How to test my mains supply is earthed properly

Discussion in 'Electrics UK' started by diy_dave, 4 Aug 2004.

  1. diy_dave

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    Hi - This might be a stooopid question, but... I want to check that my house electrics are properly grounded. When wiring up my boiler, an electrician extended an existing ground wire (previously used for an immersion that has now been removed) and bonded all the pipes. However, I would like to ensure that the wires are properly grounded.

    How can I test this? And preferably not just test the wire to the consumer unit. How can I tell if the main earth wire from the consumer unit does go to earth?

    Also, I have just bought an electric guitar and some humming is noticeable. One of many possible reasons for this is poor earthing of the circuit that the guitar is plugged into via the amplifier. So I would like to rule this out.

    Thanks
    Dave
     
  2. ban-all-sheds

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    By visual inspection. What type of earthing system do you have? If it's TT then the resistance of the earth rod can be measured, but if it's TN-S or TN-C-S you have to ask the supplier for the Ze value.
    The earth loop impedance of your installation can be measured.

    Learn the words then....
     
  3. diy_dave

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    Thanks Ban. :)

    I live in london in a terrace so I imagine it's TN-S or TN-C-S (I'll check later). What would the Ze value tell me? Will it tell me that my home electrics are connected to the electric supply companys earth?

    Is there a specific measurement I could do, say at the point my bonded pipes connect to the earth wire, to verify they are indeed adequately earthed? If so, could you give me a few hints?

    Cheers
    Dave
     
  4. kendor

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    regarding the humming, an amplifier will pickup noise on the earth and of course amplify it, with a good quality amp there will be sufficient filtering inbuilt to isolate this perhaps the amp you are using is a cheap model? also try plugging into different sockets around the house and try to keep it (and the guitar, lead) a decent distance away from other electrical equipment. You may well have to buy better quality equipment? This goes for the lead also.
     
  5. phil_ballard

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    This occasionally happens if either the controls compartment on the guitar is not adequately screened (very common on cheaper instruments, rather too common on dearer ones) and/or if the required earth connection in the guitar from the cable screen (i.e. the amplifier earth) to the bridge (and thus the strings etc) and the above-mentioned cavity screening is missing or inadequate. If the bridge/strings/machine heads are not earthed the 50Hz hum will be significant, and will reduce (usually with an audible 'click') while you are touching the strings or bridge.

    All guitar or bass setups seem to have a degree of mains hum, though, especially if you have neon lights or dimmer switches nearby

    Also, guitars with single-coil (i.e. non-humbucking) pickups, like a Fender Strat, are more prone than those with humbuckers (like most Gibsons) whose two coils are wired in reverse-parallel to cancel hum
     
  6. diy_dave

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    Thanks for the advice- I have spent sometime researching it this morning :) I have a strat with a single coil pickup so some humming is to be expected but my home electrics are a bit dodgy so it is one possiblity.

    I still want to test how effective my earthing is - even if it is just to ensure I don't get electricuted whilst in the bath (I rate getting rid of the humming as more pressing though ;) )
     
  7. phil_ballard

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    Don't play guitar in the bath (it makes the strings rust.....) :)
     
  8. kendor

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    Do you play your guitar in the bath then? :eek:
     
  9. diy_dave

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    I want to see how effective my earthing is, but preferably not using myself as a conducter. :)

    I have a steel bath and nearby are copper pipes running into the boiler closet that are bonded - that is what I would like to ensure is properly earthed.
     
  10. Lectrician

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    Well, i doubt you will be able to borrow the required test kit, and waiting for a power cut to test with a normal multimeter is not even funny :)

    Get a spark to test for ya.
     
  11. diy_dave

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    Thanks for your input.

    Amp / guitar humming dropped by about 50% when I plugged it into a different ring. I also noticed that if I have my laptop next to me when I am holding the guitar, my body channels noise into it and (as I read somewhere) when I touch an earthed part of the guitar it earths me (not the other way round).

    Cheers
    Dave
     
  12. Lectrician

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    How many plugs do you use for the amp/guiter?? if more than one, you can create 'earth loops'. You need to break these by breaking an earth in the audio screening at ONE end.

    NOT THE MAINS PROTECTIVE EARTH THOUGH :)
     
  13. AdamW

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    Never break the mains earth to an amp! This was a common practice years back before they started using balanced lines on PA systems...

    I am a guitarist and bass player, I have found that there is always some hum even on expensive instruments and equipment.

    I found one brand of lead reduced the hum considerably... I can't remember the name but the jacks at each end are slightly different for the amp end and the guitar end (weird I know, but it works). I have a cheapo 10m lead as well and the hum on that is pretty bad.

    If you have the TV on, try turning it off. I have found that the pickups are heavily affected by the EM noise of a TV.

    One last thing, try squirting a little bit of WD-40 (only a little bit!) into the pots on your guitar. Sometimes they get dirty very easily, and cause hums and crackles. Don't put a lot of WD-40 in... I did that once and it took weeks for it to evaporate/wear enough that I could play properly!
     
  14. plugwash

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    you have various options

    inserting capacitors in the screen connection of the audio cable can help but can lose bass

    you can also get audio isolation transformers

    finally you could connect the amplifier to the mains via an isolation transformer which would then make it safe to disconnect the earth to the amplifier
     
  15. phil_ballard

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    if you play LOUD ENOUGH yo won't notice the hum at all ...... :cool:
     

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