Thursday, February 11, 2010
Earthing_system
Earthing_system
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In electricity supply systems, an earthing system defines the electrical potential of the conductors relative to that of the Earth's conductive surface. The choice of earthing system has implications for the safety and electromagnetic compatibility of the power supply. Note that regulations for earthing (grounding) systems vary considerably among different countries.
A protective earth (PE) connection ensures that all exposed conductive surfaces are at the same electrical potential as the surface of the Earth, to avoid the risk of electrical shock if a person touches a device in which an insulation fault has occurred. It ensures that in the case of an insulation fault (a "short circuit"), a very high current flows, which will trigger an overcurrent protection device (fuse, circuit breaker) that disconnects the power supply.
A functional earth connection serves a purpose other than providing protection against electrical shock. In contrast to a protective earth connection, a functional earth connection may carry a current during the normal operation of a device. Functional earth connections may be required by devices such as surge suppression and electromagnetic interference filters, some types of antennas and various measurement instruments. Generally the protective earth is also used as a functional earth, though this requires care in some situations.
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[edit] IEC terminology
International standard IEC 60364 distinguishes three families of earthing arrangements, using the two-letter codes TN, TT, and IT.
The first letter indicates the connection between earth and the power-supply equipment (generator or transformer):
| T | : | direct connection of a point with earth (Latin: terra); |
| I | : | no point is connected with earth (isolation), except perhaps via a high impedance. |
The second letter indicates the connection between earth and the electrical device being supplied:
| T | : | direct connection of a point with earth |
| N | : | direct connection to neutral at the origin of installation, which is connected to the earth |
[edit] TN networks
In a TN earthing system, one of the points in the generator or transformer is connected with earth, usually the star point in a three-phase system. The body of the electrical device is connected with earth via this earth connection at the transformer.
The conductor that connects the exposed metallic parts of the consumer is called protective earth (PE). The conductor that connects to the star point in a three-phase system, or that carries the return current in a single-phase system, is called neutral (N). Three variants of TN systems are distinguished:
| TN?S | : | PE and N are separate conductors that are connected together only near the power source. |
| TN?C | : | A combined PEN conductor fulfills the functions of both a PE and an N conductor. The combined PEN conductor typically occurs between the substation and the entry point into the building, whereas within the building separate PE and N conductors are used. In the UK, this system is also known as protective multiple earthing (PME), because of the practice of connecting the combined neutral-and-earth conductor to real earth at many locations, to reduce the risk of broken neutrals - with a similar system in Australia being designated as multiple earthed neutral (MEN). |
| TN?C?S | : | Part of the system uses a combined PEN conductor, which is at some point split up into separate PE and N lines. |
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| TN-S: separate protective earth (PE) and neutral (N) conductors from transformer to consuming device, which are not connected together at any point after the building distribution point. | TN-C: combined PE and N conductor all the way from the transformer to the consuming device. | TN-C-S earthing system: combined PEN conductor from transformer to building distribution point, but separate PE and N conductors in fixed indoor wiring and flexible power cords. |
It is possible to have both TN-S and TN-C-S supplies from the same transformer. For example, the sheaths on some underground cables corrode and stop providing good earth connections, and so homes where "bad earths" are found get converted to TN-C-S.
[edit] TT network
In a TT earthing system, the protective earth connection of the consumer is provided by a local connection to earth, independent of any earth connection at the generator.
The big advantage of the TT earthing system is the fact that its clear of high and low frequency noises that come through the neutral wire from various electrical equipment connected to it. This is why TT has always been preferable for special applications like telecommunication sites that benefit from the interference-free earthing. Also, TT does not have the risk of a broken neutral.
In pre-RCD era, the TT earthing system was unattractive for general use because of its worse capability of accepting high currents in case of a live-to-PE short circuit (in comparison with TN systems). But as residual current devices mitigate this disadvantage, the TT earthing system becomes attractive for premises where all AC power circuits are RCD-protected.
[edit] IT network
In an IT network, the distribution system has no connection to earth at all, or it has only a high impedance connection. In such systems, an insulation monitoring device is used to monitor the impedance
[edit] Other terminologies
While the national wiring regulations for buildings of many countries follow the IEC 60364 terminology.In North America (United States and Canada), the term "equipment grounding conductor" refers to equipment grounds and ground wires on branch circuits, and "grounding electrode conductor" is used for conductors bonding an earth ground rod (or similar) to a service panel. "Grounded conductor" is the system "neutral".
[edit] Properties
[edit] Cost
- TN networks save the cost of a low-impedance earth connection at the site of each consumer. Such a connection (a buried metal structure) is required to provide protective earth in IT and TT systems.
- TN-C networks save the cost of an additional conductor needed for separate N and PE connections. However, to mitigate the risk of broken neutrals, special cable types and lots of connections to earth are needed.
- TT networks require proper RCD protection.
[edit] Fault path impedance
If the fault path between accidentally energized objects and the supply connection has low impedance, the fault current will be so large that the circuit overcurrent protection device (fuse or circuit breaker) will open to clear the ground fault. Where the earthing system does not provide a low-impedance metallic conductor between equipment enclosures and supply return (such as in a TT separately-earthed system), fault currents are smaller, and will not necessarily operate the overcurrent protection device. In such case a residual current detector is installed to detect the current leaking to ground and interrupt the circuit.
[edit] Safety
- In TN, an insulation fault is very likely to lead to a high short-circuit current that will trigger an overcurrent circuit-breaker or fuse and disconnect the L conductors. With TT systems, the earth fault loop impedance can be too high to do this, or too high to do it quickly, so an RCD (or formerly ELCB) is usually employed. The provision of a Residual-current device (RCD) or ELCB to ensure safe disconnection makes these installations EEBAD (Earthed Equipotential Bonding and Automatic Disconnection).
- Many 1950s and earlier earlier TT installations in the UK may lack this important safety feature. Non-EEBAD installations are capable of the whole installation CPC (Circuit Protective Conductor) remaining live for extended periods under fault conditions, which is a real danger.
- In TN-S and TT systems (and in TN-C-S beyond the point of the split), a residual-current device can be used as an additional protection. In the absence of any insulation fault in the consumer device, the equation IL1+IL2+IL3+IN = 0 holds, and an RCD can disconnect the supply as soon as this sum reaches a threshold (typically 10-500 mA). An insulation fault between either L or N and PE will trigger an RCD with high probability.
- In IT and TN-C networks, residual current devices are far less likely to detect an insulation fault. In a TN-C system, they would also be very vulnerable to unwanted triggering from contact between earth conductors of circuits on different RCDs or with real ground, thus making their use impracticable. Also, RCDs usually isolate the neutral core. Since it is unsafe to do this in a TN-C system, RCDs on TN-C should be wired to only interrupt the live conductor.
- In single-ended single-phase systems where the Earth and neutral are combined (TN-C, and the part of TN-C-S systems which uses a combined neutral and earth core), if there is a contact problem in the PEN conductor, then all parts of the earthing system beyond the break will rise to the potential of the L conductor. In an unbalanced multi-phase system, the potential of the earthing system will move towards that of the most loaded live conductor. Therefore, TN-C connections must not go across plug/socket connections or flexible cables, where there is a higher probability of contact problems than with fixed wiring. There is also a risk if a cable is damaged, which can be mitigated by the use of concentric cable construction and/or multiple earth electrodes. Due to the (small) risks of the lost neutral, use of TN-C-S supplies is banned for caravans and boats in the UK, and it is often recommended to make outdoor wiring TT with a separate earth electrode.
- In IT systems, a single insulation fault is unlikely to cause dangerous currents to flow through a human body in contact with earth, because no low-impedance circuit exists for such a current to flow. However, a first insulation fault can effectively turn an IT system into a TN system, and then a second insulation fault can lead to dangerous body currents. Worse, in a multi-phase system, if one of the live conductors made contact with earth, it would cause the other phase cores to rise to the phase-phase voltage relative to earth rather than the phase-neutral voltage. IT systems also experience larger transient overvoltages than other systems.
- In TN-C and TN-C-S systems, any connection between the combined neutral-and-earth core and the body of the earth could end up carrying significant current under normal conditions, and could carry even more under a broken neutral situation. Therefore, main equipotential bonding conductors must be sized with this in mind; use of TN-C-S is inadvisable in situations such as petrol stations, where there is a combination of lots of buried metalwork and explosive gases.
[edit] Electromagnetic compatibility
- In TN-S and TT systems, the consumer has a low-noise connection to earth, which does not suffer from the voltage that appears on the N conductor as a result of the return currents and the impedance of that conductor. This is of particular importance with some types of telecommunication and measurement equipment.
- In TT systems, each consumer has its own connection to earth, and will not notice any currents that may be caused by other consumers on a shared PE line.
[edit] Regulations
- In the United States National Electrical Code and Canadian Electrical Code the feed from the distribution transformer uses a combined neutral and grounding conductor, but within the structure separate neutral and protective earth conductors are used (TN-C-S). The neutral must be connected to the earth (ground) conductor only on the supply side of the customer's disconnecting switch. Additional connections of neutral to ground within the customer's wiring are prohibited.
- In Argentina, France (TT) and Australia (TN-C-S), the customers must provide their own ground connections.
- Japan is governed by PSE law.
[edit] Application examples
- Most modern homes in Europe have a TN-C-S earthing system. The combined neutral and earth occurs between the nearest transformer substation and the service cut out (the fuse before the meter). After this, separate earth and neutral cores are used in all the internal wiring.
- Older urban and suburban homes in the UK tend to have TN-S supplies, with the earth connection delivered through the lead sheath of the underground lead-and-paper cable.
- Some older homes, especially those built before the invention of residual-current circuit breakers and wired home area networks, use an in-house TN-C arrangement. This is no longer recommended practice.
- Laboratory rooms, medical facilities, construction sites, repair workshops, mobile electrical installations, and other environments that are supplied via engine-generators where there is an increased risk of insulation faults, often use an IT earthing arrangement supplied from isolation transformers. To mitigate the two-fault issues with IT systems, the isolation transformers should supply only a small number of loads each and/or should be protected with an insulation monitoring device (generally used only by medical, railway or military IT systems, because of cost).
- In remote areas, where the cost of an additional PE conductor outweighs the cost of a local earth connection, TT networks are commonly used in some countries, especially in older properties. TT supplies to individual properties are also seen in mostly TN-C-S systems where an individual property is considered unsuitable for TN-C-S supply.
- In Australia, the TN-C-S system is in use; however, the wiring rules currently state that, in addition, each customer must provide a separate connection to earth via both a water pipe bond (if metallic water pipes enter the consumer's premises) and a dedicated earth electrode. In older installations, it is not uncommon to find only the water pipe bond, and it is allowed to remain as such, but the additional earth electrode must be installed if any upgrade work is done. The protective earth and neutral conductors are combined until the consumer's neutral link (located on the customer's side of the electricity meter's neutral connection) - beyond this point, the protective earth and neutral conductors are separate.
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For More Refer: http://marinenotes.blogspot.com/
For More Electrical Notes:: http://marineelectricalnotes.blogspot.com/
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An Emergency Power System
An Emergency Power System
For More Refer: http://marinenotes.blogspot.com/
For More Electrical Notes:: http://marineelectricalnotes.blogspot.com/
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An emergency power system tackles all emergency conditions on ship by providing an uninterrupted supply of power.To ensure safety on ship utmost priority should be given to back up power sources and generators. Lets learn what does this emergency power system comprises of and stands for?
The Story Continues
You have read about a few basics of marine electrical systems including generators, motors and switchboards. As you now find yourself enlightened with the brief gist of the crucial elements of the ship’s electrical system, you decide to take a quick coffee break and munch on a bar of snickers, simultaneously trying to recollect and imbibe whatever you have learnt so far:
The generator is the heart of the ship. To control and use the generator we use motors, motor controls and main switch boards.
Now, these elements constitute the basic requirement of the electrical system of any kind of ship. But that doesn’t mean that the ship can run solely on them. There are few other elements and parameters that need to be taken into consideration for the continuous supply of electrical power and also to ensure safety and redundancy. Let’s learn a bit more about them.
Emergency Power System
The main intention of an emergency power system is to furnish immediate, automatic electric power to a limited number of selected vital circuits. The emergency power system includes one or more diesel-driven emergency generators (which we will learn a bit more on later) as well as an emergency switch board and a distributed system. Both emergency generator and emergency switch board are kept above the waterline to minimize danger from flooding. Emergency services also include the navigation, safety and emergency lights.
All these emergency services should be supplied from the emergency switch board. An emergency switch board has distributed panels for all the emergency services onboard. The emergency switch board as we discussed earlier is always located above the bulkhead deck. This is to ensure that at the time of any emergency such as fire or flooding, the switchboard is not affected and is easily accessible and that any such emergency does not lead to loss of lighting in escape routes and other important navigation control areas.
Thus as of now, the main components of an emergency power system are a generator and switch board.
Safety: An integral part.
The ship’s emergency electrical system and the safety of the ship and the crew are interwoven. You can say that the electrical emergency services, also known as the “emergency power system”, is the back-bone of safety on the ship.
The arrangement and the installation of the emergency services should be so smartly done that no matter what happens to the ship the emergency system should provide energy and support lives on ship until the situation can be handled.
Firstly, let us find out what does it mean by emergency power system or emergency services?
Emergency power system
The main intention of emergency power system is to furnish immediate, automatic source of electric power to limited number of selected vital circuits. The emergency power system includes one or more diesel driven emergency generator, which we will learn a bit more on later, emergency switch board and a distributed system. Both emergency generator and emergency switch board are kept above the waterline, to minimize danger from flooding. Emergency services also include the navigation, safety and emergency lights.
All these emergency services should be supplied from the emergency switch board. Emergency switch board has distributed panels for all the emergency services onboard. Emergency switch board as we discussed earlier is always located above the bulkhead deck. This is to ensure that at the time of any emergency such as fire or flooding, the switchboard is not affected and is easily accessible and that any such emergency does not lead to loss of lightening in escape routes and other important navigation control areas.
Thus the main components of an emergency power system are : Emergency generator and emergency lights.
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Emergency generator
“Black out” is a condition considered similar to a “dead ship” condition. It is a condition under which the main propulsion plant, boilers and auxiliaries are not in operation and also there is no stored energy available to restore them. This is the time for the emergency generator to take over. The emergency generator has its own prime mover and fuel supply.
As the ship’s emergency generator takes over the battle functions as soon as the main power fails, utmost care should be taken at the time of installation. It’s of extreme importance that load on the generator is at the top of the priority list while considering the factors affecting. This is because when there is a sudden transfer of load from the main generators to the emergency generator there is a high chance of the later getting overloaded. Thus the emergency generators should be of the capacity same as the main generators capacity. To equalize the capacity requirement of the emergency generator to the main generator generally more then one emergency generator should be made available.
Emergency lights
These lighting fixtures must provide an uninterrupted source of lighting in the event of power outage. The source of power for these lighting shall consist of accumulator batteries which are continuously charged from the main switch board. All these lights should operate at least for three hours.
For More Refer: http://marinenotes.blogspot.com/
For More Electrical Notes:: http://marineelectricalnotes.blogspot.com/
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