Do Residual Current Meters suitable for DIYers exist?

Discussion in 'Electrics UK' started by boringoldcodger, 25 Oct 2020.

  1. boringoldcodger

    boringoldcodger

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    There are plug-through (doubt whether that's the right word) RCDs available, an example (link):
    https://www.ryness.co.uk/13-amp-30ma-plug-in-rcd-adaptor-white

    In my minds eye I can see a Residual Current Meter looking something like (apologies for shadowy photo):
    RC_monitor.jpg

    but I can't find anything, am I throwing wrong keywords at Mr. Google?
    What I can find are clamp meters, example (link):
    https://cpc.farnell.com/multicomp-pro/mp780050/mini-leakage-current-tester/dp/IN08364?st=ac leakage

    It's my belief that to use such a clamp meter I'd have to knock up a shortish extension lead in which the earth wire is in a separate cable to the one which contains the live/line and neutral wires, is this right?

    Background: A friend has a 100mA Current Operated Earth Leakage Circuit Breaker (does the name give away its age?) which started tripping and which cuts all the electric to her house. It seems likely that its caused by the boiler but I kept on impressing on her (what I believe I've gleaned from other threads) that the boiler might be just the final straw and that other device(s) might also be faulty. The boiler is plugged into a normal 13A socket (which has been used successfully to boil a kettle) and so it seemed it would be easy to plug in a Residual Current Meter and see what it showed.
     
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  3. AdrianUK

    AdrianUK

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    I too have looked for such a device & never found it.

    In the past I have resorted to building my own. A few years back I found a variable RCD module in RS that happened to have a real time readout of the earth leakage current on a three digit LCD display. I boxed this up & put 32A ceeform in & out leads on it. This became a mainstay of our diagnostics kit used on festivals for many years - along with inline current meters for 16A & 32A ceeforms.

    I think the closest you'll get is a clamp meter similar to the one you've linked to together a modified short extension lead that separates the earth core from the other two.
     
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  4. boringoldcodger

    boringoldcodger

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    Many thanks for your speedy response.
     
  5. blup

    blup

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    Your breaker trips when the boiler is plugged in but stays on when a much larger load (the kettle) is put through it, so using clamp meters will tell you what you already suspect/know, that there is an earth fault/short circuit, all potentially dangerous as well as an inconvenience now the colder weather is upon us.

    You could call out a plumber who was familiar with boiler electrics and kill two birds with one stone. But it needs a qualified electrician urgently to check the installation overall given its age.

    There are professional electricians who post on this site who may be able to give more focussed guidance.

    Blup
     
  6. EFLImpudence

    EFLImpudence

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    In a way yes, you could just carefully remove the cable insulation for a few inches.

    Bear in mind, you clamp the Line AND Neutral; not the CPC(earth wire).
     
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  7. Harry Bloomfield

    Harry Bloomfield

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    To measure the earth leakage, you either need to measure the flow of current (mA) in the earth wire, or measure the difference in current flow between the live and the neutral again in mA. An ordinary ac multi-meter, able to measure mA, could quite easily be used for the first method, whilst taking suitable safety precautions.

    Be aware when measuring leakage, that the measurement is an instantaneous one - there might well be a much larger leakage which only appears under certain circumstances for an just instant.

    Lots of modern equipment includes mains filters, using the earth pole as a reference. The more of these there are on an RCD protected circuit, the greater the leakage, the higher the likelihood of nuisance trips of the RCD. So the filter on the boiler, might well be the last straw in leakage, rather than the main cause of the nuisance trip. This is where multiple RCD circuits have value, in reducing the total leakage seen by one RCD (or ELCB).
     
  8. AndyPRK

    AndyPRK

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  9. EFLImpudence

    EFLImpudence

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    That's not conclusive as there may be other paths to earth.

    It's called a Residual Current Device because it is the Residual Current of the Line and Neutral; not the currant in the earth wire - obviously it can and might be the same but that is not what you are looking for as far as the RCD is concerned; it knows nothing about the CPC.
     
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  11. Harry Bloomfield

    Harry Bloomfield

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    There are possible other paths to earth, but we are usually looking at an appliance's leakage to earth. Items like boilers will also quite likely have pipework forming a path to earth. So the only way to be sure, is a difference measurement between L & N.
     
  12. EFLImpudence

    EFLImpudence

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    I know; that's what I said.
     
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  13. Harry Bloomfield

    Harry Bloomfield

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    Then we have agreement :)
     
  14. JohnW2

    JohnW2

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    You do, but you are agreeing about something rather different from what you originally stated :)

    In fact, (in terms of what you originally wrote) with anything to do with plumbing/heating, measuring the current in just a CPC is definitely not appropriate, since any leakage current will, in that situation, almost inevitably be shared between the CPC and other paths to earth (even other paths back to the MET).

    Kind Regards, John
     
  15. Harry Bloomfield

    Harry Bloomfield

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    I mentioned the OP's boiler in passing, in regards to filters. The majority of equipment will be plugged in and have no other path to ground, than via the CPC. As with all things, some element of engagement of brain is required, when testing.
     
  16. JohnW2

    JohnW2

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    I'm not convinced that 'in passing' is appropriate :)

    As you imply, the OP's immediate interest was in relation to a boiler, and that is one of the situations in which measuring current in the CPC would not be appropriate (or necessarily helpful), don't you agree?

    Kind Regards, John
     
  17. boringoldcodger

    boringoldcodger

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    I did consider that but my (extremely) cheap and nasty multimeter doesn't do AC current.
     
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