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Difference between DC RCD's and Type B RCD's?

Discussion in 'Electrics UK' started by eveares, 3 Sep 2017.

  1. eveares

    eveares

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    Trying to work out if the DiLog DL9118 MFT tests Type B RCD's?

    The instructions say it tests DC RCD's of both selective and non selective types. Is this referring to Type B RCD's or just to pure DC RCD's and not AC RCD's what can detect DC bias?

    Am I also right in that 2-Wire no trip loop testing is rarely found on non Megger branded meters?

    Regards: Elliott.
     
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  3. ericmark

    ericmark

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    I can't answer as to which types it will test.

    However in the main most loop testers will test with around 9 mA which should not trip a RCD, as to if testing with such a low current will give accurate results is another question. I noted the low current used with the plug in socket testers and tried to say as a result they would never be as good as a proper tester which in the old days used something like 10 amps and with repeated tests would get rather warm, and it was pointed out to me that is no longer the case.
     
  4. eveares

    eveares

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    The question is I want to spend around £550 on a MFT later this month (including calibration costs if 2nd hand), and whether I should get new or used. Only for occasional use and when I do small non-notifiable jobs for family.

    Just worry that If I get an MFT 2nd hand on ebay, it mail fail it's calibration or not be as described/damaged.
     
  5. JohnW2

    JohnW2

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    I think it has been pointed out to you wrongly, at least in terms of some machines. My Fluke 1652 tests loop impedance with 12A for 10mS (half a cycle), not 9 mA !

    Kind Regards, John
     
  6. Iggifer

    Iggifer

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    Yea my Megger 1552 will trip the RCD every single time on the hi-current test. Definitely uses more than 9mA to test.

    Only the 175x series of Meggers do the 2-wire non trip, not the older ones
     
  7. JohnW2

    JohnW2

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    My Fluke 1652 (and I think most/all Fluke MFTs) use 12A for "non-trip" loop impedance testing (and I've virtually never known mine to trip an RCD). I can but presume that the very short duration (10 ms, half a cycle) is not long enough to trip an RCD.

    Kind Regards, John
     
  8. JohnW2

    JohnW2

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    That is obviously a risk whenever one buys anything used, on ebay or whatever. If it is listed as being in 'full working order' and doesn't 'fully work', then one hope that the seller (with ot without the help of ebay) would accept it back and would refund you. A few on ebay are calibrated and/or offer calibration for an extra charge.

    For what it's worth, all of the used MFTs I've bought on ebay (in particular, my last two Fluke 1652s) have been fine.

    However, if you're concerned, you budget is probably large enough to get you a new one. As was pointed pout to me during the recent discussions about my Fluke 1652 (no longer made), although the 'list price' is £860+VAT, ISSWWW (and probably others) are selling the current equivalent (Fluke 1662) for £550+VAT.

    Mind you, that's more than double what you'd probably pay (certainly more than double what I would pay!) for a used MFT, so it's up to you!

    Kind Regards, John
     
  9. flameport

    flameport

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    Selective normally refers to those with a delayed trip, such as would be used upstream of other non-selective RCDs to avoid both tripping in the event of a fault.

    Quite probably - however such a feature has little or no uses in the real world, and certainly isn't necessary or required.
     
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  11. Iggifer

    Iggifer

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    The point in a traditional non-trip test is that it uses the third, neutral lead to balance that load surely? The Megger will tell you the RCD has tripped if you remove the neutral lead whilst testing on no-trip.

    I do want to upgrade to the 175x series for the 2-wire no trip but they're just too much money, even used. I can only assume that it simply uses lower current and takes longer. Means you can get the Zs from a light switch with no neutral though - can be a life saver in certain situations.
     
  12. EFLImpudence

    EFLImpudence

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    What happens if you connect both the N & PE leads to the CPC?
     
  13. Iggifer

    Iggifer

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    Have never tried it. Would say I'll try it later but I don't have an RCD! Might knock one on a bit of flex just to see
     
  14. JohnW2

    JohnW2

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    Ah! My 1652 does not trip an RCD with the neutral lead disconnected, and nor does it give me any message that I understand - but it does tell be that the loop impedance is >2000Ω, which I guess may be its way of trying to tell me that it has aborted the test!

    I presume that it must detect that the N lead is disconnected by it's failure to send any current down it - since, per recent discussions regarding my old 1652, whilst it does a test for the presence/absence of L and PE leads, it does not test the N one.

    Kind Regards, John
     
  15. JohnW2

    JohnW2

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    Ah, this is interesting.

    I tried it with my 1652 and, whilst the 30mA RCD (which is working/testing fine) in the CU did not trip, the up-front 100mA TD one did (just about the first time it's ever operated in anger!). Any thoughts?

    Edit: I wonder if it's related to the fact that my installation in TT?

    Kind Regards, John
     
  16. EFLImpudence

    EFLImpudence

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    Is the 100mA one upstream of your bonding (and/or electrode) connections?
     
  17. EFLImpudence

    EFLImpudence

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    Second thoughts - does that matter on 'no-trip' setting?
     
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