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2391 practical - Earth Fault Loop impedance

Discussion in 'Electrics UK' started by gsparks101, 5 Aug 2010.

  1. Taylortwocities

    Taylortwocities

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    Then you don't remember correctly.

    You can measure Ze at the board (probably Zdb as all the earth bonds will be in place) and add R1 + R2 to it. That gets you Zs.

    These days of RCDs I prefer to do that. The "no trip" feature on many test meters gives bad results for Zs. I have seen as much as an ohm discrepancy. That can make a difference if your loop impedance is on the edge of the required values..
     
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  3. Spark123

    Spark123

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    I don't see why you can't use either method, as long as it complies with the methods in GN3
     
  4. londonboy

    londonboy

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    Erm actually I do remember correctly, you need to show the examiner BOTH WAYS OF FINDING Zs.


     
  5. Electrifying

    Electrifying

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    The reason is simple - they want to see that you can measure it....

    Safely

    Accurately

    At the correct places on the installation

    AND understand what readings you should be getting. :)

    It's a PRACTICAL assessment - not a maths test.

    My 9 year old daughter can do simple maths..... Zs = Ze + R1 + R2 -

    it's hardly rocket science, and I'm pretty sure the 2391 assessor will take

    it for granted that you can add up :D

    The calculation can be used to give you an idea of the measurement you should be getting.

    The only other reason you should be using the calculation method, in real life or the 2391, is if (after assessing risk), it's unsafe to measure it ;)
     
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  6. Electrifying

    Electrifying

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    I've just thought of another reason why you need to show that you know how to measure it -

    The 2391 practical is a PIR

    In the 'real world' there's a very good chance that the measurement you won't be doing (on a PIR), is your R1 + R2.

    So what will you add your Ze to then? :)
     
  7. Spark123

    Spark123

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    I don't see why only where it is unsafe. It is a valid method in GN3 and that doesn't say only where it is unsafe to conduct live testing and it doesn't have the same issues of tripping RCDs.
     
  8. Electrifying

    Electrifying

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    Like I said in the post above, in a PIR there's a good chance you won't have the R1 + R2 figure anyway, so you won't be able to use calculation.

    On an 'Initial Verification' you would complete all the tests in the order that they are given and there really shouldn't be a reason not to measure Zs.
    The way I see it is this -

    Calculation gives you the figure that it should be.

    Measurement gives you the figure that it is.

    I know which figure I'd rather have :)

    The problem is a lot of 'lazy' sparks are using the fact that the regs allow you to calculate the figure, as a way of cutting down on the work they have to do - you know, just sit in the van with a calculator.

    Personally, I think calculation should be used as a last resort, which is why I said what I said about risk :D
     
  9. Spark123

    Spark123

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    Not sure where you are going with the "calculation" statement.
    If you have a measured value of Ze, and a measured R1+R2 and add the two together it is still a measured value.
    If there was a major issue with using the above then surely GN3 wouldn't say it is permitted to do it this way?
     
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  11. Electrifying

    Electrifying

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    Two things,

    Firstly, like I said above, you will rarely be taking an R1 + R2 measurement during a PIR, so you aren't going to have anything to add to your Ze anyway, you'll have to measure Zs :)

    Secondly. if we want to be absolutely accurate, R1 + R2 is a measurement of line and cpc conductor resistance, prior to any connection of earth fly-leads etc. etc.

    Zs is a measurement of AC Impedance, taking into account reactance and parallel paths.

    So, really speaking, you can't expect to add an Impedance (Ze) to a resistance (R1 + R2) and get an answer as another impedance (Zs)
    It will only give you a rough guide.

    Like I said, calculation is an alternative method, but not my choice. :)
     
  12. Spark123

    Spark123

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    Impedance isn't really a big issue in small cables.
    The resistance is close enough to the impedance for a resistance measurement to be sufficiently accurate.
     
  13. STI

    STI

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    Electrifying, Why wouldnt you be doing this on a PIR. Surely its a requirement for any radial circuit and one of the tests on a ring for confirming earth continuity Or is this something else my training has taught me incorrectly!!

    Im talking domestic here
     
  14. Spark123

    Spark123

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    In a PIR it can be easier to do R2 wanderlead testing, have done it like this before however tend to do R1+R2 for a new circuit.
     
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  15. Electrifying

    Electrifying

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    Hi STI,

    A PIR is supposed to be non-intrusive - in other words they don't want (or expect) you to start dismantling circuits and disconnecting things in the CU.
    For one thing, you could create more problems than you started with.

    As Spark123 says, for continuity of Protective Bonding, exposed coductive parts etc you can use the R2 wander lead method.

    For continuity of CPCs, it's acceptable to use the Zs test at the extremity of radials and every socket outlet to prove this.

    The only time it's recommended that you do R1 + R2 during a PIR, is if there have been alterations to a ring final since last tested - or if there is a lack of previous EIC or PIR test results - then you would do the full RFC continuity testing.

    This is all detailed in the Periodic Report section of GN3, along with the testing that you should carry out.

    If you haven't got a copy, give me a shout and I'll type some of the info out for you :)
     
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  16. STI

    STI

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    Thanks Sparks123 and Electrifying. I guess our training was based around new instals rather than a PIR hence why it was used. Having said that i gather from talking to people and many posts on here a domestic is very unlikely to have the EIC or alteration certs :)

    On the GN3 I am trying to get hold of a copy, second hand as like a lot of people nowadays money is tight. Thanks for the offer of typing it out (the section not the whole book!!) and if its not too much effort that would be great, if its a lot of work dont worry i will be getting a copy it seems pretty essential.
     
  17. Electrifying

    Electrifying

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    Unfortunately, this is so true.

    It's a judgement call then on what further tests are required and how much sampling to apply.
    You also need to increase the level of inspection and tests if faults/problems are found.

    The recommended tests on pg73 (Table3.3) are as follows:

    Protective Conductors Continuity
    Between earth terminal of distribution boards and the following exposed conductive parts..

    socket outlet earth connections. (4)
    accesible exposed conductive parts of current using equipment. (4&5)

    Bonding conductors continuity

    All protective bonding conductors.
    All necessary supplementary bonding conductors.

    Ring Circuit Continuity

    Where there are proper records of previous test, this test may not be necessary.
    The test should be carried out where inspection/documentation indicates there have been changes made to the ring final circuit.

    Insulation Resistance

    Between Live conductors (with Line/s and Neutral connected together) and earth, at all final distribution boards.
    At main and sub-main distribution panels, with final circuit distribution boards isolated from mains. (6)

    Polarity

    All the following positions:

    Origin of installation.
    Distribution boards.
    Accessible socket outlets.
    Extremity of radial circuits. (7)

    Earth Electrode Resistance

    Test each rod or group of rods seperately, with test links removed and installation isolated from the supply.

    Earth fault loop impedance

    At the following positions

    Origin of installation.
    Distribution boards.
    Accessible socket outlets.
    Extremity of radial circuits. (8

    Functional tests

    RCD
    Test as per regulation 612.13.1, then operate test button.

    Manual operation of all other devices.

    The table is then followed by these notes:

    1. The person carrying out the testing is required to decide which of the above tests are appropriate by using their experience and knowledge of the installation being inspected and by consulting any available records.

    2. Where sampling is applied, the percentage used is at the discretion of the tester. However, a percentage of less than 10% is inadvisable.

    3. The tests need not be carried out in the order shown in the table.

    4. The earth fault loop impedance test may be used to confirm continuity of protective conductors at socket outlets and at accessible exposed conductive parts of current using equipment and accessories.

    5. Generally, accessibility may be considered to be within 3 m from the floor or from where a person can stand.

    6. Where the circuit includes surge protective devices (SPDs) or other electronic devices which require a connection to earth for functional purposes, these devices will require disconnecting.

    7. Where there are proper records of previous tests, this test may not be necessary.

    8. Some earth loop impedance testers may trip RCDs in the circuit.


    Hope that's helpfull, mate. :)
     
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