We know that, in case if there is a faulty insulation,or sc, the current is bypassed to earth through the earth wire that has low resistance.Actually, what will happen to that current that flows through gnd?In that case,if we come in contact with that faulty appliance, wont that current flow through us as we form a closed path by standing in gnd?Is grounding a safety measure?
Most of the time it doesn't do anything but chase worms and waste energy but this is usually the cause of "stray voltage" problems when it happens. Cows stop giving milk and people get shocked in places where they shouldn't.
If I understand your question, you are thinking about a short from a hot conductor to a grounded appliance case.
If you are grounded and touch that case, a tiny current will flow through your body. Tiny because your body's resistance is much greater than the resistance between the appliance case and ground. Some current flows through both paths in inverse proportion to the path's resistance.
Chuck
Grounding / Earthing is done to protect the equipment and people from the destructive force of lightning and to stabilize the voltage relative to the earth. Were grounding is not done the voltage relative to earth or bodies that will function in lieu or the earth can get high enough to puncture insulation and cause a ground or earth fault.
To protect people from stray electrical currents and to keep them from being electrocuted by current passing through their bodies what is done is actually bonding. The confusion comes from the use of the terms grounding or earthing for both purposes. Grounding / Earthing is done at the source of supply and such other points in the distribution system as is necessary to stabilize the voltage and provide lightning a less destructive pathway to equalize the charges involved. Bonding is the process of connecting everything that is conductive, but not intended to carry current, back to the low voltage point of the source of supply. Part of the confusion results from the fact that the low voltage point is also the point were the grounding / earthing conductors are connected.
The illustration that works for me is to picture an aircraft in flight. No grounding is done on an airplane except when it is being fueled on the tarmac. Even though no grounding / earthing is possible on that aircraft when it is in flight every piece of metal in it's construction, that is not intended to carry current, is very carefully connected together into a single equal potential assembly. Having done that assures that nothing you touch in that plane will be at a higher voltage then whatever else you may be in contact with at the same time. No difference in potential means no current flow. What the actual voltage of the skin of the aircraft is relative to the earth is unimportant to you as a passenger as long as the entire aircraft is at the same potential. It could all be at several thousand volts relative to the earth over which it flies but no current will flow through the people and so they suffer no harm.
So bonding is the process of connecting everything that could conduct current back to the low voltage point of the source of supply. A fault is an unintended contact between an insulated current carrying conductor and the rest of the conductive materials in the environment in which that electrical conductor is installed. If the bonding is complete then any current that escapes from the intended pathways of the electrical systems insulated conductors will have a low impedance path to the source.
If the fault in question occurs on the low voltage side of the load some of the current will travel back to the source via the equipotential structure that is the aircraft. If the bonding is effective then there will be very little voltage drop across the structure so no voltage will be present to force a harmful current through a human being who happens to be in contact with too different surfaces of the planes structure. The bonding has made the two surfaces behave electrically as one.
If the fault occurs on the high voltage side of the circuit prior to the load then a lot of current will flow for a few fractions of a second. The circuit Over Current Protective Device (OCPD); in other words it's fuse or circuit breaker; will open the circuit at the source of supply thus clearing the fault. During the few moments it takes to open the circuit their can be a voltage drop that may be enough to cause harm to people.
It only takes about three tenths of an amp to cause a human heart to go into a disorganized rhythm called fibrillation. For the human being to get shocked however they have to be unlucky enough to be in contact with two different parts of the environment over which so much current is flowing that the voltage drop across the connections between the two surfaces is enough to cause the current to overcome the resistance of their skin and flow through their body. That's pretty long odds.
But if the human being will be in the environment in such a condition; such as immersed in water; that they would be vulnerable to such a shock you take additional precautions. The additional precaution that many are familiar with is called by different names in different places. The name is unimportant. It does not matter if it is called a Ground Fault Circuit Interrupter (GFCI) or a Residual Current Detector. What is does is detect the fact that the current in both legs of the circuit is no longer the same. Once detection occurs the device opens the circuit so quickly that a healthy adult is unlikely to come to harm. Mind you that doesn't mean that they will find the experience pleasant.
I hope you find this helpful.
On Fri, 04 Apr 2008 11:26:53 -0500 snipped-for-privacy@aol.com wrote: | On Fri, 4 Apr 2008 07:34:17 -0700 (PDT), VIDHYA | wrote: | |> We know that, in case if there is a faulty insulation,or sc, the |>current is bypassed to earth through the earth wire that has low |>resistance.Actually, what will happen to that current that flows |>through gnd?In that case,if we come in contact with that faulty |>appliance, wont that current flow through us as we form a closed path |>by standing in gnd?Is grounding a safety measure? | | Most of the time it doesn't do anything but chase worms and waste | energy but this is usually the cause of "stray voltage" problems when | it happens. Cows stop giving milk and people get shocked in places | where they shouldn't.
I've been thinking of having a big NEMA 6-60R installed in the garage and get a 15 kVA transformer for isolation and just see what happens to earthworms and moles and grass roots and such when I "roast dirt". Maybe it would be a bit safer to do that with a generator.
oops. Make that too "low to trip the breaker". Maybe I'm getting too old for this. ;-)
Chuck
| I hope you find this helpful.
Great info, Tom!
For a phase to ground fault, the fault loop path is through the earth conductor upstream across the multiple earth neutral (MEN), into the neutral conductor terminating at the star (wye) point of the transformer.
The MEN is found between the neutral and earth bar of a switchboard or distribution board or in most times at the transformer terminals where it links the neutral and earth terminal together.
The LV protection i.e fuse of circuit breaker should be designed to trip the prospective fault current in such a time so that the thermal properties of the electrical cable are not overcome.
For LV systems, the electrical earthng is generally solidly earthed. The design of the earthing system should be such that there is a low impedance path for the fault current to flow. In Australia, the requirement is 1 ohm for a combined HV/LV earthing system. The equivalent impedance of a human is about 1 k-Ohm. Therefore if a person was to touch an earthed applicance whilst between the 0-0.4s of fault before the circuit breaker will trip, the fault current flowing through the person is minimal. Additional protection such as shoes, and clothing increase the persons impedance further thus aiding in the amount of fault current through the person.
Hope this helps,
Matthew.
| For a phase to ground fault, the fault loop path is through the earth | conductor upstream across the multiple earth neutral (MEN), into the | neutral conductor terminating at the star (wye) point of the | transformer.
Is this for a fault on the LV or MV side?
| The MEN is found between the neutral and earth bar of a switchboard or | distribution board or in most times at the transformer terminals where | it links the neutral and earth terminal together.
What if the upstream MV distribution has a broken neutral somewhere?
| The LV protection i.e fuse of circuit breaker should be designed to | trip the prospective fault current in such a time so that the thermal | properties of the electrical cable are not overcome. | | For LV systems, the electrical earthng is generally solidly earthed. | The design of the earthing system should be such that there is a low | impedance path for the fault current to flow. In Australia, the | requirement is 1 ohm for a combined HV/LV earthing system. The | equivalent impedance of a human is about 1 k-Ohm. Therefore if a | person was to touch an earthed applicance whilst between the 0-0.4s of | fault before the circuit breaker will trip, the fault current flowing | through the person is minimal. Additional protection such as shoes, | and clothing increase the persons impedance further thus aiding in the | amount of fault current through the person.
You might get 1 ohm into earth nearby to the electrode. But at some distance from the electrode, 1 ohm is not going to be likely, even in Australia. A ground fault at some random earth location will likely have many ohms due to a lot of earth to go through. The result will be fault currents somewhere between "kill human" and "trip breaker" levels. This is why RCD/GFI breakers are useful since they sense the misdirected current, rather than the high current.
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