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
>>
>> 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?
>
>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.
| |
|> 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.
>>
>>>>
>>> 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?
>>
>>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
>>
>>
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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