Easiest way to check for a good earth

Discussion in 'UK DIY' started by D.M. Procida, Feb 26, 2006.

  1. D.M. Procida

    D.M. Procida Guest

    What's the easiest way to check for a good quality earth connection at
    mains sockets?

    Am I right in thinking that mere earth continuity is not enough to prove
    that there's a decent connection to the earth provided by the
    electricity board?

    Daniele
    D.M. Procida, Feb 26, 2006
    #1
  2. "D.M. Procida" <> wrote in
    message
    news:1hbd9o0.121xowzqazejvN%...
    > What's the easiest way to check for a good quality earth connection at
    > mains sockets?
    >
    > Am I right in thinking that mere earth continuity is not enough to prove
    > that there's a decent connection to the earth provided by the
    > electricity board?
    >
    > Daniele


    Ypu will need and earth loop impedance meter, a low resistamce ohm meter and
    the skill and knowledge to operate and understand the results.
    Stephen Dawson, Feb 26, 2006
    #2
  3. D.M. Procida

    Andy Dingley Guest

    On Sun, 26 Feb 2006 12:38:07 +0000,
    (D.M. Procida) wrote:

    >What's the easiest way to check for a good quality earth connection at
    >mains sockets?


    Shove one end of John Prescott in the socket and a Megger up the other
    end. Doesn't tell you much, but it sure tells him something.

    A "plug with 3 neons" will give you the basics of which socket has the
    missing earth wire, but if you want to know about "good quality" then
    there's no option but to get a sparkie's on-site guide (tenner well
    spent) and hire a real earth impedance meter for the day, then follow
    the instructions. It's not particluarly hard, but you need the right
    tools.
    Andy Dingley, Feb 26, 2006
    #3
  4. D.M. Procida

    ukdiyuser Guest

    D.M. Procida wrote:
    > What's the easiest way to check for a good quality earth connection at
    > mains sockets?
    >
    > Am I right in thinking that mere earth continuity is not enough to prove
    > that there's a decent connection to the earth provided by the
    > electricity board?
    >
    > Daniele


    This is notoriously hard (and potentially very dangerous) to do.
    Firstly you don't have a reference against which to compare. You would
    need an independent earth point, and even then all you would be
    comparing is the quality of this independent point with the quality of
    the earth point you're wanting to test. Secondly an earth is only a
    good one if it can support a short circuit of potentially thousands of
    amps long enough to allow the circuit protection (fuse/MCB) to cut out.
    So even if you established that there was a low resistance to earth,
    this in itself doesn't mean that there isn't a weak link in the earth
    circuit that would blow before the cutout under short circuit conditions.

    Short answer - leave it to the professionals, who (should!) have the
    specialist and well maintained equipment and experience to do this
    properly and safely.
    *** Free account sponsored by SecureIX.com ***
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    ukdiyuser, Feb 26, 2006
    #4
  5. D.M. Procida

    Guest

    D.M. Procida wrote:
    > What's the easiest way to check for a good quality earth connection at
    > mains sockets?
    >
    > Am I right in thinking that mere earth continuity is not enough to prove
    > that there's a decent connection to the earth provided by the
    > electricity board?
    >
    > Daniele


    As said, checking earth impedance is not so easy. But all you need is a
    good enough earth rather than a good earth. There's no need for it to
    sink 1000s of amps, it only need sink enough to pop an RCD if your
    install is RCD protected, or enough to pop the biggest fuse/mcb if not.

    If you've got a low V bench supply, multimeter, local earth rod and a
    bit of metal rod, put the rod in the ground some distance form the main
    earth, and connect the PSU output between the 2 earths. If the output
    is 'earthed' you need to connect the non-earthed output pin to your 2nd
    E rod. Now your PSU v/i output tells you the impedance of your 2 earths
    in series. If both are smimilar rods in similar soil, you now have a
    rough measurement of twice E impedance. Since your main rod R cant be
    more than the load impedance seen by the psu, you've got a max
    impedance number you can count on as well as a best guess number.

    The old fashioned way was to connect a 40w mains bulb between live and
    earth, if it lit up properly the earth was passed. Do not do this, it
    is dangerous in some circumstances and does not ensure an adequate and
    safe earth. It does unsafely check there is an earth, and that it
    passes some current, but thats all.


    NT
    , Feb 26, 2006
    #5
  6. D.M. Procida

    tony sayer Guest

    >The old fashioned way was to connect a 40w mains bulb between live and
    >earth, if it lit up properly the earth was passed. Do not do this, it
    >is dangerous in some circumstances and does not ensure an adequate and
    >safe earth. It does unsafely check there is an earth, and that it
    >passes some current, but thats all.
    >


    I did that with a 3 kW electric fire to test the lightning system we
    have and worked a treat:))

    But don't do as I do, and don't do as I say....etc...

    --
    Tony Sayer
    tony sayer, Feb 26, 2006
    #6
  7. D.M. Procida

    Cicero Guest

    <> wrote in message
    news:...
    > D.M. Procida wrote:
    >> What's the easiest way to check for a good quality earth connection at
    >> mains sockets?
    >>
    >> Am I right in thinking that mere earth continuity is not enough to prove
    >> that there's a decent connection to the earth provided by the
    >> electricity board?
    >>
    >> Daniele

    >
    > As said, checking earth impedance is not so easy. But all you need is a
    > good enough earth rather than a good earth. There's no need for it to
    > sink 1000s of amps, it only need sink enough to pop an RCD if your
    > install is RCD protected, or enough to pop the biggest fuse/mcb if not.


    <snipped>
    >
    > The old fashioned way was to connect a 40w mains bulb between live and
    > earth, if it lit up properly the earth was passed. Do not do this, it
    > is dangerous in some circumstances and does not ensure an adequate and
    > safe earth. It does unsafely check there is an earth, and that it
    > passes some current, but thats all.
    >
    >
    > NT
    >

    =================
    This method (using a 15 Watt bulb) is described in the current edition (Pub.
    2005) of 'Modern wiring practice - Design and installation' by W.E.Steward &
    T.A.Stubbs so it's probably OK.

    It's described as a test for polarity rather than earth but the result
    should be the same.

    Cic.
    Cicero, Feb 26, 2006
    #7
  8. D.M. Procida

    D.M. Procida Guest

    Cicero <> wrote:

    > > The old fashioned way was to connect a 40w mains bulb between live and
    > > earth, if it lit up properly the earth was passed. Do not do this, it
    > > is dangerous in some circumstances and does not ensure an adequate and
    > > safe earth. It does unsafely check there is an earth, and that it
    > > passes some current, but thats all.
    > >
    > >
    > > NT
    > >

    > =================
    > This method (using a 15 Watt bulb) is described in the current edition (Pub.
    > 2005) of 'Modern wiring practice - Design and installation' by W.E.Steward &
    > T.A.Stubbs so it's probably OK.
    >
    > It's described as a test for polarity rather than earth but the result
    > should be the same.


    It will of course give any circuit breaker the willies though, won't it?

    Daniele
    D.M. Procida, Feb 26, 2006
    #8
  9. D.M. Procida

    Guest

    D.M. Procida wrote:
    > Cicero <> wrote:
    >
    > > > The old fashioned way was to connect a 40w mains bulb between live and
    > > > earth, if it lit up properly the earth was passed. Do not do this, it
    > > > is dangerous in some circumstances and does not ensure an adequate and
    > > > safe earth. It does unsafely check there is an earth, and that it
    > > > passes some current, but thats all.
    > > >
    > > >
    > > > NT
    > > >

    > > =================
    > > This method (using a 15 Watt bulb) is described in the current edition (Pub.
    > > 2005) of 'Modern wiring practice - Design and installation' by W.E.Steward &
    > > T.A.Stubbs so it's probably OK.
    > >
    > > It's described as a test for polarity rather than earth but the result
    > > should be the same.

    >
    > It will of course give any circuit breaker the willies though, won't it?
    >
    > Daniele


    neither mcb nor fuse would take any action. They dont care where the
    current goes, just that there isnt too much of it, ie >6A, >32A etc..

    Its not altogether safe to do with a 15w bulb because if your E
    impedance is high or o/c, everything in the building thats supposed to
    be earthed becomes live. This is not a safe scenario. It would be fine
    if your earthing is ok, and no E connection has come adrift anywhere,
    but if youre doing this kind of test, its probaly because your earthing
    isnt behaving as expected.

    Daniele why do you want to test earth impedance? Or did you really just
    mean you wanted to make sure your socket earths were connected, which
    is a diffrent thing? (much easier)


    NT
    , Feb 26, 2006
    #9
  10. D.M. Procida

    tony sayer Guest

    >Its not altogether safe to do with a 15w bulb because if your E
    >impedance is high or o/c, everything in the building thats supposed to
    >be earthed becomes live. This is not a safe scenario. It would be fine
    >if your earthing is ok, and no E connection has come adrift anywhere,
    >but if youre doing this kind of test, its probaly because your earthing
    >isnt behaving as expected.


    Mind you that won't be too clever either what with all the "EMC"
    suppression capacitors coupled twixt line and earth.....

    --
    Tony Sayer
    tony sayer, Feb 26, 2006
    #10
  11. D.M. Procida

    Guest

    tony sayer wrote:
    > >Its not altogether safe to do with a 15w bulb because if your E
    > >impedance is high or o/c, everything in the building thats supposed to
    > >be earthed becomes live. This is not a safe scenario. It would be fine
    > >if your earthing is ok, and no E connection has come adrift anywhere,
    > >but if youre doing this kind of test, its probaly because your earthing
    > >isnt behaving as expected.

    >
    > Mind you that won't be too clever either what with all the "EMC"
    > suppression capacitors coupled twixt line and earth.....


    Yes, not ideal. I guess the currents involved should be low enough that
    almost any form of incidental earthing would keep it close to zero. And
    the unloaded voltage would be 120 rather than 240.


    NT
    , Feb 26, 2006
    #11
  12. D.M. Procida

    D.M. Procida Guest

    <> wrote:

    > > > This method (using a 15 Watt bulb) is described in the current edition
    > > > (Pub. 2005) of 'Modern wiring practice - Design and installation' by
    > > > W.E.Steward & T.A.Stubbs so it's probably OK.
    > > >
    > > > It's described as a test for polarity rather than earth but the result
    > > > should be the same.

    > >
    > > It will of course give any circuit breaker the willies though, won't it?
    > >

    > neither mcb nor fuse would take any action. They dont care where the
    > current goes, just that there isnt too much of it, ie >6A, >32A etc..


    Really? I thought that that the circuit breakers on consumer units also
    kept an eye out for current going to earth (are they called RCDs?) and
    took action accordingly.

    Daniele
    D.M. Procida, Feb 26, 2006
    #12
  13. D.M. Procida

    Tim S Guest

    D.M. Procida wrote:

    > <> wrote:
    >
    >> > > This method (using a 15 Watt bulb) is described in the current
    >> > > edition (Pub. 2005) of 'Modern wiring practice - Design and
    >> > > installation' by W.E.Steward & T.A.Stubbs so it's probably OK.
    >> > >
    >> > > It's described as a test for polarity rather than earth but the
    >> > > result should be the same.
    >> >
    >> > It will of course give any circuit breaker the willies though, won't
    >> > it?
    >> >

    >> neither mcb nor fuse would take any action. They dont care where the
    >> current goes, just that there isnt too much of it, ie >6A, >32A etc..

    >
    > Really? I thought that that the circuit breakers on consumer units also
    > kept an eye out for current going to earth (are they called RCDs?) and
    > took action accordingly.
    >
    > Daniele


    That *is* an RCD you are thinking of - effectively measures the difference
    between live and neutral currents and if the difference gets to big (say
    30mA is the most common), it trips. It doesn't care about overload current.

    The beauty is an RCD doesn;t care where the "missing" current has gone, just
    that it isn't returning via the neutral. So it will detect current
    disappearing to this circuit's earth, another circuits's earth, another
    circuit's neutral, down the water pipe, down soem hapless person's feet
    into the wet grass etc.

    Now, more often than not, you get one RCD on the consumer unit which
    monitors a number (sometimes all) the circuits.

    The devices on each circuit are either a fuse or an MCB, teh latter look
    similar to the RCD in as much as they have a lever to operate them.

    MCBs are purely for tripping out over current faults (like fuses).

    There is a combined type called an RCBO which does both jobs in one device -
    you don't see these so often as they cost more.

    You're right, if you have an RCD protecting the circuit and you test the
    earth with anything that draws more than 30mA or so, the RCD will trip.
    Your test lamp for example.

    You can get (expensive) instruments to test the earth that try not to trip
    the RCD - electricans who don;t have an instrument with the feature have to
    bypass the RCD to perfrom the test.

    If you just want to test that the earth is vaguely connected, buy one of
    those tester plugs with the 3 lights in them. Quick and easy and safe.

    Doesn;t prove the earth is solid - but will show obviously missing earth,
    and a few other problems too.

    For "proper" testing, you really need a specialised test meter which cost
    in the order of 100's pounds new (depending on whether it only does earth
    tetsing or insulation and RCD testing too).

    Cheers

    Tim
    Tim S, Feb 26, 2006
    #13
  14. D.M. Procida

    John Rumm Guest

    D.M. Procida wrote:

    > Really? I thought that that the circuit breakers on consumer units also
    > kept an eye out for current going to earth (are they called RCDs?) and
    > took action accordingly.


    Circuit breakers trip on overcurrent and that is all[1]. Most have two
    mechanisms - a magnetic part for large overcurrents (shorts etc, this
    usually kicks in at a few times the rating of the breaker[2]). This
    opens the breaker very quickly under this type of fault. In addition
    there is a thermal mechanism that opens the breaker more slowly after a
    period of prolonged overcurrent (e.g. drawing 1.5x the rated current for
    20 mins).

    RCDs are different beasts altogether (but see [1]), and monitor for a
    difference in the current going out and returning. Hence enough[3] loss
    to "somewhere else" (e.g. you, earth etc) will cause them to trip.

    (there is a another archaic device (use now deprecated) called a Earth
    Leakage Circuit Breaker - which monitors the current flowing to an earth
    terminal)

    [1] Just to add to the confusion there is a third category of device in
    reasonably common use called a RCBO. This combines the functions of RCD
    and Circuit Breaker (MCB)

    [2] How many times greater than the rating of the breaker the fault
    current needs to be to cause an instantaneous trip depends on the type
    of breaker. Type B requiring less than types C or D.

    [3] Enough will depend again on the rating of the trip sensitivity.
    Common sizes are 30mA (required for protection of sockets that can power
    outdoor equipment) and 100mA (often used for whole house protection on
    systems with a local earth rod).

    --
    Cheers,

    John.

    /=================================================================\
    | Internode Ltd - http://www.internode.co.uk |
    |-----------------------------------------------------------------|
    | John Rumm - john(at)internode(dot)co(dot)uk |
    \=================================================================/
    John Rumm, Feb 27, 2006
    #14
  15. D.M. Procida

    Guest

    D.M. Procida wrote:
    > <> wrote:
    >
    > > > > This method (using a 15 Watt bulb) is described in the current edition
    > > > > (Pub. 2005) of 'Modern wiring practice - Design and installation' by
    > > > > W.E.Steward & T.A.Stubbs so it's probably OK.
    > > > >
    > > > > It's described as a test for polarity rather than earth but the result
    > > > > should be the same.
    > > >
    > > > It will of course give any circuit breaker the willies though, won't it?
    > > >

    > > neither mcb nor fuse would take any action. They dont care where the
    > > current goes, just that there isnt too much of it, ie >6A, >32A etc..

    >
    > Really? I thought that that the circuit breakers on consumer units also
    > kept an eye out for current going to earth (are they called RCDs?) and
    > took action accordingly.
    >
    > Daniele


    If your wiring is new enough to have an RCD, that would trip. For most
    people nothing would trip. Those with partial RCD cover could use the
    lighting circuit, which will usually be non-RC.

    In principle you could still do the lightbulb test on an all-RCD TT
    system by:
    a) ensuring equal i flow from L and N to E
    or
    b) using an isolating transformer and feeding the output of that to
    earth and a secondary earth.

    But I dont suggest anyone doing it, not a good way to test earth
    impedance.


    NT
    , Feb 27, 2006
    #15
  16. D.M. Procida

    Ian_m Guest

    "D.M. Procida" <> wrote in
    message
    news:1hbd9o0.121xowzqazejvN%...
    > What's the easiest way to check for a good quality earth connection at
    > mains sockets?
    >
    > Am I right in thinking that mere earth continuity is not enough to prove
    > that there's a decent connection to the earth provided by the
    > electricity board?
    >

    We use this at work to verify each week the earth loop impedance important
    work benches.
    http://uk.farnell.com/jsp/endecaSearch/partDetail.jsp?SKU=5057838&N=401

    I am sure this will work.
    Ian_m, Feb 27, 2006
    #16
  17. "Andy Dingley" <> wrote in message
    news:...
    > On Sun, 26 Feb 2006 12:38:07 +0000,
    > (D.M. Procida) wrote:
    >
    >>What's the easiest way to check for a
    >>good quality earth connection at
    >>mains sockets?

    >
    > Shove one end of John Prescott in
    > the socket and a Megger up the other
    > end.


    The brainwashed Tory Little Middle Englanders are still at it. Part P is
    bill.
    Doctor Drivel, Feb 27, 2006
    #17

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