The earthing is an electric communication done intentionally between an electric device or devices’ net from one side and the earth from other side. So the earthing is required for providing the safety of the electrical system and the workers in the institution. This is well known among many people but the way of earthing is not clear for great percentage of people. We can describe the earthing as deliverance arches or the landing parachute because it is very valuable when needed.
The importance and characteristic of the earthing:
- First: earthing is protecting people from the electrical shock danger which is resulted from the shortening of isolation.
- Second: it is providing from the electrical deflating danger.
- Third: it is protecting the devices from the immediate and big changes in the voltage surges.
- Fourth: it is providing suitable operation for the electrical devices and systems.
However, the person may feel that there isn’t any effect of the earthing on the electrical systems during the normal operation, and this made him think wrongful that it is possible to cut the earthing without any effects. As a result it seems (apparently) that the earthing subject is not important, but infact the earthing activity can not be known unless we made periodic tests from time to time.
We can consider that the earth is a huge block without electrical effort; it means that its effort is zero. While the electrical system’s parts have some effort if we compare them with the earth effort.
The live conductors of the electrical system parts are bearing always electrical effort during its normal operation, while the other metal parts like the shells and containers of the electrical devices do not bear any effort during its normal operation but they can be with an effort when there is an electrical disrepair and this is very danger for the workers and the institutions if there aren’t any preventive procedures like connecting these parts to the earth net.
We can have suitable earth bound for houses like using one metal bar or more and laid it in the clod in order to enable the contiguity with the earth.
There are some bars as sections that can be connected with each other in order to have bar with the required length and then it will implant in the ground through knocking in order to arrive to the earth layers with low resistance so we will have low earthing resistance. If we want to have less resistance we always use many bars and connected them with each other on parallel by using earthing connecters in order to make earthing net as a big tree in which all the electrical devices and the metal shells are connected to this net. The earthing connectors are representing the sprigs of the leaves and the main earthing cable is representing the leg of the tree, and the earthing poles are representing the roots of the tree.
The best earthing should have a resistance between 1 and 5 ohm, but having these values in a high resistance clod is not very simple especially if we use conceivable number of earthing poles, so we should calculate the highest possible value according the whole resistance of the earthing circuit which allow enough disrepair current to pass in order to operate the protection devices (circuit breaker or passing around) so that we can isolate the electrical disrepair circuit.
Definitions:
- Earth net: it is a group of connectors that we can find good electrical connection through them between the parts and uncovered metal shells and the earth block.
- Earth pit: it is a group of electrodes which are buried in ground in order to have more contiguity with less resistance so the connection instrumentality can be formed between the other earth net parts and the earth block.
- Main earthing lead: it is the main lead which connect group of electrical devices to the earthing.
- Bonding lead: it is the lead which connects the shell of the electrical device to the main earth lead.
- Functional earthing: it is the earthing of neutral point of the power transformers and the earthing of common points of the current transformers and it is used for operation reasons.
- Static earthing: it is used for guaranteeing have a leakage in the static charges which are generated in the containers and tanks as a result of the crashing of the hydro carbonic liquids in the walls of these tanks during the loading and deflating. The good earthing is leading to a leakage of the static charges to the earth and not having dangerous effort on these containers and tanks.
- Lightening Protection Earthing: it is used for have leakage in the very high currents which are resulted from the electrical deflating of the thunderbolts so that it will protect the institutions from the fire and destruction dangers which may happened if there was no protection from the thunderbolts.
General aspects of designing the earth net:
- Lessen the effort between the uncovered neighbored metal parts and also between these parts and the earth from other side. This will be done by Equal Potential Bonding between the neighbored metal parts from one side and connected them with the earth net which has low electrical resistance from other side. This will lessen the contiguity effort and Touch & Step Voltage so it will protect people from the dangerous shocks.
- Lessen the relucting of the earth pole, and this will be done by using connectors of the earthing net with suitable sizes to make their resistance low. In addition to that we must chose the type of the earthing poles that are buried in the clod and prepare them and their buried depth and the place of the burying in order to provide less suitable resistance to the earth block.
Lessen the relucting of the disrepair circuit lead to the moving of the high currents through it when there is a contiguity of the electrical circuit with the earth. And this is what we want to get because it make the protection devices feel the electricity then it will cut the voltage from the disrepair part and isolate it from the good parts of the electrical circuit during a very short time. Enough protection will be provided for the establishments from the disrepairs and fires and also protecting people from the electrical shock. The time of the cutting is between part of one second and few seconds and it is measured invertibley with the disrepair earthing voltage and contiguity effort.
The main parts of the relucting of the disrepair earthing circuit are as follows:
- In an electrical system from TT kind, the relucting of the disrepair earthing circuit has low connecting circuit resistance and earth net connectors and has the earth poles resistance on source side (the neutral point resistance of the transformer to the earth) and the consumer side. The resistance should be low and between (part of ohm till some ohms) if the earthing poles were in good situation. Finally the resistance of the disrepair place will depend on the type and nature of the disrepair.
In this kind of the systems the earthing poles resistance will form the great part of the whole resistance of the earthing disrepair, so it will play a great role in the activity of the earthing net as a whole. This resistance should be checked and repaired periodically.
- In an electrical feeding system from TN kind the relucting of the disrepair earthing circuit is composed of circuit connectors and earth net connectors in addition to the disrepair area without depending on the earthing poles resistance. So the electrical protection devices in the electrical circuits that are connected with these systems will be more sensitive and activity in isolating the earthing disrepair circuits than the other systems from TT kind.
|
