How does grounding an electrical device protect our body?
There is something I never quite understood about the concept of grounding an electrical device.
Maybe my interpretation of the concept is wrong, but:
First suppose the device is NOT grounded, and the circuit's hot wire somehow gets connected to the device housing:
The resistance of my body Rb = 1000 ohm.
The source voltage is 230 V.
This means a current of I = 230/1000 = 0.23 A will flow through my body (auch!).
Now suppose the dive is grounded. My body still forms a path from device to earth, and now so does the ground circuit. What I mean is: my body and the ground circuit are now 2 resistances placed in parallel (Rb and Rg).
V = 230V
Rb = 1000 ohm
Rg = 10 ohm
Since the resistances are in parallel, they each get a voltage of 230, which means that a 23 A current will flow through the ground wire,
but my poor body still has to bear with 0.23 A!!
The current flowing through my body is not less than before. So how does grounding protect people then??
Answer:
See http://www.allaboutcircuits.com/vol_1/ch... regarding current effects on the body for different values of current.
Observation: your value for the resistance to ground is high; less than one ohm is more practical. So the point is, the second device in the protective scheme of things is a circuit breaker. Given a short then with 1 ohm of Rg, the source will attempt to deliver 230 amperes which actually won't happen because 30 or 40 or 50 ampere breakers will have opened or tripped to protect the circuit/personnel.
Another point is that human protection is provided by the electrical resistance of human skin. Given that understanding, if you penetrate the skin, say on two fingers, a human could die from the current produced by 1.5 volt battery. Thereforeskin resistance is very important in the discussion
From Ref No. 1. Body resistance also varies depending on how contact is made with the skin: is it from hand-to-hand, hand-to-foot, foot-to-foot, hand-to-elbow, etc.? Sweat, being rich in salts and minerals, is an excellent conductor of electricity for being a liquid. So is blood, with its similarly high content of conductive chemicals. Thus, contact with a wire made by a sweaty hand or open wound will offer much less resistance to current than contact made by clean, dry skin.
Measuring electrical resistance with a sensitive meter, measures approximately 1 million ohms of resistance (1 MΩ) between two hands, holding on to the meter's metal probes between two fingers. The meter indicates less resistance when you squeeze the probes tightly and more resistance when you hold them loosely. Sitting here at acomputer,when hands are clean and dry, resistance is high. Working in some hot, dirty, industrial environment, the resistance between hands would likely be much less, presenting less opposition to deadly current, and a greater threat of electrical shock.
But how much current is harmful? The answer to that question also depends on several factors. Individual body chemistry has a significant impact on how electric current affects an individual. Some people are highly sensitive to current, experiencing involuntary muscle contraction with shocks from static electricity. Others can draw large sparks from discharging static electricity and hardly feel it, much less experience a muscle spasm. Despite these differences, approximate guidelines have been developed through tests which indicate very little current being necessary to manifest harmful effects (again, see end of chapter for information on the source of this data). All current figures given in milliamps (a milliamp is equal to 1/1000 of an amp): see Ref NO. 2
Grounding or Earthing an electrical device is done with a coil of wire (usually copper). The wire (copper) is a good conductor of electricity. As a result, in case of faulty device, the electricity directly travels to earth and get neutralised.
Suppose it is not grounded, in the same condition the electricity would not be able to flow through the earthing wire (since its not there). If you touch that device then the electricity flows through you (your body) to the ground.
When the the device is grounded, then since the copper wire is a better conductor of electricity than your body, electricity prefers to pass through the wire instead of your body. The body is safe. Thats how grounding protect people.
The grounding has to work together with a fuse in the live wire. When the circuit connects to the metal casing of the device, a large current flows through the live wire and so breaks the fuse. Once that is broken, no current can flow through anymore. Since the casing is connected to earth through grounding, the casing would not accumulate any voltage and so current would not flow through anyone touching the faulty device.
If someone is unlucky enough to come in contact with the device before the fuse breaks, the person still will have a current flow through them. Thankfully, fuses overheat extremely fast, less than a second, so not much damage can be done.
Basically, because it will always follow the path of least resistance.
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