Electric shock in the shower

Discussion in 'Electronics & Electrics' started by shmity, May 11, 2011.

  1. zero_velocity

    zero_velocity Member

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    Yes, 30mA RCDs are used for life saving circumstances on the base assumption that >30mA of current across the chest can cause fibrillation. a significantly greater rated RCD say 100mA or 150mA would be sufficient for broken neutral protection.
     
  2. mjunek

    mjunek Member

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    Makes sense then - so the broken neutral RCD should ideally pop as soon as the neutral disappears, assuming that there is significant power draw in someone's property.
    And then the subcircuit RCD would cover the issue of someone getting zapped due to exposed live conductors after that RCD.

    Without overcomplicating it (ok, maybe it is) how would this work in a property supplied by 3 phase? Wouldn't you just end up with the neutral dancing around voltage wise, but all return current going back via the incoming live phases? In this case, the RCD wouldn't be able to detect this fact.
     
  3. zero_velocity

    zero_velocity Member

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    RCDs work independant of the earth - they react to an imbalance of the active-neutral or active-active. 3p RCDs exist, so no major issues there. The RCD would trip when the property at fault draws enough to trip, irrelevant of the neighbors.

    You are right, a 3p property would be significantly harder to protect against a broken neutral. the board would essentially need to be divided into 3p connections and 1p ones, and protected against according.

    Edit: In essence an RCD works by measuring the difference between the active and neutral -the earth provides a return pathoutside of the RCD, this is what allows for the imbalance and for the RCD to trip on an earth leakage fault.

    This is why it is so important to have earthing and bonding throughout the home.
     
    Last edited: Apr 22, 2019
  4. Symon

    Symon Castigat ridendo mores

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    Plumbers are trained to put a bridge across a metal pipe when they cut into it for exactly this reason.

    I believe this has changed, or is in the process of being changed. With new installations the earth stake needs to be tested.

    On a MEN system? Unless the earth impedance is sufficiently high (ie, crap) then I can't see how this will work. You will have an MEN on both sides of the RCD so parallel paths back which will give you no end of nuisance trips.
     
  5. Sankari

    Sankari Member

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    Because it runs through the house and leaves us more vulnerable to electric shock if we touch any part of the plumbing (e.g. a tap). If my house has to be earned through any piece of metal, I'd prefer it to be metal that I don't need to touch at all. Instead, I have to just cross my fingers and hope no stray current leaks into my plumbing, which is carrying water, which also conducts electricity.

    Why does metal not connected to electricity need to be earthed? What about all the other metal in a house that's not connected to electricity? Should I start earthing my gutters now? Perhaps my cutlery as well?

    If earth is 'essentially 0V', why is 'a small amount of current' there in the first place? Why is it practically unpreventable?

    Well that didn't work for the girl who got zapped into a wheelchair, and it didn't work when my taps decided to start zapping me. My RCD didn't even blink. So although it sounds perfect in theory, in practice it's more like 'electricity is going to leak into your plumbing at some stage because we've designed it that way, and if it does there's literally no way to know if it will kill you or politely trip your RCD instead.'
     
  6. RobRoySyd

    RobRoySyd Member

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    Your thinking is the wrong way around. Assuming you have metallic water pipes they are inevitably connected to earth. The big pipe in the street is buried in the Earth and quite likely so is the pipe that runs into your premises. Disconnect the mains earth from the plumbing and connect it to an earth stake and both are still going to be connected via the Earth anyway. Running a fat wire between the earth stake and the pipe simply ensure a lower resistance to earth and between the two systems which is a good things\ as it reduces the likelihood of any potential difference.

    As for you getting a tingle from touching a tap while taking a shower how do you know it was even the tap that gave you the shock? If you've got a dodgy water heater then you could have been 'electrified' by the water and that was only felt when you touched the earthed tap. It's an unlikely scenario as tap water is a pretty poor conductor of electricity. Either way the voltage that gave you a tingle was only 4V. Unless you had an electrode surgically implanted onto your heart that's not enough to cause harm.

    Because the tap wasn't earthed. She was, the tap wasn't and somehow it was at mains potential. Long time ago a friend of mine had their dog electrocuted when it leant against a metal downpipe. The downpipe and gutter had become connected to an active conductor. If that metalwork was earthed it would tripped a breaker or more likely blown a fuse.
     
  7. Symon

    Symon Castigat ridendo mores

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    As I thought, you still don't understand.

    Earthing exposed metalwork, including the metallic piping, makes you less likely to receive an electric shock - it doesn't make you more vulnerable at all. Holding everything at earth potential means that should a fault develop (or even in the event some stray current from a nearby installation) then there will not be a step or touch voltage that may harm you.

    I suggest you read up on earth potential rise and earthing systems in general.

    To reduce the risk of electric shocks if it became live. If you have a steel framed house, then the electrician will earth the framing. There have been fatalities from people getting shocks from metallic roofing and gutters, especially if the consumers mains to the house is via an overhead service. AS3000 has specific rules about how these services are to be terminated onto the conductive supports, it is becoming less of an issue these days as new estates have underground mains.

    The newer versions of AS3000 have very specific requirements around earthing of pools, spas, and other wet areas. The steel re-enforcing in the concrete is also earthed. So it isn't just your metal pipes.

    Something that you need to get your head around is that nothing is really isolated from an electrical perspective. There are small amounts of current flowing through everything, even electrical insulators have a small (in the nano or pico amps range) amount of current flowing at all times. Even if a piece of metal is not touching something live, it can become live by capacitive or inductive coupling. A classic example of this is a metal wire fence or pipe running in parallel to an overhead line.

    Even galvanic or other chemical reactions in the earth will produce some current. We often consider 'earth' to be 0V, but the reality is if you stick two probes at any distance apart in the ground there will be some voltage detected.

    However in the context of this discussion, the main risk is a fault somewhere that increases earth currents to the point that hazardous voltages develop in the metalwork.

    That is because the fault was a broken neutral, which is on the upstream side of the RCD, so in that situation the RCD would not detect the fault. A broken neutral is the Achilles heel of the TN-C-S (or MEN) earthing system - it is the one fault that is the most hazardous and also the most difficult to detect. Apart from that fault, it is a pretty safe system.

    RCD's are an electromechanical device, so they are prone to failure. People put way too much faith in those things. A well designed earthing system will give you more reliable protection than an RCD.

    And no, the electricity will not leak into your plumbing because that is how it is designed - it will get into your plumbing because it is located on planet earth and is unavoidable. We bond it to the earthing system to reduce the risk to you if it does become energised. Many, many, lives have been saved because electricians have been doing that for a long, long time.
     
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  8. RobRoySyd

    RobRoySyd Member

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    I would have thought it's a pretty easy fault to detect, simply measure the current flowing through the neutral - earth connection and if it exceeds a threshold disconnect all power to the premises.

    In my dim memory I do recall such or a similar electrical safety device being installed in Au. Problem with them was parasitic tripping, a fault anywhere in a street could trip multiple premises etc.

    It arguably could be made safer but the law of diminishing returns kicks in. We could mandate earth resistance testing, we could go further and test under fault conditions. The costs escalate and as a society it's not unreasonable to ask if the money could be better spent.
     
  9. Sledge

    Sledge Member

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    Just HOW many heads do you have?
     
  10. Symon

    Symon Castigat ridendo mores

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    You mean this thing? https://www.tasnetworks.com.au/safety/Home-safety-and-CablePI
     
  11. RobRoySyd

    RobRoySyd Member

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    No. This 'thing' had a Bakelite housing and was installed in the switchboard. House it was in would have been built just after WWII.
    All electromechanical, just had what looked like a relay. I recall the relay coil was wound with cotton covered wire.

    Speaking of old things electrical I think I still have the grandparents B22 double adaptor. Used so Gran could have light and use the iron.
     
  12. Slug69

    Slug69 Member

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    I think there is a lot of confusion about grounding.

    Watch this video based on American Power system in homes to be further confused:
     
  13. Symon

    Symon Castigat ridendo mores

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    NFI what that would have been. From what I've seen of the CablePI (there is a teardown video online) it either works on the voltage drop due to the higher impedance return path, or somehow monitors the continuity of the L-N loop.
     
  14. RobRoySyd

    RobRoySyd Member

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    I suspect you're right about the CablePI. It's probably much the same as how the more advanced outlet testers work.


    I think though the old device I was talking about relied on the potential difference between two ground connections e.g. the house's mains ground connection and a ground stake some distance away. That would have worked fine at detecting a A-N fault in the house but it could pretty easily also trip other nearby faults.
     
  15. Symon

    Symon Castigat ridendo mores

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    Ah, I do believe that is the 'direct earthing system' that was used in the southern states some time ago. What would happen is if there was an A-E fault then the relay would operate, cutting power to the installation. I don't think it could detect N-E faults, but am happy to be corrected.

    Used to see it used for noise immunity on communications huts all the time, but not so much in domestic use.
     
  16. merlin13

    merlin13 Member

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    American Power System? Dumb bastards don't even believe in On/Off switches on their standard power points...
     
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  17. gdjacobs

    gdjacobs Member

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    There are standards for controlled impedance earthing in three phase systems. This technique allows active fault detection and ground fault current limitation but must be handled right to be safe. It also doesn't really work for systems with more than negligible phase imbalance.

    I disagree. Every standard US/Canada plug has a manual disconnect that can be engaged by pulling out the cord.
     
  18. Hive

    Hive Member

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    Their electric outlets are deathtraps. Luckily everything is piss weak 110v so no actual damage can be done.
     
  19. gdjacobs

    gdjacobs Member

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    Damage? In Canada, a little mains voltage through the body is great on a cold winter day!
     

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