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  1. Sintra

    Sintra The Innkeeper Staff Member Admin

    Safe isolation

    The procedure for proving dead should be by use of a test lamp or two pole voltage detector as recommended in HSE Guidance Note GS38.
    Non-contact voltage indicators (voltage sticks) and multi-meters should not be used. The test instrument should be proved to be working on a known live source or proprietary proving unit before and after use. All phases of the supply and the neutral should be tested and proved dead.

    Test sequence and descriptions

    The following tests are carried out with the Consumers main switch isolated

    1. Extenal earth fault loop impedance
    Reason: To establish that a good earth exists at the installation in order for the remaining tests to go ahead.

    Method: Disconnect the main earthing conductor from the main earthing terminal. An earth fault loop impedance tester is connected at line and earth (main earthing conductor) at the supply side of the installation and a test performed. Reconnect the main earthing conductor. The result is Ze and recorded on the sheet. The prospective fault current is measured at the same time after the reconnection of the main earthing conductor.

    2. Continuity of protective and bonding conductors
    Reason: To check that all circuit protective conductors (green and yellow cables) are continuous and are present at every electrical accessory on the circuit. Also to check that the main earthing conductor and main bonding conductors are continuous and correctly connected.

    Method 1: The line conductor is connected to the circuit protective conductor of the same circuit at the consumer unit and a measurement taken at ALL accesories on that circuit between line and c.p.c. The highest measurement obtained is recorded on the test report.
    Test result is R1 + R2. The line conductor and neutral conductor are then connected and the above repeated to obtain R1 + Rn

    Method 2 (used for main earth and main bonding conductors): A wandering lead is connected to one end of the conductor to be tested and a measurement taken between the other end of this lead and the other end of the conductor.
    Test result is R2.
    During this test polarity can be checked as well. The continuity of the neutral conductor can also be checked.

    3. Continuity of ring final circuit conductors
    Reason: This test ensures that all ring final circuits (sockets usually) are indeed a continuous ring with no interconnects or breaks within it.

    Method: The line, neutral and earth conductors of the circuit are identified and a measurement from one end to the other end of each is taken. These results are r1, r2 and rn.
    The incoming line conductor is then connected to the outgoing earth conductor and the outgoing line conductor is connected to the incoming earth conductor. A measurement is then taken at ALL socket outlets on the ring. The highest of which is recorded on the report.
    This result is R1+R2 for that circuit. The above is then repeated using the neutral conductor instead of the earth conductor. This test provides R1+Rn which does not need to be recorded on the report but is essential to check the circuit correctly.

    4. Insulation Resisitance
    Reason: This test checks whether the insulation around a cable is still intact and has not broken down over time. It is a good indicator of the age of an installation.

    Method: An insulation resistance tester is connected across line and neutral tails at the origin of the supply. 500V are then pumped down the conductors to see if any voltage leaks across from one conductor to the other. The same is then done for the line and earth and the earth and neutral conductors.

    5. Polarity
    Reason: To check that all accesories are correctly connected to line, neutral and earth and that all switches and circuit breakers are connected in the line conductor only.

    Method: The method for this is the same as for continuity and is usually done at the same time by operating switches etc whilst conducting the test.

    6. Earth electrode resistance
    Reason: To make sure that any earth electrode used is of a sufficiently low impedance to allow the timely operation of the RCD protecting the installation.

    Method: An earth fault loop impedance tester is connected between line and earth at the origin of the supply and a test performed. The result of which is considered the resistance of the electrode (Ra).

    The following tests are carried out with the Consumers main switch energised

    7. Live polarity test
    Reason: To verify polarity of supply authorities system.

    Method: An approved voltage indicator shall be used or test lamp to GS38. Using the approved voltage indicator, one probe shall be placed on the incoming neutral, and the other on the incoming line conductor, on the main breaker. The indicator should show it is live. One probe shall now be placed on the CPC and the other on the incoming line conductor. The indicator should show it is live. A test shall be preformed between CPC & incoming neutral. The indicator should show that it is not live.

    8. Earth fault loop impedance
    Reason: This test is done at the furthest point on a circuit in order to make sure the impedance of the earth path is not too high even at the furthest point so that sufficient current will flow under fault conditions to take out the circuit breaker protecting the circuit.

    Method: An earth fault loop impedance tester is connected to line and earth at the furthest point on the circuit and the test performed.

    9. RCD test
    Reason: To make sure RCD's trip within the correct time

    Method: An RCD tester is connected and a test at 1/2 times, 1 times and 5 times the trip current is performed on each side of the cycle and a time of trip obtained. Usually milli-seconds with the highest being recorded. The manual test button is then pressed.

    10. Functional testing
    Reason: To make sure all switches, isolators, MCB's etc. work as they should.

    Method. Self explanatory.
    • Like Like x 9
  2. Steve D

    Steve D Guest

    Good post - Cheers

    My only comments are that you have missed out live polarity using an approved voltage indicator. Strictly, live polarity should be the first test you carry out with the supply switched on.

    Also, a reference to safe isolation should be on there.

    I know, I'm a picky little b*****d !! :)
    Last edited by a moderator: May 13, 2010
  3. acat

    acat Guest

    Excellent post mate we should have a lot more of these as a refresher for the old boys and a great help to those just starting out.

    Last edited by a moderator: May 13, 2010
  4. WayneL

    WayneL Guest


    Should also have mentioned that the earthing conductor is disconnected from the MET for the Ze measurement and subsequently re-connected to measure PFC.
  5. Sintra

    Sintra The Innkeeper Staff Member Admin

    Thanks Steve good points all modified now.

    Thanks acat

    Thanks for that Wayne now included.

    If anyone else has a comment please feel free to add.
  6. Phil

    Phil Guest

    some good stuff on there. it would have made my course a lot easier with a site like this to look trough and saved a small fortune on books :( mind i would have only spent it on tools :)
  7. SafetyFirst

    SafetyFirst Guest

    Sintra some useful info there for people like myself currently studying the 2391, however i believe the first part could be mis-informative to people who may have interpretated it as i have. Ze (external earth fault loop impedance taken at the origin) is not a dead test as described. Granted all outgoing sides of the DB main breaker should be open (in OFF position). However it is a live test and therefore will not be done after you perform a safe isolation.
  8. Sintra

    Sintra The Innkeeper Staff Member Admin

    Beg to differ here IMO the safe isolation must be done prior to the Ze test as you are removing the main earthing conductor to the installation. I stated that the Ze test had to be done with the supply switched off not that it was a dead test
    • Like Like x 1
    • Agree Agree x 1
    Last edited: May 20, 2010
  9. SafetyFirst

    SafetyFirst Guest

    The actual supply is not switched off, the installation past the main breaker is switched off granted but not the supply. Anyone just about to take the 2391 practical may be confused as books including GN3 and OSG use the CRIPLER method and this is the way i've been taught to test within my 2391 course;

    Dead tests:

    Continuity of circuit protective conductors
    Ring final circuit conductor continuity
    Insulation resistance testing

    Live Tests:

    Loop impedance test external (Ze)
    Earth fault loop impedances (Zs)
    RCD testing
    • Like Like x 3
  10. Sintra

    Sintra The Innkeeper Staff Member Admin

    Have changed some of the terminology to try and make it clearer.
    Thanks for your input.
  11. SafetyFirst

    SafetyFirst Guest

    No probs Sintra i knew what you meant but i think the way it is now is more clearer for the likes of myself who are new(ish) to the testing game and studying 2391. Wa hey my first receieved thanks! Well thank you Sintra! :) Cool bike!
    Last edited by a moderator: May 20, 2010
  12. WayneL

    WayneL Guest

    Hi Safetyfirst,

    You are right to a degree.

    The sequence you stated above applies to 'Initial Verification' only.

    On a PIR, as you will do on your 2391 practical, there is no set sequence of testing, you can do them in any order that you like:)
  13. SafetyFirst

    SafetyFirst Guest

    Yes but i personally believe thats the most logical way the cripler method anyhow, each to there own but i personally would carry testing out that way the majority of the time, PIR or no PIR.
  14. WayneL

    WayneL Guest

    That's all very well, but on the 2391 time is short - and in the real world, time is money.

    Consider this:

    Whilst performing safe isolation of the installation you have your voltage tester in your hand -

    so you might as well perform the same tests on the incomming side of the main switch and confirm

    supply polarity.

    Then, as you have the cover off the board anyway, disconnect the earthing conductor and measure


    Re-connect the earthing conductor and measure your PFC

    These three tests have took you 5 minutes and you haven't moved.

    Then onto your sequence of tests - continuity, ring final, IR, Polarity

    Now, really speaking, you should put the cover back on the board before re-energising. (HSE and all

    that) - but that's okay, because your finished in there and shouldn't have to take it off again:D

    On with the remaining live tests - Zs, RCD and the functionals.

    Job done.

    Your not doing anything wrong - it's just more efficient:)
  15. SafetyFirst

    SafetyFirst Guest

    In the 2391 the 1st thing you do is safe isolation anyhow so you can't do a live test first as an isolator feeds your board (at least mine did) but generally at an occupied building of course the way you said is the choice a lot of sparks would command to. Though i believe however you do them you still take the same amount of time doing the individual tests any which way as you don't go quicker with individual tests doing them in different orders.
    Anyways its each to there own, we all have our own way of doing things. My main worry was for anyone reading this article as it was before being edited that are about to sit there 2391 practical, not for those who test day in day out, they are confident and can do it however they feel at ease to.
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