# can anybody help me i have three quetions ?

Discussion in 'General Electronics Chat' started by justin2015, Mar 6, 2015.

1. ### justin2015 Thread Starter New Member

Mar 6, 2015
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• If the presence of a ground point in the circuit provides an easy point of contact for someone to get shocked, why have it in the circuit at all? Wouldn't a ground-less circuit be safer?
• The person getting shocked probably isn't bare-footed. If rubber and fabric are insulating materials, then why aren't their shoes protecting them by preventing a circuit from forming?
• How good of a conductor can dirt be? If you can get shocked by current through the earth, why not use the earth as a conductor in our power circuits?

2. ### Hypatia's Protege Distinguished Member

Mar 1, 2015
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1) Only if the distribution system is ungrounded (a practical impossibility)
2)They [their shoes] may or may not depending on construction, moisture, etc...
3) The electrical properties of soil are sufficient to conduct lethal currents at residential EMFs--- "you can't get deader than dead "

Best regards
HP

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3. ### #12 Expert

Nov 30, 2010
16,703
7,341
1) That's why we use isolation transformers.
2) They are...up to a point.
3) Dirt can be a very good conductor. We don't use it to wire up circuits because it crumbles and we would have to keep it wet.

Don't think the two of us are arguing. Everybody thinks differently and you will get several points of view, depending on how a person interprets your questions.

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4. ### wayneh Expert

Sep 9, 2010
12,388
3,245
You and I are generally in contact with Earth ground to varying degrees. So we can touch any other object at ground without risk of a shock. We might suffer a static shock, but once that has passed there is nothing to continue driving current through our bodies. Perhaps you are asking about getting a shock from a circuit common, which doesn't have to be at earth ground.

But I think you may be right that intentionally grounding the user is slightly less safe than letting the user "float". I mean, I can grab the hot wire in my household wiring without much of a shock if I'm not well grounded. But if I'm also touching ground, yikes!
They absolutely help. Contact with the mains while standing barefoot in wet mud could easily kill you. Wearing shoes and standing on carpet, you'll feel it but you'll be able to respond.
Not all dirt is the same, and the moisture level has a big impact. In a way, we do use "dirt". Every home has a ground stake to provide an earth ground.

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5. ### WBahn Moderator

Mar 31, 2012
18,087
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1) It's a tradeoff. If you have a floating system and you get a transformer fault then you can send 10kV to 50kV into someone's home which is a serious safety risk, floating or not. With a grounded system, such a fault immediately results in high current flow in a way that immediately blows breakers and/or fuses.

2) It does protect them to some degree. But it is unreasonable to expect clothing to provide the necessary level of protection all the time. They might be bare-foot, they might be kneeling on the ground, the might have one hand on the ground, etc., etc..

3) In a small region it's usually a pretty poor conductor, but in a large region is can be an excellent conductor. Typical values of soil conductivity are between 10 mS/m and 100 mS/m, though they can be an order of magnitude lower or higher, too. So let's look at a couple of scenarios. What would the resistance be of a square bar of soil that is one meter long and one centimeter on a side? Using the lower conductivity of 10 mS would make the effective resistance (along the long axis) 1 MΩ and if we wanted to limit the current to 10 mA, then we could get tangled with voltages as high as 10kV via this conductor and not exceed this. But now let's ask about the resistance between the faces of a cube that is 1 meter one a side (imagine standing on such a cube that was, in turn, sitting on a steel plate that was electrified and you were in contact with the other electrode). That would be 100Ω and now our max voltage would be just 1V (which would mean that, in practice, the resistance of the soil is so low that it provides no meaningful protection).

Some circuits DO use the earth as a conductor in power circuits. There are high voltage DC monolines that do just that. Let's ask a hypothetical question about a 1MV monoline that transmits 100MW of power over a distance of 100 km. How large would the return path need to be if we model it as a rectangular solid that is 100 km long, L on a side, and we want no more than 1% of the power to be lost in the ground return path? If you crank the numbers, you get that the return path only needs to be about 30m on a side.

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