[Physics] Is that ground, wall conductive


If i put my foot exposed to ground and put my finger to the USB power, i will be electrocuted. So the electron(s) from the USB power is passing over my hand, run to foot and go to ground ? As i might thinking, so ground must be an object that can pull excess electron toward them and the pulled electron will spread out to the ground or go to somewhere that more positive, so that reaction can continues forever (if i put a LED light with one terminal to power and one terminal go to ground it will emit continuously). With this argued, i think the ground or wall must be conductive. But ironically, the truth is not as i expected. Can someone please explain for me what happened inside these reaction ?

Best Answer

What we call 'earth' or 'ground' in electrical / electronic circuits is a 'reference' point for electrostatic potential. There are generally several 'types' of 'earth' used in this sense. An 'instrument earth' or 'signal ground' is used as a reference for low level signals, such as those used in communications devices. So when we say 'the USB power pin has 5V', we mean it has a 5V potential difference with respect to 'ground'. The USB port has a 'GND' pin which is connected to a common 'ground' in your computer system (chassis).

The electrical power system which supplies your house mains power is typically connected to 'protective earth' at the main switchboard. All external conductive components such as chassis are generally connected to this 'protective earth' for safety reasons, to prevent electrocution in case an exposed conductive components such a chassis becomes 'live' through faulty wiring etc, by tripping an over-current protection device (fuse or circuit-breaker) when such a 'fault current' flows to earth, returning to the supply via an earth connection on the neutral conductor in the main switchboard in your house.

So the earth must be able to conduct, otherwise this 'protective earth' function will not work.

In fact, although not as good a conductor as metal, the earth is used as the 'return' conductor to supply power in some remote regions of Australia (see: en.wikipedia.org/wiki/Single-wire_earth_return). The earth was also used as the return conductor in some early telephony systems.

A brick on the other hand, like most ceramics, is not conductive. The wall of your house therefore does not form part of the 'protective earth' system, although the concrete footings which make up the foundations of your house DO conduct (portland cement used in concrete is actually more conductive than brick).

So to answer your question 'is the earth conductive', the short answer is 'yes' but 'not as much as metal'.

Exactly how conductive? Well, that depends on the condition of the soil (pH, moisture, composition (eg: clay, sandy, salty), temperature, etc) as well as how you measure it.

The literature indicates that the values of earth resistivity vary from less than 1 Ω-m for sea water up to 109 Ω-m for sandstone. The resistivity of the earth increases slowly with decreasing temperatures from 25 °C to 0 °C. Below 0 °C, the resistivity increases rapidly. In frozen soil, as in the surface layer of soil in winter, the resistivity can be exceptionally high.

Using a standard multimeter 'resistance' setting to measure earth resistance is not effective. The reason for this is that the multimeter applies a low voltage (typically no more than 5VDC) to the component being tested via an internal series resistor of known resistance and then measures the current flow (from several micro-Amps to several Amps) to the tested device. Although this allows measurements from several milli-ohms to several mega-ohms, it is not recommended for earth soil resistivity testing, since it is too unreliable and inaccurate for this purpose.

To properly measure earth resistance, a special 'earth ground impedance' device is used, which apply a voltage (up to 50VDC) and/or current (up to 10mA) to two, three or four 'earth electrodes', inserted at a given (minimum) depth, at strategic locations in the ground. A dedicated earth-tester will also be able to perform ac resistivity measurements by applying an AC voltage (from 1kHz to 3.4kHz) to the electrodes, which is important for eliminating interference from 'electrical' ground noise.

There are a number of 'standardised' test methods for determining soil resistivity for design of protective earthing systems:

Dead earth method - uses 2 electrodes: the ground electrode under test and a convenient reference point such as a water pipe, metal fence post which is some distance away. A standard, two-lead multimeter or ohmmeter is limited to this test method.

Fall-of-potential method - uses 3 electrodes: a voltage is applied to a 'potential' electrode, setting up a current between it and a current electrode (some distance away). The voltage is then measured on the test electrode test electrode (inserted somewhere between the two).

Wenner method - uses 4 electrodes: spaced equal distances $d$ apart and inserted at a depth of $d/20$ into the ground. This gives a measurement of average soil resistivity to a depth equal to the electrode separation.

Since soil is not homogeneous, the earth soil test is typically repeated at multiple locations across the site.

A 'good' (conductive) earth has soil resistivity in the range 10-15,000 Ohms.cm.

Measurements over 50,000 ohm.cm are generally too high and considered 'poor earths' and not suitable for 'protective earth' systems. Measurements in between (15,000 - 50,000 Ohm.cm) can sometimes be 'treated' to improve soil resistivity.

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