Why is grouding the prefered safety principle?

Discussion in 'General Electronics Chat' started by meemoe_uk, Aug 29, 2012.

  1. meemoe_uk

    Thread Starter New Member

    Jul 31, 2012
    Hi all, there was an accident in china recently where some workers shorted a high voltage cable by grounding it.

    It reminded me that I've never fully understood grounding a circuit from a safety perspective.

    I can fully understand why it's a good idea to electrically connect unpowered objects to stop voltages from building up.
    It's practical to use the Earth as a general connection for this purpose.

    However it makes no sense to me why we would then also connect a high powered device such as a power station to the ground.
    This greatly increases the risk of accidents I think.
    It's common sense now-a-days to not touch in-use electrical cables. But by Earthing a power station you've effectively connected a neutral cable into every persons foot, so that if they touch a mains live cable, they will get a shock.

    Remove the power station's Earth electrode, and a person would have to touch a live cable and a return path cable to get a shock.

    In short, why do we use grounding as our standard electrical safety principle instead of isolation ?
    I assume its a 'cache 22' situation with each safety principle having its own drawbacks. If so, isolation must have a worse drawback than grounding. What is it?

    "For an isolated device, touching a single powered conductor does not cause a severe shock, because there is no path back to the other conductor through the ground." - wikipedia
    Last edited: Aug 29, 2012
  2. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    True, assuming at each and every point along the entire power grid absolutely no device has less then perfect isolation from earth ground.

    By Earth grounding one line you have a deterministic system where the required isolation voltage is defined (it's minimum is the line voltage itself).

    Additionally, you have ensured when touching a wire you have a minimum 50% change of touching a safe Earth ground potential.

    This is not inconsequential if you look at the layout of an incandescent light base: the large screw area closest to the open end is neutral, and the hot end is the small tab furthest from the open end.
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  3. vk6zgo

    Active Member

    Jul 21, 2012
    Earthing one side of the supply dates from the earliest days of power distribution,& has several advantages.

    First,let's examine a fully isolated system/

    Historically,most equipment operated from the mains had a metal outer body or case.
    Providing full insulation for both leads of the mains supplied to it was extremely difficult.
    In the possible event of one of the active lines shorting to the case,all would be well,except if some other piece of equipment had a short of the other lead to the case,when you could get a shock by touching both devices.
    This may sound unlikely,but was a serious possibility before the advent of modern insulating materials.

    When the neutral line is connected to earth at the mains inlet to the house,& all the equipment cases connected to earth,if the active circuit becomes shorted to the case,it will blow the fuse,& not endanger the operator.
    If the neutral line becomes shorted to the case,it will not blow the fuse,but will not constitute a danger.

    There is now no chance of two units ending up with their cases connected to different sides of the incoming supply.

    Now,however,if someone fails to connect the earth to the case,& the case becomes shorted to active,it is possible to get a shock between that case,& a correctly earthed one,or a water pipe,which are commonly connected to mains earth.

    It is obvious,that with such a system,it is extremely important that the internal earth connection of the equipment is good.

    More recently,the idea of "Double Insulation" has arisen.
    With this system,not only is the actual wiring insulated,but a second independent layer of insulation separates the working parts from the outside world.
    This has only become possible due to the development of modern plastics.
  4. JMW

    Active Member

    Nov 21, 2011
    Navy Shipboard electrical systems are isolated from ground. The reasoning is that if a cable takes a "hit", 1/2 the voltage will get to the equipment and it may work in a limited fashion. Even so, you still read a voltage to the ships hull. Sometimes there is an equipment problem, but most often it is capacitance leakage to ground. Even this "leakage" is lethal. In short, there is no practical way to isolate a large system from earth ground.
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  5. Rleo6965

    New Member

    Jan 23, 2012
    Grounding connection was important with computer with hardwired connection with each other. All computer must have common ground potential near 0V. If other connected computer have no ground connection or was removed. That computer casing voltage willdangerously raised above 0V ( 50% of AC supply source ) and will cause overheating of interface board or components and damage will result.

    Other might left casing ground not connected to ground and let the casing voltage float. This will be dangerous if somebody accidentally hold the casing and the person was not insulated from ground and will be electrocuted. More computers interconnected more shock current and more devastating..
  6. panic mode

    Senior Member

    Oct 10, 2011
    in general good and reliable connection is safer than poor connection. isolation is no connection that will sooner or later become poor (or possibly good) connection when a failure occurs.

    yes, isolated systems are safer - while isolated...

    but that is not what happens in real world, at least does not last forever. the problem is one cannot guarantee they would remain that way nor how long (maybe it can be done reasonably cost-sensitive and reliable for small, closed system but not for something as large as power grid). dust, rust, moisture etc all do their magic and eventually you got conductivity. same happens when tree branches grow or fallen trees lean on power lines, circuits grow whiskers etc. there are many ways failures occur. in real life any random part of the circuit can become reference (connected to ground) and any time. if the circuit is not already referenced, the point of failure becomes reference.

    the scary thing is that in an event of a failure, circuit remains powered and - continues to operate, seemingly intact:
    the voltages in rest of the circuits will simply shift but there is no fault current, no fuse gets blown and no breaker gets tripped. one believing that circuit is safe because it is expected to be isolated would likely be charred and crisped before realising that safety of isolated system was just illusion.

    by grounding one side of the circuit, failures shorting "hot" side of circuit to ground will cause over-current device to trip. as a result, circuit is no longer powered (this is a safer condition). also tripped breaker or blown fuses will get attention so someone will investigate and remedy reason for the trip or the trip will happen again until fault is removed. isn't this safer?
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