An insulated system is one that is completely electrically isolated from the earth or in our case the hull of the vessel.
The supply neutral point is connected to earth in an earthed system and main low voltage (LV) (440 V AC) systems are generally shielded from the ship’s hull. Ashore, similar systems are earthed to the ground. To limit earth fault current, high voltage systems (>1000 V) are normally earthed to the ship’s hull through a neutral earthing resistor or a high impedance transformer.
Example of earth fault indication diagram
The key requirement on board ship is to keep the electrical supply to critical equipment running in the case of a single earth fault, while ashore immediate isolation of the faulty circuit is a priority and where this can be achieved by an automatically earthed system.
An earth fault in the system is caused by a conductor insulation failure (breakage) , allowing the live conductor to get in contact with the hull or an earthed metallic enclosure.
In order to safeguard against the danger of electric shock or fire that may result form an earth fault, the electrical systems must be earthed, through a conductor which connects the metal enclosure to the ship’s hull. This will prevent to attain a dangerous voltage with respect to earth, as such earth connection of equipment ensures that it always remain 0 V.
Example of a grounded metallic enclosure
Onboard vessels, if a single earth fault occurs on the live line of an earthed distribution system, this would be an equivalent to a short circuit fault across the generator through vessel’s hull. The large earth fault current that would result from this, would immediately cause the protective device to trip the faulty circuit and that particular electrical circuit would be immediately isolated from the supply . However, this would lead to loss of power that could create a hazardous situation, especially on the essential equipment (steering gear, emergency fire pump, emergency compressor etc.) and more than that, the large fault current could cause arcing damage at the fault location.
Therefore, if a single earth fault occurs on the line of an insulated system will not cause any protective equipment to operate and the system will continue to normally operate with a single earth fault as it doesn’t provide a complete circuit so no earth fault current will flow.
If the second earth fault occurs on another line in the insulated system, the two earth fault together would be similar to a short circuit fault, via the ship’s hull, and the resulting large current will operate the protection devices and cause disconnection of the system involved, creating a risk for the ship’s safety if this is an essential system.
Therefore, an insulated system requires two earth faults on two different lines to cause an earth fault current. As opposite, an earthed system requires only one earth fault on the line conductor to create an earth fault current which will trip the faulty circuit.
For that reason, onboard vessels, an insulated system is more effective than an earthed system in maintaining the essential services supply continuity.
The High Voltage (HV) systems onboard vessels are normally earthed via a resistor connecting the generator neutrals to earth.
Each earthing resistor’s ohmic value is typically chosen to limit the maximum earth fault current to no more than the generator’s full load current. Such a Neutral Earthing Resistor (NER) is usually assembled from metallic plates.
When using an earthed HV system, a single earth fault causes current to flow in the neutral connection wire, which is monitored by an earth fault relay, which can activate an alarm and/or trip functions.
Certain critical loads (steering gear) can be supplied by a transformer with the secondary unearthed to ensure supply security in the case of a single earth failure.
SOLAS and Class regulations requires that tankers have only insulated distribution systems as is intended to reduce the danger from earth fault currents circulating in the hull within hazardous areas which may cause an explosion of the flammable cargo. Exception by regulation is where tanker has a 3.3 kV earthed system which doesn’t extend forward of the engine room bulkhead and into the hazardous area.
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 and can be either a set of indicator lamps or an instrument to show the system insulation resistance value to earth.
Earth indication lamps have been the most common method used for many years, being an inexpensive installation and easy to understand, but unable to indicate the presence of high impedance earth fault.
The earth indication instrument are increasily used nowadays and they connects a small DC voltage to the distribution system. Any resulting DC current is a measure of the insulation resistance of the system.
The earth fault monitor on the switchboard shows the presence of an earth fault on the distribution system, but the engineers must trace the exact location of the fault and remediate it as soon as possible.
The most simple and practical method is to open the circuit breakers feeding the loads, one at a time, and by monitoring the the earth fault instrument while observing which circuit breaker, when tripped, clears the earth fault. The earth fault must be on that particular circuit.
The method of tracing the earth fault is essentially that of continuously breaking down the circuit into smaller and smaller sections until is finally located.
In real life, circuits cannot be randomly disconnected, as some vital service may be interrupted causing the important equipment or even main engine to stop. It is advisable to do this when the vessel is in port or anchorage in safe waters.
Tracing and repairing the earth fault must be carefully coordinated with the operational requirements of the vessel’s electrical services.
When located, the damaged insulation must be repaired and the method depends upon the cause of the earth fault and this is determined by visual examination.
If the insulation has been mechanically damaged or weakened by overheating must be made good again. if surface dirt is the cause, a through cleaning will cure the fault. A lamp fitting that is damaged must be replaced. Dampness in insulation must be dried out by gentle heat and measure must be taken to prevent future ingress of moisture.
If you have any questions regarding above, please feel free to use our existing forum Seafarer’s World and will try to answer to all your queries.
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Source and Bibliography:
- Ship’s Electrical Systems