0
\$\begingroup\$

I don't quite understand what the situation is with the safety ground.

In theory, it evens out the voltage to zero but only if the resistance is exactly zero.

Even if the ground has only a resistance of say 3 ohms then that means a person making contact with the line will still get 230V, and the voltage causes the current. In two resistors connected in parallel, the magnitude of the current does not affect the current of the other. Is there as much current flowing through that person as it would be without the grounding?

Does the grounding only make sense if a ground fault circuit interrupter is also installed?

Addendum: The situation is basically like two resistors mounted in parallel: the voltage over both is the same and the current over each resistor is only caused by this voltage. The current over one resistor does not change if a resistor is added in parallel, whatever its value.

\$\endgroup\$
7
  • 2
    \$\begingroup\$ What do you mean "ground has resistance"? You mean, connection to earth has 3 Ohm resistance? \$\endgroup\$
    – Ilya
    Mar 3, 2022 at 8:07
  • 3
    \$\begingroup\$ Does this answer your question? Is ground connection in home electrical system really necessary? \$\endgroup\$
    – Andy aka
    Mar 3, 2022 at 8:26
  • \$\begingroup\$ Ilya: yes, which a low value for a common safety earth \$\endgroup\$ Mar 3, 2022 at 8:38
  • \$\begingroup\$ Andy: no that is a different question. Consider my person grounded, like the installation. If the resistance of the safety ground would zero, ideally, there would be no potential difference between the person touching a grounde faulty insulation. But because the safety earth connection ALWAYS had at least a tiny resistance the voltage over the person could the maximum the installation has, in other words the safety ground has no use apart from triggering the differential breaker \$\endgroup\$ Mar 3, 2022 at 8:43
  • \$\begingroup\$ @KantonBubahsky a tiny resistance (like 1 ohm) would cause hundreds of amps to flow before the person touched the installation. That would trip the breaker in milliseconds and be a self-revealing fault long before someone could get near enough to receive a shock. \$\endgroup\$
    – Andy aka
    Mar 3, 2022 at 9:13

2 Answers 2

3
\$\begingroup\$

Even if the ground has only a resistance of say 3 ohms then that means a person making contact with the line will still get 230V.

Yes, correct, for the first few milliseconds.

The earth connection allows a large current to flow in the event of a line fault to chassis, in this case 240/3 = 80 A, which blows the supply fuse or breaker, cutting off the power. All supplies have some sort of over-current fuse or breaker, whether or not they have GFCIs as well.

The chance of a person touching the chassis during that live few milliseconds is small. As soon as the fault happens, the power goes off. Without the earth connection, the chassis could stay live, a fault waiting for somebody to touch it.

True story from my childhood (60ish years ago). We had a refrigerator in our kitchen that was not earthed, and developed just such a silent live-to-chassis fault. Moving around on a linoleum floor in shoes or slippers, we were typically not earthed either, so the fault went almost unnoticed. I felt a slight tingle if I touched the bare metal handle lightly enough, but, being small, this was interesting rather than dangerous, and thought no more of it. One day I put a metal milk pan onto our metal gas stove, that was connected to ground through the metal gas pipe, while closing the fridge door with the other hand. Milk went everywhere, and I was traumatised, but otherwise unharmed, as the contractions caused by the shock broke my contact with the earthed stove immediately.

\$\endgroup\$
3
  • \$\begingroup\$ Wow, beautiful story. And a bit scary. The reason for my question is not that I doubt the usefulness of a ground in combination with breakers but if a ground only, as some texts, including wikipedia, would suggest \$\endgroup\$ Mar 3, 2022 at 12:46
  • \$\begingroup\$ @KantonBubahsky There is some sort of fuse in all supplies. \$\endgroup\$
    – Neil_UK
    Mar 3, 2022 at 13:04
  • \$\begingroup\$ If you had an RCD, this would've stopped it still. So tl;dr, an RCD offers yet another layer of safety. Even if grounding fails, RCD would still spring in and save your life. \$\endgroup\$
    – user47093
    Feb 9 at 18:23
2
\$\begingroup\$

No, earthing is not pointless.

Earth wire is connected to Neutral somewhere in your electrical system.

If your electrical device has a metal case and it is connected to earth wire, a fault such as the live wire getting loose and touching the earthed case, a large fault current flows and the fuse blows.

If there is no earth then the metal case would be live and dangerous to touch.

The GFCI or RCD will work without earth too, like with hair dryer. If there is an imbalance between current to hair dryer and back from it, it means it goes via an unintended path and must be shut down.

Edit:

So if wires have 3 ohms of resistance, humans can be approximated with 1000 times that. The point is that if live comes in contact with earthed metal case there is mains over 6 ohms and that's 40A, so a 10A breaker should trip or fuse blow rapidly. Typically wiring should have less resistance.

If an ungrounded device chassis comes live, and human touches only it, very little current flows as humans are never fully isolated due to capacitive coupling. But if human touches live part and some other earthed part at the same time, milliamps of dangerous current can flow, not enough to blow a fuse but enough to kill a human. This difference in live and neutral wires is detected by the GFCI/RCD, the equipment (e.g. hair dryer) does not need to be earthed for the detection to work.

\$\endgroup\$
6
  • \$\begingroup\$ This doesn't answer my question: The same voltage still causes the same current through the person, with or without ground. Unless there is diff.breaker. \$\endgroup\$ Mar 3, 2022 at 8:48
  • 1
    \$\begingroup\$ This answers the question because the principle of earthing is based on stopping the fault before a person touches it through the generation of currents to ground, not by detecting a current through a person. The use of RCDs to detect faults leaking to outside the system such as a person are completely orthogonal to the safety provided by earthing. So no the same voltage will not be present if the earthing worked, because the current will have already returned to earth triggering a breaker. \$\endgroup\$ Mar 3, 2022 at 9:11
  • \$\begingroup\$ The question was it pointless without a breaker device? Not is it pointless with a breaker device? \$\endgroup\$ Mar 3, 2022 at 13:01
  • \$\begingroup\$ Earthing is not pointless, with or without a breaker device. They solve completely different problems. Earthing is very relevant as it prevents metal case to become live to begin with, un-earthed metal case can become live but an RCD can trip when you touch the live metal case. RCD works even when your appliance has no earth connection, i.e. using plastic hair dryer with wet hands. \$\endgroup\$
    – Justme
    Mar 3, 2022 at 13:10
  • \$\begingroup\$ But if the earth has a (small) resistance, the live metal case will have a voltage. \$\endgroup\$ Mar 3, 2022 at 13:11

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.