Isolated and Earthed neutral systems on ships | Neutral Earthing Resistor

Insulated neutral system or isolated neutral system (INS)

Insulated system

An insulated system is the one that is totally electrically insulated from earth (ship’s hull)

Reason for using an insulated system

The priority requirement on board ship is to maintain continuity of the electrical supply to equipment in the event of a single earth fault occurring.  On an insulated system, one earth fault does not interrupt the supply but raises a warning on the earth's detection system.  This allows the operator to locate and clear the fault.

Advantages of insulated neutral system

Avoids the risk of loss of essential services like steering gear

One earth fault doesn’t interrupt the supply but an earth fault detection system will give warning

Earthed system

An earthed system has one pole or the neutral connected to earth

On board ships, the main system is normally insulated for the bus bar voltage of 440 V

When the voltage exceeds 1000 V, then the neutral of the alternator has to be earthed.

Neutral earthing

As mentioned earlier, for the systems having more than 1000 V, the neutral will be earthed.  To protect the system against the high fault current, normally the neutral is earthed either through a resistor or a transformer.  In the case of an earth fault, it is preferable that the fault current should not exceed the full load current of the alternator.

What is Earth fault?

An earth fault is due to a break in the insulation, allowing the conductor to touch the hull or an earthed metal enclosure.

What is an Open circuit fault?

An open circuit fault is due to a break in the conductor so that the current cannot flow.

What is the Short circuit fault?

It is due to a double break in the insulation, allowing both conductors to be connected so that very large current passes or short circuit occurs.  The fault current depends on the overall impedance in the circuit at the time of the fault.

What are the Causes of short circuit?

A short circuit may occur because of the breakdown of insulation due to overheating or ingress of moisture/water.

Terminal connections become loose due to vibration or any other reasons, thereby two cables come into contact.

Cables exposed to fire, mechanical damage, cuts, etc. may lead to short circuits.

Earthing of electrical equipment

In order to protect against the dangers of electric shock and possible fire hazard, the metal enclosures, and other non-current-carrying metal parts of the equipment must be earthed.  This enables the potential at the time of earth fault to become zero and also provides the least resistance path for the current to flow to the earth.  (Ship’s hull).

CORE BALANCE CURRENT TRANSFORMER

Significance of earth faults

If an earth fault occurs (assuming an earthed system), it would be equivalent to a short circuit fault via the ship’s hull.  The resulting large current would immediately blow the fuse and supply to the equipment is cut off.  If this happens to essential equipment like Steering gear, then the ship will be having loss of control.  The large current could also cause arcing damage at the fault location.

What are the Causes of earth faults?

• Dampness
• Mechanical damage
• Contamination due to dirt
• Temperature rise
• Ageing

Prevention of earth faults

• Appropriate types of the enclosure should be used to prevent the ingress of moisture and dust
• Make sure protection against the mechanical damage
• Exact cable glands and seals are to be used on cable ends
• Insulation tests are to be performed on a regular basis
• Maintaining the equipment strictly as per PMS

Effect of a single earth fault

A single earth fault occurring in an insulated distribution system, will not cause any protective gear to operate and the system will continue to function normally. The single earth fault does not provide a complete circuit so no earth fault current will exist.

Effect of two earth faults

If an earth fault occurs at B on another line, the two earth faults would be equivalent to a short circuit via ship’s hull and cause protective gears to operate

An insulated system is, therefore, more effective than an earthed distribution the system, because the insulated system requires two earth faults on two different lines to cause the tripping of the equipment.

Earth fault indicators

Regulations require that earth fault indicators are fitted to the MSB to indicate the presence of an earth fault on each isolated section of a distribution system.  Earth fault indicators can be a set of lamps or an ohm meter calibrated in k Ω to show the system insulation resistance value to earth.

Earth fault indicator using lamps

If the system is healthy (no earth faults) then the lamps will glow with equal brilliance.  If an earth fault occurs on one line, the lamp connected in that line is dim or extinguished and the other lamps glow brighter.

The disadvantage of this system is that it is not very sensitive to indicate the presence of high resistance/impedance faults.

Earth fault indicator using ohm meter

This system can incorporate a switch which will give off an alarm when the insulation resistance falls to the set value.  A small DC voltage is injected into the distribution system.  Resulting current indicates the insulation resistance.  The maximum earth fault monitoring current is 250 μ A.

An alternative arrangement for k Ω meter

Common locations of earth faults in the ship

• Lamp fittings on the open deck.
• In the laundry, washing machines drenched with water
• In the galley, the ovens, hot plates, etc.
• Dripping of water over electrical machinery.
• Condensed moisture in the motor terminal box.
• Overheated motors leading to the melting of varnish on the windings.
• Dirty electrical apparatus with surface tracking (leakage current)
• Terminal lock nuts becoming loose due to vibration etc., leading to cable touching the body.
• Due to aging, the insulation may crack due to brittleness, leading to possible earth faults.

Treatment of earth faults

• Earth faults should be eliminated when located.
• Damaged conductor insulation has to be repaired or renewed.
• Dampness or moisture has to be removed by gentle and gradual heating using lamps.
• Machinery should be kept clean of dirt and dust.

An insulation meter with 3 modes

• Monitoring
• Fault finding.
• Test.

What is the difference between monitoring and fault finding?

Application of three modes of an insulation meter:

Monitoring: to check the healthiness of the insulation of motors, Cables, etc. by measuring the IR values.

Fault Finding: insulation meter/megger can also be used to check the continuity of cables, motor windings, etc. To find the O/C fault.

Test: to check the insulation meter is ok or not. Is there 1000V or 500V is available or not. Sometime battery/cell may be discharged of digital Meter.

What is a Neutral Earthing Resistor (NER)?

The neutral earthing resistors are commonly used to handling the fault currents. NER is also called Neutral Grounding Resistors. NER is used in the AC distribution system to limit the transient overvoltages that flow through the neutral point of the alternator to a safe level during a fault condition.

Generally, NER is connected between the neutral of the generator and the ground. NERs limit the fault currents to a value that keeps away from damage to equipment, so far permit the adequate flow of fault current to activate protection devices.

NERs must withstand a huge amount of energy for the duration of the fault condition as per IEEE32 standards. Therefore the selection of an NER is highly important to ensure the equipment and personnel safety and continuity of power supply.

Neutral earthing resistor made of?

NER is made from stainless steel. Because

• Less corrosion
• High-temperature performance
• Economic reasons

Specifications of Neutral Earthing Resistor

Temperature rise: 

the utmost short time temperature rise for the resistive component is 760°C, as per IEEE32

Rated Voltage: 

line- to neutral voltage | unit Voltage divided by root3

Rated Current: 

The initial current that will flow through the NER when it is cold. Generally, Full load current value is the same as Rated current.

Time Rating: 

It is the duration of time for which NER must be tolerate the rated voltage.

Short Time Rating: 

Generally, it is 10 or 30 or 60 sec. depending on the design parameters of the protection system. IS-3043 recommends 30 sec. rating.

Continuous Rating: 

It is normally 10 % of full load current for a healthy system NER to be designed for the continuous rating of 5 % to 10 % of full load current.