Earth / Ground safety

Discussion in 'General Electronics Chat' started by NZMikeV, Jun 23, 2010.

  1. NZMikeV

    Thread Starter New Member

    Jun 18, 2010
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    Dick Smith's Fun Way into Electronics Volume 1 (first published 1979) has the following warning at the end of the Crystal Receiver Project... "NEVER use as an earth connection, the 'earth' pin from a household power outlet. The earth in a mains power system is there for your protection, and should a fault develop while you are using your crystal set, etc. YOU MAY NEVER GET THE CHANCE TO BUILD ANOTHER PROJECT."

    Is this correct? Surely, you are only going to get fried if you are BETWEEN earth and phase. To get done from your crystal set, phase would have to come into contact with earth AND earth would have to be 'un-earthed' (as in someone removing the ground rod).

    Can anyone clarify this?
     
  2. retched

    AAC Fanatic!

    Dec 5, 2009
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    Sure.

    During a fault, there could be more potential on the ground line that on your crystal radio. If I remember correctly, we would wrap the antenna around some plumbing pipes for better reception.

    So in that case, you have just provided an additional path to ground with lower potential that the faulting ground wire.

    Electricity flows from higher to lower potentials.
     
  3. NZMikeV

    Thread Starter New Member

    Jun 18, 2010
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    That makes sense. To clarify further, if you were standing outside and touched the earth wire (during a fault) you would be safe unless you also touched something (like the plumbing) that was 'earthed' at a lower potential. Or if you were standing in a puddle and the ground was wet.

    But what if you were inside, say in your living room and holding the earth wire (which phase had leaked onto), you'd generally not be at a lower/higher potential than the earth? But you could be if the floor was concrete?

    Do I think too much? :D
     
  4. retched

    AAC Fanatic!

    Dec 5, 2009
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    That is correct. If you create a path to lower potential, there is a problem.

    If you are outside your home, at the ground rod, and you drive another ground rod into the dirt 10ft away, you can hook a circuit up, using the original rod as + and the new rod as -. Wouldn't do anything until a fault. Then when a fault occured, the original ground rod would become the + and the circuit would operate...for the duration of the fault, at least ;)
     
  5. NZMikeV

    Thread Starter New Member

    Jun 18, 2010
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    I guess a more realistic scenario would be the outside of a metal toaster faulting phase down to ground. Touch that while your other hand is in the kitchen sink and WHAMO! :eek:

    I think metal sinks are usually also grounded in NZ, but it depends how far the plumbing is from the ground rod, and how deep it goes.

    Sometimes 'Earth' really is the world's most dangerous planet! :D
     
  6. retched

    AAC Fanatic!

    Dec 5, 2009
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    thats funny...and right ;)
     
  7. t_n_k

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    Mar 6, 2009
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    There is normally no risk to you unless the earth bond connection at the appliance has been compromised and the active lead comes into contact with an internal point which has a conductive path to any exposed metallic part on the outside. Then you could be in real trouble. That's why it's good idea to have appliances checked regularly to ensure the earth bond connection is satisfactory. [There is no issue with double insulated appliances.]

    There have been unfortunate cases where just this problem has arisen - particularly with older appliances which haven't been checked for many years. A typical fault might be a badly corroded earth connection - say on a washing machine - which doesn't ensure a good low resistance connection back to the the household common earth point.

    If the household earth connection itself is compromised the use of MEN [Multiple Earthed Neutral] systems can give some level of protection in such circumstance. You probably use the MEN system in NZ.

    The thought of plugging a crystal set 'ground' wire into the mains wall outlet sends shivers down my spine - just think if one inadvertently pushed the wire into the active point by mistake. That's definitely dangerous and not much "fun". It's often children who are experimenting with crystal sets and likely to end up making the wrong connection. Hence this might also be on the manufacturer's mind with respect to the warning on the Dick Smith kit instructions.
     
    Last edited: Jun 23, 2010
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