4
    2011
    Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,550 Mod ✭✭✭✭2011


    This is to be expected.
    If the power is off the "earth loop" is open. Therefore a high earth fault loop impedance makes sense.
    Draw it out and it will make sense.


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    #5
    grousedogtom
    Registered Users Posts: 463 ✭✭grousedogtom


    2011 wrote:
    This is to be expected. If the power is off the "earth loop" is open. Therefore a high earth fault loop impedance makes sense. Draw it out and it will make sense.


    I've only switched off power until esb connect link, all the previous tests were carried out with power on


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    #6
    2011
    Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,550 Mod ✭✭✭✭2011


    grousedogtom wrote: »
    I've only switched off power until esb connect link, all the previous tests were carried out with power on

    If you have the power switched off for the earth fault loop the impedance test the reading will be high as the loop you are testing is open circuit.
    Was the power on or not during this test?


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    #7
    grousedogtom
    Registered Users Posts: 463 ✭✭grousedogtom


    2011 wrote:
    If you have the power switched off for the earth fault loop the impedance test the reading will be high as the loop you are testing is open circuit. Was the power on or not during this test?


    Power was on during all fault loop impedance tests I carried out


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    #8
    brightspark
    Registered Users Posts: 2,637 ✭✭✭brightspark


    I've had a couple of glasses of wine, but where is the earth loop circuit with no neutralising link fitted?


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    #9
    Tuco88
    Registered Users Posts: 1,052 ✭✭✭Tuco88


    I'd expect all your other final circuit impedance values to be high also with no neutralising link in place.


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    #10
    Risteard81
    Registered Users Posts: 2,567 ✭✭✭Risteard81


    brightspark wrote: »
    I've had a couple of glasses of wine, but where is the earth loop circuit with no neutralising link fitted?
    It will be through the earth electrode, through the general mass of earth, and back to the star point of the transformer.


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    #11
    2011
    Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,550 Mod ✭✭✭✭2011


    Risteard81 wrote: »
    It will be through the earth electrode, through the general mass of earth, and back to the star point of the transformer.

    Exactly.

    With the dry weather we have been having results for a TT installation would be higher, but the OP's value is far too high. In fact it is beyond the range of the test instrument.


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    #12
    grousedogtom
    Registered Users Posts: 463 ✭✭grousedogtom


    2011 wrote:
    With the dry weather we have been having results for a TT installation would be higher, but the OP's value is far too high. In fact it is beyond the range of the test instrument.


    Yes agreed
    To check my meter I carried out the same test at my own house this morning by temporarily disconnecting the n link and got a reading of 60 ohms which seems about right, I'm confused as to why the reading was so high at the other house with a new earth rod driven


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    #13
    Risteard81
    Registered Users Posts: 2,567 ✭✭✭Risteard81


    2011 wrote: »
    the OP's value is far too high. In fact it is beyond the range of the test instrument.

    Absolutely. It shouldn't be anything like as high as that.


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    #14
    Bruthal
    Closed Accounts Posts: 13,422 ✭✭✭✭Bruthal


    Where was the tester connected to?


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    #15
    grousedogtom
    Registered Users Posts: 463 ✭✭grousedogtom


    Bruthal wrote: »
    Where was the tester connected to?

    At the consumer unit with power on, green lead earth bar, black lead main neutral bar, red lead main switch terminal, I also plugged the 13 amp plug lead into a live socket to ensure my leads were ok with the same high reading


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    #16
    brightspark
    Registered Users Posts: 2,637 ✭✭✭brightspark


    Aren't there only really two possibilities, either the supply earth electrode or the customer earth electrode is not making good contact with the soil etc.


    (I'm assuming that the conductor linking the new earth rod to the consumer unit is ok, was it checked?)


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    #17
    Bruthal
    Closed Accounts Posts: 13,422 ✭✭✭✭Bruthal


    grousedogtom wrote: »
    At the consumer unit with power on, green lead earth bar, black lead main neutral bar, red lead main switch terminal, I also plugged the 13 amp plug lead into a live socket to ensure my leads were ok with the same high reading

    Is it a housing estate, or a single house connected to a small pole transformer etc?


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    #18
    grousedogtom
    Registered Users Posts: 463 ✭✭grousedogtom


    Yes I've confirmed the conductor is good, it's only a few meters from the consumer unit, this house is a terraced house with houses all around


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    #19
    brightspark
    Registered Users Posts: 2,637 ✭✭✭brightspark


    Only for test purpose obviously, but have you tried soaking the electrode and its surrounding soil?

    Can you test any of the neighbours? If everyone is high it might indicate a problem on the supply side?


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    #20
    grousedogtom
    Registered Users Posts: 463 ✭✭grousedogtom


    brightspark wrote: »
    Only for test purpose obviously, but have you tried soaking the electrode and its surrounding soil?

    Can you test any of the neighbours? If everyone is high it might indicate a problem on the supply side?

    Ill be back at the house next week and will try soaking the surrounding area in water to check if reading on meter improves. Not sure about neighbours houses but will try as this would tell a lot.


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    #21
    2011
    Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,550 Mod ✭✭✭✭2011


    The problem could be at the other end.
    Can you neutralize or is it a TT system ?

    Edit: I see it is a terraced house. Most likely you can neutralize and the problem will disappear.


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    #22
    grousedogtom
    Registered Users Posts: 463 ✭✭grousedogtom


    OK
    I was back today second fixing and checked the fault loop impedance again with the neutralising link disconnected and got a reading of about 600 omhs , i poured a bucket of water down over the earth rod and this reading improved to about 450 omhs,still pretty high.
    There is a gas supply in the house fed from copper pipework outside after i bonded the hotpress with my 10sq earth i took another reading and got a value of 29 omhs . This must indicate an earth path through the incoming pipework.


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    #23
    salmocab
    Registered Users Posts: 11,309 ✭✭✭✭salmocab


    just having a read thru here and if the neutralizing link is in then surely the fault loop impedance back to the star of the esb transformer is via the Neutral of the supply system? this could be an ESB issue or am I way off?


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    #24
    grousedogtom
    Registered Users Posts: 463 ✭✭grousedogtom


    salmocab wrote: »
    just having a read thru here and if the neutralizing link is in then surely the fault loop impedance back to the star of the esb transformer is via the Neutral of the supply system? this could be an ESB issue or am I way off?

    I only disconnected the neutralising link temporarily to check earth rod on its own,neutralising link is back in place at the moment


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    #25
    salmocab
    Registered Users Posts: 11,309 ✭✭✭✭salmocab


    grousedogtom wrote: »
    I only disconnected the neutralising link temporarily to check earth rod on its own,neutralising link is back in place at the moment

    can you do the test from the meter point? if your still getting high readings with the neutralizing link in then Id say it might be an ESB issue. Or am I getting the wrong end of the stick, I had a long day!


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    #26
    grousedogtom
    Registered Users Posts: 463 ✭✭grousedogtom


    salmocab wrote: »
    can you do the test from the meter point? if your still getting high readings with the neutralizing link in then Id say it might be an ESB issue. Or am I getting the wrong end of the stick, I had a long day!

    No ,readings with the neutralising link in place are grand .4 of an omh , checking without the link was just checking the earth rod on its own .


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    #27
    brightspark
    Registered Users Posts: 2,637 ✭✭✭brightspark


    grousedogtom wrote: »
    OK
    I was back today second fixing and checked the fault loop impedance again with the neutralising link disconnected and got a reading of about 600 omhs , i poured a bucket of water down over the earth rod and this reading improved to about 450 omhs,still pretty high.

    Doesn't that prove your earth electrode isn't in an ideal place?


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    #28
    grousedogtom
    Registered Users Posts: 463 ✭✭grousedogtom


    brightspark wrote:
    Doesn't that prove your earth electrode isn't in an ideal place?


    Yes, I'm sure it does, not many options with a terraced house and concrete back and front,


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    #29
    brightspark
    Registered Users Posts: 2,637 ✭✭✭brightspark


    grousedogtom wrote: »
    There is a gas supply in the house fed from copper pipework outside after i bonded the hotpress with my 10sq earth i took another reading and got a value of 29 omhs . This must indicate an earth path through the incoming pipework.

    Of course you know that you cannot consider that an alternative to the earth electrode.


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    #30
    2011
    Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,550 Mod ✭✭✭✭2011


    brightspark wrote: »
    Doesn't that prove your earth electrode isn't in an ideal place?

    This only "proves" that there is high impedance in the loop.
    There are a number of points within the loop that could be causing this including the OP's earth electrode an the ESB's electrode.

    It is possible that the resistivity of the soil around the OP's earth rod is particularly high. This often happens when the soil is dry, stoney and sandy.
    If the earth rod is installed close to the foundations of the house it may be going into hardcore and only making minimal contact with the general mass of earth.


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    #31
    2011
    Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,550 Mod ✭✭✭✭2011


    grousedogtom wrote: »
    There is a gas supply in the house fed from copper pipework outside after i bonded the hotpress with my 10sq earth i took another reading and got a value of 29 omhs . This must indicate an earth path through the incoming pipework.


    A metal gas pipe in contact with the ground connected to the MET will provide an additional parallel return path.

    If the gas pipe provides a conductive path to the neighbouring houses then there will be multiple additional parallel paths, all of which will contribute to a lower earth fault loop impedance value. Under these circumstances some of the return paths may be through a neighbours neutralising link.

    My understanding is that there is a plastic section of pipe used on all gas supply lines to all modern installations. This may be different in some older homes.


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