Demystifying The Isolated and Earthed Neutral System

The function of an electrical system is to safely convey the power from the source to the various consumer equipment connected to it. A ship's electrical system can take any of the two forms:

• Neutral Insulated system, or

• Neutral Earthed system.

Ship Electrical System

The electrical system consists of a source and different consumer equipment. The source may either be a single phase or three phase. In a single phase system, there happens to be two leads emanating from source(Generator/Alternator), The line and The neutral. In a three phase system, 3 or 4 leads may emanate from the source depending upon the type of system used.



For single phase system :

In neutral insulated system, the neutral of the source is insulated. In other words, the neutral is not connected to the earth.
In neutral earthed system, the neutral of the source is connected to the earth.

Insulated and Earthed Systems

For three phase system :

In neutral insulated system, the star point of the source is not connected to earth. Hence there happens to be only 3 leads (3 phase wires R,Y,B) emanating from the source.
In neutral earthed system, the star point is earthed to ground. So 4 wires emanating from source (3 phase and 1 neutral).

In an electrical system, 3 different types of fault may occur:

• Open circuit fault (A) : due to break in the conductor so that current cannot flow.
• Short circuit fault (C) : due to break in insulation so that two conductor touches each other giving a short path to current and allowing a very large current to bypass the load.
• Earth Fault (B) : This is also due to break in insulation, but the conductor touches the metal enclosure or the hull indirectly.

Faults in Electric Circuit

The discussion of the two types of electrical system(Insulated or earthed neutral) depends on a large extent to the earth fault occurring in the circuit.

If a single earth fault occurs in the line of an earthed neutral system, then it would be equivalent to a short circuit fault. The reason being, the earthed neutral creates a closed path for the earth current to flow through the hull of the vessel.Since this path has a minimal resistance, the earth current can increase to a very large extent.

Earth Fault in Neutral Earthed System

Onboard ship, if the earth current increases beyond the current rating of the generator, the entire system may collapse causing irreparable damage. To limit this earth current, a Neutral Earthing Resistor is connected to the earthed neutral of the source. This resistor is of sufficient ohmic value to limit the earth current within rating of the generator. However, the magnitude of earth current is sufficiently large to operate the tripping mechanism of the faulted equipment immediately isolating it from supply and rendering it safe.

Neutral Earthing Resistor (NER)

In a neutral insulated system, a single earth fault occurring in a system would not cause any earth current to flow in the ship's hull.This is because a single earth fault current does not provide a complete circuit for earth current to flow. So no protective trip will operate and system will continue to function normally.

Single Earth Fault in Neutral Insulated System

However, if a second earth fault occurs on another line in the insulated system, the two earth faults together will be equivalent to a short circuit and the resulting earth fault current will operate protection devices and cause disconnection of services.

Two Earth Faults in Neutral Insulated System

Onboard a ship, the priority requirement is to maintain continuity of the electrical supply to essential equipment in event of a single earth fault occurring. The priority requirement ashore is the immediate isolation of earth-faulted equipment which is automatically achieved by an earthed system.

To understand this better, a case study is used;  If the earth fault occurs in an essential system like that of steering gear, then in case of:
Insulated neutral system, no earth fault current will flow and the steering gear will continue to operate until there is a second earth fault in any equipment present onboard. Hence, even with a single earth fault, the essential service will continue to operate.
If the system would have been an earthed neutral, then a single earth fault would have caused heavy earth current to flow and operate the tripping mechanism causing shutting down of steering gear. This will seriously compromise with the safety of navigation.

Keeping this in mind, it is well understood that onboard a ship, a neutral insulated system is to be used. And, in shore installations neutral earthed system is used.

Shipboard main LV systems at 440 V are normally provided with neutral insulated system. On the other hand HV system (1000 V to 3.3 KV) are usually provided with neutral earthed system via a neutral earthing resistor.
In a HV system, certain essential loads can be supplied by a transformer with it's secondary insulated to ensure no earth fault current flows in the equipment. This maintains the continuity of service.

SOLAS regulations require that the hazardous areas of tanker viz. cargo area, pump room should have a neutral insulated system to prevent any stray earth current from flowing in the hull and causing explosion hazard.
However an exception is included in case if tanker has a 3.3 KV system, the earthed system is permitted provided that the earthed system does not extend forward of engine room bulkhead and into the hazardous area.

Both the Insulated neutral and Earthed neutral system have got their own advantages and disadvantages. Where it is easier in the earthed neutral system to detect any earth faults in system, it is easier in isolated neutral system to maintain the continuity of service.

Do Share this article, If found useful.