Insulated and earthed neutral system both have been explained in this article in a detailed manner. We will also discuss the merits and demerits of individual systems.
For critical equipments, we often use the insulated earth neutral system on ship. But earthed system is also important. And we will learn as how earthed system works.
Earthing a system is to protect equipments, and personals from the electric shocks. If equipment is having insulation break at two places, then there are chances of a short circuit. It is same like joining two live wires together. This generally leads to flow of large current across the hull and equipment. Insulated and earthed neutral systems are explained below.
Insulated Earth Neutral System
As name suggest, in such earth system it is completely insulated from the ship Hull.
Two earth faults in two different lines are needed in insulated earth neutral system to cause a earth fault current to flow. So basically if single earth fault occurs, it will not cut or operate any protection equipment and hence parent machine will keep on working. On ship such system is preferred as we need equipment to keep working if single earth fault occurs like steering gear, or other critical machinery. So in order to make the electrical continuity of the equipments, insulated earth neutral system is much better.
Note: An electric steering gear motor can be made earthed by using a transformer in the line. But its secondary will not be connected to earth. So in case of Earth fault, it will not cut off the motor.
Earth Neutral System
In such kind be of system single earth fault will lead to a short circuit to take place and will lead to flow of large current across the hull and equipment.
This will lead to blow of fuse as shown in the figure below. And we can see the arrangement is much different from insulated system we read before.
Earth neutral system will cut off the equipment immediately as soon as the fault is detected. And the equipment is safe with such kind of action.
High voltage systems like generator are generally earthed. Such high voltage equipments are earthed by using a resister.
Let’s take an example of a generator whose neutral is earthed without using resistor in between. So if any earth fault occurs, the current will flow equal to the full load current of the generator. If this happens that it will damage the machinery.
So we connect a resistor in between the neutral and Earth, it is also called as neutral earth resistor -NER. Ohmic value of such resistor is well below the maximum load current that a generator can take. So in the event of the earth fault, this resister will limit the current and protect the machinery.
For disconnection of three phases motor in case of Earth fault is by using a current transformer. When no earth fault is there, current transformer will read zero voltage across the phases. But as soon as earth fault occurs it will detect current. And it will be detected by protective equipment and will cut of the motor.
Advantages of isolated earth neutral over earth neutral
1) It maintains the continuity of the electrical equipments. Hence critical equipments are not cut off in case of Earth fault.
2) Single earth fault does not cause a short circuit.
Advantages of Earth neutral over isolated earth neutral
1) Cuts off the electrical equipment as soon as earth fault occurred. This saves the instrument.
2) Only one earth fault is enough to cause the earth fault and hence the short circuit.