If a single earth fault occurs on the live line of an earthed ship distribution system it would be equivalent to a short-circuit fault across the generator through the ship’s hull.
The resulting large earth fault current would immediately cause the line protective device (fuse or circuit breaker) to trip out the faulty circuit.
The faulted electrical equipment would be immediately isolated from the supply and so rendered safe.
However, the loss of power supply could create a hazardous situation, especially if the equipment was classed essential, e. g. steering gear.
The large fault current could also cause arcing damage at the fault location.
In contrast, a single earth fault “A” occurring on one line of an insulated distribution system will not cause any protective trip to operate and the system would continue to function normally.
This is the important point: equipment continues to operate with a single earth fault as it does not provide a complete circuit so no earth fault current will flow.
If a second earth fault at “B” occurred on another line in the insulated system, 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 therefore requires two earth faults on two different lines to cause an earth fault current to flow.
In contrast, an earthed distribution system requires only one earth fault on the line conductor to create an earth fault current which will trip out the faulty circuit.
An insulated system is, therefore, more effective than an earthed system in maintaining continuity of supply to essential services. Hence its adoption for most marine electrical systems.
Double-poles witches with fuses in both lines are necessary in an insulated single-phase circuit.
High voltage systems (3.3 kV and above) on board ship are normally earthed.
Such systems are usually earthed via a resistor connecting the generator neutrals to earth.
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 usually assembled from metallic plates.
The use of such an earthed HV svstem means that a single earth fault will cause current to flow in the neutral connection wire. This is monitored by an earth fault (E/F) relay to create alarm and trip functions.
Regulations require that an earth fault monitor is fitted to the main switchboard to indicate the presence of an earth fault on each isolated section of a distribution system, e.g. on the 440 V and 220 V sections.
An earth fault monitor can be either a set of indicator lamps or an instrument (calibrated in k ohms) or M ohms) to show the system IR value to earth.
Earth indication lamps is a 3-phase a.c. system.
When the system is healthy (no earth faults) then the lamps glow with equal half brilliance. If an earth fault occurs on one line, the lamp connected to that line goes dim or extinguished.
The other lamps experience an increased voltage so will glow brighter than before. Earth indication lamps have been the most common method used for many years, being an inexpensive installation which is easy to understand.
Their major disadvantage is that they are not very sensitive and will fail to indicate the presence of a high impedance earth fault.
This has led to the development of instrument type earth fault indicators.