Question

With reference to a three phase shipboard electrical distribution system:

(a) Enumerate the advantages of an insulated neutral system,

(b) Enumerate the disadvantages of an insulated neutral system;

(d) Compare the use of an insulated neutral system as opposed to the use of an earthed neutral system with regard to the risk of electric shock from either system.

An insulated system is one that is electrically insulated from earth.

Advantages

If an earth fault occurs on the insulated pole of an ‘EARTHED DISTRIBUTION SYSTEM’ it would be equivalent to a ‘short circuit’ fault across the load via the ship’s hull.

The resulting large earth fault current would immediately ‘blow’ the fuse in the line conductor. The faulted electrical equipment would be immediately isolated from the supply and so rendered SAFE, but the loss of equipment could create a hazardous situation, especially if the equipment was classed ESSENTIAL, e.g. loss of steering gear. The large fault current could also cause arcing damage at the fault location.

An earth fault ‘A’ occurring on one line of an ‘INSULATED DISTRIBUTOIN SYSTEM’ will not cause any protective gear to operate and the system would continue to function normally. This is the important – equipment still operates. The single earth fault does not provide a complete circuit so no earth fault current will exist.

If an earth fault ‘B’ developed on another line, the two earth faults together would be equivalent to a short-circuit fault (via the ship’s hull) and the resulting large current would operate protection devices and cause disconnection of perhaps essential services creating a risk to the safety of the ship.

An insulated distribution system requires TWO earth faults on TWO different lines to cause an earth fault current.

An earthed distribution system requires only ONE earth fault on the LINE conductor to create an earth fault current.

An insulated system is, therefore, more effective than an earthed system in maintaining continuity of supply to equipment.

Hence its adoption for most marine electrical systems.

Disadvantages

Although fault current is very small / negligible over voltage are high

Voltage to earth is 1.73Vph [√3Vph]

Tracing the earth fault is more difficult although the selective tripping may trace the faulty circuit, the actual position of the fault may still be difficult to locate

Earthed neutral system. Neutral point connected to earth. Usually for HV system e.g. for 3.3Kv system. Main priority is immediate isolation of earth faulted equipment

Earth fault current is very high but over voltage due to earth fault are lower. The earthed system is chosen to limit overvoltage and give automatic earth fault location and disconnect

High voltage systems (3.3 kV and above) on board ship are normally ‘earthed’. Such systems are normally earthed via resistor connecting the generator neutrals to earth as shown below.

The ohmic value of each earthing resistor is usually chosen so as to limit the maximum earth fault current to not more than the generator full load current. Such a Neutral Earthing Resistor (NER) is often assembled with metallic plates in air but liquid (brine) resistors have also been used. The use of such an earthed system means that a single earth fault will cause that circuit to be disconnected by its protection device. Certain essential loads (e.g. steering gear) can be supplied via a transformer with its secondary unearthed to maintain security of supply in the event of a single-earth fault. Regulations insist that tankers have only insulated distribution systems. This is intended to reduce danger from earth fault currents circulating in the hull in hazardous zones which may cause an explosion of the flammable cargo.

An exception allowed by regulating bodies occurs where a tanker has a 3.3 kV earthed system. Such a system is permitted providing that the earthed system does not extend forward of the engine room bulkhead and into the hazardous zone area. Electrical supplied forward of the engine room bulkhead are usually 3-phase 440V insulated and obtained from a 3-phse 3.3 kV/440V transformer.

In a insulated neutral system, very small current will flow through an earth fault on one phase, because there is no easy path for it to come back to the electrical system

In an earthed neutral system, the result of not isolating the electrical system from the hull is that current flow from an earth fault on any phase has a path through the hull steel and earthed neutral back to the system.

The availability of path encourages higher fault current flow. Equipment with an earth fault, where the system is earthed must be disconnected immediately because high fault current may cause shock and fire.

To protect against danger of electric shock and fire that may result from earth fault the metal enclosure and other non current metal parts of electrical equipment must be earthed. The earthing conductor connects the metal enclosure to earth the ships hull to prevent it from attaining a dangerous voltage with respect to earth. Such earth bonding of equipment ensure that it always remains at zero volts

Notes