Insulation resistanceTest

Discussion in 'Electricians' Talk' started by tonk, Sep 11, 2018.

  1. tonk

    tonk New Member

    Hello all.
    Very often I work on electrical minor works and always struggle with this test especially as this not like a new build meaning appliance switched off, etc - dead. During minor works: conventional lights and down lights connected - qty 20 to 30, appliance on etc
    How do you guys deal with this situation.
  2. Alan sherriff

    Alan sherriff Member

    Difficult one to answer text books connect line and neutral together and test to earth, you can do at consumer unit on each circuit . Problem is finding the neutral in a cramped consumer unit, alternataly test whole installation with link on secondary side of main switch line to neutral .
    And test to earth . This dead test will not descriminate you minor work so insulation low it may not be your circuit. A 230 volt test on the circuit will give you an indication of any issues , this can be done with links out . Or you could leave that test and make a note on Mwc on issues that may occour doing this test . Any low insulation via neutral or line to earth will trip rcd if fitted. As for line to neutral then 2 meg value will not trip or blow fuse unless a dead short be interested in any other comments
    FatHands likes this.
  3. Bob Rathbone

    Bob Rathbone Well-Known Member

    Given the large amount of electronic transformers and other electronic kit now found in installations, my personal view is that the 500V insulation test should be discontinued. However, it is still a requirement and must be done. As stated by others, just test between a linked phase and neutral and earth at the consumer unit main switch, do 2 tests, one before you start to identify any issues that may be present, you will then know that any faults showing on the final test, assuming your job is good, are in the existing installation. You will have made the customer aware of these faults found in the first test before you proceeded to connect the new installation.
  4. unphased

    unphased Screwfix Select

    Insulation testing on an existing installation is very time consuming to do correctly. On a lighting circuit, for example, if there are any electronic ballasts you have to disconnect them all else you will damage them, even doing L/N to E there is a risk of damage. Florescent lights are a nightmare to Insulation test. Similarly RCDs. They get damaged by them. Its no use saying "oh just do a 230V test", that's not in accordance with the Regs. You do 500V insulation tests else they are pointless. You need to ensure all switches are 'on', you need to remove any connections to neons and remove all lamps. IMHO it is a worthless test on existing circuits. The only true value in an insulation test is on a newly installed circuit where you know where everything is and you can prepare it for the test. That's my opinion and not expecting anyone to agree with it, they usually don't.

    230V ins test is useful when fault finding.
    Last edited: Sep 12, 2018
  5. mykjonlow

    mykjonlow New Member

    I was always taught to test L-N, L-E, N-E for single phase systems and also between phases for 3 phase. Why no testing between live conductors?

    I also agree for numerous reasons that testing at operating voltage proves very little.
  6. unphased

    unphased Screwfix Select

    The theory is, myk, if you have any items connected that are vulnerable to a 500V insulation test, then by shorting out the L and N and testing that between E you are avoiding any dangerous voltages across the L and N terminals and just testing both against the E. Its a fudge because you don't know if there are any faults between the Land N by doing that. That's why I don't think insulation testing an existing installation tells you much. There is far too much to disconnect to do it correctly. I do my insulation testing on new circuits and sometimes do them in sections as I go. Nothing wrong with that.
    FatHands likes this.
  7. mykjonlow

    mykjonlow New Member

    How do you get around not completing a full test if not testing between phases? is it stated it test limitations?
  8. seneca

    seneca Screwfix Select

    On an EIC in the boxes for ins. I just write L+N-E >500 megohms (or whatever it happens to be) when in this situation, usually on a c/unit change.
  9. Bob Rathbone

    Bob Rathbone Well-Known Member

    It is the presence of sensitive electronic devices embedded within the installation that has lead to the P,N test being omitted. I agree with Unphased on this but until the regs change we are stuck with this inappropriate method of testing insulation values. On an existing installation, an insulation test will only tell you how bad the insulation is, if the rats have eaten the PVC on a twin & earth cable and air is separating the conductors, it will not show on an insulation resistance test, but the installation is not safe and you may not be able to access the cables for a visual inspection. I taught my students in College to test between all live conductors and live conductors and earth with 500VDC, but that was 10 years ago, things have changed greatly in that time but the regulations have not kept pace with these changes.
  10. retiredsparks

    retiredsparks Well-Known Member

    On SWA whack 1000 V down it to see if it's up to the job.:)
  11. Elmoakash

    Elmoakash New Member

    ...and now the whole issue gets even more complicated with USB sockets , Wi-Fi extender sockets, LED lamps as well as the 'traditional' do not megger items like PIR's and dimmer switches. Rodents....
  12. peter palmer

    peter palmer Well-Known Member

    Surge protection extension leads are the bane of my life, whats the reason for having a path to earth in these leads. At least I know the Megger reading now when there is one plugged in.
  13. Mr Rusty

    Mr Rusty Well-Known Member

    Just for curiosity, how do you guys get on with high tech lighting systems with touch dimmers etc? and other home automation systems like wifi controlled sockets?
  14. Bob Rathbone

    Bob Rathbone Well-Known Member

    Yes, a nice long run of 95mm will charge up like a capacitor, and give the unwary a belt.
  15. FatHands

    FatHands Well-Known Member

    I agree with @unphased
    In a populated installation with gawd knows how many things connected i tend to connect my earth leakage current meter across the tails and load the circuits up.
  16. retiredsparks

    retiredsparks Well-Known Member

    hey fats ,,,,where you been...i guess in prison..for some terrible nefarious saying you want your Beautiful BRITISH culture in Uk.#
    you know you are not allowed to say that....cause they will murder you if you do.
  17. FatHands

    FatHands Well-Known Member

    lol. still out in the sun drinking all day RS ;)
  18. sparky Si-Fi

    sparky Si-Fi Well-Known Member

    If its an add on and the job in hand has an RCD upfront and if you know what you're doing, any addition will be fine.
  19. retiredsparks

    retiredsparks Well-Known Member

    Hi Fats
    Glad you are back.
    Will post on Just talk to avoid going off track here .

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